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Moisture Barriers

There are two types of moisture that affect buildings. The first type is bulk water like rain and ground water. The second type is water vapor that is water in a gaseous state suspended in the air. This discussion addresses bulk water. Moisture barrier projects are planned to control the movement of bulk water. Water attempting to penetrate the building envelope must be kept out or managed by allowing the water to evaporate without causing damage.


The primary role of the moisture barrier in the foundation assembly is to keep groundwater from penetrating through the foundation and into the basement. Water goes through a foundation by capillary movement. Capillary movement is when water is absorbed through the small pores in a substance, which act as a passageway through the substance.

Installing an effective moisture barrier for a foundation is accomplished by using a combination of construction techniques:

  • Using gutters and rainspouts to direct roof run-off away from the ground next to the foundation.

  • Sloping the finished grade away from the foundation so water runs off away from the building.

  • Installing impermeable barriers under the surface of the ground to prevent water from penetrating and encourage run-off.

  • Installing an effective drainage system outside of the foundation and backfilling the drainage area with fill that allows water to easily drain down to the drainage pipes.

  • Properly waterproofing a foundation, which includes using capillary breaks between parts of the foundation (the footing and the wall for example).

  • In retrofit or failed situations, providing an impermeable barrier and drainage system inside the foundation and a means of discharging the water outside.


Moisture barriers on walls are primarily used to prevent rainwater from entering the wall assembly. The approach for managing rainwater is to prevent as much water as possible from being deposited on the walls and controlling the water that does get deposited.

            There are four major forces that move water through a substance:

  • Air Pressure Difference from one side of the substance to the other

  • Momentum of the water

  • Surface tension of the water

  • Gravity

 The primary methods for controlling water that does get past the siding are:

Pressure Equalized Rain Screens

This is a system where strapping is attached to house wrap covered sheathing and the siding is attached to the strapping. This creates an air barrier in back of the siding. This air barrier removes any pressure differences between the front and back of the siding and therefore eliminates one of the forces that drives moisture through the wall assembly. The air space also allows any moisture that does get through to either evaporate or run down the house wrap to flashing and back outside.

Redundant barriers

This is a system that uses moisture impermeable house wraps (like Tyvak and Typar) and flashing to direct any water that gets through the siding down to the bottom of the wall assembly and back outside. For this method to be effective, the house wrap must be properly installed and sealed so that no holes are present. Flashing must also be properly installed to work in conjunction with the house wrap so that any water draining down the house wrap can be directed back outside


Roof assemblies are designed to shed water and moisture rather then act as a barrier as a house wrap does on wall assemblies. Typical shingle roof construction can include plywood sheathing covered with overlapping layers of roofing paper, which then get covered with shingles. Building a roof assembly this way works well as long as water is allowed to run off the roof. But if conditions such as ice dams exist along the edge of a roof or in a roof valley, water can get backed up under the layers of shingle and roofing paper and leak through the roof into an attic or living area.

Closed-cell foam is effectively waterproof:

  • Foam will not be damaged by roof leaks, foundation leaks, or condensation.

  • Foam can be used below grade and in masonry construction without being damaged from water penetration.

  • Foam sealants can help to protect structures against wind-driven rain penetration.


Bynum, Richard, 2001. Insulation Handbook, McGraw-Hill, New York, NY

Lstiburek, Joseph and John Carmody, 1993. Moisture Control Handbook, Van Nostrand Reinhold, New York, NY

Lstiburek, Joseph, 1998. Builders Guide: Cold Climates, Building Science Corporation, Westford, MA

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