United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPLRP581

Performance of

Back-Primed and

Factory-Finished

Hardboard Lap Siding

in Southern Florida

Charles Carll

Mark Knaebe

Vyto Malinauskas

Peter Sotos

Anton TenWolde
Abstract 

Because of performance problems with hardboard siding in 

southern Florida, the U.S. Department of Housing and Urban 

Development (HUD) proposed a local standard requiring 

prefinishing of siding and priming of all siding surfaces, 

including the back. However, the effectiveness of these 

practices was questioned. To determine if back-priming or 

factory finishing improves durability and performance of 

hardboard siding, we installed factory-finished and factoryprimed siding on two buildings in southern Florida. The 

buildings were identical except that one had gutters and no 

overhangs and the other had overhangs and no gutters. Half 

the siding was back-primed and half was not. Moisture 

content, temperature, and air pressure difference across the 

siding were continuously monitored for 2 years. Condition 

and thickness of siding boards were recorded every 

3 months. After removal from the buildings, siding was 

inspected and final moisture contents were determined. The 

siding was in excellent condition after about 21⁄2 years of 

outside exposure. There was no evidence that back-priming 

the siding reduced its in-service moisture content. Whether 

the siding was from the overhang building or the guttered 

building did not seem to make a difference, but inspection of 

the windows and final moisture contents of the trim strongly 

suggested that overhangs provided additional protection on 

the gable ends (gutters were only present on the sidewalls). 

Keywords: hardboard, siding, moisture, moisture content, 

air pressures 

February 2000 

Carll, Charles; Knaebe, Mark; Malinauskas, Vyto; Sotos, Peter; 

TenWolde, Anton. 2000. Performance of back-primed and factory-finished 

hardboard lap siding in southern Florida. Res. Pap. FPL–RP–581. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 36 p. 

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

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endorsement by the U.S. Department of Agriculture of any product or 

service. 

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an equal opportunity provider and employer. 

Acknowledgments 

The following people were members of the research team for 

this project: Charles Carll, Research Forest Products Technologist; Mark Knaebe, Chemist; Vyto Malinauskas, General 

Engineer; Joseph Murphy, Supervisory General Engineer; 

Peter Sotos, Physical Science Technician; Anton TenWolde, 

Research Physicist (PI); and Sam Williams, Supervisory 

Research Chemist. The authors also thank Earl Geske and 

Gary Larson for their efforts and expert advice during the 

development, testing, and installation of the data acquisition 

system and for their assistance during data acquisition for 

this project. 

Contents 

Page 

Research Summary ...............................................................i

Introduction.......................................................................... 1

Approach ............................................................................. 1

Siding Selection ............................................................... 1

Building Construction...................................................... 2

Wall Construction............................................................ 4

Siding Finishes................................................................. 4

Siding Installation and Trim ............................................ 5

Instrumented Siding Boards............................................. 5

Interior Conditions........................................................... 5

Instrumentation and Data Acquisition ............................. 7

Site Inspections.............................................................. 11

Siding Removal ............................................................. 11

Laboratory Measurements ............................................. 12

Results From Inspections and Measurements During 

Siding Removal, May 1997 ............................................... 13

Visual Inspection ........................................................... 13

Final Moisture Content of Siding .................................. 13

Final Moisture Content of Plywood Sheathing.............. 16

Inspection of Walls ONa and OWd ............................... 16

Results of Site Inspections................................................. 17

General Observations..................................................... 17

Thickness Measurements............................................... 17

Results of Automated Data Collection............................... 19

Outdoor Humidity and Temperature.............................. 19

Rainfall .......................................................................... 19

Wind Speed and Direction............................................. 20

Air Pressure ................................................................... 20

Moisture Content ........................................................... 27

Results of Laboratory Measurements ................................ 33

Linear Expansion ........................................................... 33

Residual Thickness Swell .............................................. 33

Lateral Nail Resistance .................................................. 34

Discussion.......................................................................... 34

Conclusions........................................................................ 36

References.......................................................................... 36
Research Summary 

The goal of this project was to determine if back-priming or 

factory-finishing improves the durability and performance of 

hardboard siding when installed according to recommended 

practice. The total project length was 34 months with a 

planned data collection period of 24 months. The work was 

performed under a Sponsored Research and Development 

Agreement between the USDA Forest Service, Forest 

Products Laboratory (FPL), and Masonite Corporation. 

In December 1994, hardboard siding from Masonite Corporation’s Laurel, Mississippi, mill was installed on two test 

buildings in Delray Beach, Florida. The buildings were 

identical, except that one was constructed with 0.3-m (12-in.) 

overhangs without gutters and the other with gutters without 

overhangs. The siding boards were installed by an independent contractor’s crew, following manufacturer’s recommendations. Factory-finished and factory-primed (subsequently 

site-finished) boards were installed; half were back-primed 

and half were not, which resulted in four finishing treatments. The condition and thickness of the boards were 

recorded during site inspections every three months. Continuous monitoring of moisture content, temperature, and air 

pressure difference across the siding started in April 1995 

and was terminated on May 5, 1997. The siding was removed and inspected in May 1997. We determined the 

final moisture content of the siding as well as that of the 

wood trim. 

The siding used in this study had better moisture absorption 

and edge swell related properties than the minimum values 

required in the industry standard. This siding was statistically 

indistinguishable from other hardboard siding manufactured 

at the Laurel, Mississippi, plant between 1992 and 1995, but 

the data suggest that it had properties slightly better than the 

average for the other boards produced at that plant from 

1992 to 1995. Our conclusions therefore cannot be extended 

to other hardboard siding without additional information 

about the properties of that siding. 

The siding was in excellent condition after 21⁄2 years of 

exposure to southern Florida weather. The moisture content 

of the siding remained at about 8% or lower, and in-place 

thickness swell of the siding was less than 3% during the 

entire exposure. There was no appreciable difference in 

lateral nail resistance between unexposed boards and siding 

that was on the building for 21⁄2 years. There is no evidence 

that back-priming this siding lowered in-service moisture 

content. Siding on south-facing walls tended to be drier, but 

the type of wall construction or weather barrier had no 

detectable effect on moisture content of the siding. 

There was no difference in siding moisture content between 

the two buildings. The moisture content of the trim, however, was generally lower on the overhang building. The 

overhangs also significantly reduced water leakage around 

windows. There was evidence of water leakage behind the 

window trim below more than half of the windows on the 

guttered building. The leakage occurred between the window 

unit and the bottom trim, even though the trim was carefully 

caulked with high-quality urethane caulk. In May 1997, the 

moisture content of the wood trim was generally between 

9% and 12%, but we found several locations with moisture 

contents more than 20%. The leakage did not lead to decay 

of the siding or elevated siding moisture content. We found 

decay in a pine doorjamb on the guttered building. 

The pressure data in this report indicate that conventionally 

installed lap siding allows for substantial air pressure equalization across the siding. The argument in favor of an air 

space behind the siding may still be valid but primarily 

because it probably provides better drainage of water that 

may penetrate to the back of the siding (especially around 

windows and doors). An air space would provide less opportunity for this water to penetrate the weather barrier and wet 

the sheathing. An air space may also increase the rate of 

drying. 

There was evidence of air leakage past the top plate into the 

wall cavity. This air bypassed the weather barrier and created 

larger than expected air pressures across the gypsum board. 

This air leakage led to periods when the cavity was pressurized with respect to both the inside and outside. Finally, we 

found that wind-induced air pressures across the exterior 

walls were predominantly exfiltrative (that is, inside pressure 

higher than outside pressure), even on the windward side of 

the building. 

Performance of Back-Primed and 

Factory-Finished Hardboard 

Lap Siding in Southern Florida 

Charles Carll, Research Forest Products Technologist

Mark Knaebe, Chemist

Vyto Malinauskas, General Engineer

Peter Sotos, Physical Science Technician

Anton TenWolde, Research Physicist

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

Performance problems with hardboard siding have been 

reported in southern Florida. Investigations have shown that 

most of these problems were due to improper installation or 

design, lack or deterioration of caulking, improper flashing, 

or lack of maintenance (HUD 1992) (Keplinger and 

Waldman 1988, unpublished data, available from HUD, 

Washington, DC). In response to these problems, the 

Department of Housing and Urban Development (HUD) 

promulgated a Local Acceptance Standard that requires 

prefinishing of the siding and priming of all surfaces 

(including the back surface). However, there was disagreement and uncertainty about the necessity and effectiveness of 

these measures. This study was aimed at resolving this 

uncertainty. 

The goal of this 34-month project was to determine if backpriming or factory finishing improves the durability and 

performance of hardboard siding when installed according to 

recommended practice. The work was performed under a 

Sponsored Research and Development Agreement between 

the USDA Forest Service, Forest Products Laboratory (FPL), 

and Masonite Corporation. 

Approach 

Hardboard 0.2-m (8-in.) lap siding was installed on two test 

buildings in Delray Beach, Palm Beach County, in southern 

Florida. All siding was manufactured at Masonite Corporation’s Laurel, Mississippi, mill. A large number of boards 

were instrumented and monitored for 2 years. There were 

four finish treatments consisting of combinations of backprimed versus not back-primed and factory prefinished 

versus site finished. The siding was exposed for approximately 29 months. 

Temperatures, moisture contents (MCs), and time of wetting 

were monitored. In addition, information on wind and air 

pressures was collected to assess the potential for wetting by 

wind-driven rain. During regular site inspections, the condition of each board was noted and the thickness of the bottom 

edge was measured. Samples of the siding were used to 

determine linear expansion and residual thickness swell. At 

the time of siding removal, we cut a large number of samples 

and determined their final MC gravimetrically. At that time, 

we also thoroughly inspected the back of the siding as it was 

removed, as well as windows and other building details. We 

measured final MC of the trim with an electric moisture 

meter. The test buildings were demolished shortly after 

siding removal. 

Siding Selection 

All boards used in this study were provided by one manufacturer (Masonite Corporation) and came from a single mill 

(Laurel, Mississippi). To establish whether these boards were 

typical for boards manufactured at this mill, we compared 

our measurements of residual thickness swell, linear expansion, and lateral nail resistance to data from limited independent third-party random sampling and testing of siding 

produced from the same mill from 1992 through 1995. Our 

test procedures are described later in this report. The data are 

summarized in Table 1. 

The siding used in this study, as well as the other siding 

manufactured at the plant, had properties that were substantially better than the minimum values required in the industry 

standard. The variation in the measured properties was such 

that the siding used in this study is statistically indistinguishable from the sample of boards manufactured at the same 

plant between 1992 and 1995. However, the data suggest that 

our siding had properties slightly better than the average for 

boards produced at that plant from 1992 to 1995. 
2 

Table 1—Comparison of test results on boards used in this study, test results on boards manufactured at the 

same mill (Laurel, Mississippi), and industry minimum standards 

Number of specimens 

Residual 

swell 

Linear 

expansion 

Lateral nail 

resistance in 

machine 

direction 

Lateral nail 

resistance in 

transverse 

direction 

Residual 

swella

, average 

(and range) 

(%) 

Linear expansionb

, average 

(and range) 

(%) 

Lateral nail resistance, average 

(and range) 

(kN [lb]c

) 

Boards used in 

this study, 1995 

48 24 0 64 4.3 (1.6–7.4) 0.23 (0.21–0.27) 2.0 (1.6–2.3) 

[441 (360–518)]d

Boards from 

Laurel mill, 1992e

3 3 9 9 6.0 (4.0–8.7) 0.24 0.25) 2.7 (2.6–2.8) 

[607 (585–621)] 

Boards from 

Laurel mill, 1993e

3 3 9 9 3.6 (2.0–4.5) 0.32 (0.27–0.38) 2.2 (2.0–2.3) 

[503 (458–526)] 

Boards from 

Laurel mill, 1994e

3 3 9 9 9.8 (5.0–13.2) 0.26 (0.24–0.30) 2.5 (2.4–2.7) 

[576 (546–598)] 

Boards from 

Laurel mill, 1995e

4 4 12 12 6.2 (3.6–8.4) 0.23 (0.18–0.25) 2.0 (1.8–2.1) [455 

(412–482)] 

Industry standardf

 Not 

specified 

Not 

specified 

Not 

specified 

Not 

specified 

20 maximum 0.40 maximum 0.67 minimum 

[150 minimum] 

a

Weatherability of substrate test. 

b

30% to 90% RH, machine direction. 

c

Data are for measurements in machine direction and transverse direction, except for FPL data, which were made in transverse 

 direction only. 

d

FPL lateral nail tests were performed according to ASTM D1037 part A, with a 3.2-mm- (0.125-in.-) diameter pin. The other lateral nail 

 tests were performed perpendicular to machine direction according to ANSI/AHA A135.6, with a 3.3-mm- (0.131-in.-) diameter pin. 

 These differences make it difficult to directly compare FPL results with the other results. 

e

Test samples from the Laurel, Mississippi, mill were collected and tested by an independent third party. 

f

ANSI/AHA A135.6-1990 minimum requirements. 

Table 2—Equilibrium moisture content (EMC) at different relative 

humidity (RH) levels

RH 

(%) 

EMC of hardboard 

sidinga

 (%) 

EMC of southern 

pineb (%) 

EMC of construction 

plywoodb (%) 

30 3.9 5.3 4.8 

43 4.7 7.0 6.3 

65 6.8 10.8 9.7 

79 8.9 14.8 13.4 

90 10.4 20.0 18.5 

a

Sorption only. 

b

Richards and others (1992). 

We measured equilibrium moisture content (EMC) of the 

hardboard siding (Table 2). The siding had lower EMC than 

solid wood or plywood at corresponding relative humidity 

(RH) conditions. Values for hardboard in Table 2 are in 

general agreement with previously published values 

(Bristow and Back 1969) for heat-treated hardboards. 

Building Construction 

One building was built with 0.3-m (12-in.) overhangs, 

including the gable ends, but without gutters (the overhang 

building; walls in this building are designated with an O). 

The other building was built without overhangs but with 

gutters (the guttered building; walls in this building are 

designated with a G) (Figs. 1 and 2). The buildings were 

identical in all other construction details. Dimensions of the 

buildings were 9.8 by 9.8 m (32 by 32 ft) with a slab-ongrade foundation. The top of the slab was about 0.10 to 

0.15 m (4 to 6 in.) above grade. The buildings had balloonframed gable ends with a 4/12 pitch roof with asphalt shingles. The rest of the roof was framed with standard roof 

trusses. One gable end faced NNE (35° from north). The 

attic of each building was vented with gable end louvers, and 

the attic of the overhang building received additional venting 

from perforated aluminum soffits. Ceilings and exterior 
3 

Figure 1—Test building with overhang (foreground) and test building with gutters (background). 

Figure 2—Test building with gutters (side view). 
4 

walls were insulated with fiberglass batt insulation (unfaced 

in the walls, faced in the ceiling). The buildings had interior 

electrical wiring, phone service, and air ducts for distribution 

of cooled air, with ceiling fans for additional air distribution. 

The air ducts were within the conditioned interior (that is, 

not in the attic), and there was no evidence that the air conditioner or distribution system pressurized or depressurized the 

interior to any significant degree. 

Wall Construction 

The walls were wood-framed, standard 38- by 89-mm (nominal 2- by 4-in.) construction. For both buildings, each side of 

the building was divided into four wall sections, designated 

a, b, c, and d. Three different wall construction techniques 

were used on each building: 

• no sheathing, 15-lb felt building paper (OF); 

• plywood sheathing (0.5-in. CDX), 15-lb felt (PF) 

• plywood sheathing, woven polyolefin (Barricade, 

Simplex Products Division, Adrian, Michigan) (PT) 

Each wall section was 2.4 m (8 ft) long. Adjacent wall sections were separated with a chromated copper arsenate 

(CCA)-treated pine standard 38- by 140-mm (nominal 

2- by 6-in.) stud sandwiched between the end studs of the 

wall sections, except at the corners. The outside face of the 

treated 2 by 6 separator protruded beyond the face of the 

siding as can be seen in Figure 2. The separators (or air 

dams) prevented air movement behind the siding between 

wall sections. The outside corner walls were joined with 

standard vertical outside-corner trim consisting of western 

redcedar strips (Fig. 2). Wall framing was installed so that 

the outside surface of siding was flush; on wall sections 

without plywood sheathing, this was achieved by moving the 

framing outward by 13 mm (0.5 in.). Wall construction on 

the gable ends was identical to that of the wall sections 

directly below. 

Two doors and twelve windows were installed in each building. The doors were prehung steel-door units with pine (not 

pressure treated) jambs. Exterior trim for these units was 

site-fabricated from western redcedar strips. The windows 

were aluminum single-hung (lower sash sliding vertically) 

units with side nailing flanges (the top and bottom flanges 

were narrow and unsuitable for fastening the units in place). 

These were installed over the sheathing or framing, sometimes in a bed of caulk. The weather barrier (felt or polyolefin) was applied after window installation, generally over the 

side flanges, but just missing the narrow top and bottom 

flanges; in some cases, the weather barrier below the window was caulked to the framing or sheathing. Rectangular 

trim units were site-fabricated from western redcedar, which 

were then installed over the flanges. The piece of cedar at the 

bottom of the trim unit was sloped to the exterior. The trim 

unit was caulked to the window unit with urethane caulk. 

Joints in the trim unit were also caulked. Aluminum head 

flashing was installed over the top cedar trim piece above all 

window and door units. An additional sheet of weather 

barrier was installed over the upper leg of the aluminum 

head flashing. 

Figure 3 shows the orientation of the buildings, designation 

given to each wall section, and distribution of construction 

types around the building. As built, the buildings were rotated 180 degrees relative to the orientation specified in the 

plans. Thus, wall sections designated N actually faced 

southwest, sections designated S faced northeast, sections 

designated E faced northwest, and sections designated 

W faced southeast. 

Siding Finishes 

All siding was embossed Masonite 0.2-m (8-in.) hardboard 

lap siding. All siding was factory-primed on the front side. 

Topcoat finishes were factory-applied or applied on-site. 

FPL personnel primed the back surface of roughly half the 

boards with an oil-based primer in an unconditioned warehouse in Delray Beach about 2 months prior to installation. 

The primer spread rate was 7.6 m2

/L (308 ft2/gallon). The 

rest of the boards were left unprimed, although all siding had 

some incidental paint on the back side, which was how it 

was received from the mill. The following siding finishing 

combinations were installed: 

• back-primed, factory finished (BF) 

• not back-primed, factory finished (NF) 

• back-primed, site finished (two coats) (BS) 

• not back-primed, site finished (two coats) (NS) 

Figure 3—Orientation of test buildings, with wall construction type and location of pressure taps (N, S, E, W 

designate direction; a, b, c, d designate wall section; PF, 

plywood sheathing with felt; OF, felt without sheathing; 

PT, plywood sheathing with polyolefin weather barrier). 
5 

Site-applied finish was applied within 10 days of siding 

installation. Wet application weight of site-applied finish 

averaged approximately 190 g/m2

 (0.039 lb/ft2). This was 

equivalent to a spread rate of approximately 6.6 m2

/L 

(270 ft2

/gallon) of paint. We used Porter Paint (Louisville, 

Kentucky) acrylic gloss exterior paint, applied by brush by a 

professional painter. The paint color was similar to that of 

the factory-finished siding. Care was taken that good coverage was obtained, and a mirror was used to inspect the drip 

edge to ensure full coverage. 

Siding Installation and Trim 

The siding was installed on the test buildings during the last 

week of November and first two weeks of December 1994. 

The guttered building was sided first. The contractor’s carpenter crew installed the siding, but instrumented boards 

were installed under the direction and with assistance of FPL 

personnel. Siding was face-nailed with hand-driven galvanized nails. Any damage from installation was repaired. The 

minimum overlap between boards was 25.4 mm (1 in.). 

Siding crosscuts were left unpainted, and their location on 

the building was marked (uncut primed edges are marked P 

in Figs. 4–11). Joints between separators and siding were 

caulked with urethane caulk. (The detail resembled a standard inside corner detail.) Window, door, and outside-corner 

trim were installed before the siding. All joints between 

siding and trim were caulked with urethane caulk. Gable end 

cuts of the siding were protected from the weather. On the 

overhang building, they were covered by aluminum soffit, 

and on the guttered building, they were covered with 

redcedar gable end trim. The metal roof edge covered the top 

edge of the gable end trim; the gable end trim was slid up 

under the roof edge and nailed in place. The drip edge of the 

bottom siding course was about 0.10 to 0.15 m (4 to 6 in.) 

above grade and had a metal termite shield below it. 

The four different combinations of siding finish treatments 

were alternated vertically. A different treatment was used on 

the bottom of each wall section, and the same sequence was 

used moving up. This sequence is shown in Table 3. Table 4 

shows which types of boards were installed as bottom boards 

in which wall unit. 

As siding pieces were installed, end-cut pieces were saved 

for determining siding MC as installed. Disks were cut from 

these pieces with a hole saw, loose fibers were removed by 

sanding, and the disks were weighed on a digital balance. 

The disks were transported to FPL, ovendried, and again 

weighed with a digital balance. Moisture content of all 

specimens at installation ranged between 5% and 7% 

(Fig. 12). Back-primed specimens were consistently somewhat drier at the time of installation; all were under 6% MC. 

Instrumented Siding Boards 

A total of 46 siding boards were monitored on each building. 

These boards were placed in strategic locations to represent a 

variety of exposures. Placement was identical on both buildings, except for two short boards that had been cut too short 

for their intended location on the guttered building. These 

two boards were moved to other locations on the same or 

adjacent wall section. Most instrumented boards were located in potential moisture stress locations. Potential moisture stress locations were abutting a window or door, bottom 

of wall (backsplashing of rainwater), and top of sidewalls. 

Boards at the bottom and top of the wall ran the full width of 

the wall section (2.4 m (8 ft)), whereas the boards abutting 

windows and doors were considerably shorter. Instrumented 

boards were distributed around the building as follows 

(Fig. 4–11): 

• Gable ends (total of 12 sites per wall) 

• 6 window sites (short) 

• 2 door sites (short) 

• 2 bottom sites (long) 

• 2 top sites (long) 

• Sidewalls (total of 11 sites per wall) 

• 6 window sites (short) 

• 4 bottom sites (long) 

• 1 top site (long) 

The distribution of instrumented boards was such that all 

board finish combinations were represented on each side of 

the building, the distribution of board finish combinations by 

type of site (window–door, top, or bottom) was as even as 

possible, and the distribution of board finish combinations by 

type of wall construction was as even as possible. 

Interior Conditions 

Interior conditions were selected to simulate a typical southern Florida home environment. The thermostats controlling 

the air conditioners in both buildings were set at 24°C 

(75°F). Measurements indicate that the guttered building 

actually was maintained between 25°C and 28°C (77°F and 

82°F), and the overhang building between 22°C and 26°C 

(72°F and 79°F). Because the buildings were unheated, 

indoor temperature was uncontrolled for short periods during 

winter. Indoor RH was maintained at 50% with humidistatcontrolled humidifiers. Water for the humidifiers was obtained from the air conditioner drip pans. Excess water from 

the drip pans was drained to the outside through sub-slab 

drains. Humidity occasionally rose above 50% during winter 

periods when the air-conditioners did not run. 
6 

Figure 4—Overall layout of southwest-facing wall (wall sections GNa–GNd) of guttered building. 

Figure 5—Overall layout of northeast-facing wall (wall sections GSa–GSd) of guttered building. 
7 

Instrumentation and Data Acquisition 

The instrumentation monitored the following parameters on 

an hourly basis: 

• moisture content of the siding 

• time of wetness (presence of liquid water on surface), 

front and back of the siding 

• temperature of siding 

• outdoor temperature and RH 

• indoor temperature and RH 

• wind speed 

• wind direction 

• humidifier run-time 

• humidifier water supply status 

Full-length siding boards (18 per building) had the 

following sensors (Fig. 13): 

• 2 thermocouples 

• 3 pairs of MC pins 

• 4 time-of-wetness (TOW) sensors (2 on the front, 2 on the 

back surface) 

Short boards (next to windows and doors, 28 per building) 

had the following sensors (Fig. 13): 

• 1 thermocouple 

• 1 pair of MC pins 

• 2 TOW sensors (1 on the front, 1 on the back surface) 

Figure 6—Overall layout of northwest-facing wall (wall sections GEa–GEd) of guttered building. 

Figure 7—Overall layout of southeast-facing wall (wall sections GWa–GWd) of guttered building. 
8 

Figure 8—Overall layout of southwest-facing wall (wall sections ONa–ONd) of overhang building. 

Figure 9—Overall layout of northeast-facing wall (wall sections OSa–OSd) of overhang building. 
9 

The total numbers of sensors per building were as follows: 

• 64 thermocouples 

• 82 pairs of MC pins 

• 128 TOW sensors 

• 1 weather station (temperature and RH) 

• 1 indoor RH and temperature sensor 

Type T thermocouples were embedded in the boards, 

through holes drilled from the back of the board. After installation, the holes were caulked. Thermocouples were connected directly to a computer data acquisition card. 

The insulated MC pins were installed in predrilled holes and 

carefully caulked to prevent water entry into the holes. After 

attachment of the leads, the pins were covered with silicone 

and urethane caulking for protection from the rain. The pins 

were calibrated at FPL for the specific siding used in this 

study. Calibration was achieved by preconditioning the 

hardboard at various MCs, measuring MC with pins, and 

then determining actual MC gravimetrically. Leads were 

connected to linear amplifiers, which in turn fed into computer data acquisition cards. 

The Forest Products Laboratory developed TOW sensors 

especially for this study. The TOW sensors indicate when 

Figure 10—Overall layout of northwest-facing wall (wall sections OEa–OEd) of overhang building. 

Figure 11—Overall layout of southeast-facing wall (wall sections OWa–OWd) of overhang building. 
10

Table 3—Sequence of siding boards, by treatment, as 

installed on the buildings starting from the bottoma

Back-priming Finish Code 

Yes Site BS 

No Site NS 

Yes Factory BF 

No Factory NF 

a

The board used to start at the bottom of each wall varied, 

 but the sequence moving up the wall was the same. 

Table 4—Type and location of bottom starting siding 

boards in each wall 

Wall 

Wall 

section 

Board 

type Instrumented 

Southwest Na NS Yes 

Southwest Nb NF No 

Southwest Nc BF No 

Southwest Nd BS Yes 

Southeast Wa NF Yes 

Southeast Wb NS Yes 

Southeast Wc BS Yes 

Southeast Wd BS Yes 

Northeast Sa NS Yes 

Northeast Sb NF No 

Northeast Sc NS No 

Northeast Sd BF Yes 

Northwest Ea BF Yes 

Northwest Eb NF Yes 

Northwest Ec BS Yes 

Northwest Ed BF Yes 

the surface is wet, either from rain or dew. The sensor consisted of a pair of parallel lead alloy foil strips that were 

attached to the surface. Each strip was approximately 10 by 

51 mm (3/8 by 2 in.), and the distance between the strips was 

approximately 0.4 mm (1/64 in.). Connection wires, attached 

to each strip with silver solder epoxy, connected the TOW 

sensor to an oscillating circuit. When liquid water between 

the two strips reduced the electric resistance between the 

strips below a critical value, the oscillating circuit produced 

a signal that was recorded hourly with the data acquisition 

system. The TOW sensors were placed on the front and back 

surface of instrumented boards. The strips on the front surface were placed horizontally at mid-height (Fig. 13). Foil 

strips on the back surface were oriented vertically, with the 

lower ends of the strips ending 13 mm (0.5 in.) above the 

drip edge so that there would be at least 13 mm (0.5 in.) of 

sensor in the siding overlap (Fig. 13). In October 1996, we 

checked the TOWs on the exterior surface by spraying them 

with water. The TOWs that were not functioning were disconnected. The TOWs on the back surface of the siding 

Figure 12—Initial moisture content of the siding at time 

of installation, by finish type. 

Figure 13—Location of instrumentation on instrumented boards on full-length 

and short boards (1 ft = 0.3048 m, 1 in. = 25.4 mm). 

4

5

6

7

Moisture content (%)

BS NS BF NF
11 

that we suspected of malfunctioning were checked in a similar manner in May 1997, just before removal of the siding. 

However, testing of the TOW sensors in the laboratory in 

high RH and temperature after the conclusion of monitoring 

revealed that some TOW sensors registered “wet” at high 

RH without the presence of liquid water. We therefore cannot trust the data from the TOW to always indicate liquid 

water and decided to reject the data. However, we have since 

found a method to avoid the problem of false positive readings and intend to use this sensor in the future. 

Air pressure differences across the siding were monitored 

continuously at eight locations on each building. The location of the pressure taps is indicated in Figure 3. The pressure tubes were installed at mid-height of the wall section 

(that is, approximately 4 ft (1.2 m) from the ground), which 

minimized the inclusion of any stack effect in the pressure 

readings. Pressure at each location was measured with an 

individual differential pressure sensor, which had a range of 

±200 Pa with a resolution of 0.1 Pa (1 Pa = 1.45×10–4 lb/in2

). 

The zero-pressure offset was recalibrated every hour. Pressure differences were recorded continuously every 5 s, with 

pressure data collection suspended for roughly 5 min each 

hour for reading MC pin, TOW, and thermocouple data (the 

computer in the guttered building also collected wind speed 

and direction data). The detailed pressure data were usually 

discarded every hour after an hourly average, maximum, 

minimum, and standard deviation were computed for each 

pressure tap. However, we instructed the computer by phone 

several times to store and transmit the detailed pressure data. 

During the early months of data collection, we experienced 

problems with insects blocking pressure tubes and with 

malfunction of the pressure sensor hardware. After these 

problems were corrected, reliable pressure data were collected on the guttered building starting mid-September 1995 

and on the overhang building starting early February 1996. 

On October 8, 1996, the pressure tap configuration in the 

overhang building was changed as shown in Table 5. This 

change allowed us a more detailed look at the pressures 

inside the cavities and pressures across the entire wall of 

wall sections OWd and ONa. Many of these data were collected at 15-s intervals. The pressure taps in the guttered 

building were left unchanged. 

Hourly wind speed and direction data were collected starting 

in the middle of September 1995. The orientation of the wind 

vane was verified at the time of installation by the position of 

the sun at local solar noon. Both wind speed and direction 

were recorded as 10-min averages at the beginning of each 

hour. Instantaneous direction at the end of each 10-min 

period was also collected. On March 2, 1996, the wind vane 

failed, and on June 20, the anemometer also failed. No reliable wind speed and direction data were received between 

June and early October, when a new wind measurement 

system was installed. 

All hourly and wind data were collected and stored on a 

personal computer (one for each building) and transferred 

automatically to FPL in Madison, Wisconsin, by phone each 

day. The data acquisition system was installed and activated 

in February 1995. After some adjustments and corrections 

were made, data acquisition officially commenced on May 1, 

1995, and ended on May 6, 1997. 

Precipitation was measured manually with an on-site rain 

gauge, generally at weekly intervals. These data were 

periodically mailed to FPL. 

Site Inspections 

The siding condition was visually inspected every 3 months. 

Thickness of the lower left and right edges of each siding 

board was measured in-place during each visit, using a specially designed tool (Fig. 14). At each inspection, photographs were taken to document any visual changes in the 

siding. In total, 10 site inspections were performed between 

December 1994 and May 1997. The last inspection took 

place just before the siding was removed in May 1997. 

Siding Removal 

On May 5 and 6, 1997, prior to removal of the siding, we 

measured the MC of all wood trim, except the fascia of the 

guttered building, with an electric resistance moisture meter. 

On May 6 to 9, 1997, the siding was removed and visually 

inspected. Water stains were noted and photographs taken. 

For each building, we cut and weighed 128 0.2-m- (8-in.-) 

wide siding specimens (eight per wall section) for MC 

determination. However, three specimens were lost, so we 

ended up with 126 samples from the guttered building and 

127 samples from the overhang building. The samples were 

ovendried at FPL and weighed again to determine MC. In 

addition, we cut three 25-mm (1-in.) horizontal slices from 

ten 0.10-m- (4-in.-) wide samples for determining vertical 

MC gradients. An additional sixty-four 0.08-m- (3-in.-) wide 

specimens (four per wall section) were collected from each 

Table 5—Pressure tap configuration in overhang 

building before and after changes in configuration 

on October 8, 1996 

Sensor 

number 

Pressure before 

October 8, 1996 

Pressure after 

October 8, 1996 

1 Across siding of wall ONd Between cavity and 

outside, wall ONa 

2 Across siding of wall OWa Across entire wall OWd 

3 Across siding of wall OWd Unchanged 

4 Across siding of wall OSa Between cavity and 

outside, wall OWd 

5 Across siding of wall OSc Unchanged 

6 Across siding of wall OEa Unchanged 

7 Across siding of wall OEd Across entire wall ONa 

8 Across siding of wall ONa Unchanged 
12

building for measuring lateral nail resistance. We also cut 

several circular specimens out of the plywood sheathing for 

MC determination. On May 9, we opened the Na and Wd 

wall sections of the overhang building for inspection. These 

were the wall sections for which we collected detailed air 

pressure data. Finally, two windows from each building (one 

from the southwest-facing wall and one from the northeastfacing wall) were cut out of the wall and removed with the 

wall sections attached for later disassembly and inspection. 

Laboratory Measurements 

Linear Expansion 

In early November 1994, samples from the lot of siding used 

on the buildings were selected by FPL personnel and transported from Delray Beach to FPL in Madison for determining linear expansion (LE) characteristics. These were kept in 

storage until late spring of 1995, at which time LE measurements were made in general accordance with sections 107 to 

110 of ASTM D 1037 (ASTM 1996). Deviations from D 

1037 were as follows: (a) linear expansion was measured 

between equilibrium at 30% and 90% RH as specified in 

ANSI/AHA A135.6-1990 (AHA 1990), (b) temperature at 

each of these conditions was 26.7°C (80o

F) rather than 

21.9°C (70o

F), and (c) measurement was made across 

0.25 m (10 in.). 

The device used to measure length consisted of two magnifying eyepieces with crosshairs and a traveling stage equipped 

with a linear variable differential transducer (LVDT) with 

digital readout. Reference marks were placed on the specimen by placing scratch marks on the heads of short brass 

pins that were driven into predrilled holes approximately 

0.25 m (10 in.) apart. Test specimens were placed on the 

traveling stage and weighted with a lead weight to prevent 

movement of the specimen relative to the stage. A measurement was made by adjusting the stage until one of the crosshairs lined up with one of the scratch marks, zeroing the 

LVDT, then moving the stage until the other crosshair lined 

up with the other scratch mark, and reading the stage movement from the digital readout. 

There were 24 linear expansion specimens, six specimens 

from each of the four finish treatments (NF, BF, NS, and 

BS). Because the sample boards were collected prior to 

installation (and thus prior to site finishing), specimens 

representing treatments NS and BS did not actually have a 

finish topcoat. Within each finish treatment, specimens were 

selected at random. With the room maintained at 30% RH, 

the specimens rapidly reached equilibrium after which the 

first set of measurements was made. With the room maintained at 90% RH, time to reach equilibrium was substantially longer, and measurements were made after weight gain 

from vapor sorption became negligible. Specimens were 

weighed at both equilibrium conditions. After the LE measurements were completed, specimens were ovendried and 

MCs at each RH condition were determined. 

Residual Thickness Swell 

From the same sample boards transported from Delray 

Beach to FPL in early November 1994, specimens were 

prepared and tested for residual thickness swelling as per 

section 4.1 of ANSI/AHA A135.6-1990 (Weatherability of 

Substrate). Tests were performed during the summer of 

1995. Forty-eight specimens were tested, 12 randomly selected specimens from each of the four finish treatments. As 

with linear expansion tests, specimens representing treatments NS and BS did not actually have a finish topcoat. 

Because of size limitations of the soaking tank, the specimens were tested in two batches, with 24 specimens in each 

batch. 

Deionized water was used in the soak tank. The tank was 

equipped with a float valve to maintain 25-mm (1-in.) immersion depth of the lower edge during the 18.5 h of soaking. Immersion depth was checked roughly 7 h into each 

soak period, with readjustment of the float valve made as 

necessary. Immersion level and water temperature were 

maintained within the limits specified in ANSI/AHA 

A135.6–1990. The only minor deviations from the 

ANSI/AHA specifications occurred when cycling was 

stopped during the weekend after the fourth or fifth cycle 

and when samples were stickered at ambient temperature 

during the weekend rather than put in a plastic bag, as 

Figure 14—Thickness measurement device. 
13 

specified in the ANSI/AHA standard. We believe that these 

slight irregularities were unlikely to appreciably affect the 

results. 

Lateral Nail Resistance 

We performed lateral nail resistance (LNR) tests to determine if exposure on the test buildings changed the resistance 

of the boards to lateral nail movement. We speculated that 

the drip edge was probably more subject to degradation than 

any other part of the panel, and therefore, we performed 

LNR tests near the drip edge. The standard procedure for 

performing this test is outlined in sections 41 to 46 of ASTM 

D 1037. We used ASTM D 1037 procedures for our tests, 

with the following exceptions: (a) We used a tempered high 

carbon steel pin of 3.2-mm (0.125-in.) diameter rather than a 

common 6d nail, because of the tendency of the 6d nail to 

bend when testing hardboard (see Note 17 of ASTM D 

1037). (b) All specimens were tested with the loading stress 

perpendicular to the long dimension of the siding boards (so 

as to pull the pin downward towards the drip edge). 

ASTM D 1037 stipulates that half of the test specimens be 

tested in this direction and half in the perpendicular direction 

to determine whether the boards have directional properties. 

(c) We did not test any specimens in the soaked condition. 

Our test procedure was also slightly different from the procedure described in Standard ANSI/AHA A135.6 for lateral 

nail tests for hardboard siding. The ANSI/AHA procedure is 

based on ASTM D 1037 but prescribes an 8d nail (3.3-mm 

or 0.131-in. diameter) rather than a 6d nail (2.9-mm or 

0.113-in. diameter) and a loading speed of 3.2 to 

4.4 mm/min (0.125 to 0.175 in/min) rather than 6.4 mm/min 

(0.25 in/min). These differences make it difficult to compare 

our results with those produced according to ANSI/AHA. 

Test specimens were conditioned to equilibrium at 23°C 

(74°F) and 65% RH prior to test. Tests were done at pin 

spacing (from panel edge) of 6, 9, and 12 mm (1/4, 3/8, and 

1/2 in.). Three test specimens were cut from each of 193 

larger samples to provide one specimen for each test. In this 

manner, we collected three sets of 64 specimens from each 

of the buildings, plus three sets of 64 specimens from unexposed boards. The specimens represented each of the finishing treatments (BF, BS, NF, and NS) from each of the 16 

wall sections. Inadvertently, we collected and tested an 

additional three BS specimens from the overhang building 

and lost a specimen from the unexposed boards, ending up 

with LNR measurements for 578 specimens (191 unexposed, 

192 from guttered building, and 195 from overhang building). Lastly, we found (after testing, tallying, and inspection) 

that the finish system on the 11th course of wall section OEa 

on the overhang building was NS rather than BS. 

Results From Inspections and 

Measurements During Siding 

Removal, May 1997 

Visual Inspection 

In general, we found the siding to be in excellent condition. 

We did not notice any decay of the siding, edge swell, or 

paint failures. In some more shaded locations on the 

buildings, some mildew was apparent on the siding and 

wood trim. 

We found water stains on the back of siding below at least 

half of the windows on the guttered building. In contrast, on 

the overhang building, we did not see any staining of siding 

below windows. On the guttered building, stained siding 

backs were observed below windows in walls GNa, GNc, 

GNd, GSc, GWa, and GWd. Another two windows (GEb 

and GWb) may also have leaked. Water leakage apparently 

occurred between the window unit and the bottom trim or 

lower portions of side trim. This occurred despite careful 

caulking with high quality urethane caulk. Caulk adhesion 

failed at some wood–wood joints (trim) and at some wood–

metal joints (window), which led to the window water leakage. Although the water stains on siding backs indicated that 

water leakage into the siding system had occurred, there was 

no evidence of damage to the siding. Tracing of stains across 

the backs of successive siding courses indicated that the 

water eventually drained to the front of the siding or 

evaporated. 

Four windows (ONc, OSc, GNc, and GSc) were completely 

removed from the buildings, including the adjacent wall 

assembly, and were shipped to Masonite’s West Chicago, 

Illinois, laboratory for disassembly. Evidence was found of 

leakage behind the weather barrier on window ONc (overhang building) and GNc (guttered building). The plywood 

sheathing below window GNc was severely discolored and 

perhaps would have decayed in several years. The staining of 

the plywood under window ONc was less severe. Window 

trim in the lower left corner of window GSc was discolored. 

Window OSc showed no signs of leakage or deterioration. 

Only four windows were inspected in this fashion, so this 

type of leakage or deterioration may have gone undetected 

on other windows where we did not remove the weather 

barrier. 

We found decay in the lower jamb of the door in wall GNb 

(guttered building) but not in any other doorjambs or trim. 

Final Moisture Content of Siding 

We gravimetrically determined the MC of samples cut from 

the siding at the time of siding removal (May 1997). The 

126 samples from the guttered building varied between 5% 

and 8% MC, with an average MC of 6.4%. This corresponds 

with an equilibrium MC at RH between 60% and 70% 

(Table 2). The 127 samples from the overhang building also 
14

varied between 5% and 8% MC, with an average of 6.3%. 

Siding MC of both buildings was essentially the same. These 

averages agree well with the hourly MC data collected by 

computer. The final average MC of the siding was 6% to 7% 

for both buildings. 

To assess the influence of wall orientation and finishing 

type, we sorted the data by direction and finishing type. In 

most cases, this significantly reduced the standard deviation 

in the data. The results are tabulated in Table 6. 

Effect of Wall Orientation 

We determined if there was a statistically significant influence of wall orientation by applying the t-test to differences 

in average MCs in Table 6, with a confidence coefficient of 

0.95. Table 7 shows the results for the effect of orientation. 

The following conclusions can be drawn about the effect of 

wall orientation on siding MC at time of removal: 

• The effect of wall orientation on final siding MC was 

pronounced and masked the effect of other parameters. 

• On both buildings, boards facing southwest were drier 

than those facing northeast at time of removal. 

• On the overhang building, and probably also on the guttered building, boards facing southeast were drier than 

those facing northeast at time of removal. 

• On the overhang building, boards facing southwest were 

drier than those facing northwest at time of removal. 

Effect of Finish Treatment Type 

We determined the effect of the finish treatment type with 

the same statistical approach we used to determine the effect 

of wall orientation. The results are tabulated in Table 8. The 

effect of finish treatment type on average MC at the time of 

removal was not as strong as the effect of wall orientation. 

Boards without back-priming tended to be drier than backprimed boards, especially on the overhang building. Significantly, at the time of removal, back-primed boards were 

never drier than boards without back-priming facing in the 

same direction. 

Effect of Wall Construction 

The data in Table 6 suggest that siding over walls with felt 

and without sheathing (OF) was slightly drier at the time of 

removal than siding over the other two wall construction 

types, but statistical analysis revealed no significant differences in final MC of siding over the three types of wall 

construction. 

Table 6—Average moisture content (MC) of siding at the time of removal from building, by finish treatment type, 

wall construction, and orientation 

Finish treatment Average MC (%) [standard deviation (%)/number of specimens] 

or wall construction typea

 East (northwest) North (southwest) South (northeast) West (southeast) All 

Guttered building 

BF boards 7.16 [0.22/8] 6.11 [0.35/8] 7.71 [0.16/8] 5.96 [0.24/9] 6.71 [0.77/33] 

BS boards 6.99 [0.30/8] 6.00 [0.34/8] 7.71 [0.24/7] 5.96 [0.33/7] 6.65 [0.78/30] 

NF boards 6.14 [0.23/7] 5.63 [0.52/9] 7.20 [0.40/8] 5.72 [0.82/8] 6.10 [0.76/32] 

NS boards 6.20 [0.47/9] 5.44 [0.38/7] 7.00 [0.16/8] 5.60 [0.74/7] 6.06 [0.77/31] 

PF walls 6.61 [0.48/16] 5.84 [0.35/16] 7.31 [0.33/7] 5.62 [0.29/8] — 

PT walls 6.89 [0.51/8] 6.16 [0.43/8] 7.41 [0.46/16] 6.38 [0.58/8] — 

OF walls 6.39 [0.64/8] 5.36 [0.42/8] 7.23 [0.43/8] 5.53 [0.47/15] — 

All 6.62 [0.56/32] 5.80 [0.48/32] 7.35 [0.41/31] 5.77 [0.59/31] 6.38 [0.83/126] 

Overhang building 

BF boards 7.14 [0.33/8] 6.28 [0.24/8] 7.56 [0.17/8] 6.21 [0.27/8] 6.80 [0.63/32] 

BS boards 6.93 [0.18/7] 6.09 [0.28/7] 7.34 [0.17/8] 6.05 [0.26/8] 6.61 [0.60/30] 

NF boards 6.52 [0.31/8] 5.27 [0.37/8] 6.89 [0.19/8] 5.48 [0.27/8] 6.04 [0.74/32] 

NS boards 6.26 [0.32/9] 5.17 [0.44/8] 6.70 [0.34/8] 5.48 [0.42/8] 5.91 [0.71/33] 

PF walls 6.81 [0.42/15] 5.87 [0.51/16] 7.14 [0.35/8] 5.70 [0.40/8] — 

PT walls 6.75 [0.46/8] 5.63 [0.58/7] 7.21 [0.39/16] 6.15 [0.45/8] — 

OF walls 6.40 [0.41/8] 5.36 [0.64/8] 6.39 [0.46/8] 5.68 [0.38/16] — 

All 6.69 [0.46/32] 5.69 [0.60/31] 7.12 [0.41/32] 5.80 [0.45/32] 6.33 [0.77/127] 

a

BF, back-primed, factory finished; BS, back-primed, site finished; NF, no back-priming, factory finished; NS, no back-priming, 

 site finished; PF, plywood with felt; PT, plywood with polyolefin weather barrier; OF, felt, no sheathing. 
15 

Moisture Content Gradient Within Boards 

To determine if there was a vertical moisture gradient 

within the boards at the time of removal, we collected ten 

101.6-mm- (4-in.-) wide specimens, cut three horizontal 

25- by 102-mm (1 by 4 in.) slices from the drip edge up, and 

determined the MC of each slice gravimetrically. The average MC of these specimens was 6.6%, and there was no 

evidence of a vertical MC gradient within boards. 

Moisture Content of Siding Below Windows 

Seven siding specimens were cut from directly below windows, mostly below windows that showed signs of water 

leakage. The average MC of these specimens was 6.9%. One 

water-stained specimen taken from below window GSc 

registered 9% MC. The rest of the specimens were all 6% or 

7% MC (including one water-stained specimen from below 

window GWa), with an average of 6.6%, which is similar to 

the MCs of other siding. 

Table 7—Effect of wall orientation on average MC of siding at time of removal (results of statistical comparison of differences between average MC by finish treatment type (0.95 confidence coefficient))a

Comparison of final siding MC by wall orientation 

Finish treatment SW<NW? NW<NE? SE<NW? SW<NE? SE<SW? SE<NE? 

Guttered building 

BF yes yes 

BS yes yes yes yes yes 

NF yes yes yes yes yes 

NS yes yes 

All types yes yes 

Overhang building 

BF yes yes yes yes 

BS yes yes yes yes yes 

NF yes yes yes yes 

NS yes yes yes 

All types yes Yes 

a

SW, southwest facing; NW, northwest facing; SE, southeast facing; NE, northeast facing; BS, back-primed 

 and site finished; NS, no back-priming, site finished; BF, back-primed, factory finished; NF, no back-priming, 

 factory finished. 

Table 8—Effect of finish treatment type on average MC of siding at time of removal (results of statistical 

comparison of differences between average MC by wall orientation (0.95 confidence coefficient))a

 Comparison of final siding MC by finish treatment 

Wall orientation BS<BF? NF<BF? NS<BF? NF<BS? NS<BS? NS<NF? 

Guttered building 

Northwest yes yes yes 

Southwest 

Northeast yes yes 

Southeast 

All 

Overhang building 

Northwest yes yes 

Southwest yes yes yes yes 

Northeast yes yes yes yes 

Southeast 

All 

a

BS, back-primed and site finished; NS, no back-priming, site finished; BF, back-primed, factory finished; 

 NF, no back-priming, factory finished. 
16

Final Moisture Content of Trim 

Most of the wood trim had MCs between 9% and 12%, as 

indicated by moisture meter measurements taken in May 

1997. One reading was greater than fiber saturation (window 

trim, wall GNc), and a few were more than 20%. These 

included the lower end of a doorjamb on wall GNb 

(23% MC), where decay had been observed, and doorjambs 

on walls GSb (25%) and OSb (22%). Other readings of 20% 

to 23% were near ends of window trim (GNc, GNd, ONd) 

and near one end of the gable end trim above wall ONa 

(20%). All observations of 20% MC or higher were on gable 

ends. Table 9 shows the distribution of all MC readings for 

each building. 

The MC readings of the wood trim on the overhang building 

tended to be lower, with none more than 22% MC. The 

readings on the overhang building also showed less 

variation. 

Since all trim MC readings of 20% or more were on gable 

ends and most were on window or door trim (including pine 

doorjambs), we sorted the moisture meter data for window 

and door trim (including pine doorjambs) by wall type (gable-end wall or eave wall). The data are shown in Table 10. 

With the exception of one reading taken above the decayed 

region in a doorjamb, moisture meter readings on window 

and door trim were taken in identical locations on each 

building. Therefore, paired t-tests could be performed on the 

data summarized in Table 7 (pairing by individual location) 

to compare trim MCs between buildings. On gable-end 

walls, the difference in window and door trim MCs between 

buildings was highly significant (confidence coefficient of 

0.995), with the overhang building having the lower MC 

readings. Even when readings of 20% MC or more were 

deleted from the data set, a statistically significant (confidence coefficient of 0.99) difference in gable-end trim MC 

between buildings remained. Thus, the roof overhang helped 

to keep window and door trim dry on gable-end walls. On 

eave walls, however, the difference between buildings was 

not significant even when we used a confidence coefficient 

criterion of only 0.9. This suggests that at the eaves, where 

there is roof runoff, rain gutters are as beneficial as roof 

overhangs, provided the gutters are functional. 

Final Moisture Content 

of Plywood Sheathing 

We took a total of 13 specimens of plywood sheathing from 

both houses and determined their MC gravimetrically. All 

measured between 7% and 9%, with an average of about 

8.2% MC, higher than that of the siding (approximately 

6.4% MC). This is best explained by the fact that at the same 

RH, the equilibrium moisture content (EMC) of plywood is 

higher than that of hardboard. The higher MC reflects the 

sheathing’s higher EMC values at comparable RH levels. 

Final in-place MCs of all sheathing specimens were within 

1.5% MC of their sorption EMC value at 50% RH (which 

was equal to indoor RH). As with siding specimens, sheathing specimens from walls facing southeast and southwest 

were drier than specimens from other walls. 

Inspection of Walls ONa and OWd 

We opened one stud bay each in wall sections ONa and 

OWd in the overhang building. We did this from the interior, 

cutting away the drywall with a drywall saw and removing 

the insulation. These were the two stud bays for which we 

collected detailed air pressure data (see Instrumentation and 

Data Acquisition). Wall ONa had felt building paper over 

plywood sheathing, and wall OWd had a polyolefin weather 

barrier over plywood sheathing. In wall ONa, a firestop had 

been installed just below ceiling level between the (balloonframe) studs. The firestop had been caulked to the plywood 

sheathing. 

We found no evidence of excessive air leakage or moisture 

in wall ONa. Moisture meter readings of the wood framing 

in wall ONa were between 8% and 9%. In wall OWd, we 

found a gap of approximately 6 mm (1/4 in.) between the top 

plate and the sheathing, and we could feel air movement 

through this gap. The top of the insulation had collected dirt, 

which was further evidence of significant air movement. 

Thus, there was direct air moving through the eave vents 

into the top of the wall, bypassing the weather barrier and 

Table 9—Distribution of moisture content (MC) 

readings on wood trim taken in May 1997. 

MC 

(%) 

Guttered building, number of 

readings 

(percentage of 

readings) 

Overhang building, number of 

readings 

(percentage of 

readings) 

μ30 1 (0.6) 0 

25 [ MC < 30 1 (0.6) 0 

20 [ MC < 25 3 (1.7) 3 (1.6) 

15 [ MC < 20 20 (11.6) 12 (6.6) 

10 [ MC < 15 122 (70.9) 150 (82.4) 

<10 25 (14.5) 17 (9.3) 

Total 172 (100) 182 (100) 

Table 10—Average moisture content (MC) of window 

and door trim in May 1997 by building and wall type 

 Average MC (%) [standard deviation 

(% MC)/number of specimens] 

Type of wall Guttered building Overhang building 

Gable-end 12.84 [4.10/78] 11.63 [2.33/78] 

Eave 11.95 [1.92/54] 11.67 [1.74/54] 
17 

sheathing. Moisture meter readings of the wood framing 

were between 8.5% and 11%, with the highest readings near 

the top. 

Results of Site Inspections 

General Observations 

A total of 10 quarterly visual inspections were performed. 

At the last inspection in May 1997, all boards appeared to be 

in excellent condition. During the May 1995 inspection, a 

total of 40 caulking failures were discovered on both buildings. All but three of these occurred next to air dams. It is 

plausible that the failures adjacent to air dams were due to 

shrinkage of the air dams, which were made from treated 

lumber and were fairly wet at installation. Because air dams 

are not part of typical construction, Masonite Corporation 

and FPL agreed that the failures next to air dams should be 

repaired, but that the other three failures should not be repaired. However, in our September 1995 inspection, we 

discovered that the contractor had repaired all caulking 

failures. The repairs of the three caulk failures not adjacent 

to air dams 6 months after installation would represent an 

exceptional level of maintenance by a homeowner. Subsequent caulk failures were not repaired. 

During the September 1995 inspection, heavy mildew was 

noted on some of the caulking, especially on lower window 

trim joints. This mold remained present throughout the remainder of the test. One lower window caulk seam had 

failed. Summer 1995 was unusually wet in southern Florida, 

with rainfall about 580 mm (23 in.) above normal. 

During the December 1995 inspection, mildew was noted on 

portions of 46 boards on the guttered building and on portions of 71 boards on the overhang building. The vast majority of these boards were factory-finished, suggesting that the 

paint used for site finishing had a more effective mildewcide. 

Most of the mildew was on the northwest and northeast sides 

of the buildings, the sides that receive the least amount of 

direct sunlight. 

Iron staining of part of wall section B on the northwest side 

of the guttered building was present at the September 1995 

inspection. This was caused by an irrigation sprinkler wetting the siding during August and September 1995. A guard 

dog had dug the sprinkler head out of the turf, which made it 

fall over and spray in the direction of the building. This was 

corrected on September 10, 1995. We suspect that wetting by 

the sprinkler occurred for roughly a month. The irrigation 

water hit one end of an instrumented board, and we have 

identified a 1-month period during which it appeared to have 

been periodically wetted by the sprinkler. At the September 

1995 inspection, boards that were iron-stained were not 

thicker than adjacent boards that were not iron-stained. 

The May 1997 inspection found the buildings and siding in 

essentially the same condition as in December 1995, when 

mildew was first noticed. In respects other than mildew, the 

siding was in virtually the same condition as in December 

1994. Although the siding was obviously exposed to very 

high moisture and humidity conditions, all boards appeared 

to be in good condition. This observation is supported by the 

measurements. 

Thickness Measurements 

Thickness measurements on all boards were performed at the 

time of installation in December 1994 and at the nine subsequent quarterly inspections. Summaries of these data are 

shown in Figures 15 to 20. 

Initial Board Thickness 

The average initial thicknesses of site-finished boards (NS 

and BS) on the guttered building, as measured in December 

1994, were 0.13 to 0.15 mm (0.005 to 0.006 in.) greater than 

those on the overhang building. Gutters were installed after 

siding installation, leaving the boards on that building more 

exposed to rain than the boards on the overhang building 

(gutters were installed in late December 1994 or January 

1995). We therefore suspect that boards on the guttered 

building, especially boards that were left unpainted for a 

short period of time, may have been wetter at the time of first 

thickness measurement. This would explain the greater 

initial thickness of site-finished boards on the guttered building than on the other building. However, it is reasonable to 

assume that average board thickness was the same on both 

buildings at the time of installation. 

We measured the thickness of 48 board samples, collected at 

the building site at the time of installation, after conditioning 

at 50% RH at FPL. The average thickness was about 10 mm 

(0.4 in.), but the standard deviation within each of the four 

board groups (grouped by finish treatment) was 0.08 to 

0.15 mm (0.003 to 0.006 in.). This variation did not allow us 

to distinguish between the four finish treatments. Moreover, 

half the boards in this sample were site-finished but actually 

had been primed only and had not received their finish coating. Only boards actually installed on the building were sitefinished after installation. 

In the following analysis, we assume that the initial average 

thickness of boards is 10 mm (0.4 in.). This choice is conservative, because back-priming obviously added to the board’s 

thickness and using this number to calculate thickness swell 

produces somewhat conservative estimates (that is, actual 

swelling may be slightly less than reported). 

Thickness Swell 

Figures 15 through 20 show that the increase in thickness of 

all boards was very small. Assuming initial board thickness 

of 10 mm (0.4 in.), average board thickness increased by 

about 0.20 mm (0.008 in.), or 2%, from December 1994 to 

May 1997. As mentioned earlier, this is a conservative 
18

Figure 15—Results of thickness measurements on 

guttered building, by wall orientation (northeast, 

northwest, southeast, southwest). 

Figure 16—Results of thickness measurements on 

overhang building, by wall orientation (northeast, 

northwest, southeast, southwest). 

Figure 17—Results of thickness measurements on 

guttered building, by wall construction type (OF, no 

sheathing with felt; PF, plywood with felt; PT, plywood 

with polyolefin weather barrier). 

Figure 18—Results of thickness measurements 

on overhang building, by wall construction type 

(OF, no sheathing with felt; PF, plywood with felt; 

PT, plywood with polyolefin weather barrier). 

Figure 19—Results of thickness measurements on 

guttered building, by siding finish type (BS, back-primed, 

site finished; NS, not back-primed, site finished; 

BF, back-primed, factory finished; NF not back-primed, 

factory finished). 

Figure 20—Results of thickness measurements on 

overhang building, by siding finish type (BS, backprimed, site finished; NS, not back-primed, site finished; 

BF, back-primed, factory finished; NF not back-primed, 

factory finished). 

10.15

10.25

10.35

10.45

10.55

Thickness (mm)

Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95 Mar-96 Jun-96 Sep-96 Dec-96 Mar-97 Jun-97 

sw

sw sw

sw

sw

sw sw

sw sw

sw

ne ne

ne

ne ne

ne

ne ne

ne

ne

nw nw

nw

nw

nw

nw

nw

nw

nw

nw

se

se se

se

se

se

se

se

se

se

Gutter

10.15

10.25

10.35

10.45

10.55

Thickness (mm)

Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95 Mar-96 Jun-96 Sep-96 Dec-96 Mar-97 Jun-97

sw

sw sw

sw

sw

sw sw

sw

sw

sw

ne

ne

ne

ne ne

ne

ne

ne

ne

ne

nw

nw nw

nw

nw

nw

nw

nw nw nw

se

se

se

se se

se se se se se

Overhang

10.15

10.25

10.35

10.45

10.55

Thickness (mm)

Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95 Mar-96 Jun-96 Sep-96 Dec-96 Mar-97 Jun-97 

OF

OF

OF

OF

OF

OF OF

OF OF

OF

PF

PF

PF

PF

PF

PF PF PF

PF

PF

PT PT

PT

PT

PT

PT PT

PT

PT

PT

Gutter

10.15 

10.25 

10.35 

10.45 

10.55 

Thickness (mm)

Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95 Mar-96 Jun-96 Sep-96 Dec-96 Mar-97 Jun-97 

OF

OF

OF

OF OF

OF OF

OF OF

OF

PF

PF

PF

PF

PF

PF

PF

PF

PF

PF

PT

PT

PT

PT

PT

PT PT

PT

PT

PT

Overhang

10.15

10.25

10.35

10.45

10.55

Thickness (mm)

Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95 Mar-96 Jun-96 Sep-96 Dec-96 Mar-97 Jun-97 

BS BS BS

BS

BS

BS

BS

BS

BS

BS

NS

NS

NS

NS

NS

NS

NS NS

NS

NS

BF BF

BF

BF

BF

BF BF BF BF

BF

NF

NF

NF

NF NF

NF

NF

NF

NF

NF

Gutter

10.15 

10.25 

10.35 

10.45 

10.55 

Thickness (mm)

Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95 Mar-96 Jun-96 Sep-96 Dec-96 Mar-97 Jun-97 

BS

BS BS

BS

BS

BS

BS

BS

BS

BS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

BF

BF

BF

BF

BF

BF BF

BF

BF

BF

NF

NF

NF

NF

NF

NF

NF

NF NF

NF

Overhang
19 

estimate and actual average thickness swell is probably 

somewhat lower. The data for the overhang building 

(Figs. 16, 18, and 20) suggest that, after some initial swelling 

immediately after installation, very little additional swelling 

took place. All figures show seasonal variations, but net 

increases after March 1995 were very small, 0.1 mm 

(0.004 in.) or less, and after June 1996, no long-term 

increase in thickness is apparent. 

Effect of Orientation—The effect of orientation seemed 

consistent on both buildings (Figs. 15 and 16) during the first 

2 years of exposure, with northeast-facing boards the thickest 

and southeast-facing boards the thinnest on both buildings. 

However, on the overhang building, this trend did not continue and northwest-facing boards ended up with the most 

thickness swell, with the other orientations roughly equal. 

Thus, the thickness swell data do not show the effect of 

orientation as consistently and clearly as the final MC data 

taken from siding samples. 

Effect of Wall Construction—Figures 17 and 18 suggest 

that on both buildings, siding on walls without sheathing 

(OF) measured slightly thicker than boards on other wall 

types throughout the measurement period. Assuming an 

initial average thickness of 10 mm (0.4 in.) at installation, 

the average thickness swell of boards installed on walls 

without sheathing (OF) on the guttered building was about 

0.23 mm (0.009 in.), or 2%, at the time of dismantling in 

May 1997 and 0.30 mm (0.012 in.), or 3%, on the overhang 

building. The corresponding average thickness swell of 

boards installed over plywood and felt (PF) was about 

0.20 mm (0.008 in.), or 2%, on both buildings. The final 

thickness swell of boards over the polyolefin weather barrier 

(PT) on the guttered building was only 0.10 mm (0.004 in.), 

or 1%, and about 0.20 mm (0.008 in.), or 2%, on the overhang building. 

Effect of Back-priming and Site Finishing—We used the 

same initial thickness for the analysis of effects of finishing 

treatment, although we realized that the site and factory paint 

coatings are probably not exactly equal in thickness. The 

finish treatments are as follows: site-finished without backpriming (NS), site-finished with back-priming (BS), factoryfinished without back-priming (NF), and factory-finished 

with back-priming (BF). Figures 19 and 20 show few consistent differences in thickness swell by treatment type, except 

that on both buildings, boards that were site-finished without 

back-priming (NS) seemed to swell the least. On the guttered 

building, factory-finished boards without back-priming (NF) 

swelled somewhat less than back-primed boards (BS and 

BF), but this is not confirmed by the data for the overhang 

building. In addition, part of these differences can be ascribed to differences in initial thickness. Nevertheless, the 

data do indicate that back-priming did not decrease thickness 

swell. 

Effect of Gutters and Overhangs—The thickness measurements do not reveal significant differences between the 

two buildings in board thickness swell. 

Results of Automated 

Data Collection 

Outdoor Humidity and Temperature 

Data from the two weather stations show that daily maximum summer temperatures generally were between 65°C 

(85°F) and 76°C (105°F), and maximum RH was between 

75% and 95%. During winter, maximum daily values 

dropped to between 54°C (65°F) and 68°C (90°F) with RH 

between 50% and 90%. 

Rainfall 

A summary of rain data collected on-site, as well as data 

from the nearest National Weather Service (NWS) station at 

West Palm Beach, Florida, about 26 km (16 mi) from the test 

site, are shown in Table 11. The NWS data and the sitecollected data are in general agreement. Using site-collected 

data for the period April 28, 1995, through May 5, 1997, 

(approximately 2 years of data) and normalizing for the 

number of days in this period gives annual rainfall of 

approximately 1,500 mm (59 in.) per year at the site. This is 

close to the historical average (years 1961–1990) of 

1,550 mm (61 in.) per year at the NWS site in West 

Palm Beach. 

Using NWS data, we calculated the climatic decay hazard 

index (Scheffer 1971) during the period that the siding was 

exposed. We used NWS data because calculation of decay 

hazard index requires daily rain data. Site-collected data did 

not contain this information, since it was collected on weekly 

intervals. The index value was developed as an indicator of 

the potential for decay of wood exposed on building exteriors above ground. The calculated value for the exposure 

Table 11—Summary of rainfall data (December 4, 1994 

to May 9, 1997) 

 Cumulative rainfall 

(mm (in.)) 

Period NWSa data Site data 

Dec. 12, 1994 to Apr. 14, 1995 287 (11.3) Not collected 

Apr. 14, 1995 to Dec. 29, 1995 1,293 (50.9) 1,494 (58.8) 

Dec. 29, 1995 to July 5, 1996 518 (20.4) 582 (22.9) 

July 5, 1996 to Jan. 3, 1997 511 (20.1) 494 (19.4) 

Jan. 3, 1997 to May 9, 1997 450 (17.7) 475 (18.7) 

a

NWS, National Weather Service. 
20

Figure 21—Average daily rainfall for April 1995 

through May 1997. 

period was 154. This is an exceptionally high value, 

indicating that climatic conditions during the exposure period were very conducive to decay of wood products on 

building exteriors. Index values in excess of 65 (based on 

historic average data) indicate climates of high decay hazard. 

Index values for the continental United States (based on 

historic average data) exceed 110 only for the Florida peninsula (Scheffer 1971). 

Figure 21 shows the average daily rainfall, calculated from 

the rain gauge data, from April 1995 through May 1997. 

It shows that the summer of 1995 was especially wet. 

Wind Speed and Direction 

As mentioned previously, hourly wind speed and direction 

data were collected starting in the middle of September 

1995. Figure 22 shows the daily maximum and average wind 

with the dominant direction for November 11 through December 8, 1995. The data generally agreed with patterns of 

air pressure differences across the siding (that is, positive 

pressure differences on the windward side, negative on all 

other sides), increasing our confidence in the accuracy of 

pressure and wind data. 

Air Pressure 

Air Pressures Across Siding 

Figures 23 to 30 show maximum, minimum, and average air 

pressure difference across the siding on the eight locations 

on the guttered building for each hour between November 11 

and December 8, 1995, the same time period as in Figure 22. 

This period was chosen as representative of the entire data 

set. Positive pressure means that the air pressure exterior to 

the siding exceeds the air pressure in the airspace between 

the siding and the building paper (or polyolefin air barrier), 

and a negative pressure means air pressure behind the siding 

exceeds air pressure exterior to the siding. Designations on 

the plots (GNa, GNd, GEa, GEd, ONa, etc.) are the designations given to wall sections (Fig. 4). Because the buildings 

were oriented 180 degrees from the planned orientation, wall 

sections Na through Nd actually faced southwest. 

Figure 22—Average and maximum daily wind speed and dominant wind direction for 

November 11 through December 8, 1995. 

0

5

10 

15 

20 

25 

30 

35 

Apr-95 Oct-95 May-96 Dec-96 Jun-97 

Rainfall (mm/day)

0 

1 

2 

3 

4 

5 

6 

Windspeed (m/s) 

11-Nov 13-Nov 15-Nov 17-Nov 19-Nov 21-Nov 23-Nov 25-Nov 27-Nov 29-Nov 01-Dec 03-Dec 05-Dec 07-Dec

SE

N

E

WNW

NNE

E

E

NE

E

NNW

NNE

E

SE

NNW

NE

SE

ESE

SE

N

W W

W

W

NEENE

W

E

Daily average 

windspeed Maximum windspeed
21 

Figure 23—Average, maximum, and minimum air 

pressure difference across siding on wall section GNa 

(southwest-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

Figure 24—Average, maximum, and minimum air 

pressure difference across siding on wall section GNd 

(southwest-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

Figures 23 to 30 reflect the gusty and turbulent nature of 

wind pressure, as well as changes in wind direction. During 

all hours, both positive and negative air pressure differences 

occurred across the siding on all sides of the building. As the 

figures show, peak pressure differences of 20 Pa are common on windy days. During the afternoon of November 14, 

1995, extreme negative pressures of −40 to −60 Pa or lower 

were recorded in some locations. This was a particularly 

windy day with WNW winds. With WNW winds, pressures 

would be expected to be positive on wind-facing walls 

(northwest) and primarily negative on the other walls. 

Figure 25—Average, maximum, and minimum air 

pressure difference across siding on wall section GWa 

(southeast-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

Figure 26—Average, maximum, and minimum air 

pressure difference across siding on wall section GWd 

(southeast-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

On November 14, pressures on northwest-facing walls GEa 

(Fig. 29) and GEd (Fig. 30) had the highest positive values, 

while southwest- and northeast-facing walls parallel to the 

wind direction (GNa, GNd, GSc, and GSd) registered the 

most extreme negative pressures. 

Maximum negative pressures usually exceeded maximum 

positive pressures and the average pressure (average of 

slightly more than 600 individual 5-s readings per hour) was 

generally slightly negative on most sides of the building. 

Both buildings produced very similar air pressure data. This 

slight pressurization of the back of the siding was most 

prominent when the wind was strong. We have not found a 

convincing explanation for this phenomenon. 
22

Figure 27—Average, maximum, and minimum air 

pressure difference across siding on wall section GSa 

(northeast-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

Figure 28—Average, maximum, and minimum air 

pressure difference across siding on wall section GSc 

(northeast-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

It has been argued in recent years that, to properly function 

as a rain screen, wood-based siding should be spaced out 

from the sheathing and ventilated to provide air pressure 

equalization. The pressure data suggest that conventionally 

installed lap siding provides some air pressure equalization, 

although relatively large exfiltrative pressure peaks occasionally occur. These exfiltrative pressures do not cause 

concern for water penetration, and therefore, an air space 

does not appear necessary for that purpose. (When we dismantled the building in May 1997, we removed and inspected most of the siding. There was no evidence of water 

staining on the back of the siding or on the building paper 

other than some evidence of leakage that had occurred 

Figure 29—Average, maximum, and minimum air 

pressure difference across siding on wall section GEa 

(northwest-facing) of guttered building, November 11 

through December 8, 1995 (positive values indicate that 

exterior air pressure exceeded air pressure on back of 

siding). 

Figure 30—Average, maximum, and minimum air 

pressure difference across siding on wall section GEd 

(northwest-facing) of guttered building, November 11 

through through December 8, 1995 (positive values 

indicate that exterior air pressure exceeded air pressure 

on back of siding). 

due to caulk failures around windows.) An air space may still 

be beneficial but primarily because it provides better drainage of water that may penetrate to the back of the siding 

(especially around windows and doors). An air space would 

reduce the chance of this water penetrating the weather 

barrier and wetting the sheathing. 

Air Pressures in Walls ONa and OWd 

On October 8, 1996, pressure taps were rearranged as described in Table 5. This allowed a more detailed look at the 

pressure differences across wall sections OWd and ONa, 

specifically pressure differences across the sheathing–siding 

combination, across the entire wall, and across the siding. 
23 

Figure 32—Hourly average pressure difference across 

siding, between the outside and the cavity, and across 

the entire wall for wall ONa on October 18, 1996 (positive 

values indicate that air pressure was infiltrative). 

To demonstrate the pressure behavior of these walls, we 

focus on data collected on October 18, 1996, a day with 

typical windspeeds, and November 15, 1996, a very 

windy day. 

Figure 31 shows the wind speed (10-min average) and predominant wind direction on October 18, 1996. This day was 

of interest because of the rapidly changing wind direction in 

the early afternoon. Figures 32 and 33 show the hourly average pressure difference across the siding, between the outside and the cavity, and across the entire wall for walls ONa 

and OWd, respectively. Wall section ONa (Fig. 32) was 

located on the west corner and faced southwest. The wall 

Figure 33—Hourly average pressure difference across 

siding, between the outside and the cavity, and across 

the entire wall for wall OWd on October 18, 1996 (positive 

values indicate that air pressure was infiltrative). 

experienced exfiltrative (negative) pressures all day, even 

with south-southeast winds. Wall OWd (Fig. 33) was located 

on the east corner and faced southeast. It experienced infiltrative (positive) pressures with east to southeast winds, 

which abruptly changed to exfiltrative (negative) pressure 

when the wind shifted to the north. This illustrates that only 

wall sections near the windward corner experienced infiltrative pressures, while all other wall sections were under exfiltrative pressure. Average pressures across the siding were 

small (0.5 Pa or less) and relatively insensitive to wind 

speed. Hourly average pressures across the sheathing– 

weather barrier–siding combination were in the order of 0 to 

0.5 Pa in both walls most of the time but increased to almost 

Figure 31—Hourly windspeed (10-min average) and direction on Oct. 18, 1996. 

0 

1 

2 

3 

4 

5

10-minute average windspeed (m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour

SW

NE SE SE SE SE

SE

E

ESE

SE

ESE E

SSE N

N

N

N

N

N NW

NNW

NNW

NNW

NNW
24

2 Pa in wall ONa near midnight (Fig. 32). Hourly average 

pressures across the gypsum board (∆pcavity – ∆pwhole wall) 

were about 0.5 Pa most of the time but approached 1 Pa in 

OWd during the middle of the day. 

Figures 34 and 35 show instantaneous pressures between 

8:45 and 9:00 a.m. on October 18, 1996. Winds were from 

the east to southeast at 0.9 to 1.3 m/s (2 to 3 mi/h) (10-min 

average). Data are shown as 15-s averages. Pressures across 

the siding on wall ONa (Fig. 34) remained well below 1 Pa, 

Figure 36—Instantaneous pressure difference between 

cavity of wall ONa and outside and inside air, 

respectively, between 8:45 and 9:00 a.m. on October 18, 

1996 (winds were from the southeast; positive values 

indicate that the air pressure was infiltrative). 

Figure 37—Instantaneous pressure difference between 

cavity of wall ONa and outside and inside air, 

respectively, between 12:15 and 12:30 p.m. on October 

18, 1996 (winds were from the south-southeast; positive 

values indicate that the air pressure was infiltrative). 

while exfiltrative pressures across the wall at times reached 

3 Pa. Pressures on the windward wall OWd (Fig. 35) were of 

the same magnitude but fluctuated between infiltrative (positive) and exfiltrative (negative). This demonstrates that even 

a windward-facing wall can experience significant periods of 

exfiltrative airflows. The data also suggest that hourly average pressure data can be quite misleading when infiltrative 

and exfiltrative flows cancel each other in the averaging 

process. Figures 36 and 37 show the difference between 

pressure in the cavity and outside and inside pressure. 

Figure 34—Instantaneous pressure difference across 

siding, between the outside and the cavity, and across 

entire wall for wall ONa between 8:45 and 9:00 a.m. on 

Oct. 18, 1996 (winds were from the southeast; positive 

values indicate that the air pressure was infiltrative). 

Figure 35—Instantaneous pressure difference across 

siding, between the outside and the cavity, and across 

entire wall for wall OWd between 8:45 and 9:00 a.m. on 

Oct. 18, 1996 (winds were from the southeast; positive 

values indicate that the air pressure was infiltrative). 
25 

Figure 38—Instantaneous pressure difference between 

cavity of wall OWd and outside and inside air, 

respectively, between 8:45 and 9:00 a.m. on October 18, 

1996 (winds were from the southeast; positive values 

indicate that the air pressure was infiltrative). 

Figure 39—Instantaneous pressure difference between 

cavity of wall OWd and outside and inside air, 

respectively, between 12:15 and 12:30 p.m. on October 

18, 1996 (winds were from the south-southeast; positive 

values indicate that the air pressure was infiltrative). 

It appears that in both walls, pressure drops across the 

sheathing and siding were often similar in magnitude to 

pressure drops across the gypsum board, even though the 

sheathing with the weather barrier had been expected to 

resist most of the pressure. The large gap at the top plate that 

was revealed during inspection of wall section OWd explains 

the results for that wall, but in wall ONa, air apparently also 

bypassed the exterior sheathing and barrier to a considerable 

degree. Windward wall OWd also shows periods when the 

cavity appears simultaneously pressurized compared with 

inside and outside. We first observed and reported a similar 

phenomenon in walls with vents to the outside in a test building in Madison (TenWolde and others 1995). The current 

data indicate that the same effect takes place in conventional 

wood-framed walls. Figure 38 shows that 31⁄2 h later, with 

slightly more southerly winds, the cavity of wall ONa also 

Figure 40—Instantaneous pressure difference between 

cavity of wall ONa and outside and inside air, 

respectively, between 11:05 and 11:20 p.m. on October 

18, 1996 (winds were from the north-northeast; positive 

values indicate that the air pressure was infiltrative). 

Figure 41—Instantaneous pressure difference between 

cavity of wall OWd and outside and inside air, 

respectively, between 11:05 and 11:20 p.m. on October 

18, 1996 (winds were from the north-northeast; positive 

values indicate that the air pressure was infiltrative). 

was pressurized for short periods of time. The cavity of wall 

OWd continued to be pressurized at times as well 

(Fig. 39). This pressurization was probably due to air infiltrating through the top of the wall. Finally, with winds from 

the north at 11:00 p.m., both walls were on the leeward side 

of the building and under exfiltrative pressure (Figs. 40 

and 41). Pressurization of the cavity of either wall rarely 

occurred during that period. 

Figures 42 and 43 present similar pressure data for November 15. Around 11:00 p.m., winds were strong from the east 

to northeast with windspeeds of around 8.1 m/s (18 mi/h) 

(10-min average). Pressure across the siding was still relatively modest, usually less than 2 Pa, with some spikes of 

10 to 15 Pa. Some pressure spikes of 15 to 20 Pa occurred 

across the gypsum board, especially in wall ONa. 
26

Figure 42—Instantaneous pressure difference across 

siding, between the outside and the cavity, and across 

entire wall for wall ONa between 10:45 and 11:00 p.m. on 

Nov. 15, 1996 (winds were from the north to northeast; 

positive values indicate that the air pressure was 

infiltrative). 

Figure 43—Instantaneous pressure difference across 

siding, between the outside and the cavity, and across 

entire wall for wall OWd between 10:45 and 11:00 p.m. 

on Nov. 15, 1996 (winds were from the east to northeast; 

positive values indicate that the air pressure was 

infiltrative). 

Figure 44 shows pressures averaged across the same 15-min 

period. It shows that wall ONa had an average exfiltrative 

pressure of about 6 Pa, and the sheathing–weather barrier–

siding combination exhibited a pressure of almost 4 Pa. This 

means that the gypsum board sustained an average pressure 

of about 2 Pa. The siding pressure difference was slightly 

greater than 1 Pa. The average pressure across wall OWd 

was in the inward direction but was only about 2 Pa, 

Figure 44—Fifteen-minute average pressure difference 

across siding, between the outside and the cavity, and 

across entire wall for walls ONa and OWd between 10:45 

and 11:00 p.m. on Nov. 15, 1996 (winds were from the 

east to northeast; positive values indicate that the air 

pressure was infiltrative). 

Figure 45—Ten-minute average pressure difference 

across siding, between the outside and the cavity, and 

across entire wall for walls ONa and OWd between 10:05 

and 10:15 p.m. on Nov. 15, 1996 (winds were probably 

from the east to northeast; positive values indicate that 

the air pressure was infiltrative). 

primarily because there were many pressure reversals (OWd 

was on the windward corner of the building). Pressure across 

the gypsum board was about 2 Pa (inward direction). Finally, 

Figure 45 shows averages for data taken 45 min earlier. 

These average pressures for the downwind wall (ONa) were 

very similar, but the pressures across wall OWd are reversed 

in direction, indicating a slightly different (probably more 

northerly) wind direction during that period. 

-7

-6

-5

-4

-3

-2

-1

0

Pressure difference (Pa)

Siding Cavity Wall































Wall ONa



 Wall OWd

-7

-6

-5

-4

-3

-2

-1

0

Pressure difference (Pa)

Siding Cavity Wall



























Wall ONa  Wall OWd
27 

Several observations can be made from all the data 

presented: 

• There was reasonably good pressure equalization across 

the lap siding. 

• Pressures across the gypsum board were greater than 

expected, probably because of air leakage through the top 

of the wall. 

• Most of the walls experienced exfiltrative wind pressures 

most of the time; only walls on the windward corner experienced extensive periods of inward pressure, but even 

then, many pressure reversals occurred. 

• Windward walls sometimes experienced short periods of 

pressurization of the cavity, with pressures that were 

higher than outside and inside pressures. This most likely 

occurred because of air infiltrating through the top of the 

wall. 

The data as well as the inspection of the walls indicate that 

exterior air barriers have limited value if the top plate is not 

carefully sealed. 

Moisture Content 

Data Reliability 

In-place MCs of the siding on the test building during exposure were indirectly measured by electric resistance in the 

board between two pins. The pin readings were calibrated at 

FPL for the specific siding used in this study. The electric 

resistance became too large to measure when boards dropped 

below 7% MC, and therefore, 7% was the lowest measurable 

MC. Because of the resolution of our data acquisition system 

and the noise in the system, our resolution was 10 mV, 

which translates into an uncertainty of about ±2% MC between 7% and 10.5% MC. Uncertainty in temperature correction factors provided an additional error, which we estimate at 1% to 2% MC. We therefore believe that in the MC 

range of 7% to 10.5%, the total error of individual MC 

measurements was in the range of 3% to 4% MC. Above 

10.5%, errors decreased and resolution improved. However, 

averaging the results from many sensors improved accuracy, 

even in the low MC range. Indeed, a comparison of final 

gravimetrically obtained MCs with hourly MC pin data 

suggested that the error in average MCs was about 1% MC. 

Moreover, when differences between MCs were considered, 

much of the error in the temperature correction tended to be 

canceled out (that is, the error in both MCs was probably of 

the same magnitude and direction). Therefore, when considering differences between MC levels, we ignored the 

temperature correction error. 

An additional error in hourly MC readings occurred during 

periods of rain. Apparently, with the dry siding providing 

high electrical resistance, liquid water on the pins provided a 

sufficient shunt resistance to cause spikes of higher readings. 

These spikes are evident in Figures 46 to 58. However, MC 

readings would quickly return to their previous values 

afterwards. 

During a site visit in early February 1996, we repaired and 

cleaned a number of MC pins. We also found defective 

amplifier circuits for five MC pins on the guttered building. 

We rerouted two pin pairs to unused amplifiers and disconnected defective circuits. During this visit, computer thermocouple boards were also recalibrated. The MC data in 

Figure 46 show that these repairs and recalibrations had a 

profound influence on the MC readings. After resumption of 

data collection on February 9, 1996, the average of all MC 

readings dropped to about 6% to 7% on both buildings, 

which is the lowest measurement limit of the MC pins. These 

readings were verified with several measurements from a 

hand-held electric resistance moisture meter, which indicated 

MCs between 6% and 7.5%. The defective amplifier circuits 

that were repaired or disconnected during the February visit 

apparently had an undue influence on the MC results for the 

guttered building prior to the visit. Recalibration of temperature circuits is responsible for the much smaller decrease in 

MC readings on the overhang building. 

Differences Between Buildings 

There was no significant difference between measured average MCs of the two buildings after the repairs of the MC 

pins in February 1996 (Fig. 46). 

Effect of Back-Priming and Finishing 

Figures 47 to 50 show no discernable effect of finishing 

treatments on the MC readings after February 15, 1996. 

All boards were essentially dry. 

Effect of Wall Construction 

Figures 51 to 53 show no discernable effect of the type of 

wall construction (open with felt, plywood with felt, or 

plywood with polyolefin weather barrier) on the MC readings after February 15, 1996. All boards were essentially dry. 

Effect of Orientation 

Figures 54 to 57 show no discernable effect of wall orientation on the MC readings after February 15, 1996. All boards 

were essentially dry. 

Bottom Boards 

After February 15, 1996, the data show no discernable 

difference in MC between bottom boards and boards in other 

locations (Fig. 58). 
28

Figure 46—Daily average moisture content of all siding on both buildings for April 1, 1995, 

through May 5, 1997. 

Figure 47—Daily average moisture content of back-primed, site-finished (BS) siding on both 

buildings for April 1, 1995, through May 5, 1997. 
29 

Figure 48—Daily average moisture content of back-primed, factory-finished (BF) siding on 

both buildings for April 1, 1995, through May 5, 1997. 

Figure 49—Daily average moisture content of site-finished siding without back-priming (NS) 

on both buildings for April 1, 1995, through May 5, 1997. 
30

Figure 50—Daily average moisture content of factory-finished siding without back-priming (NF) 

on both buildings for April 1, 1995, through May 5, 1997. 

Figure 51—Daily average moisture content of siding on walls with plywood sheathing and 

polyolefin weather barrier (PT) on both buildings for April 1, 1995, through May 5, 1997. 
31 

Figure 52—Daily average moisture content of siding on walls with plywood sheathing and 

felt (PF) on both buildings for April 1, 1995, through May 5, 1997. 

Figure 53—Daily average moisture content of siding on walls without sheathing (open) with 

felt (OF) on both buildings for April 1, 1995, through May 5, 1997. 
32

Figure 54—Daily average moisture content of siding on northeast-facing walls on both 

buildings for April 1, 1995, through May 5, 1997. 

Figure 55—Daily average moisture content of siding on southeast-facing walls on both 

buildings for April 1, 1995, through May 5, 1997. 
33 

Results of Laboratory 

Measurements 

Linear Expansion 

Linear expansion of the specimens ranged from 0.21% to 

0.27%, well within the maximum 0.40% specified in the 

ANSI/AHA standard. The mean linear expansion was 0.23% 

with a sample standard deviation of 0.016%. There was no 

apparent influence of finish system on linear expansion, 

indicating that the measurements were indeed taken at 

equilibrium. 

Residual Thickness Swell 

Residual thickness swell after reconditioning to 50% RH 

averaged 4.3%, substantially less than the maximum allowable of 20% specified in the product standard. Average 

residual thickness swell in the first batch of 24 specimens 

was 4.6%, and in the second batch, it was 4.0%. 

Figure 56—Daily average moisture content of siding on southwest-facing walls on both 

buildings for April 1, 1995, through May 5, 1997. 

Figure 57—Daily average moisture content of siding on northwest-facing walls on both 

buildings for April 1, 1995, through May 5, 1997. 
34

Because standard deviations of residual thickness swell 

within each batch were 1.3% and 1.6%, there is no statistical 

difference between these two batches. 

Lateral Nail Resistance 

We conducted lateral nail resistance tests on 578 siding 

specimens following slightly modified ASTM D 1037 

procedures. A summary of the results is shown in Table 12. 

Although the average values for the unexposed siding are 

slightly higher, they cannot be statistically differentiated 

from the exposed samples. Further categorization by wall 

orientation and finishing treatment did not reduce 

Table 12—Average lateral nail resistance of siding before and after exposure on the buildings 

 Average lateral nail resistance (kN (lb))a

 

at three distances from edge of board 

 6.3 mm 

(1/4 in.) 

9.5 mm 

(3/8 in.) 

12.7 mm 

(1/2 in.) 

Unexposed 

siding 

1.93 [0.20] 

(433 [45]) 

1.96 [0.17] 

(441 [38]) 

2.14 [0.20] 

(480 [46]) 

Guttered 

building 

1.80 [0.15] 

(405 [33]) 

1.85 [0.18] 

(415 [40]) 

2.05 [0.18] 

(461 [41]) 

Overhang 

building 

1.82 [0.16] 

(410 [36]) 

1.87 [0.20] 

(420 [44]) 

2.00 [0.16] 

(449 [36]) 

a

Standard deviation shown in brackets for both units. 

the standard deviation in the data and did not produce consistent and significant differences between groups. We therefore conclude that the 21⁄2-year exposure did not significantly 

reduce the lateral nail resistance of the siding. 

Discussion 

This study involved only lap siding from one particular mill. 

The quality of this siding as measured with several tests 

(Table 1) was very high, especially compared with the industry minimum standards for hardboard siding. The applicability of our results is therefore limited to siding from that 

particular mill and cannot be extended to other hardboard 

siding without additional information about the properties of 

that particular siding. Siding with similar properties should 

give similar performance. The 29 months of exposure was 

also too short to use the results of this study to draw broad 

conclusions about the long-term durability of hardboard 

siding in this climate. However, the 29 months of exposure 

had no significant effect on the condition of the siding. 

The final MCs of the siding were below 8%, and the hourly 

data show that this was typical for the entire 29 months of 

exposure. The siding showed no deterioration after 

29 months of exposure. The MC data of the trim and the 

outdoor humidity and rain gauge measurements show that 

the building was exposed to a great amount of rain and very 

high humidity conditions. It appears that the siding did not 

absorb liquid rainwater and was essentially in moisture 

equilibrium with the surrounding air, especially on the northfacing walls, while some of the wood trim was more prone to 

absorbing rainwater. 

Figure 58—Daily average moisture content of bottom boards on both buildings for 

April 1, 1995, through May 5, 1997. 
35 

The final MC data showed that boards facing south were 

drier than boards facing north, and the in-place thickness 

measurements provide us with a weak indication that at least 

the southeast-facing boards were indeed drier throughout 

the test. 

At the time of removal, boards without back-priming tended 

to be drier than boards with back-priming. However, these 

differences did not necessarily exist during the entire 

21⁄2 years of the test. The hourly MC data did not provide us 

with the ability to check if these differences existed throughout because all the MCs were too close to the lower limit of 

the measurement system. The full coverage of primer on the 

backs of back-primed specimens apparently retarded drying 

of the siding through the back surfaces. This is supported by 

the drying curves for 203-mm (8-in.) squares of siding specimens with and without back-priming (Figs. 59 and 60), 

which show that specimens without back-priming dried more 

rapidly than back-primed specimens of similar size, at either 

103°C (217°F) or 27°C (81°F). The temperature of siding in 

direct sunshine on the test buildings was between these two 

levels. 

Figure 60 illustrates the effect of back-priming under cyclical conditions. We placed four 203-mm (8-in.) specimen 

squares (retrieved from the test buildings) in humidity rooms 

and monitored their MC. The rooms were at 27°C (80°F) and 

at 30% and 90% RH, respectively. Samples remained in one 

room for 2 weeks and were then moved to the other room. 

Figure 60 shows drying and wetting curves for the first full 

exposure cycle, along with MCs at the ends of the 2-week 

exposure periods during the second cycle. Figure 60 shows 

that back-priming can significantly retard both vapor sorption and desorption, that is, back-priming retards both wetting and drying. Our site-collected data suggest that unusually heavy rains occurred in the months prior to removal and 

dry weather occurred for the week prior to removal. In light 

of this, it seems likely that back-primed siding had a slightly 

but consistently higher MC at time of removal because it had 

not yet fully dried out. However, all the siding, including that 

not back-primed, had at least some primer or paint coverage 

on the back at the laps. 

The in-place thickness measurements provided us with a 

weak indication that NS boards experienced the least thickness swell for most of the period, but the thickness swell data 

for the NF boards are more ambiguous. Taken together, the 

final MC data and thickness swell data support the conclusion that back-priming this type of hardboard siding did not 

result in lower MCs and may have been somewhat counterproductive. 

The MC data for the siding do not show an effect of wall 

construction type or a difference between the overhang 

building and the guttered building. However, the inspection 

of the windows and the final MCs of the trim strongly suggest that, while gutters provide protection from rain on the 

sidewalls, overhangs provide additional protection because 

of their presence on the gable ends. 

It has been argued in recent years that, to properly function 

as a rain screen, wood-based siding should be spaced out 

from the sheathing and ventilated to provide air pressure 

equalization. The pressure data in this report suggest that 

conventionally installed lap siding provides substantial air 

pressure equalization. An air space may still be beneficial, 

primarily because it probably provides better drainage of any 

water that penetrates to the back of the siding (especially 

around windows and doors). An air space would reduce the 

chance of this water penetrating the weather barrier and 

wetting the sheathing. 

Wind pressures across the two intensively monitored wall 

sections were predominantly exfiltrative (that is, inside 

pressure was higher). Infiltrative pressures only occurred in 

the wall sections near the windward corner. This generally 

only happened for short periods of time because wind direction continually varied. We found that our measured pressures were considerably different from published pressure 

data based on wind tunnel experiments. 

Figure 59—Weight loss compared with drying time of 

203-mm (8-in.) squares of siding at approximately 7% 

initial MC in drying oven at 103°C. 

Figure 60—Average moisture content of siding with and 

without back-priming when exposed cyclically to 30% 

and 90% RH. Water loss (g)

0 

5 

10 

15 

20 

0 2 4 6 8 10 12 14 

Drying time (h)

Backprimed

Not backprimed

5 

6 

7 

8 

9 

10 

11 

12 

Moisture content (%)

0 20 40 60 80 

Time (h)

Not backprimed

Backprimed
36

Conclusions 

Our conclusions apply only to siding from one particular mill 

and cannot be extended to other hardboard siding without 

additional information about the properties of that siding. 

• The siding was in excellent condition after 29 months of 

exposure to southern Florida weather conditions. Weather 

during the test period was typical for that area, which 

means that it presented an extreme decay hazard for wood 

on the exterior of buildings. The MC of the siding remained low at around 8% or lower. There was no appreciable difference in lateral nail resistance (measured near 

the drip edge) between unexposed boards and boards that 

were on the building for 21⁄2 years. 

• There is no evidence that back-priming this siding lowered 

in-service moisture content. 

• Siding on south-facing walls tended to be slightly drier. 

• The type of wall construction or weather barrier had no 

detectable effect on MC of the siding. 

• The MC of the solid wood trim on the gable ends was 

lower on the overhang building. The data also suggest that 

gutters, when functioning properly, can provide protection 

of wood trim on sidewalls similar to that of overhangs. 

There also was evidence that the overhangs significantly 

reduced water leakage behind the window trim. There was 

no difference in siding MC between the two buildings. 

• There was evidence of water leakage behind the window 

trim below more than half of the windows on the guttered 

building. The leakage occurred between the window unit 

and the bottom trim, even though the trim was carefully 

caulked with high-quality caulking. Leakage was due to 

caulk adhesion failure. The leakage had not led to decay 

of the siding or elevated siding MC at the time of siding 

removal. 

• In-place thickness swell on the drip edge of the siding was 

less than 3%. 

• We found no decay in redcedar trim but did find decay in a 

pine doorjamb on the guttered building. In May 1997, the 

MC of the wood trim was generally between 9% and 12%, 

but we found several locations with MCs greater than 

20%. 

• The lap siding provided substantial air pressure equalization across the siding. 

• There was evidence of air leakage past the top plate into 

the wall cavity. This air bypassed the weather barrier and 

created larger than expected air pressures across the gypsum board. This air leakage led to periods when the cavity 

was pressurized with respect to both the inside and 

outside. 

• Wind-induced air pressures across the exterior walls were 

predominantly exfiltrative, even on the windward side of 

the building. Infiltrative pressures only occurred near 

windward corners of the building during short periods of 

time. We found that actual pressures were considerably 

different from published pressure data that were based 

on wind tunnel experiments. 

References 

AHA. 1990. American National Standard for hardboard 

siding. ANSI/AHA A135.6–1990. Palatine, IL: American 

Hardboard Association. 

ASTM. 1996. Standard test methods for evaluating the 

properties of wood-base fiber and particle panel materials. 

ASTM D1037–96a. Sec. 107–110. Vol. 04.09. Annual Book 

of Standards. West Conshohocken, PA: American Society 

for Testing and Materials. 

Bristow, A.; Back, E.L. 1969. Moisture content of fibre 

building boards. Svensk Papperstidning. 72: 367–374. 

HUD. 1992. Field inspection of residences with hardboard 

siding in south Florida. Atlanta, GA: U.S. Department of 

Housing and Urban Development. 

Richards, R.F.; Burch, D.M.; Thomas, W.C. 1992. Water 

vapor sorption measurements of common building materials. 

ASHRAE Transactions 98(2). Atlanta, GA: American 

Society of Heating, Refrigerating, and Air-Conditioning 

Engineers. 

Scheffer, T. 1971. A climate index for estimating potential 

for decay in wood structures above ground. Forest Products 

Journal. 21(10): 25–31. 

TenWolde, A.; Carll, C.; and Malinauskas, V. 1995. 

Airflows and moisture conditions in walls of manufactured 

homes. In: Airflow performance of building envelopes. 

ASTM STP 1255. West Conshohocken, PA: American 

Society for Testing and Materials: 137–155.