Continuing to move forward with our attic remodel. Now that we're done running all of the plumbing, electrical and HVAC through the walls, the next step is to cover it all up. Moving on to air sealing and insulation!
This is the part where we really start to enclose everything and make it a habitable conditioned space; which means keeping the outside air, water and temperatures separate from the inside. Air sealing keeps air and water vapor from moving through the building envelope, while insulation keeps heat either inside or outside depending on the season. However, there is a product which does both; closed cell spray foam.
Spray foam comes in a number of different formulations and sizes, depending on the application. For our remodel, we needed the highest R value per inch, (a measure of an insulation's efficiency) plus an air sealing technique that was easy to get right. For this application, a large, 2 part spray foam kit looked like the right option. It would allow us to fill the roof cavity with spray foam and easily seal around the edges and penetrations where air gaps normally exist.
The only problem with this option is the price. Closed cell spray foam can cost twice as much or more than the next most efficient option, which was going to keep us from being able to use it as the only insulation material. Luckily, there is a system that is supposed to combine the air sealing qualities of spray foam with the cost savings of traditional batt insulation: Flash & Batt.
This system is pretty much just what it sounds like: you, "flash" on a layer of closed cell spray foam to air seal around the outside of the building envelope, then fill the remaining cavity with fiberglass, rock wool or cotton insulation batts. This minimizes the amount of spray foam you have to use and maximizes the performance of the batts because of the superior air seal.
However, there is a potential downside.
When designing a building envelope, it's important to consider where the, "dew point" will be inside of the wall. This is the point where the temperature is low enough and the humidity high enough to cause condensation. If that point is past the vapor barrier, the moisture can't escape back to the outside and can cause any number of problems. Because of this, the flash & batt system recommends a certain thickness of spray foam to ensure that the dew point falls somewhere inside the spray foam layer; in my region it's about 2-3 inches.
That being said, I'm not sure I'm going to sleep well trusting that this rule of thumb will hold true during the cold nights of winter and humid nights of summer. So, if I accept the potential of moisture developing inside my building envelope, how do I let it out? If it's unable to move through the vapor barrier to the outside, the only place it can go is to the inside. However, since water vapor moves from high pressure to low, that means it moves from the wall to the inside during the summer, but that it can travel in reverse during the cold winter months and cause the same problem from the opposite direction.
Into this conundrum steps a new building product.
Polyamide film is a new type of membrane that is installed at the innermost part of the wall, directly underneath the drywall. When the humidity is high, the pores in the membrane open up, allowing for water vapor to leave the cavity to the lower humidity inside the house. During the winter those pores close up, preventing the more humid inside air from migrating into the wall cavity.
What I've learned through all of this is that no one product offers a perfect solution to the design on a building envelope. All of the elements have to work together as a system, sometimes balancing each other out. As new products and ways of doing this come along, it's always important to remember how it works as a whole.