Housewrap
Purpose: To explain the function of housewrap, examine its
attributes, and address common problems associated with its use.
Key Issues Housewrap has two functions: to prevent airflow
through a wall and to stop (and drain) liquid water that has
penetrated through the exterior finish.
Housewrap is not a vapor retarder. It is designed to allow
water vapor to pass through.
The choice to use housewrap or building paper depends on the
climate and on specifier or owner preference. Both materials can
provide adequate protection.
Housewrap must be installed properly or it could be more
detrimental than beneficial. Proper installation, especially in
lapping, is the key to successful housewrap use.
Purpose of Housewrap
Housewrap serves as a dual-purpose weather barrier. It not only
minimizes the flow of air in and out of a house, but also stops
liquid water and acts as a drainage plane. Housewrap is not a vapor
retarder. The unique characteristic of housewrap is that it allows
water vapor to pass through it while blocking liquid water. This
permits moist humid air to escape from the inside of the home, while
preventing outside liquid water (rain) from entering the home.
When Should Housewrap Be Used?
Almost all exterior finishes allow at least some water
penetration. If this water continually soaks the wall sheathing and
framing members, problems such as dryrot and mold growth could
occur. Housewrap stops water that passes through the siding and
allows it to drain away from the structural members. In humid
climates with heavy rainfall, housewrap is recommended to prevent
water damage to the framing. Use in dryer climates may not be as
critical, since materials are allowed to adequately dry, although
housewrap also prevents air movement through the wall cavity, which
is beneficial for insulating purposes.
Housewrap or Building Paper?
To answer this question, it is important to know what attributes
are most important for a particular climate. Five attributes
associated with secondary weather barriers are: Air permeability
ability to allow air to pass through
Vapor permeability ability to allow water vapor (gaseous
water) to pass through
Water resistance ability to prevent liquid water from passing
through
Repels moisture ability to prevent moisture absorption
Durability resistance to tearing and deterioration
Installing Housewrap
No matter what product is used (housewrap or building paper),
neither will work effectively if not installed correctly. In fact,
installing housewrap incorrectly could do more harm than not using
it at all.
Housewrap is often thought of and installed as if it were an air
retarder alone. A housewrap will channel water and collect it
whether the installer intends it to or not. This can lead to serious
water damage if the housewrap is installed in a manner that does not
allow the channeled water out of the wall system.
The following are tips for successful installation of
housewrap:
Follow manufacturers instructions.
Plan the job so that housewrap is applied before windows and
doors are installed.
Proper lapping is the key the upper layer should always be
lapped over the lower layer.
Weatherboard-lap horizontal joints at least 6 inches.
Lap vertical joints 6 to 12 inches (depending on potential
wind-driven rain conditions).
Use 1-inch minimum staples or roofing nails spaced 12 to 18
inches on center throughout.
Tape joints with housewrap tape.
Allow drainage at the bottom of the siding.
Extend housewrap over the sill plate and foundation joint.
Install housewrap such that water will never be allowed to flow
to the inside of the wrap.
Avoid complicated details in the design stage to prevent water
intrusion problems.
When sealant is required:
use backing rods as needed, use sealant that is compatible
with the climate,
use sealant that is compatible with the materials it is being
applied to,
surfaces should be clean (free of dirt and loose material), and
discuss maintenance with the homeowner.
Avoid These Common Problems
Incomplete wrapping Gable ends are often left unwrapped,
leaving a seam at the low end of the gable. This method works to
prevent air intrusion, but water that gets past the siding will run
down the unwrapped gable end and get behind the housewrap at the
seam. Also, it is common for builders to pre-wrap a wall before
standing it. If this is done, the band joist is left unwrapped. Wrap
the band joist by inserting a strip 6-12 inches underneath the
bottom edge of the wall wrap. In addition, outside corners are often
missed.
Improper lapping This often occurs because the housewrap is
thought of as an air retarder alone. When applying the housewrap,
keep in mind that it will be used as a vertical drainage plane, just
like the siding.
Improper integration with flashing around doors and windows .
Relying on caulking or self-sticking tape to address improper
lapping Sealant can and will deteriorate over time. A lapping
mistake corrected with sealant will have a limited time of
effectiveness. If the homeowner does not perform the required
maintenance, serious water damage could occur when the sealant
eventually fails. Therefore, do not rely on sealant or tape to
correct lapping errors.
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