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Cathedral Ceiling under Steel Roof

EtQyFdEVpC | Posted in Energy Efficiency and Durability on

I am new to this site and have seen a lot of great communication on proper insulation and vapor barrier techniques. I am in the process of building a house in Seattle WA and could use some advice.

My architect just put in fiberglass batt under the roof sheathing to meet local code. House is roofed with standing seam steel laid directly on top of the roof sheathing (1/2″ CDX). The roofer did install what he called a “ice barrier” between the CDX and steel. This barrier is Interwrap Titanium PSU-30 material. The roof is a cathedral style meaning the rafters are 2×12 with conditioned rooms directly underneath. So far, the steel roof, Interwrap ice barrier, and CDX are installed. We are quickly approaching the point at which we’ll be insulating and this is where I need advice.

Given the fact that the steel and ice barrier have very low permeability the roof is going to dry to the interior. I am trying to figure out whether I need to create a venting space directly under the roof decking but above the insulation? If so, what is the best stack up that would achieve this and are there design images out there showing it?

I have been told that installing a closed cell foam (about 3″) directly onto the lower surface of the roof decking (and, therefore, not going with a vented space) is best. The remainder of the rafter bay could then be filled out with something like wet spray cellulose or batting. Thoughts on which of these techniques is best?

Lastly, we’re planning on installing in ceiling canned lighting so looking for insulation build ups compatible with having this type of lighting in the rafter bay.

Thanks for any helpful advice!


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  1. EtQyFdEVpC | | #1

    Thanks Armando. Would you recommend leaving a vented space under the decking or install the cellulose right up to the decking?

  2. Expert Member
    ARMANDO COBO | | #2

    There are mmany folks here at the GBA that advocate for vented cavities, I don't in this application.

  3. Expert Member
    ARMANDO COBO | | #3

    I would not install closed cell foam, that would "sandwich" your plywood sheathing between two impermeable layers. I would air-seal the cavities on all edges with a bit of calking, mastic or foam, then fill the cavities with packed cellulose (R39) and finished with an air tight drywall. That will allow the roof assembly to dry well to the inside.

  4. GBA Editor
    Martin Holladay | | #4

    Armando's suggestion will probably work if it is well executed, but it won't meet code in most areas. Most building codes require venting under the roof sheathing when using air-permeable insulation like cellulose, unless the roof sheathing has rigid foam on top.

  5. EtQyFdEVpC | | #5

    Thanks Martin. Are you suggesting that I should go forward with the 3" of closed cell foam approach I outlined in the entry post at the top of this thread? Or with a vented space? Just trying to figure all of this out as there seems to be a lot of conflicting opinions.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Are you suggesting that I should go forward with the 3" of closed cell foam approach I outlined in the entry post at the top of this thread? Or with a vented space?"

    A. Either way will work.

  7. EtQyFdEVpC | | #7

    Any suggestions as to how to best deal with canned lighting in the rafter bays? Do I need to segragate the can from the insulation material?

    Thanks again.

  8. Foamer | | #8


    With an exterior vapor barrier, you should either vent or use a non-air and vapor permeable insulation material. What you cannot do is install fiberglass in a non-vented system. In your climate, a couple of inches of closed cell spray foam on the deck will do the trick.

    Install as thick a layer of polyisocyanurate insulation board between the top of your can lights and the roof deck as you have room for. Extend the foam board 6-12 inches past the cans and let your foam contractor tie into the board. He will leave an air space around the can and you will be set.

  9. davidmeiland | | #9

    I would recommend against the can lights in an insulated ceiling like that, because they are real heat leakers, but if you're committed to it I would look for the lowest-profile models out there (some fit into 2x6 bays) and you need to use insulation contact models (best to use air-tight also... designation is IC/AT).

    Re the venting, the best thing to do IMO is install baffles made of thin plywood or possibly rigid foam (2" polyiso would be ideal), leaving a 1" vent space between the baffles and the roof deck. Nail 1" spacers along the sides of the rafters, and then push the baffle material into place against the spacers and nail it in. Air-seal the baffles to the rafters along the full length (and where they butt/overlap each other) and make sure to detail them at the top and bottoms so there aren't air leaks there. Doing this right is labor intensive and takes a clear understanding of how air leaks happen.

    Once you have baffles in place, you can install blown-in cellulose or maybe fiberglass. Spray foam would also work, but I would not spray it directly to the deck.

    I'm in the same region as you, a bit north, and am not familiar with the roofing underlayment you mention. We use ASTM D226 30# felt under metal roofing with CDX plywood decking, and there are no problems. In some regions it is common to use something like Grace Ice & Water Shield over part or all of the roof, but that's hateful stuff if you ever have to get it off to remodel or repair.

    Here's an infrared image of a roof with can lights in it. Every house with cans in a cathedral ceiling looks like this. In general, cathedral ceilings leak a lot of air and heat because they are rarely insulated and sealed well.

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