Combobulating the Perfect Wall - The Basics of Control Layers
Have you thought about the best wall to build a wall? In my Building Science 101 article, I mentioned that buildings need to control the flow of heat, air, and moisture to perform well. Does the standard stick-built, wall with its cladding, housewrap, sheathing, framing, and cavity insulation, do the job well enough?
A couple of years ago, I got to hear Joe Lstiburek speak at Greenprints in Atlanta on what he called the Perfect Wall. It was one of those Aha! moments for me because he elucidated perfectly the functions of the various components in our wall assemblies and laid out the perfect way to put it all together to get the best performance. Plus, it works in all climate zones! He gives credit to the Canadians for originally developing the idea, but I think he's the one who applied the perfect name to it - The Perfect Wall. (The link takes you to his paper on this topic at the Building Science Corporation website. You should definitely read it.)
As I mentioned in my post last year about Lstiburek's Building Science Summer Camp, Dr. Joe broke the wall down into its basic components: structure and control layers (plus finishes, electrical, and mechanicals, but those aren't important for what we're talking about here). The control layers do the following:
- Rain control
- Air control
- Vapor control
- Thermal control
As Joe says in his article, these four control layers are listed in order of importance. If rain is getting into the wall, who cares about infiltration? If you don't have a good air barrier, worrying about the details of the vapor retarder is pointless.
So, if a wall is made of structure and control layers, what's the perfect way to assemble them? With all four control layers on the outside of the structure, of course. As Lstiburek puts it:
Remember we really do want to protect that darn structure—especially for the sake of making the structural engineers life more happy. Expansion, contraction, corrosion, decay, ultra violet radiation, and almost all bad things all are functions of temperature.
So, not only do the drainage plane, air barrier, and vapor retarder go on the outside, so does the insulation. In fact, if you're putting the vapor retarder on the outside, you have to put the insulation there, too. Otherwise, you could end up with condensation inside the wall cavity in cold climates.
Below is a diagram of the Perfect Wall. I've shown the insulation in light blue and the drainage plane, air barrier, and vapor retarder in black.
I think a lot of our problems in buildings is that we intermingle structure and control layers, and we don't pay enough attention to the details at the interesections of roofs, ceilings, walls, and floors. As my friend and former colleague, Mike Barcik, likes to say, think of the house as a beer cooler - it needs continuous control layers all the way around. In Lstiburek's article, he discusses how the Perfect Wall laid horizontally becomes the Perfect Floor or Perfect Ceiling, with proper attention paid to the intersections.
I don't know that we'll see houses adopt the Perfect Wall concept en masse, but builders who qualify their homes in ENERGY STAR Version 3 are being pushed in that direction by the requirement for reducing thermal bridging. The Perfect Wall is a concept that makes perfect sense - if your goal is to create a building that works.