By Ted Winslow
While the quest for increased R-values has led to dramatic reductions in air movement through building assemblies, it has also reduced their ability to dry when they get wet. In-wall systems allow this moisture to dissipate while maintaining the desired degree of airtightness. One way to create moisture escape routes is through construction techniques. Another is through the use of products that employ advanced air/moisture barrier technology.
The “Moisture Sandwich”
The intended consequence of the tight-envelope construction techniques mandated by various building codes is significantly less air leakage. The unintended consequence is that moisture, which finds its way inside the wall cavity – and moisture will always find a way into wall cavities – can get trapped there, creating a “moisture sandwich.”
There are four primary physical mechanisms by which moisture infiltration takes place: water flow, airborne moisture flow, vapor diffusion and capillary suction.
Water flow is the most basic of these mechanisms. As rain or snow falls, gravity pulls it down a building’s exterior, where improperly installed flashing can divert it into the building. Wind-driven rain can penetrate openings in the exterior cladding at imperfect mortar joints, laps, utility and electrical cutouts, etc., and gain access to a wall’s interior.
Vapor diffusion occurs when water vapor flows through building materials due to differences in air pressure on either side of the wall. The flow direction is from the high-pressure side to the low-pressure side. The direction and intensity of this flow vary with season and geographic location. Higher temperatures inside a building increase vapor diffusion to the outside in cold climates, while the direction is reversed when outside temperatures are warm/humid and the inside of a building is cool and dry (due to air conditioning).
Whichever the direction of the flow, moisture can accumulate when this vapor condenses on cold surfaces inside the wall, and if there’s no mechanism for it to dry quickly, problems like wood rot and mold can occur.
Capillary suction is a result of surface tension and adhesive forces between water and the vertical plane of the wall. Moisture held there can be driven through tiny pores in the weather-resistant barrier and permeable wall sheathing and into the wall’s interior by the heat of the sun — a phenomenon known as solar or vapor drive.
Masonry products like brick and cultured stone are sometimes referred to as “reservoir” cladding because they absorb and store moisture. As soon as this moisture is driven inside the wall and encounters a cold surface, it condenses, and the potential for mold growth and damage to wood framing increases the longer the area stays damp.
Helping Moisture Escape
Exterior wall and roof systems are the first line of defense against rain, snow and subtler problems like condensation and vapor drive. To protect against the heavy moisture flows caused by precipitation, exterior finishes should be backed by a drainage plane, which redirects water that breaches the outside of the wall away from the building.
Ventilating airflow – which enables dampness behind the exterior cladding to dry more quickly – can be facilitated by the use of rainscreen systems that create a capillary break, a space that disrupts the surface tension that holds water in place on a wall surface and allows it to drain out of the assembly. A 1-mm gap will facilitate drainage; bigger gaps may be called for to create ventilating airflow to promote drying in wetter climates.
It’s also important to plan a moisture escape path for condensation and water vapor on the inside of the wall. Cold air contains less water than hot air, and diffusion will carry moisture from a warm place to a cold place. In order to allow the inside to dry, poly sheeting, vinyl wall coverings and low-perm paint should be replaced with more advanced vapor retarders.
Choosing the Optimum Vapor Retarder
In “mixed-climate” regions like most of North America, moisture travels into and through walls in different directions from winter to summer. Typical polyethylene vapor barriers do well at keeping moisture out of the wall cavity in the cold season, but can trap it there when summer heat reverses the direction of vapor drive – a problem that can be exacerbated when airtight construction techniques and moisture-retaining reservoir cladding products are used.
An ideal approach is to incorporate a material that can vary its vapor permeance. “Smart” vapor retarders do just that. These materials are specially engineered to accommodate the effects of seasonal weather changes on moisture flow by altering their physical structure in response to changes in relative humidity (RH). During winter, when RH is low, smart vapor retarders provide high resistance to vapor movement from the interior into the wall cavity. But when RH increases to 60 percent or higher, the material opens up, allowing water molecules to pass through and preventing moisture from condensing inside the wall.
Moisture issues require appropriate products and solutions based on climates, environmental conditions and building codes.
Smart vapor retarders can be a particularly valuable component of wall systems in areas with mixed climates. Incorporating ventilation drying in a wall cavity would be another tactic to combat the damaging effects of moisture. The key is to make sure that moisture that gets into the wall system has a path to get out.
Ted Winslow is product manager, Building Science, Systems & Technical Marketing for CertainTeed Insulation.