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Community and Q&A

Roof replacement and insulation

7echo | Posted in General Questions on

I plan to replace my roof shortly. I am in zone 2A. Roof is 1″ in 12″, so low slope, and about 24 squares. House looks like an Eichler sort of, but not as nice. Only has 5 penetrations so roofing should be relatively easy. Current assembly from outside to inside-
Torch down mod bit, roofing felt, 5/8″ plywood, 2 1/2″ x 6 1/2″ beams 4′ on center, foil sheet, 2x4s strapped to the bottom of the beams, drywall. Some of the ceilings have 6″ batts, poorly installed.
Shell is brick, air gap, exterior gypsum panel, 2×4 wall studs. At the top of the brick wall there is wood siding for about 12″ up to the roof deck. There are 2″ holes in that siding that open into the joist bays as vents.
I plan to leave the existing roof, add 3″(maybe a little more) of polyiso, then maybe a recover board, then 80 mil TPO.
Is it worth while to cut the roof open at the rim joist(not sure if that is the correct term for this assembly?) and spray foam that short area from top of the walls to the roof deck? That would seal those vent holes and give me something close to an insulated and sealed envelope.

Comments and options are appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Q. "Is it worthwhile to cut the roof open at the rim joist (not sure if that is the correct term for this assembly?) and spray foam that short area from top of the walls to the roof deck? That would seal those vent holes and give me something close to an insulated and sealed envelope."

    A. Yes, that would be worthwhile.

    Q. "Comments and options are appreciated."

    A. Three inches of polyiso gives you about R-18 or R-19. In your climate zone, building codes call for a minimum of R-38, which is twice as much as you are planning. So I urge you to consider installing 6 inches of polyiso.

    For more information, see "Insulating Low-Slope Residential Roofs."

  2. Expert Member
    Dana Dorsett | | #2

    Code min on a U-factor basis is U0.030, or R33.3 "whole assembly R". Roofing polyiso suitable for walkable torch-down roofs is usually R17 @ 3". The interior & exterior air films add another R1, and the roof deck & drywall add up to another R1, so now you're at R19.

    To that it's legitimate to add the additional air films & reflectivity of the sheet aluminum can add up to R8-R9 when shiny-new and dry, but with even a skim coat of condensation (you air condition the house, do you, bringing the temperature of the aluminum down to the outdoor dew point sometimes) that drops to ~R3, so worst-casing it at R3 you're really at R22, not R33.3.

    Blocking the vents and blowing the 6.5" space above the foil full of 1lb density fiberglass would yield another R24 at center cavity, and will definitely be enough even after factoring in the thermal bridging. With R17 above the roof deck you have more than adequate dew point control at the roof deck.

    Whether 6" of polyiso or 3" polyiso + 6.5" fluff, the question remains as to whether the foil on the bottom plus the TPO on top creates a moisture trap once the vents are closed. I believe it does. If its possible to remove at least most of the foil prior to filling the cavity it would be more moisture resilient.

    Venting the brick to the exterior with weep hole vents in the vertical mortar every 18-24" on a course of brick near the bottom, and a corresponding set of vents in a course of brick near the top will convection-dry the space behind the brick. There are purpose-made bug-screens that t wedge into the weeps & vents.

  3. 7echo | | #3

    Thanks for the replies.
    I am David.
    I will try to edit my user information and have my name appear.
    2 issues with meeting code-
    1)Doubling the foam thickness is going to add some dollars, I will get that cost from the roofer.
    2)The face of the eave is currently 9 3/4" thick. Adding 3" is going to start looking too thick, adding another 3" to that will likely look bad. I realize these are aesthetic issues but that does factor in for re-sale and such. However, I will mock up a section with 6" of added thickness and see how it looks.

    I think it will be too difficult/expensive to fill the joist bays with blown in insulation. If I were removing and replacing all the decking I would consider adding batts or blown in. But that is going to be too expensive. I was also concerned about the moisture issue you mentioned. Of course it would be easy to remove the foil if I were re-decking.
    I like the idea of venting the bricks. It seems like a DIY project I can handle. I will search for some tips on the procedure.

    I reckon this project is like many others and has to be balanced between cost, effort, and return. Going from essentially no R value to R-18 is probably the best I can manage but I will consider options.

  4. Expert Member
    Dana Dorsett | | #4

    The moisture trap problem issue exists whether or not the bays are insulated with fiber, or whether there's 3" of foam up top or 6". It was designed to be vented from below, which is a thermal bypass for the insulation, but gives an air-drying path to the outdoors. With the foil in there preventing drying via diffusion to the indoors it's a moisture trap once the vents are blocked. That's fine if the wood is dry when it's sealed up, provided the roof never leaks.

    Using reclaimed roofing foam 6" is cheaper (and greener) than 3" of virgin-stock polyiso. Many building supply salvage outfits will have it, many will advertise in the local craigslist:

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