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Insulating a flat roof with open rafters

kmir | Posted in General Questions on

I have a small concrete block warehouse, located in Nashville, TN, with concrete floors (hopefully soon to be wood, I’ve researched that topic extensively and have some experience with it, so I feel relatively confident that my plan will work). The ceilings are 17 – 18 feet high (1 foot drop over 50 feet for the flat roof). I attached a sketch I made of the ceiling situation. There is some medium quality EPDM roof on top, a layer of rigid insulation of unknown type and thickness (I saw it once briefly three years ago but didn’t bother to note the details back then), then probably some other layers of stuff (? maybe, probably?), and then the plywood that is visible on the inside of the building. The plywood is in good shape for being over 20 years old. I do not see any rot or other forms of decay etc. The whole thing is being held up by two rows of rafters, which are connected by trusses.

I would like to insulate this ceiling/roof a little more. I’ve thought about the different options, and spray foam is certainly the standard solution most people mention first. I do not like the idea of spray foam for a couple of reasons:

1) Health concerns related to spray foam; I am not aware of specific evidence, but I just don’t trust it all the way.
2) Moisture concerns: The plywood will be sealed to the space below, but the roof is not perfect and there might be moisture intruding. I really want some build up for a larger slope and a metal roof later on!
3) Price point: Estimated $10000 with DIY

I really like rockwool. It insulates well and also provides sound proofing. The attached sketch shows what I have in mind. I would love to leave a gap between the rockwool and the plywood. I am prepared to monitor conditions through a few easy to remove panels distributions throughout the ceiling.

Also, I expect to require a new roof in 2 to 3 years, and I am hoping to change the whole assembly at that point, converting the roof from flat to not flat and ideally using a metal roof.

How does all of this sound?

Thank you for your time!

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  1. GBA Editor
    Martin Holladay | | #1

    You have an unvented low-slope roof. Unless you know the R-value of the rigid insulation above the roof sheathing -- and you don't know that -- you can't install any fluffy insulation like mineral wool under the roof sheathing (because the result might be moisture accumulation and sheathing rot).

    Your only options are (a) Installing closed-cell spray foam on the underside of the roof sheathing, or (b) adding a known thickness of rigid foam insulation on the exterior side of the roof sheathing, followed by new roofing.

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

    (Note: Even though the article has the word "residential" in the title, the advice applies to commercial buildings as well as residential buildings.)

    You are in Climate Zone 4. If you want to create an unvented roof assembly that combines exterior rigid foam insulation with interior fluffy insulation, the R-value of the exterior rigid foam insulation must be equal to at least 31% of the total R-value of the roof assembly. For more information on this point, see "Combining Exterior Rigid Foam With Fluffy Insulation."

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