Moisture Barrier Systems
Home Builder’s Guide to Coastal Construction
FEMA 499/June 2005 
Technical Fact Sheet No. 9

Purpose: To describe the moisture barrier system, explain how typical wall 
moisture barriers work, and identify common problems associated with moisture 
barrier systems.

Key Issues
--A successful moisture barrier system will limit water infiltration into 
unwanted areas and allow drainage and drying of wetted building materials. 
--Most moisture barrier systems for walls (e.g., siding and brick veneer) are 
“redundant” systems, which require at least two drainage planes (see page 2).
--Housewrap or building paper (asphalt-saturated felt) will provide an adequate 
secondary drainage plane.
--Proper flashing and lapping of housewrap and building paper are critical to a 
successful moisture barrier system.
--Sealant should never be substituted for proper layering.

The purpose of the building envelope is to control the movement of water, air, 
thermal energy, and water vapor. The goal is to prevent water infiltration into 
the interior, limit long-term wetting of the building components, and control 
air and vapor movement through the envelope.

Locations and Causes of Common Water Intrusion Problems
--Poor water shedding from roof: use moderate overhangs of 12 to 16 inches, drip 
edges, and a gutter system.
--Roof/wall intersection: install effective kick-out flashing at roof-to-wall 
intersections, diverter flashing around trapped-valleys, and rake flashing.
--Flashing around windows: proper lapping is key to leak prevention. Do not 
depend on sealant for sustained protection. Protect flashings with overlapping 
wrap.
--Door sills: use pan flashing to prevent damage to subfloor.
--Improper flashing over doors: proper lapping is key to leak prevention. Do not 
depend on sealant for sustained protection. Protect flashings with overlapping 
housewrap or building paper. 
--Improper flashing and damaged housewrap or building paper at wall 
penetrations: follow window flashing techniques at every wall penetration.
--Damaged or improperly installed siding: follow manufacturer’s guidelines. 
Prime all surfaces of wood siding (back-priming) before applying top coats.
--No housewrap or building paper used, or improperly lapped: virtually all 
siding leaks. Use housewrap or building paper to shed water. Properly lap 
material so water flows without bucking seams. Water must be allowed to drain 
out of walls.

The location of water entry is often difficult to see, and the damage to 
substrate and structural members behind the exterior wall cladding frequently 
cannot be detected by visual inspection.

Proper Lapping Is the Key…
Proper lapping of moisture barrier materials is the key to preventing water 
intrusion. Most water intrusion problems are related to the improper lapping of 
materials. Usually, flashing details around doors, windows, and penetrations are 
to blame. If the flashing details are right and the housewrap or building paper 
is properly installed, most moisture problems will be prevented. Capillary 
suction is a strong force and will move water in any direction. Even under 
conditions of light or no wind pressure, water can be wicked through seams, 
cracks, and joints upward behind the overlaps of horizontal siding. Proper lap 
distances and sealant help prevent water intrusion caused by wicking action.

How a Redundant Moisture Barrier Works
--Siding: the siding is the first line of defense, but by no means should it be 
the only protection from outside moisture. Sidings shed most of the water, but 
some does get through, especially in coastal areas where high winds can drive 
rain.
--Housewrap or Building Paper: Housewrap or building paper is a dual-purpose 
protection layer. It sheds water that gets through the siding and limits air 
intrusion from the outside. A unique feature of this barrier is that it sheds 
water, but allows water vapor to pass through. This permits water vapor from the 
inside to pass through without condensing on the framing.
--Sheathing: If structural sheathing is used, it should be protected from 
moisture. Prolonged wetting, especially without the ability to drain the 
moisture and dry out, will damage the sheathing.
--Vapor Retarder: In cold regions, a vapor retarder is often used on the warm 
side of the wall cavity to minimize the movement of vapor from the inside of the 
building into the wall cavity, where it will condense on the cool framing 
members. Vapor retarders are typically used only where the predominant vapor 
drive is from the inside to the outside (cold climates).

[End of Fact Sheet No. 9]