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Vaulted ceiling, open attic

pshyvers | Posted in Green Building Techniques on

I have two vaulted ceilings that I am trying to work out how to insulate. I have read all the material here on insulating cathedrals. However these are essentially a variant on an unconditioned, vented flat attic. If the roof is 6/12, the ceiling is 3/12. Unfortunately while they are not a classic cathedral with only a stud’s depth, they still have very little space between the ceiling joist and rafter, ruling out fluffy insulation. (The R-38 batts already brush the roof sheathing halfway down the slope) Dense pack, of course, I hear is a very bad idea due to moisture.

Access is also extremely poor, while the space is open & connected to the regular flat attic, only a small child can enter. You’d really either need to go through the sheathing or drywall, like with a cathedral.

So we are left with rigid foam, I believe.

If I apply continuous rigid foam outside the sheathing, I need to seal off the vaults from the flat attic and remove the venting.

If I cut and cobble between the rafters, I would want to make 1″ vent channels above the right and still seal off the vault from the flat attic.

But in either of these cases I have appreciable dead air between the Batts and the rigid foam for convection. Is this an issue?

Are these all of my options? Is one clearly superior? Are there options that do not require sealing the vault from the flat? Lastly, can anyone tell me what this type of ceiling attic combo is properly called? 🙂

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  1. pshyvers | | #1

    Ah, I am in climate zone 5, so I'd be aiming for R-60, with at least R-20 in foam if placed outside the sheathing. I also witness noticable thermal bridging through a thermal camera, so the batts I have today leave a lot to be desired despite their thickness.

    (Also an issue, the top plates likely need lots of air sealing!)

    I hope I have not left off other important details.

  2. GBA Editor
    Martin Holladay | | #2

    This type of ceiling is often referred to as a scissors truss ceiling. I'll attach an image of one type of scissors truss.

    If you want to combine rigid foam with fluffy insulation like fiberglass or cellulose, the fluffy insulation must be installed in direct contact with the rigid foam (or with the underside of the roof sheathing in the case of rigid foam installed above the roof sheathing). You don't want any intervening air space.

    There are several ways to proceed, none of them easy or cheap. One way would be to put all of the insulation on the exterior side of the existing roof sheathing. Another way would be to remove the drywall ceiling for better access.


  3. GBA Editor
    Martin Holladay | | #3

    In Comment #1, you wrote, "I am in climate zone 5, so I'd be aiming for R-60, with at least R-20 in foam if placed outside the sheathing." In Zone 5, a foam-plus-fluffy roof requires at least 41% of the total R-value to be in the form of foam insulation. R-20 foam would be 33%, not 41%. You'll need a minimum of R-25 in the form of rigid foam.

    For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

  4. Jon_R | | #4

    You could create a tray ceiling on the interior edges using polyiso foam and rely on only fiberglass/cellulose in the center portion. Ie, add insulation below the trusses.

    For others with new builds, such trusses are available with raised heels to solve this problem.

  5. pshyvers | | #5

    Martin, thank you. Yes, it looks like there are all kinds of options, none a clear winner! The rigid foam outside the sheathing seems to be a popular choice for cathedrals, but it's unclear if crews around my neck of the woods in northern Colorado have any experience hiding the thicker roof. I'd also be converting from vented to ventless, I'd need to fill the current dead space with something, AND I'd need to seal off the vaults from the flat attic. Are there any hidden-rot risks with exterior rigid foam, as with interior spray foam? Or is the 41% rule pretty bulletproof.

    Are you familiar with methods like described here:

    I wasn't planning on pulling drywall, but if I assume I have at least 12 inches for the R-38 batts, plus baffle, today- then I can achieve around R-48 at the eaves with dense pack, loose-fill the middle, and keep a straightforward vented scheme w/ mostly fluffy insulation. I'd also be able to seal top plates while the drywall is off.

    I was initially attracted to exterior rigid sheathing for thermal bridging concerns, but reading more about scissor trusses I realize the problem in my ceiling is more likely the simple fact the ceiling trusses are completely uninsulated (batts between, nothing on top) rather than thermal bridging. Loose fill would dramatically improve that problem.

    Jon, are you imagining a flat tray, or simply an inch or two of polyiso screwed to the trusses? The former I feel would significantly close the openness of the rooms, but a foot or two of the latter might pass as an interesting architectural detail.

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