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Community and Q&A

Insulating a 1-1/2 Storey with little Opportunity for Ventilation.

CynthiaV | Posted in Green Building Techniques on

We are the owners of a house that we’d like to upgrade the insulation to code that it turns out is very difficult to do properly. Ice dams and leaks in the valleys made it urgent to take action on this. Essentially it is a cross gable 1-1/2 storey house, with full width gable additions on front and back. The rear addition is only one storey with a properly ventilated attic, so that’s simple enough, but the original house and front full width gable addition are all built with 2×4 framing. The old insulation in the knee walls has been extremely ineffective, with lots of air leaks and drafts everywhere. Our building inspector when we got the place recommended above deck rigid insulation under a new roof, but because the roofing contractors in the area were so busy, no-one was willing to take on the project with the insulation component, so faced with leaks, we had to re-roof without it. As a matter of fact a contractor said they would do it, we bought a great deal of 2.5″ unfaced EPS board, and then cancelled on us.

If we are in climate zone 6 in Ontario , and need R28 on the sloping ceiling and R50 in the attic and there’s no way I’d be able to do that with the existing framing, if it needs reworking anyhow I would like to move the insulation to the roof line and reclaim the knee wall space for storage, especially since I’ve discovered plumbing in the knee walls.

The front full width gable extension is a single room, which we can vent most of the roof from soffit to ridge, excepting the rafter cavities in the valleys and one that has a chimney going through it, but the original house, with additions spanning the whole width of both sides of the building, doesn’t really have any opportunity for ventilation. 

For the ventilated part, I was considering either furring an extra 1.5″ on the rafters and installing Durovent baffles and R14 Rockwool, and then attaching 2x4s horizontally on edge under the rafters with another layer of R14 between them. Or instead of a baffle just installing the 2.5″ unfaced EPS R10, which is readily available here, spaced an inch away from the sheathing for ventilation and sprayfoamed for airtightness all the way into the attic area, and then a 2×6 horizontally on edge from the rafters with R22 rockwool? I guess a structural engineer would have to okay all that weight on the rafters. Would this not cause issues unless I had a smart vapour retarder? All foam is possible I guess but 3 Layers of R10 foam on the inside seems really impractical to drywall on though!

In the original building I’m not sure what I can do with all the ventilation paths blocked aside from connecting holes in the attics in the front and rear vented additions to this attic. I was just considering R14 rockwool, cross 2x4s and another layer of R14 again.

In whatever case the knee walls are going to have to be rebuilt to keep the insulation consistent.

Personally I don’t know if it’s superstition or well founded, but I feel like for fire risk foam on interior, once it’s gotten through a fire barrier is less survivable than exterior foam because of all the thick smoke, but maybe I’m wrong and there’s no difference, but I’ve tended to like Rockwool’s properties on the interior, especially with the house having a history of roof leaks! And after having cleaned out multiple trailerloads of wet cellulose, I’m a little biased against blown in.

Really would appreciate any and all consideration of these options!

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