Sunday, October 27, 2013

Insulation in Kneewalls

Kneewalls are a challenging part of a home because they are a little bit hard to build and insulate. We chose kneewalls because of the expanded space and better, though challenging, insulation.

Here is a drawing from Shelter Kit showing the kneewall extension. So rather than having the rafters meet the floor joists directly, there is a extension of the wall that supports the rafters. This gives you more usable space.

Kneewall End View
Shelter Kit - Drawing of Kneewall 
The next drawing shows the increase in space in the upstairs with kneewalls.

Kneewell Side
Comparison of space with and without kneewalls

The difficulty arises with the funky cavities made by these kneewalls. How do you insulate this space?

Dormer framing with wall framing behind it

I posted a question on Green Building Advisor about it because I just didn't think the insulation company would be able to blow-in cellulose effectively in these spots. I decided to fill them with Roxul mineral wool insulation and then I'll cover that with the Intello Plus smart vapor barrier. Then we will insulate with the cellulose over that. Does that seem like a lot of insulation for this area? Well, it is, but this is what I call "a vulnerable area" a seam between sections of the house (the roof, the wall and the rim joist below) that is very prone to air leakage. 

Also the details of adding exterior insulation to this short section of wall would have been very challenging. So we omitted it from this section of wall. And we built this wall before I started going crazy about air sealing so the inside of this cavity is not air-sealed as almost every other cavity in the house is. The outside is taped though. 

Now you can see my efforts at insulating the kneewalls. This might not meet Grade 1 insulation standard, but the voids are filled. Remember that there will be at least 5 inches of cellulose interior to this insulation to make sure the wall is well insulated.

Intello Plus membrane will be stapled between the Roxul mineral wool insulation and the dormer wall framing in front. That space will be filled with dense-packed cellulose.

Bedroom kneewall stuffed with Roxul mineral wool insulation.

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