Risks of insulating kneewalls before ceiling?
I live in Zone 5A, and the weather is starting to hit the high 70s, almost 80. That means my upstairs, with kneewall attics, is going to get hot. I’d like to add rigid foam to the back of the studs, which are 2x4s, and the bays have kraft faced fiberglass batts. If I understand correctly, kneewalls with unconditioned attics are treated as exterior, so I’ve calculated the appropriate thickness (https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing), and I’ll need at least 2 inches. The purpose of this is primarily comfort.
Assuming I properly detail this job, what benefit, if any, do I stand to gain if I start by doing the kneewalls and don’t immediately add insulation to the sloped ceiling portion and upper attic?
The big question, are there any risks to delaying dealing with the upper portion, like unwanted moisture accumulation?
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Replies
Mat,
Q. "Are there any risks to delaying dealing with the upper portion, like unwanted moisture accumulation?"
A. No. By the way, one of your statements -- "If I understand correctly, kneewalls with unconditioned attics are treated as exterior" -- isn't accurate. The best way to deal with a triangular attic behind a kneewall is to bring that triangular attic into the home's conditioned space. The way to do this is to install insulation that follows the sloped roofline above the attic.
For more information, see this article: “Two Ways to Insulate Attic Kneewalls.”
Martin,
I know the approach I'm describing isn't optimal, I just feel a bit stuck. To get enough R value to insulate the roofline, it seems like I'd have to go unvented and add rigid foam above the roof sheathing. I'm not in a position to replace the roof right now, so that approach seems unrealistic right now.
My immediate goals right now are to increase comfort without putting the structure at risk, and reducing energy usage. It sounds like starting with the kneewalls won't add risk.
So then the question is, will it do anything increase comfort and reduce energy use before I can complete the upper portion?
Mat,
Q. "Will it do anything increase comfort and reduce energy use before I can complete the upper portion?"
A. Yes, it will -- although you may still be unsatisfied with the results. Ideally, you'll come up with a plan to finish the work. Here are links to two relevant articles:
"Insulating a Cape Cod House"
"Sliding Insulation Between Rafters From Above"
Thanks for your help Martin, as always. Is there a good book I could read to learn about the the home as a system? I understand certain things about insulation on a functional level, but I'd like to learn more about moisture control and things like the stack effect. It might help me so I don't have to rely so much on the generosity of strangers on the internet.
Mat,
Q. "Is there a good book I could read to learn about the the home as a system?"
A. You might want to buy my book, Musings of an Energy Nerd.
Other possibilities:
- Continue to learn more by using the resources on Green Building Advisor. You can use the search box to find relevant articles, or you can educate yourself by using the links on this index page: "How to Do Everything."
- If you are a real nerd, and want to dive deep into areas usually of interest only to home performance contractors, you should buy and read this book: Residential Energy by John Krigger.
It somehow escaped me that you had a book. I will get my hands on that right away.