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Community and Q&A

Insulating cathedral ceiling w/polyiso, etc.

mikhailwatt | Posted in Energy Efficiency and Durability on

Hello all,

I’m preparing to re-insulate my pitched cathedral ceiling(s) with polyiso board (2 layers x 3″ for R40 or thereabouts) and just looking for a sanity check on the foil-side orientation… will have a 2″ vented air gap above – assuming I want the top layer foil side facing the vent space, and assuming the bottom layer orientation doesn’t matter much.

Roof construction is (1989 tract home) 18″ wood web joists @ 24″ oc – will use 1/2″ foam board plus spray foam to insulate the top joist chord and maintain an air seal between the ceiling bays. Filling the web spaces with rockwool.

Going for an “exposed beam” look, wrapping the bottom section of the joists with wood of some kind, while trying to keep the insulation assembly airtight. Layer of GWB over the polyiso board, then decorative wood decking.

Austin, TX – hot/humid climate zone 2b.

Thanks for any/all advice or comments,
(Self-professed DIY hack)

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Michael,

    First of all, are you sure the insulation you are using has foil on only one side, many of the commonly available polyiso products have foil on both sides.

    Anyway, there are two reasons to be specific about the direction that the foil faces. The first is to protect the insulation from water, but this isn't relevant in your situation. The second is to get the extra performance of the foil as a radiant barrier. To do so, the foil needs to face an air space. So, if you are using foil faced polyiso to create your ventilation channel in your assembly, face the foil towards the air space. Likewise, if the insulation at the bottom of the assembly will not be in contact with the dryway, face the foil towards that air space too.

    Other than being able to offer that, I'm a bit confused by your post.

    It sound like you have I-joists for roof rafters and you plan to install polyiso rigid foam against the top flange to create an air space, spray foam the polyiso rigid foam in place for an air seal, and then fill the flange with mineral wool. From there I get lost with your intended design. Are you describing another layer of polyiso and drywall between the I-joists so you can wrap the bottom flange with trim?

    Maybe another GBA member will understand better and can help, or you could post a section drawing showing what you are trying to describe. I'd be glad to try to help more once I can understand better.

  2. mikhailwatt | | #2

    Thanks - not i-joists, they're open web wood trusses. Cross section sketch attached, hopefully clarifies the assembly.

    Using R-Max Thermasheath 3, actually might have foil both sides, will find out tonight.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Michael.

    Thanks for the drawing. That's a creative assembly.

    You're probably aware that multiple layers of foil faced polyiso are not the best from a drying perspective. In other words, if the assembly gets wet, you have foil facing, blocking drying by diffusion, in both directions.

    Have you considered a layer of EPS to create the ventilation channel followed by fibrous insulation? Or an unvented assembly with a layer of spray foam against the roof deck and over the top chords of the trusses to mitigate thermal bridging followed by fibrous insulation? Seems like you both would still leave you some depth to wrap the bottom of the trusses with your trim detail.

  4. mikhailwatt | | #4

    Thanks Brian. I did consider both methods, but the EPS+rockwool option actually came out more expensive, with a somewhat fussier installation - and would have reduced the aesthetic of the exposed beams too much.

    Unvented w/ spray foam was cost-prohibitive, and I had concerns about roofing temps and possible moisture issues if it wasn't installed perfectly.

    I would expect the ventilation channel to allow drying to the outside... not sure about any issues with interior moisture getting into the assembly. The old ceiling (GWB & fiberglass w/ vent space) showed no signs of getting/remaining wet, and it was definitely not an airtight assembly.

    1. GBA Editor
      Brian Pontolilo | | #5

      I agree with you. I don't think your proposed assembly is too risky, because of the ventilation, R-value, and because you are going to do plenty of air sealing. Just wanted to make sure that you understood what the risks are.

      Good luck and please share a finished photo of the trim detail.

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