Occurrence of Mold 

in a Two-Story WoodFrame House Operated at 

Design Indoor Humidity 

Levels

Carol A. Clausen

Jessie A. Glaeser

Samuel V. Glass

Charles Carll

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–186
October 2009

Clausen, Carol A.; Glaeser, Jessie A.; Glass, Samuel V.; Carll, Charles. 

2009. Occurrence of mold in a two-story wood-frame house operated at 

design indoor humidity levels. General Technical Report FPL-GTR-186. 

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

Products Laboratory. 12 p.

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Abstract

Mold growth was observed in limited areas in a two-story 

contemporary wood-frame house during the seventh heating 

season of operation at or near design indoor humidity levels. 

The house was in a cold climate (Madison, Wisconsin). 

Humidity levels were estimated as being exceeded in one 

of 10 homes at this location. Moderate amounts of window 

condensation were observed during each of the seven heating seasons. Areas of observed mold growth were limited 

to the lower extremities of windows (near the lower edge of 

glass panes) and the edges of lower panels of a wood panel 

entry door that was not equipped with a storm door. Mold 

growth was also present in sprayed-on cellulose insulation 

in close proximity to the rim joist (board) in the basement, 

although this mold was not visible unless the insulation was 

disturbed and was probably present during previous heating 

seasons. Mold from these locations was cultured, isolated, 

and identified morphologically or by DNA sequence. Seven 

genera of common ascomycetes and deuteromycetes were 

detected, all of which are commonly associated with indoor 

air. Penicillium was the most common genus, with at least 

six different species. The greatest variety of genera (five) 

occurred in samples taken from interior wood millwork (the 

edges of panels in the entry door and from a wood window 

sash). Only two genera were found in a sample taken from 

the interface between the rim joist board and cellulose 

insulation. Total spore counts taken in April revealed that on 

the first and second stories, mold spores were less prevalent 

than in outdoor air but that mold spores were more prevalent 

in the basement than in outdoor air.

Keywords: mold, spore, indoor air quality, relative humidity, 

condensation

Acknowledgments

The authors acknowledge the support of Windsor Building 

Systems of Madison, Wisconsin, APA—The Engineered 

Wood Association, the Southern Forest Products Association, and the Cellulose Insulation Manufacturers Association. We thank Vina Yang, microbiologist, for conducting 

mold isolations and spore counts; Robert Munson, engineering technician, for data acquisition and monitoring moisture 

conditions in the house; and Dan Lindner, botanist, for conducting DNA analysis on mold isolates.

Contents

Page

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

 The Building and Its Operation........................................1

 Visually Observed Mold Growth .....................................2

Objective ..............................................................................4

Methodology........................................................................4

 Mold Sampling and Culturing .........................................4

 Mold Identification...........................................................5

 Quantification of Airborne Spores...................................5

Results..................................................................................5

 Mold Culturing.................................................................5

 Mold Identification...........................................................5

 Total Spore Count ............................................................5

Discussion............................................................................6

Conclusions..........................................................................9

References..........................................................................10

Appendix—Surface Temperature and Relative Humidity 

Calculations........................................................................12
Occurrence of Mold in a Two-Story 

Wood-Frame House Operated at Design 

Indoor Humidity Levels

Carol A. Clausen, Supervisory Research Microbiologist

Forest Products Laboratory, Madison, Wisconsin

Jessie A. Glaeser, Research Plant Pathologist

U.S. Forest Service, Northern Research Station, Madison, Wisconsin

Samuel V. Glass, Research Physical Scientist

Charles Carll, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin

Introduction

Predicting operational conditions that are conducive to mold 

establishment has historically not been a critical consideration in house design. Mold spores that are ubiquitous in the 

environment require a simple food source, proper temperature, oxygen, and moisture for germination and growth to 

occur. Due to the ready availability of the first three requirements, moisture becomes the controllable limiting factor 

in the initiation of mold growth. Interior humidity is one of 

the most important controllable moisture sources in a house 

(Christian 1994). Moist building materials, such as framing lumber, wet-applied insulation, and especially recently 

poured concrete, can be a very significant moisture source in 

the first year after building construction. Following dissipation of moisture from construction materials, humidity levels during normal operation vary greatly and are primarily 

dependent on number of building occupants, building volume, occupant behavior, air exchange rate, water vapor content of outdoor air, and the manner in which air exchange 

occurs. A large potential source of indoor humidity that is 

not always considered is the building foundation, which is 

in contact with earth that commonly is damp or even wet 

(Christian 1994).

The Building and Its Operation

A dual-purpose research and demonstration house, located 

on the campus of the U.S. Forest Service Forest Products 

Laboratory, was constructed in 2001. The building is not 

occupied but generally has been humidified during the heating season to simulate occupancy. Carll and others (2007) 

describe the building’s construction history, operation, and 

performance through the first five heating seasons. The 

primary research effort thus far has been to identify issues 

associated with house operation in a climate with approximately 7,000 Fahrenheit heating degree days and with high 

internal humidity. Relative humidity within the building was 

targeted to exceed the relative humidity of 9 out of 10 houses at this location. The target humidity was calculated by an 

ASHRAE standard (ASHRAE 2009a) based on assumed 

moisture release rates as affected by the number of occupants, assumed air exchange rate between the house and the 

outside, water vapor content of outdoor air, and an assumed 

indoor temperature of 21 °C (Carll and others 2007, TenWolde and Walker 2001). The design relative humidity 

(RH) values that were calculated for this house are shown 

in Table 1. Design values were intentionally higher than the 

wintertime value of 35% to 40% often cited as risky to exceed in residential buildings of normal construction in cold 

climates (Powell 1994; ASTM 2008).

For the first two heating seasons, the building was not actively humidified, but indoor humidity levels moderately 

close to design levels were nonetheless reached. The probable reasons for this were discussed in an earlier manuscript 

(Carll and others 2007). The building was humidified to 

Table 1—Design and measured values 

of indoor relative humidity by montha

Month

Design

indoor RH 

(%)

Measured 

indoor RH 

(%)

November 2007 50.5 54.5 

December 2007 45.2 45.9 

January 2008 42.4 43.0 

February 2008 41.9 40.1 

March 2008 47.4 44.0 

April 2008 60.8 54.8 

a

Design indoor RH values are calculated according 

to ASHRAE Standard 160, Section 4.3.2, Indoor 

Design Humidity, Intermediate Method (ASHRAE 

2009a), using mean monthly outdoor water vapor 

pressures (NCDC 2007, 2008). The calculations 

assume an indoor design temperature of 21 °C, an 

air exchange rate of 0.2 air changes/hour, a house 

volume of 515 m3

 (18,200 ft3), and a moisture 

production rate of 15 kg/day. Measured indoor RH 

values are monthly averages from the living room 

on the first floor of the house.
2

design levels for the third and fourth heating seasons, generally at substantially lower rates of moisture release from 

humidifiers than the assumed release rate from which design 

humidity levels were calculated. For the fifth, sixth, and seventh heating seasons, a set amount of moisture (10 kg/day) 

was released by three free-standing humidifiers placed in the 

first and second stories of the house. This release rate was 

two-thirds of the design load of 15 kg/day, based on the assumption of five occupants for the four-bedroom house, but 

commonly resulted in design RH levels being reached and, 

in some cases, exceeded. No sole-purpose dehumidification 

equipment was operated in the house in any season, but the 

house was air-conditioned in summer months and the airconditioning undoubtedly provided some dehumidification. 

Humidifiers were used October through April but were inactive May through September. Hourly RH conditions during 

the 2007–2008 heating season are shown in Figures 1 

and 2 for the house living room (great room) and basement, 

respectively. Figure 1 shows substantial variability in hourly 

RH in the living room. Monthly average RH conditions are 

shown in Table 1; over the course of the heating season, indoor humidity levels more or less coincided with the design 

levels.

Visually Observed Mold Growth

During all heating seasons, some window condensation 

was observed. Through the fifth heating season, we noticed 

intermittent window condensation during winter months 

but did not observe the presence of mold. In January 2008, 

visible mold was observed on areas of the lower portions of 

all windows and on areas of the building’s front entry door, 

which faces WNW. The front entrance consisted of a sixpanel wood door and was not provided with a storm door. 

An identical wood panel entry door on the opposite side of 

the same interior room was equipped with an exterior storm 

door and was devoid of mold.1

Mold growth on a window and the panels of the entry door 

are shown in Figures 3 and 4, respectively. The mold was restricted to edge areas of door panels or to areas on windows 

near the lower edge of a glass pane. Most of the windows in 

the house were casement windows. The window sash had 

1Interior surfaces of the door lock hardware of the front entry door 

regularly showed appreciable condensation in cold weather. In 

contrast, we never observed condensation on the lock hardware of 

the rear entry door (which was equipped with a storm door).

0

10

20

30

40

50

60

70

80

1-Nov-2007

Date

7-day running average RH

Hourly temperature

Hourly RH

Temperature (°C) / Relative humidity (%)

15-Nov-200729-Nov-200713-Dec-200727-Dec-2007

10-Jan-200824-Jan-2008

7-Feb-2008

21-Feb-2008

6-Mar-2008

20-Mar-2008

3-Apr-2008

17-Apr-2008

Figure 1—Temperature and RH measured in the living room on the first floor of the 

house from November 2007 through April 2008.

General Technical Report FPL–GTR–186
3

wood cores, with all surfaces of the sash encased in polyvinyl chloride (PVC). These windows had interior wood stops 

at the sides (jambs) and top (head) that contacted the sash 

when it was closed and latched. At the bottom (sill) of the 

window was a piece of wood stop trim. This was machined 

with the same outer profile as the jamb and head stops but 

was hollow-backed to conceal the window operating hardware. The two windows in the basement were of “hopper” 

style—hinged at the bottom, opening inward at the top. All 

surfaces on these windows were PVC. The entry door was 

flanked on each side with a multi-lite side-lite panel. Each 

panel consisted of a 0.36- by 2.0-m wood sash containing 

five vertically stacked panes, with wood muntins between 

panes. All three window types showed mold growth in January 2008. In all cases, mold was visible on the window sash 

but was restricted to the bottom rail of the sash or to muntins 

between panes in the sash. In all cases, the bulk of the mold 

was very close (within 8 mm) to the lower edge of a glass 

pane.2 Mold was also present on the sill window stop trim of 

2The glass in the casement and side-lite windows was insulating 

glass (double-pane sealed insulating glass units (IGUs) with metal 

edge spacers). The glass in the hopper-style basement windows 

was dual pane, without a seal between panes; the outer pane in 

these windows can be removed by removing retainer hardware. 

Because the basement windows did not have sealed IGUs, there 

the casement windows, especially at the corners where the 

stop trim interfaced with the jamb stops. Corridors of mold 

growth, which sometimes coincided with presence of condensed water, were regularly observed between the sash and 

the stop trim. A limited amount of mold was also present on 

interior wood trim below one of the basement windows, but 

this did not appear to be associated with run-down of condensed water from the window sash.3

were no metal edge spacers between the two panes of these windows. Although the glass in the basement windows almost 

certainly had higher U-value (measured at glass center) than the 

glass in the casement units (used on first and second stories of the 

building), and although the basement windows regularly showed 

more condensation and condensation at greater distances from 

the lower glass edge, the prevalence of mold on the sash of these 

windows was not perceptibly greater.

3This window had sweep weather stripping at sash stiles and at 

the head sash rail. It had a ridge in the window frame and a mated 

groove in the bottom rail of the sash. This design is probably very 

effective with regard to prevention of water intrusion but probably 

is less effective than a functional sweep or compression gasket at 

restricting air movement. There were no stain trails between the 

sash and the interior trim. The mold-growth pattern on the trim was 

diffuse. These observations, along with the lack of an obviously effective air seal at the sash bottom rail, led us to suspect that cooling 

of the trim surface by cold air infiltration below the bottom sash 

rail was the probable cause of the moisture that resulted in mold 

growth on the trim.

0

10

20

30

40

50

60

70

80

Hourly RH

Hourly temperature

Temperature (°C) / Relative humidity (%)

1-Nov-2007

Date

15-Nov-200729-Nov-200713-Dec-200727-Dec-2007

10-Jan-200824-Jan-2008

7-Feb-2008

21-Feb-2008

6-Mar-2008

20-Mar-2008

3-Apr-2008

17-Apr-2008

Figure 2—Temperature and RH measured in the basement of the house from 

November 2007 through April 2008.

Occurrence of Mold in a Two-Story Wood-Frame House Operated at Design Indoor Humidity Levels
4

Objective

The objective of this study was to identify mold in the different areas of the building. Identification of species is 

sometimes used as an indicator regarding potential health 

concerns. An associated objective was to document the 

prevalence of airborne fungal spores in indoor air compared 

with their concurrent prevalence in outdoor air.

Methodology

Mold Sampling and Culturing

Following observation of mold on window sash and trim 

and on the front entry door in January 2008, mold was isolated from the entry door and from at least one window on 

each level of the building. Sampling locations are listed in 

Table 2. In addition, mold was sampled from one location 

that was associated with neither a window nor the front 

entry door; this location was at the interface between a rim 

joist board (in the basement) and spray cellulose insulation 

that had been installed over it. 

Sampling was performed with sterile cotton swabs. Swabs 

were streaked on 2% malt extract agar (MEA) in sterile Petri 

plates and incubated at 27 °C and 80% RH. Fungal growth 

was monitored daily. When individual colonies were visible, 

they were subcultured until pure and incubated at 

27 °C and 80% RH.

Figure 3 (a, b)—Mold growth on interior surfaces of 

lower rails of two representative vinyl clad window 

sash. Mold growth is most prevalent near pane edges 

and at corners, where condensate has collected. 

(Photo by Robert Munson, Forest Products Laboratory, 

January 10, 2008.)

(a)

(b)

Figure 4—Mold growth on the edges of panels of the 

front entry door. Evidence of run-down of condensate 

(seen as staining and mold growth) is also apparent 

on the rail below the panel, particularly on the profiled 

edge of the rail. (Photo by Robert Munson, Forest 

Products Laboratory, January 10, 2008.)

Table 2—Mold sampling locations 

Sample 

ID

Structure 

level 

Room

identifier 

Geographic

sample 

location Description Substrate 

1 Main Living 

room

ESE Window

sash 

PVC 

2 Main Living 

room

ESE Window

stop

Wood 

3 Main Front

entry 

WNW Bottom 

door panels

Wood 

4 Main Front

entry 

WNW Center door 

panels 

Wood 

5 Second Bedroom WNW Window

sash 

PVC 

6 Second Bathroom ESE Window

sash 

PVC 

7 Second Bathroom ESE Window

stop

Wood 

8 Lower Basement WNW Rim joist / 

insulation 

interface 

Cellulose 

9 Lower Basement NNE Window

sash 

PVC 

10 Main Front

entry 

WNW Window

stop

Wood 

General Technical Report FPL–GTR–186
5

Mold Identification

Microscopy

Fungal cultures were identified using light microscopy to 

analyze morphological characteristics, including the size 

and shape of spores and spore-bearing structures (Samson 

and others 2000, Wang 1990). If cultures contained more 

than one spore type or colony morphology, cultures were 

subcultured on 2% MEA (Difco, Detroit, Michigan) until 

pure. Identification keys by Samson and others (2000) and 

Wang (1990) were useful for identifying most of the fungi 

to the species level. Penicillium isolates that could not be 

identified microscopically were placed into morphological groupings based on growth characteristics on four differential media—Czapek Dox agar, Creatine sucrose agar 

(CREA), Czapek yeast extract agar (CYA), and yeast extract 

sucrose agar (YES) (Samson and others 2000). Representatives of each morphological grouping were then identified 

using DNA sequencing.

DNA Analysis

Selected Penicillium cultures that could not be identified 

microscopically were subjected to DNA sequencing analysis, following the protocol of Lindner and Banik (2008). 

Small pieces of mycelium from MEA plates were prepared 

for DNA extraction by grinding in a cell lysis buffer. DNA 

was cleaned using GeneClean III kits (Qbiogene, Carlsbad, 

California) and internal transcribed spacer (ITS) sequences 

were amplified by polymerase chain reaction (PCR) using the fungal-specific primer pair ITS1F and ITS4. PCR 

products were cleaned using ExoSap (USB Corporation, 

Cleveland, Ohio), followed by sequencing using the BigDye 

sequencing protocol (ABI Prism). Sequencing products 

were analyzed at the University of Wisconsin Biotech Center (Madison, Wisconsin) and final sequences were aligned 

using Sequencher 4.2 (GeneCodes Corporation, Ann Arbor, 

Michigan). Sequences were assigned a putative identification based on BLAST comparisons to GenBank (NCBI) 

sequences.

Quantification of Airborne Spores

Each level of the structure was sampled three times consecutively at five locations for total spore counts using a 

VersaTrap® Spore Trapping Cassette (SKC, Eighty Four, 

Pennsylvania) in conjunction with a bioaerosol QuickTake®

sampling pump (SKC, Eighty Four, Pennsylvania). The 

sampling pump pulls a sample of air through a narrow slit 

in the spore trapping cassette and deposits particles onto a 

clear glass slide coated with an optically clear sticky collection substrate. The sampling pump operated at 15 L/min 

for 2 min. Collected particles are held by the substrate in a 

well-defined rectangular-shaped particulate deposition trace. 

After sampling, the glass slide is removed from the cassette 

and examined microscopically. Sampling occurred near the 

conclusion of the heating season in April 2008. Three consecutive samples were collected immediately following the 

interior sampling at each of five exterior locations to establish a background spore count. Based on a defined field area 

and defined sampling area(s), a multiplier defined as the 

ratio of the total field area (μm2) to the total sampling area 

(μm2) divided by total volume (L) of air was determined. 

The resulting multiplier (1.17) was used to estimate the average spore count for each location, which was expressed as 

colony forming units (CFU) per unit volume (m3) of air.

Results

Mold Culturing

All areas of visible mold growth are described in Table 2 

and defined according to the structure level, room description, geographic direction, product description, and substrate 

material. Figures 3 and 4 are representative of the extent of 

the mold growth. Mold was cultured from visible locations 

on all three levels of the house from window and door surfaces facing all geographic directions. Substrates supporting 

mold growth included PVC, cellulose insulation, and wood. 

Mold is capable of growth on virtually any substrate under 

favorable moisture conditions by using nutrients from detritus that has collected on surfaces, or in the case of cellulosebased building materials, by digesting simple sugars found 

in the wood component. Mold fungi can also adhere to, 

colonize, and change the physiochemical properties of plastic polyvinyl chloride (Bochkareva and Orchinnikov 1977; 

Webb and others 1999, 2000).

Mold Identification

Table 3 summarizes the genus and species of each isolate 

cultured. A total of seven genera were identified that generally represent commonly isolated mold genera from homes. 

Penicillium and Aureobasidium were most frequently isolated, followed by Alternaria and Aspergillus. In a report 

on mold biodiversity in homes, Cladosporium, Penicillium, 

and Aspergillus were the most frequently encountered genera (Beguin and Nolard 1994). In a study of Danish homes 

infected with mold, Penicillium chrysogenum was found to 

be dominant in spring and autumn, followed by fewer numbers of Aspergillus versicolor. Cladosporium was commonly 

isolated during summer (Bech-Anderson and Elborne 2003). 

Aureobasidium pullulans was detected in most samples 

throughout the house. This is a very common fungus in most 

homes and is often associated with high moisture areas, 

including bathrooms, kitchens, and windowsills. The six 

species of Penicillium, identified by DNA sequencing, are 

all common species frequently associated with indoor air 

(Samson and others 2000). Isolate 8-2, Penicillium chrysogenum, matched the identification of a yellow mold isolated from wet cellulose insulation in the basement of the 

structure in 2001. The isolate was designated PH02 (Clausen and Yang 2003).

Total Spore Count

Total interior spore counts for each level of the structure are 

compared to the exterior spore count in Table 4. Five sampling locations per structural level revealed small variations 

Occurrence of Mold in a Two-Story Wood-Frame House Operated at Design Indoor Humidity Levels
6

in spore count estimations. The structure is conditioned, 

so windows remain closed and only the front entry door is 

routinely opened for access to the structure. While five of 

the eight genera were cultured from the front entry door and 

side-lite windows, the main level had the lowest average 

spore count (251 CFU/m3) compared with the exterior spore 

count (499 CFU/m3). The indoor/outdoor ratio of spores, 

an indicator of indoor air quality, was 1.52 for the basement 

level, 0.50 for the main level, and 0.66 for the second level. 

Ideally, the ratio for indoor to outdoor spore levels should be 

less than 1.0 and a problem is indicated for a ratio well over 

1.0.

Numeric standards for acceptable concentrations of mold 

spores in the home environment are currently lacking 

(Stetzenbach and others 2004). Likewise, environmental 

sampling methods are not capable of characterizing the sequence of events resulting in mold growth. Rather, they are 

intended to provide data at a given moment in time. In the 

absence of numerical guidelines, sampling data provide an 

indication of humidity loads, indoor/outdoor ratios of fungal 

concentrations, and the presence of a species that may indicate a health concern.

Discussion

Two factors provide a likely explanation for the presence of 

noticeable mold growth in the 2007–2008 heating season. 

The first factor is progressive accumulation of airborne detritus over time that would serve as a nutritional source for 

the mold growth. The second factor is that surface temperature and RH conditions in the 2007–2008 heating season 

were more conducive to mold growth than they had been in 

previous heating seasons (discussed below).

Mold was present on the lower portions of the sash of every 

window in the house by January 2008. Because the overwhelming majority of windows had sash with PVC surfaces, 

much of the observed mold occurred on PVC surfaces. 

Even without the presence of detritus as a food source, 

mold growth can occur on inert surfaces (Bochkareva and 

Orchinnikov 1977; Webb and others 1999, 2000). In cases 

of prolonged high humidity or condensation, some species 

of Penicillium, Aspergillus, and Alternaria can grow on 

glass and plastic (Zabel and Morrell 1992). The house was 

unoccupied, although it was entered periodically for tours 

and research work. A large amount of particulate matter was 

found in the basement air samples. The duct system for the 

house is equipped with an electrostatic filter. The ducts in 

the basement are sealed with a combination of metal foil 

tape and mastic; the basement has neither supply nor return 

registers. The first and second stories of the building appear 

to be realizing a beneficial effect from the electrostatic filter. 

In contrast, the basement, by reason of its 

isolation from the duct system, apparently realizes no or 

minimal removal of airborne particulates by the filter.

Dust accumulation on window sashes on the first and second 

stories was largely imperceptible. Some progressive collection of airborne detritus on them would nonetheless be 

expected over time, even if the accumulation was not readily observable. In contrast to the interior surfaces of window 

sashes, those portions of sash edges that were outboard of 

the weather strips collected noticeable amounts of dirt, silk 

cocoons, and dead insects over time.

Table 3—Mold isolate identification 

Sample ID 

Isolate

number Identification 

1 (Window 1 Aureobasidium pullulans var. melanogenum

 sash) 2 Aureobasidium pullulans 

2a Aureobasidium pullulans var. pullulans 

2b Penicillium glabrum 

3 Aureobasidium pullulans var. melanogenum 

2 (Window 1 Aureobasidium pullulans 

 stop) 2 Penicillium funiculosum 

3 Penicillium funiculosum

4 Penicillium funiculosum; Byssochlamys nivea

5 Byssochlamys nivea

3 (Bottom 1 Penicillium corylophilum 

 door 3 Penicillium corylophilum 

 panel) 3a Penicillium corylophilum

3b Aureobasidium pullulans var. pullulans

4 (Center 1 Penicillium corylophilum 

 door 2 Penicillium corylophilum

 panel) 3 Aureobasidium pullulans; Penicillium 

corylophilum

5 (Window 1 Cladosporium cladosporioides 

 sash) 2a Aureobasidium pullulans var. pullulans

2b Aspergillus niger

3 Aspergillus niger

4 Alternaria alternata 

5 Aureobasidium pullulans var. melanogenum

6 (Window 1 Penicillium corylophilum 

 sash) 2 Aureobasidium pullulans var. melanogenum

3a Aureobasidium pullulans var. pullulans

3b Penicillium purpurpogenum 

4 Byssochlamys nivea 

7 (Window 1a Penicillium sp. A 

 stop) 1b Aspergillus niger

2a Penicillium mineoluteum 

2b Penicillium mineoluteum 

8 (Rim 1 Phialemonium dimorphosporum 

 joist) 2 Penicillium chrysogenum 

9 (Window 1 Penicillium corylophilum 

 sash) 2 Aureobasidium pullulans var. melanogenum

3 Cladosporium cladosporioides 

4 Bacteria and/or yeasts 

10 (Window 1 Penicillium corylophilum 

 stop) 2 Alternaria alternata 

3 Aureobasidium pullulans var. melanogenum

4 Alternaria alternata 

5 Aureobasidium pullulans var. melanogenum

Cladosporium cladosporioides 

Penicillium sp. B 

6 Alternaria alternata 

General Technical Report FPL–GTR–186
7

Outdoor temperatures during the various heating seasons 

are compared in Table 5. The table shows the mean outdoor 

temperature and number of heating degree days for the 

three-month period from December through February for 

each of the years since the house was constructed. Corresponding values from monthly climate normal data for the 

period 1971–2000 are shown for reference. It is apparent 

from the table that the first six winters were milder than the 

30-year average, whereas the seventh winter (2007–2008) 

was colder than average. Further examination of the weather 

records (not shown) indicates that the 2007–2008 heating 

season was consistently cold; each of the months from November 2007 through March 2008 ranked as the coldest or 

second coldest of the seven heating seasons.

The surface temperature on the interior edges of the front 

entry door panels was calculated from hourly measured 

values of indoor temperature and outdoor temperature. The 

interior surface RH was then calculated from hourly indoor 

vapor pressure, based on indoor temperature and RH measured in the living room, and from hourly calculated surface 

temperature. The combination of surface temperature and 

RH were supportive of mold growth.

The panels in the entry door were 8 mm thick at their edges; 

the thermal resistance of the panels was thus low, particularly at their edges. As a consequence, calculated values of 

interior surface temperature are largely dominated by the 

thermal resistance of the air layer in contact with the exterior surface of the panel. This in turn depends on the local 

air speed, for which we lacked measurements. We therefore 

considered two extremes: (1) we assumed the thermal resistance of the exterior surface film to be the value for still air, 

the “high exterior thermal resistance” case; (2) we assumed 

the thermal resistance of the exterior surface film to be 

the value corresponding to air moving at a speed of 

3.4 m s–1, the “low exterior thermal resistance” case. The 

value of 3.4 m s–1 was the average wind speed from November 2007 through April 2008 measured at the nearest 

National Weather Service (NWS) station at a height of 

10 m (NCDC 2007, 2008). The NWS station was 10 km 

from the house. The wind instrument at the NWS station is 

in an open, unshielded location. Further details of the calculation methodology are given in the Appendix. Figures 

5 and 6 show calculated temperature and RH values at the 

interior surface of the wood panel door from November 

2007 through April 2008 for high and low exterior thermal 

resistance cases, respectively.

Figures 5 and 6 indicate that at either assumed extreme 

estimate for local wind conditions, there would have been 

prolonged periods during mid-winter when surface temperature and RH conditions failed to meet the criteria listed in 

ASHRAE Standard 160 (ASHRAE 2009a) as “necessary to 

minimize mold growth.” ASHRAE Standard 160 lists three 

sets of conditions, each consisting of a temperature and 

surface RH combination, coupled to a time duration 

(Table 6). Under the high exterior thermal resistance case, 

two of the three sets of conditions were not met (conditions 

over 30 days and 24 hours). Under the low exterior thermal 

resistance case, all three sets of conditions were not met.

Table 4—Total airborne spore count by location

Airborne spore count at five sampling locationsa

Structure level 1 2 3 4 5 Average (S.D.)b

Interior 

 Basement 761 570 566 665 671 756 (95) 

 Main 225 245 191 221 191 251 (27) 

 Second 251 296 341 270 251 329 (44) 

Exterior 420 405 341 530 440 499 (80) 

a

Expressed in CFU/m3, colony forming units per cubic meter of air. b

Average of five sampling locations per structure level, n = 3; S.D., standard deviation. 

Table 5—Mean outdoor temperature and number 

of heating degree days for the three-month 

period from December through Februarya

Period

Mean 

outdoor

temperature 

(°C)

Heating 

degree

daysb

December 2001–February 2002 –1.3 3,159 

December 2002–February 2003 –6.0 3,912 

December 2003–February 2004 –5.5 3,880 

December 2004–February 2005 –4.0 3,595 

December 2005–February 2006 –4.1 3,612 

December 2006–February 2007 –5.0 3,753 

December 2007–February 2008 –7.5 4,211 

30-year average (1971–2000) –6.2 3,991 

a

Calculated from NCDC (2007, 2008) data. 

b

65 °F basis. 

Occurrence of Mold in a Two-Story Wood-Frame House Operated at Design Indoor Humidity Levels

Table 6—Conditions necessary to minimize mold 

growth as outlined in ASHRAE Standard 160 

(ASHRAE 2009a) 

Period for running 

averages 

Running

average 

surface RH 

(%)

Running average 

temperature 

30 days (720 hours) <80 Between 5 °C and 40 °C 

(41 °F and 104 °F) 

7 days (168 hours) <98 Same as above 

24 hours <100 Same as above 
8

The rim joist area has been identified as an area that, along 

with windows, is subject to condensation during cold 

weather (Cautley 2004). The spray cellulose insulation was 

not covered with an interior vapor retarder, increasing the 

susceptibility of the rim joist area to moisture accumulation 

during the heating season. The rim joist board is a composition rim board (essentially an oriented strandboard (OSB) 

of approximately 30 mm thickness) of the same depth dimension as the I-joists. There was mold on the surface of 

the rim board and the insulation that contacted it. Mold was 

cultured from a sample of the insulation. We suspect that 

mold was present in previous heating seasons based on its 

presence within the insulation of a wall that was opened for 

inspection during the fourth heating season (Carll and others 

2007). This wall, like the rim joist area, did not incorporate 

an interior vapor retarder. 

Borate additive in cellulose insulation has been postulated as 

inhibitory to mold or to possibly select for certain mold species (Carll and others 2007). Borates at high loadings have 

been found to be marginally effective at inhibiting growth 

of mold fungi (Barnes and others 1989). Clausen and Yang 

(2003) reported that 5% disodium octaborate tetrahydrate 

(DOT) did not substantially inhibit Penicillium chrysogenum, Aspergillus niger, or Trichoderma viride. They estimated a minimum fungicidal concentration (MFC90) of 

3.7 mg/mL for boric acid (Clausen and Yang 2005). Micales-Glaeser and others (2004) reported that 8.5% DOT 

controlled growth of A. niger, but 15% DOT provided only 

sporadic control of Penicillium brevicompactum, Stachybotrys chartarum, and Cladosporium cladosporioides. Too 

few samples were taken in this study to draw conclusions 

about mold diversity or selectivity on various substrates.

0

10

20

30

40

50

60

70

80

90

100

Hourly surface RH

30-day running average surface RH

Hourly surface temperature

30-day running average surface temperature

Temperature (°C) / Relative humidity (%)

1-Nov-2007

Date

15-Nov-200729-Nov-200713-Dec-200727-Dec-2007

10-Jan-200824-Jan-2008

7-Feb-2008

21-Feb-2008

6-Mar-2008

20-Mar-2008

3-Apr-2008

17-Apr-2008

Figure 5—Calculated temperature and RH at the interior surface of a panel edge in 

the entry door from November 2007 through April 2008 for the case in which high 

exterior thermal resistance is assumed.

General Technical Report FPL–GTR–186
9

Conclusions

Mold growth occurred in an unoccupied wood-frame house 

in a cold climate after roughly 6-1/2 heating seasons of operation near or at design indoor humidity levels. The sites 

were primarily areas where intermittent condensation had 

been observed through all previous heating seasons, specifically on windows in close proximity to the lower edges 

of window panes. The other areas were in places where 

condensation had not previously been noted but where it 

probably had nonetheless occurred. These areas included 

the edges of lower panels on a wood panel entry door that 

was not equipped with a storm door and in the rim joist area 

in the basement. Seven genera of common mold fungi were 

isolated and identified. The diversity of species was similar 

on wood interior millwork (12 species in five samples) and 

PVC (10 species in four samples). Two mold species were 

isolated from the single sample of cellulose insulation. The 

indoor/outdoor ratio of fungal spores, measured in April, 

was appreciably below 1.0 in the normal living areas of 

the house where supply and return registers for a forced-air 

heating and cooling system were present. These parts of the 

house received conditioned air that was filtered through an 

electrostatic furnace filter. In contrast, the indoor/outdoor 

ratio of fungal spores was 1.5 in the basement. The basement had neither supply nor return registers, and the ducts in 

the basement were sealed. Minimal circulation of basement 

air through the furnace filter would thus have been expected. 

The presence of general particulate matter, as well as spore 

concentration, was distinctly higher in basement air than in 

air on first- and second-story levels.

-20

0

20

40

60

80

100

Hourly surface RH

30-day running average surface RH

Hourly surface temperature

30-day running average surface temperature

Temperature (°C) / Relative humidity (%)

1-Nov-2007

Date

15-Nov-200729-Nov-200713-Dec-200727-Dec-2007

10-Jan-200824-Jan-2008

7-Feb-2008

21-Feb-2008

6-Mar-2008

20-Mar-2008

3-Apr-2008

17-Apr-2008

Figure 6—Calculated temperature and RH at the interior surface of a panel edge in 

the entry door from November 2007 through April 2008 for the case in which low 

exterior thermal resistance is assumed.

Occurrence of Mold in a Two-Story Wood-Frame House Operated at Design Indoor Humidity Levels
10

Weather conditions were colder during the seventh heating 

season than they had been during all the previous heating 

seasons experienced by the house. Calculations indicated 

that at the edges of wood panels in the entry door, which 

was not equipped with a storm door, the combination of 

outdoor temperature and indoor humidity levels would have 

resulted in conditions supportive of the observed 

mold growth.

References

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ASHRAE. 2009b. Chapter 1: Psychrometrics. In: 2009 

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Occurrence of Mold in a Two-Story Wood-Frame House Operated at Design Indoor Humidity Levels
Appendix—Surface Temperature 

and Relative Humidity Calculations

The temperature Tsurf at the interior surface of the wood 

panel door at its location of least thermal resistance (its narrowest profile) is calculated as follows:

(A–1)

where Ti and To are the hourly measured indoor and outdoor 

temperatures, respectively; Rsurf,i is the thermal resistance of 

the interior surface film; and Rtotal is the sum of the thermal 

resistances of the interior surface film, the wood panel, and 

the exterior surface film.

The following assumptions are made in calculating Tsurf:

1. The thermal resistance of the interior surface film Rsurf,i is 

the value for still air, 0.12 m2 K W–1 (ASHRAE 2009c).

2. The thermal resistance of the wood panel door at its narrowest profile, having a thickness of 8 mm, is 0.063 m2 

K W–1, as calculated from equation (3–7) of the Wood 

Handbook (FPL 1999) for an assumed moisture content 

of 12% and an assumed specific gravity of 0.45.

3. In the “high exterior thermal resistance” scenario, the 

thermal resistance of the exterior surface film is the value 

for still air (same as Rsurf,i).

4. In the “low exterior thermal resistance” scenario, the 

thermal resistance of the exterior surface film is 

0.052 m2 K W–1. This value corresponds with an 

air speed of 3.4 m s–1 (ASHRAE 2009c).

The relative humidity RHsurf at the interior surface of the 

wood panel door is calculated as follows:

(A–2)

where pi is the indoor water vapor pressure and psat(Tsurf) is 

the water vapor saturation pressure corresponding to temperature Tsurf. Values of psat(Tsurf) are calculated using standard psychrometric relationships (ASHRAE 2009b). Values 

of pi are calculated from hourly measured values of indoor 

temperature and relative humidity.

The following assumptions are made in calculating RHsurf:

1. The water vapor content of air at the interior surface of 

the door is equivalent to that measured in the living room 

on the first floor.

2. The finish coating on the interior surface of the door is 

impermeable and non-hygroscopic; that is, water vapor 

does not diffuse into the door.

3. When the surface reaches a condition of 100% RH, 

condensation is not tracked. Accumulation, runoff, and 

evaporation of condensate are ignored. Similarly, the 

( ) Rsurf,i

Rtotal

12

General Technical Report FPL–GTR–186

effect on Tsurf of the enthalpy of vaporization is ignored. 

RHsurf may drop below 100% at the next hour simply 

from changes in indoor or outdoor conditions.

Tsurf = Ti – (Ti – To)

RHsurf = × 100%

pi

psat(Tsurf)