A-Frame insulating
I just bought a 76′ A-Frame that has 2×6 roof/wall supports. I have it gutted down to the studs right now. I am having the hardest time trying to find out how to properly insulate an A-Frame! I am not into using the spray foam technology. I live in the mountain of SW Montana so it’s cold in the winter. I was thinking about using 2 layers of ridged 2″ foam board. Leaving a 1.5″ space between the roof sheathing and the insulation. The insulation would be flush with the roof/wall 2×6’s. Then a vapor barrier over all that. Tongue and groove pine over that then. Also, adding 2″ vents between each bay of rafters, which are 2′ OC, that vent out my soffits. I made a small 10″ attic at the peak which I could vent on either side of Aframes vertical walls too. The roof is a fairly new metal roof.
Is my idea a good one or is there another option where condensation won’t be an issue. Please help, thank you.
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Replies
Korey,
Your A-frame doesn't have any exterior walls (except at the two triangular gables). The long slopes of the A are roofs, and need to be insulated as roofs.
In Montana, you want to aim for at least the minimum R-value required by code (R-49).
The method you suggest is called the "cut-and-cobble" method. It's an unconventional method. To learn more about this technique, see this article: Cut-and-Cobble Insulation.
The main problem with your suggested insulation method is that you are planning a low R-value (only R-16 or R-20, depending on what type of rigid foam you use). You really need to try to hit R-49.
There are a couple of other issues to keep in mind as you plan the details of this ceiling assembly. One issue is that polyethylene vapor barriers aren't used very much these days (because they can cause problems in the summer if your home is ever air conditioned). Another issue is that you may need a more robust air barrier on the interior side of the insulation (because tongue-and-groove boards are not an air barrier).
I suggest that you read this article, which explains many approaches to insulating this kind of roof assembly: How to Build an Insulated Cathedral Ceiling.
Montana is all US climate zone 6B. IRC 2012 code min for zone 6 is R49 between rafters/joists for roofs/attics or U0.026 max,and R13 + R10 continuous insulation or U 0.048 max .
Ideally the foam would have gone on the exterior, between the sheathing and the metal roofing, but that ship has sailed. With 2x6 rafters you can install starting at the roof deck
*1.5" standoff strips (foam or wood) along both sides of each rafter to accomodate...
* ....a 1/2" thick cut'n'cobbled unfaced XPS or unfaced EPS air barrier (NO FACERS, and no thicker than 1" or it'll be too vapor-tight) ...
* ...high density R15 fiberglass or rock wool bringing the fiber insulation flush with the stud edges...
*...a continuous layer of 2" of rigid foil faced polyiso on the interior side of that, taping the seams with a high quality foil tape making it a continuous air & vapor barrier...
*...1x4 furring (1x3s would split) through-screwed to the rafters with 5" pancake head timber screws on which to mount the t & g pine.
This stackup beats code-min for walls with some margin, with ~R17 in cavity and ~R12 continuous insulation, plus another ~ R1 of performance from the foil facer & 3/4" gap w/air films before the t & g pine, though it's quite a bit shy of the U0.026 U-factor for roofs/attics. It eats up 2.75" more interior space than with out the foam & air gap, but if you can tolerate losing a bit more space that that, taking it to 3" of foam would be better. The timber screws can be spaced 24" o.c., but for flatness you may want to opt for 16" o.c.