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Housewrap
Home Builder's Guide to Coastal Construction
FEMA 499/June 2005
Technical Fact Sheet No. 23

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

As shown in the following table, the climate where the house is located
determines the importance of the attribute.

Table: Product Attribute Rating
Attribute: air permeability
When it is important: windy and cold climates
Building paper performance: fair
Housewrap performance: good

Attribute: vapor permeability
When it is important: hot, humid climates
Building paper performance: fair
Housewrap performance: good

Attribute: water resistance
When it is important: windy and rainy climates
Building paper performance: good
Housewrap performance: excellent

Attribute: repels moisture
When it is important: high rainfall
Building paper performance: good
Housewrap performance: good

Attribute: durability
When it is important: windy, with possible extended exposure
Building paper performance: fair
Housewrap performance: good

Attribute: cost
When it is important: owner preference
Building paper performance: excellent
Housewrap performance: fair

In general, housewrap is a good choice for coastal homes.

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 - See Fact Sheet No.
22.

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.