Closed-cell foam in a 65-year-old Cape Cod roof?
We recently removed all of the vinyl siding and homasote from our cape. We installed R15 fiberglass in the walls – there was not insulation previous to this, just homasote. In addition, we replaced all of the windows with double glazed argon gas filled low E windows.
The house is much more comfortable that before, however, our bedroom is on the top floor. This area was once the attic. I was able to slide proper vents between the rafters from behind the knee walls and add a ridge vent. The insulation in the roof is original and although the knee walls now contain R-30 fiberglass insulation, the roof from the collar ties to the top of the knee walls is either very cold in winter or very hot in summer – not a surprise.
I have thought many time about removing the sheet rock, insulation and proper vents from under the collar ties and to both knees walls and spraying closed cell. In addition, I thought about adding 1″ of rigid foam to the under side – conditioned side – of the rafters before installing new sheet rook or bead board.
This may read as an insane venture from an investment perspective but its really about our comfort. Assuming this is a good idea sans the cost, I am worried about properly venting the knee wall space…we do have soffit vents…thoughts?
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...I should add that the floors in the knee wall space - ceiling for the first floor rooms - have all new fiberglass insulation between 2x10 joists. I then simply added a layer of R-21 fiberglass on top of the plywood floor. So this portion of the knee wall space has close to R50.
I also did write that the body of the house was constructed with 1/2" plywood, Tyvek, and pre-dipped cedar shingles 6" to weather.
Michael,
Start by reading this article: Two ways to insulate attic kneewalls.
After reading the article, you can post any follow-up questions on this page.
Hi Martin,
Thank you. After reading the article I think the best way to insulate is to insulate the knee wall space and eliminate is from the 'attic' . We do have AC and heat ducts in those spaces and although there is a ton of insulation around those, insulating the knee wall space makes sense.
The debate between my wife and I is one of comfort, not necessarily the cost of installing closed cell. We can, by way of her arguments, simply add a window AC unit - on top of central air - and put on another blanket in winter. In the end efficiency and long term operating costs are important to me. and although I've been told that I'm making this 65 year old house great for the next guy, modern technology has to be considered. My only hesitation is removing all of the sheet rock and reinstalling it. But I have to imagine that filling the entire 2 x 8 rafter void, adding 1" of rigid to the bottom of the rafters and then adding back sheet rock or bead board is going to change how this space feels in peak seasons.
Michael,
You wrote, "I think the best way to insulate is to insulate the knee wall space and eliminate it from the 'attic.'"
I'm not quite sure what you mean by that.
If you mean, "I think that the best way to insulate is to install insulation in the sloped roof assembly, all the way down to the soffit area, so that the triangular attic behind the kneewall is within the home's conditioned space," then I agree with you.
I agree with your approach -- to add insulation even if the work is difficult to do, or hard to justify from an economic perspective. You will gain comfort and save energy if you insulate your roof assembly.
Filling 2x8 rafters with closed cell foam to go with an unvented roof is a crime against the planet, and a waste of foam. The severe thermal bridging of the R1.2/inch wood ruins the average performance despite a high center cavity R, and the HFC245fa blowing agent is a powerful greenhouse gas (about 1000x CO2.)
Leaving the code minimum 1" vent gap under a 2x8 rafter would give you room for 6.25" of space for insulation. If you used cut'n'cobbled unfaced 1" EPS as the exterior side air barrier for fiber insulation ((held off from the roof deck by 1" x 1" strips of EPS on each side of the rafters as spacers), you could then install R21 fiberglass or R23 rock wool compressed into the remaining 5.25", and 2" of foil-faced polyiso on the underside of the rafters for about a ~R38 center-cavity R. With the ~R12 of polyiso as thermal break over the rafters it will perform much better than a thermally bridged 7" of closed cell foam, and comparably to 7" of closed cell foam with only 1" of thermal break on the rafters. If for head-banging reasons you want to limit the thermal break to 1" polyiso on the finished sloped ceiling sections it's still going to be almost as good as R38 between joists in that spot.
For the cut'n'cobble it's easier to cut them a 3/8" -1/2" narrower than the rafter bay width and tack them in place and use can-foam to seal the gaps at the edges rather than trying to cut them for a snug fit.
This is lot more detailing than standing back and blasting away with foam gun, but the material costs are less, as is the environmental damage. Using unfaced EPS on the exterior is important for it's relative vapor open-ness so the assembly can dry into the vent spaces, and foil face polyiso is greener and higher R/inch than XPS on the interior, and it's easy to reliably air-seal with aluminum tape. Both EPS and polyiso are blown with relatively benign pentane (about 7x CO2), to the HFC134a used for XPS ( about 1400x CO2.)