In working out the design for our new home, I spent many years researching home building. But I am admittedly no expert, nor do I have any professional training or carpentry skills. Which is why I’m particularly proud of the wall system I specified for our new house. The ideas are not mine, nor are they new, but they are new in my part of the world and they represent a first for our local building department. Importantly, I’ve been receiving complements from our crew on the wall design, which is very validating.
My wall system incorporates two main features: the first is an extra layer of insulation on the outside of the sheathing. Insulating above the levels prescribed by building codes makes sense, so long as one recognizes that the curve for added insulation vs. cost-savings does level off at some point (and the milder your climate, the sooner you reach that point). I wanted to make sure our new house was properly insulated, but given our relatively mild winters I didn’t want to go overboard, either. Beyond a certain point, spending more money on insulation is not going to translate into more savings in heating costs (I should mention that our summers are not hot enough that mechanical cooling is a necessity; air conditioners are uncommon here).
My wall system incorporates 5.5 inches of Roxul mineral wool batts between the studs and another 3 inches of Roxul comfortboard on the outside of the plywood sheathing. This gives us an R-value of approximately 34 (our local building code calls for 22).
You might be wondering why the extra insulation is on the exterior of the sheathing. That relates to the second main feature of my wall design and the one I am most excited about: we’ve eliminated the vapour barrier. My walls can breathe.
The idea of a breathable wall is not new, especially not in green building circles. Cob and strawbale walls breathe – which is to say they allow water vapour to flow freely across the wall, and when built properly they contain no internal surfaces upon which water can condense. However, building a breathable wall using conventional stick-frame construction is a fairly new concept, at least in my part of the world.
I have a strong dislike for vapour barriers. The point of using them is to make sure that any moisture generated inside the home (due to respiration, humidity from running a dryer, hot water, etc.) doesn’t pass into your wall system where it can condense and cause the wood framing and sheathing to rot.
However, I’m a firm believer that water is Force of Nature and will eventually get into just about everything, including the wall structure. Rather than staple a sheet of plastic around the inside of my walls and hope to goodness that it never fails, I prefer to accept the fact that water vapour will get into the walls, and then make sure that it can also get out.
My focus on this particular issue is largely due to our mild wet winters of endless rain and relatively high humidity. In my current home, which admittedly has horrible insulation, hung towels take days to dry, and I buy two pairs of running shoes each winter because they take so long to dry out between runs. We regularly get condensation inside closets and at the back of shelves and dressers. Mildew is the scourge of my housekeeping endeavours, and the smell of it permeates everything. A dehumidifier helps, but is insufficient to fully solve the problem. Our double-paned windows are full of condensation in the winter such that I can barely see out of them most of the time.
Even if you’re not living in a crappy mobile home, we bring a lot of moisture into our homes here, especially in winter. Without a mechanical dehumidifying system, that water vapour has nowhere to go. To me, it seems crazy to prevent the vapour from exiting the house passively and then spend money on mechanical equipment that needs fuel to run and regular maintenance. All you have to do is make sure the vapour doesn’t condense inside the wall.
Water vapour condenses inside walls when it encounters cold surfaces (typically the sheathing) as it passes from the warm inside towards the cooler outside of the wall. Sheathing typically sits on the outside of the wall, so it gets cold and that’s where water tends to accumulate.
In my wall system, the plywood sheathing is sandwiched between 5.5 inches of batt insulation on the inside and 3 inches of board insulation on the outside. Thus, the temperature of the plywood never drops to the dew point (the point at which water condenses), and any water that gets into the wall system stays in its vapour state and passes harmlessly to the outside. To do this, you need to use components that are unaffected by moisture and vapour-permeable. Mineral wool (also called stone wool) is perfect for this application. I also chose it for its resistance to fire and insect or rodent damage. Contractors like it because it installs easily: it’s more rigid than fibreglass insulation so it’s easy to cut to shape, and it doesn’t lose performance when compressed so they can fit it nice and tight between the studs.
Although the manufacturer of our mineral wool insulation, Roxul, has already tested, documented, and approved the use of their product in this manner for wall systems without vapour barriers, such a thing has not been done in my area before. Consequently, we had to get an engineer to sign off on my wall assembly design before the building inspector would approve it. Engineering fees are typically in the thousands of dollars, but we negotiated a very good price because I had done most of the work already and he just needed to approve my design. Due credit also goes to George, our fantastic contractor and project manager, who did his own research into fastening systems that would work with our wall structure and not create thermal bridges (a thermal bridge occurs when a material with poor insulation spans an insulated structure, creating a route by which heat can readily escape).
Our framers are very impressed with the wall design, and I’m looking forward to sharing it with other people in our area who are interested in breathable wall systems built using conventional materials. Because the wall construction uses standard techniques, any competent builder can use this design without the need for additional training. I will be sure to document the humidity levels in our home over our first few winters in the hopes of providing real-life evidence that this wall system works as designed. Hopefully, we will soon see an end to the folly of vapour barriers!