The finish line

A Forest Products Laboratory finishing factsheet

Forest

Service

Forest

Products

Laboratory

United States

Department of

Agriculture

Moisture accumulation and

extreme fluctuations in moisture

levels can adversely affect the

service life of components, such

as wood siding and windows.

Adverse moisture conditions can

induce checking, warping, paint

failure, and in severe cases,

rotting of the wood.

Proper building design and

construction can help prevent

moisture accumulation or excessive moisture fluctuation within

building components. The following are among the well-known

practices to prevent moisture

accumulation within exterior

siding:

• Use dry materials during construction,

• Provide adequate clearance to

grade and drainage at grade,

• Design with adequate roof

overhang,

• Install appropriate flashings, and

• Install an interior vapor retarder

in cold climates.

An additional less-used technique

that can improve performance of

wood-base horizontal lap sidings

is to install vertical furring strips

between the sheathing and siding.

Studies performed at the USDA

Forest Service, Forest Products

Laboratory (FPL), during the

1930s and early 1950s indicated

how rain can wet the back of

horizontal wood lap siding. Rain

water was shown to reach the

space between the siding and the

sheathing, which confirmed that it

was appropriate to use a waterresistant barrier, such as asphalt

felt, over sheathing to prevent

further penetration of rain water

into the wall. Studies in the 1970s

and 1980s suggested siding could

be dried by solar heating, with

some water moving from the

siding into the wall. This may

raise moisture levels within the

wall, but limited data suggest that

this vapor migration does not

usually cause serious moisture

problems.

During the 1980s, foam sheathing

became popular because of its

superior insulating properties.

Foil-faced sheathing and extruded

polystyrene sheathing retard

movement of water vapor; their

vapor permeability is much lower

than sheathing systems such as

asphalt felt applied over lumber,

wood fiberboard, or plywood.

Thus, water that wets the back of

horizontal wood siding installed

over foam sheathing is expected

to stay in the siding for longer

periods, particularly if the finish on

the face of the siding retards

evaporation. Although not conclusive, some experimental evidence

shows that in warm, humid

climates wood-based siding

installed over foam sheathing

stays at a higher moisture level

and undergoes greater moisture

fluctuation than similar woodbased siding installed over wood

fiberboard sheathing. We expect

that installation of wood-based

siding applied over furring strips

accelerates drying of rain-wetted

siding, particularly when installed

over foam sheathing.

During the 1950s, some U.S.

builders began installing wood

siding over wood furring strips

(usually plastering lath). These

builders reported improved paint

retention when siding was installed in this manner. The

practice of installing wood siding

over furring strips is a tradition in

Scandinavian countries, where

the climate is damp and relatively

nondecay-resistant woods (e.g.,

spruce and pine) are used for

siding.

A limited amount of experimental

data indicates that ventilating the

siding results in lower moisture

levels in the siding. The Moisture

Control Handbook, published by

Oak Ridge National Laboratory

(Oakridge, TN), describes the

rain screen design (Fig. 1) as a

way to reduce the amount of

rainwater entering into the walls.

In addition to furring strips, a rain

screen design with wood siding

consists of a relatively airtight

sheathing and an airspace

between the sheathing and the

siding that is open at the bottom

and allows unrestricted air

exchange with a ventilated soffit

or overhang. Adequate airtightness of the sheathing can be

attained by a variety of methods.

The Moisture Control Handbook

indicates that careful installation

of asphalt felt over plywood or

Before You Install Exterior

Wood-Based Siding
siding boards by inserting spaced

wedges or shims under each

course of siding. This increases

the width of the opening, thereby

reducing capillary rise and

facilitating drying of the back of

the siding after rain showers.

However, wedges also provide

larger openings for wind-driven

rain and may not be appropriate

for some locations.

In summary, ventilating horizontal

lap siding may improve its service

life. If the siding is installed using

a rain screen design, the rain

water will probably not penetrate

past the siding. Installing horizontal lap siding using the rain screen

method is also reasonably

inexpensive and easy to execute.

_________________

Mark Knaebe, Chemist, and Charles

Carll, Forest Products Technologist,

both from the USDA Forest Service,

Forest Products Laboratory, One

Gifford Pinchot Drive, Madison, WI

53705-2398.

The use of trade or firm names is

for information only and does not

imply endorsement by the U.S.

Department of Agriculture of any

product or service.

December 1995

Air space open at bottom

(insect screen may be

desirable at this location)

Wood siding (leaky)

Air space formed

by furring strips

(open top and bottom)

Building paper/wind barrier

(housewrap) sheathing (tight)

Batt insulation

(faced or unfaced)

Figure 1—Wood frame wall designed as a rain screen

oriented strandboard should

provide adequate airtightness.

Other sealing methods include

using foam sheathing and tape or

synthetic sheet air barriers,

commonly called “housewrap”.

Some architects and one lumber

trade association advise priming

the backside of solid-wood siding

with paint or a water-repellent.

Research performed at the FPL

during the 1950s conclusively

indicated that back priming the

siding with a water-repellent

improved paint retention and

overall performance of horizontal

wood lap siding. However, the

benefits of back priming the siding

with paint has not been experimentally verified. If the lower

portion of the siding back is

primed, it is likely that the siding

will absorb less moisture. Note

that if the entire siding back is

primed with paint, water that has

been absorbed by the siding will

be retarded from evaporating.

If your siding has already been

installed, you can still reduce

capillary rise of water between lap