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Attic insulation for partial cathedral?

pgh_sean | Posted in Energy Efficiency and Durability on

I’m attempting to air seal and insulate my attic and ran into something I’ve not read about before. I’m in climate zone 5.

The exterior walls on the top floor of my house are partially “cathedraled”. By that I mean, the wall runs straight up then follows the roof line for 27 or 19 inches (depending on side) then hits the ceiling. Above the ceiling is an attic. Above the cathedral are 2×8 joists. In other parts of this house, I have big cathedral ceilings and another Q&A post helped me sort out what to do there. My plan for the attic portion was to add site made baffles, air seal and insulate with cellulose up to R-60. Now with these little “mini-cathedrals”, what do I do! I’m thinking a site made baffle with some blow cellulose could be in order?

Build the baffles with 1 inch foam board and fill the rest of the cavity with insulation? This kind of blows the idea of any soffit dam. Am I better off just foregoing any insulation in the mini cathedrals and sealing up nicely with a soffit dam?

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  1. Expert Member
    Michael Maines | | #1

    Sean, this detail shows up regularly on my projects. The sloped sections need to be treated like any other cathedral ceiling, with ventilation baffles and insulation. In zone 5 you should reach at least R-30 in these sloped sections, and meeting the full IRC requirement of R-49 (or more) is better, depending on how much space you have to furr the sloped section down.

    1. pgh_sean | | #2

      Thanks Michael. My main concern was with building baffles and blowing cellulose down there with no soffit dam. The cellulose is just going to clog up the soffit making it useless.

      I suppose the best approach is going to be...

      2" of Baffle using 1" foam board (1" vent gap)
      5" of foam board (cut and cobble)
      Insulation/Soffit dam where the cathedral meets the attic. The idea being to limit air flow into the insulation from the cathedral.

      Wondering if that last bit is really necessary but if I'm going to such lengths to get it done the best of my ability, suppose that's just icing on the cake!

      I'd rather not use so much foam board and use a greener material. I'm not sure that's an option though. I'll attempt to get recycled foam board.

      Now that I think a bit more... I wonder if I can get some rockwool batts down there instead of all foam.

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