Anyone who has been in the construction industry for a while has heard the phrase – “you don’t want to build it too tight.” This is a reference to how buildings used to dry out, BEFORE air conditioning and insulation. Some still refer to allowing your home to breathe, but we completely changed how we build from balloon framing to platform framing, from using windows to catch a breeze to cool your home to mechanical systems that filter, clean the air, and cool your home. Your house no longer needs to breathe through random gaps in the exterior thermal envelope. It needs to be as tight as you can make it, with a proper ventilation system to complement the heating and cooling system.
Architects and builders who understand building science work to protect your home and the indoor air quality problems that might occur if you “let your home breathe.” In our mixed humid climate, one of the biggest problems could be dew point. This is the exact temperature at which airborne moisture transitions from gas to liquid. If that conversion happens inside a wood-frame wall, you have a recipe for mold, rot, and potentially structural failure.
Achieving an efficient, healthy, and resilient home requires understanding how to avoid some common construction mistakes.
Air Leakage
Air leakage is the highway for moisture transport through your thermal envelope. In our mixed humid climate, on a hot, humid day, that moist air wants to push inside your air-conditioned home. In the winter, warm air escapes to the dry cold air. Your home is always trying to reach equilibrium with the outside air. Air pressure forces movement through any cracks around plumbing, electrical outlets, framing, doors, and windows. It carries gallons of water vapor along with it. When this air hits a cold surface inside the wall cavity, it instantly hits the dew point and becomes liquid.
Insulation
Not all insulation works the same, and not all insulation can be used for all wall
assemblies. We often use open-cell spray foam because it is airtight and vapor
permeable. Closed-cell spray foam looks very similar, but it is a vapor barrier. Using this product in a wall above grade can work, but you must make sure it is done exactly right. If it pulls away from framing, allowing even the smallest leak, you could end up with a dew point issue. Also, if you use a vapor barrier and there is a liquid leak in the wall, you might not find it until you have a much larger problem. Using fiberglass batts and cellulose can also work, but the air sealing process is labor intensive and often does not last as long.
Vapor Barriers
We don’t want a vapor barrier in our climate due to the vapor drive that happens in different directions in the different seasons. Adding a vapor barrier can stop airflow on the cool surface you are trying to protect. Using the right vapor permeable solutions allows vapor to pass through the material while your HVAC system works to dry out the air to further protect your walls. Did you know that OSB, some paints and wallpaper, and many house wraps are vapor barriers?
A healthy, durable, comfortable home with good indoor air quality controls the dew point. You have to have the right HVAC system and fresh air as part of the holistic solution. Focusing on proper air sealing, insulation location and values, and HVAC can solve the challenges that many modern homes face of high humidity, mold, and rot.
