DIY High-Performance Wall Assembly
If you were a competent DIYer building and acting as GC for your own house, what would your wall assembly be? Let’s assume the structure is a 1400 sqft house, net-zero to PHIUS level of performance (probably won’t pursue certification, but want to maintain that possibility for now) in Zone 5A. Also assume that the green building panacea of dense pack cellulose is off the table, because that might be a reality. The driving criteria are ease of construction and material cost in that order.
My research leans heavily towards a double-stud with batts, a membrane with above-average intelligence, and plywood sheathing. However, I remain uncomfortably wary of the wet sheathing controversy. Would using a gimmicky “denim” batt provide much benefit over mineral wool (my preferred batt option) in this regard?
Thanks in advance for your thoughts.
Derek
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Replies
Derek,
I would lean towards conventionally framed walls but use 2"x8"s. For the roof, a vented attic with raised-heel trusses. All the details are simple and commonly known to the trades. I would also include a shallow (1/2") rain-screen to mitigate the dangers of cold sheathing and make the walls more resilient, and would avoid exterior insulation. It is too labour intensive and difficult to detail properly.
Thanks Malcolm,
I forgot to mention that I was definitely going to do a rain screen under LP SmartSide.
I don't think denim batts would really gain you anything over mineral wool. Mineral wool may actually help to reduce the chances of moisture retention. If you go with a double stud wall, you end up with a bit more framing work to do, but you can easily put in batts yourself and with some careful planning of stud placement, you can make the batt installation pretty easy. You'll end up with a wall that performs well without having to do anything really fancy.
Mineral wool is easy to install well because it is somewhat rigid, it has shape. You can carve it to fit and not have the need to "fluff" it the way you would with fiberglass batts. I've never worked with denim batts, so I can't comment on their installability.
If you want to keep your sheathing dry, you need to keep it warmer. The easiest way to do that is to use exterior continuous insulation -- rigid foam. A layer of even 1" polyiso makes a big difference, but I'd use 2" myself. You can use however much you want as long as you stay above the minimum required for your climate zone, but going beyond 2" is where some of the trim details can get trickier.
Bill
Derek,
What is the heating degree days (HDD) for your location? I like your wall stack up as you describe. With proper exterior drainage plane details a double wall is hard to beat for price and performance. Another often overlooked feature of a double wall is minimal sound transmission. Guess what? A wall that does not transmit much sound will also not transmit much heat if detailed correctly.
For cold climates, HDD divided by 180 for wall R- value and by 120 for ceilings. This will get you to superinsulation for your area.
Hey Doug,
Thanks for the reply. What is the best source for HDD? I've googled this in the past and I get some different responses depending on where I look. We are presently living in Chicago, and our future location won't be dramatically far away from that location.
The super insulation formula is really interesting. I'm sure it's a rule of thumb, but how was that formula determined?
Derek, there are quite a few, but this one is pretty intuitive: https://www.huduser.gov/portal/resources/UtilityModel/source/select_Geography.odb?data=2007
And Doug, I had never seen that calculation before. It’s really interesting. However, when I did that for my hdd in Boise it gave me less than code minimum for the roof. The wall was way above though.