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Insulating a Roof over a Vented, Unconditioned Attic

AHarmer | Posted in GBA Pro Help on

I am having my low pitched roof changed from tar and gravel to asphalt shingles. When the contractor removed the existing tar we discovered that, (surprise!) the roof decking used (plywood) when the roof was put on was far too thin and is now sagging on most of the roof and rotting in others. So, we have to replace the plywood at least OR, our contractor recommends, fix just the rotting spots, put polyiso rigid boards right on top of the current decking, and then another layer of plywood over that and then shingles. I am very interested in addressing some of the significant heat loss/cooling issues of our home (it has no insulation in the walls, NONE!) but am wondering if it will actually do anything to solve the issue given how our home is built: we have an attic with blown in insulation on the floor, I do not know the R-value. There are vents at the soffits but no vents on the roof  (our contractor wishes to add a ridge vent). The attic is completely sealed from the rest of the house (unconditioned) with drywall. So, with this set up, given the soffit vents and unconditioned, vented attic will insulating the roof with rigid ployiso boards do anything to assist us with keeping the house cooler in summer and warmer in winter or should we just replace the plywood, put on the shingles and be done with it?

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Replies

  1. Expert Member
    KYLE WINSTON BENTLEY | | #1

    AHarmer,

    The question below is very similar to yours, maybe give it a look over and see if it provokes any thoughts:

    https://www.greenbuildingadvisor.com/question/unvented-attic-retrofit-with-2x4-rafters

    1. AHarmer | | #2

      Hi Kyle,

      Thank you for the helpful thread! I appreciate it. The two questions I come away with after reading the thread are: (1) if my roof is vented at the soffits and we’re adding a ridge vent (the town I live in requires venting, they do not like hot roofs, apparently) do I need to air seal the roof deck before layering the polyiso board? From what I have read and what seems to make sense here the answer is yes, but my roofing contractor keeps insisting that the polyiso boards go directly on the roof deck. If air sealing is the correct path, what product can I point my contractor to to use as an air barrier, and if you have any tips/resources on convincing him he has to do it, I’m all ears, and (2) you mentioned that with this path you have to get creative with retrofitting facia boards - if we are building up an existing tar and gravel roof to the edge of existing fascia boards and without the polyiso boards we would otherwise have to cut down the fascia boards, does the retrofitting problem go away? It seems like it does. Thank you, in advance!

  2. Expert Member
    PETER G ENGLE PE | | #3

    IMVHO, you probably won't see enough energy savings from adding rigid insulation to the top of the sheathing if the attic is well ventilated. This does depend on how much insulation you have on the attic floor. FWIW, you would probably see as much reduction in heat transmitted through the sheathing just by using high reflectance shingles, at a substantially lower cost.

    1) That said, if you do go ahead and install polyiso, you do not have to air seal the roof deck and you do not have to tape the seams. There is no point in air sealing the deck when the attic below is ventilated.

    2) You may or may not have to adjust the fascia boards depending on how thick the new roof system is. This is not a big deal, but does potentially add some cost.

    Finally, I come back to your initial project description. You say you are replacing a low-slope tar & gravel roof with shingles. Is the pitch adequate for shingles? Most asphalt shingles require special underlayment for pitches of 2:12-4:12, and prohibit use at lower than 2:12.

    1. AHarmer | | #4

      Thank you for your reply! Not knowing what to do and needing to move forward this morning, I went ahead and asked the roofer to air-seal the roof deck. I can understand why the roof deck would not need to be air sealed over a vented attic, but I hope it doesn't hurt - at the very least I imagine it will help force air to circulate as we intend to, i.e. - in through the soffit vents and out through the ridge vent and help keep the roof deck warm in winter. Perhaps this is misguided but it is too late now. I'm also not sure the attic is all that well vented to begin with.

      I get that insulating a roof that has insulation at the attic floor level is probably not the best bang for our buck but for a number of other reasons it made sense, not the least of which is that the insulation on the attic floor is not great. The insulation at the attic floor level appears to be a mix of blown-in and some sort of paper-backed fiberglass and I have no idea what R-value it all amounts to. For air quality reasons, I am considering removing the blown-in and adding bats underneath the roof deck at some point but we will see.

      We do have enough pitch for shingles, thankfully, but thank you for checking.

  3. luke_p | | #5

    I personally don't see any point in insulating the roof deck if the attic is vented. Also polyiso is expensive. Increase your attic insulation by blowing more insulation in _making sure to keep the soffit vents clear) and improving the air tightness of your ceiling.
    If it is a true low slope roof, have you considered a light coloured roof membrane, such as white modified bitumen or white TPO? It won't help with heating but it would certainly help with cooling loads in the summer.

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