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Community and Q&A

Insulating an old flat roof

rjmhudson | Posted in Green Building Techniques on

I’ve been planning and talking with a roofing contractor about reroofing a small, 2-story apartment building (total building footprint is 2700 sq ft) in climate zone 5. About a third of the roof is a flat roof with EPDM membrane, so we’re going to add insulation on top of the existing roof deck. The existing roof is unvented and looks to be 6″ thick. It looks like it has 2″x6″ rafters, so I’m guessing that the existing insulation is R-19 fiberglass plus another R-2-3 for the ceiling material and tiles. We’re looking at adding about R-30 in foam board over the existing roof, mostly in a flat section plus some tapered foam to give it some slope with a membrane on top.

The contractor proposed to use polyiso to make an R-30 layer, but I learned reading GBA previously that polyiso loses R at low temperatures. So I’ve been discussing the possibility of using 3-4″ of EPS on top of 2″ foil-faced polyiso to make up the R~30 section. Then the contractor wants to add OSB over the EPS for strength. Finally, a sloped layer would add another inch or so of poly on average.

It looks like this balance of insulation above/below the old deck would be okay, based on your piece on low-slope residential roofs (

So I’d love to hear advice. My specific questions are:
1) Is pushing for the hybrid insulation worth it? It’d be simpler to just let him use poly throughout.
2) Is using foil-faced polyiso next to the existing roof a good approach? (I have a bunch of it on hand already). I’ve seen discussion of using foil-faced polyiso as the top layer. Perhaps that is for warmer climates?
3) Should the old membrane be left in place or removed?
4) Is an EPDM or TPO membrane preferable? A lot more energy goes into heating than AC in that building (lots of shade in summer), so I thought black EPDM would be better. However, the contractor seems to prefer TPO and I’m not sure the energy difference is enough to push him on it. (I’m using LBL’s home energy online calculator).

That’s it for now. Reactions appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Robert,

    I believe it would be fine to use the foil-faced polyiso that you already have on the top of this roof. Unvented roof assemblies dry inward. As long as there is no interior vapor barrier, you shouldn't have a problem.

    I don't have experience or knowledge to help with your EPDM vs TPO question. Hopefully a GBA user who does will be along soon to help. If you don't get a helpful answer on this, shoot me an email and I'll see if I can find someone to help you.

    [email protected]

  2. rjmhudson | | #2

    I'm not clear whether you mean by using the poly-iso on the top of this roof. Do you mean as top layer off the whole thing or on top of the old roof? Here's a sketch of the design I have in mind.

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