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Roof Insulation for an A-Frame House

DerekR | Posted in General Questions on

I can’t figure out what will be most effective to keep this house cool that’s almost all roof

the roof is all 2×12’s, this gives me enough room for r38 rock wool and a 1.5 air gap roof baffle

im looking at the ADO styrofoam baffle that has support on the center and sides

should I put a radiant barrier on the underside of the roof before I put in my baffles? The barrier would lose some effectiveness where the baffle touches it on the sides and the center but still could preform well where it’s not touching it?

or should I just do the r38 rockwool and baffles and not worry about the barrier?

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Replies

  1. plumb_bob | | #1

    You could add furring to the bottom of your rafters to add insulation, or cross strapping to both add insulation and reduce thermal bridging. In either situation you could increase the air gap to facilitate better ventilation. The steep nature of an A frame should make for good air flow from top to bottom if the appropriate ventilation is provided at the peak and the bottom.

    1. DerekR | | #2

      Well the baffles I’m getting are designed with a 1.5 inch air gap already, the radiant barrier is what I’m not sure if I should use or not before I put in the baffles because the baffle will touch the radiant barrier on the sides and a little in the center

  2. plumb_bob | | #3

    Sorry, I do not have experience with keeping houses cool, I live in a cold place. I assume you are in a warm place. The answer above would help with heat radiating into the house, but I am unsure about radiant barriers.

    Good luck!

  3. Expert Member
    Akos | | #4

    It feels like a radiant barrier will make a big difference, but in reality it adds only R1 to your roof (assuming installed with air space and stays clean). When you compare that to your R38 batts, it is pretty much noise. Unless already there, I would not spend money and effort on installing it.

    Rockwool is not sensitive to wind washing, if you are careful with install and keep the batts flush to the interior, you can also skip the vent baffles. I never install vent baffles with high density batts.

    The important detail with A frame houses is air sealing. This is especially important as typical wall (ceiling?) finish is T&G. If this is the case for you, make sure to get a proper air barrier installed. This can be something as simple as taped and caulked 6 mil poly, one of the fancier membranes or much better use 1" of foil faced polyiso installed over the rafters with the seams taped. The 1" of polyiso will make a big difference in this case as it reduces thermal bridging from the rafters and bumps your assembly R value up from around R30 to R36.

    The other big air leak in A frame is your floor joists as these poke through your air barrier to connect to the rafters. This is fussy work to air seal, pieces of cut and cobble rigid insulation sealed in place with canned foam is your best bet.

    Getting your air sealing right is well worth your effort. It will make a much much bigger difference in comfort and energy use than fussing around with radiant barriers.

    1. DerekR | | #5

      I won’t hurt to use the baffles though will it? I’m drywalling the inside, not tongue and groove, I’m not sure what wind washing is

      You don’t think the radiant barrier will help much if I have r38 though? I guess I’ll leave it out then I haven’t bought it yet

      I’m also putting rockwool in the crawl space, it’s a crawl space foundation

      1. Expert Member
        Akos | | #6

        The baffles won't hurt, no issue with installing them. Wind washing happens when you get air flow across exposed insulation such as in a vented roof. If exposed to wind, some insulation like chopped loose fill fiberglass looses some of its R value, having it protected by the vent baffles helps with this. High density batts such as mineral wool are effected but the it is only a very minor drop.

        With drywall as your main air barrier, make sure to caulk around the perimeter of the walls and use vapor tight device boxes for all your electrical. Vapor tight device boxes have a gasketed flange that will seal against drywall, the are pretty cheap but might be special order. Important to make sure your drywall goes down to the bottom of wall/roof and there is plate there it can seal against. If not already there, install blocking between the rafters and seal it to the subfloor.

        As for insulating crawlspace, I would read through this:

        http://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-115-crawlspaces-either-or-out

        for some reason the main site is down, here is the archive version of it:

        https://web.archive.org/web/20210909140629/http://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-115-crawlspaces-either-or-out

        You can insulate with a combination of rockwool and rigid insulation in the floor but the better option is to seal it up, insulate the stem walls with rigid or spray foam install poly on the floor.

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