Using vapor retarders to manage airflow and reduce moisture

by Ted Winslow
Industry codes are tightening the building envelope and increasing the required R-value of walls. This is a good thing for energy savings and thermal comfort. Yet, one change to a building’s system sets forth a series of other changes.

The tight-envelope construction techniques to which architects and builders are now required to adhere have led to a steep reduction in air movement through walls. This means moisture gets trapped inside wall cavities without sufficient means for it to escape, leading to reduced drying potential for a wall’s interior. Therefore, the airtight standards for energy efficiency have created new challenges for moisture management that cannot be neglected. This ‘moisture sandwich’ is occurring with increasing frequency as architects design walls and incorporate newer insulation practices to enhance energy efficiency.

How moisture enters a building envelope
The first step is to understand how moisture penetrates wall assemblies. Generally, there are four forces driving moisture through the building envelope:

• gravity;
• capillary suction;
• water vapor diffusion; and
• airborne movement of water vapor.

Gravity moves rainwater down a building’s exterior surface. When there are openings in exterior wall assemblies, such as downward-sloped openings, water will pass through them. This force is typically countered by roof systems designed with shingles and flashings. Also, overlapping, sealing, or covering exterior wall joints in a manner that diverts rain from the building helps keep gravity-drawn water out of the wall.

Another mechanism of water penetration is capillary suction, which is a result of the surface tension of water. Water is drawn inside through tiny pores in building materials, often so small they are invisible to the eye. To hinder this moisture flow mechanism, it is best to break the continuity of materials from the exterior through to the interior to obstruct moisture’s path. Breaks in components can be accomplished by incorporating small cavities that prevent moisture from migrating through all the layers of materials. Specifying moisture-tolerant exterior wall materials like concrete and masonry is also helpful.

A third way moisture enters a building envelope is through water vapor diffusion. Water vapor will pass or diffuse through building materials whenever areas of high and low vapor pressure exist on opposite sides of that material. This movement is from the material’s high vapor pressure side to the low-pressure side.

Water vapor permeance of a building material can be determined through ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, which measures diffusion using two possible means: the dry cup method (also known as Method A or the desiccant method) and the wet cup method (also referred to as Method B or the water method).¹

Air movement is yet another way moisture gets inside a building. In fact, air can bring a large amount of moisture into a building if it is not impeded by good construction practices. Compared to moisture entering a building by water vapor diffusion, moisture carried into a building by air can be up to 100 times greater. For example, a 1.2 x 2.4-m (4 x 8-ft) sheet of gypsum board will permit up to 285 ml (9.6 oz) of water to pass through it over a heating season in a cold climate. If, however, a 25-mm (1-in.) hole were to open in that board to permit airflow, airborne moisture flow could add up to 28 L (7.5 gal) of water over the same period. This phenomenon creates an excellent case for making wall assemblies airtight and preventing moisture from condensing on cold surfaces.

Naturally, moisture can enter the cavity from either the inside or outside of the wall—an average family of four can create two to three gallons of water vapor per day from cooking, bathing, and washing dishes and laundry. Over a heating season in colder climates, these moisture loads are driven toward a building’s exterior and penetrate into the wall cavity. Air can transport moisture quantities up to 28 L through holes in the building envelope’s interior side that are as small as 645.16 mm² (1 si). This has led to an increased focus on developing more airtight wall systems.

While an airtight wall is important, there is need for a means of escape for moisture that gets trapped in the assembly. If there is no established escape plan, then the building has a high likelihood of creating a moisture sandwich inside its walls.