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Flat roof assembly in a hot humid climate

We are going to construct a very modern home with low pitched roofs (1/4" per foot on 5 separate roof levels) being constructed of 18" trusses, utilizing Advantech sheathing for the deck, then a vapor permeable fluid or adhered membrane, then rigid Polyiso board varying from a minimum 1" thick to 6" to create the slopes, then Densdeck Duraguard and finally a fully adhered, highly reflective PVC membrane. Underneath the deck, we intend to use 6-8" of open cell spray foam.

This will leave an air gap of 10-12" between the top of the dry wall on the ceiling and the bottom of the spray foam insulation below the deck. Will this avoid/decrease the likelihood of condensation issues? Would you advise filling the 10-12" air gap with insulation or is that over-kill?

We haven't been able to access some of your prior articles due to website issues (have notified customer service) and have seen some examples from Joseph Lstiburek that show this gap open and some that show it filled depending on the website, and without regards to what climate zone.

Please advise.

Asked by Norman Luke
Posted Sun, 02/09/2014 - 00:42
Edited Sun, 02/09/2014 - 07:00


4 Answers

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Six inches of open-cell spray foam will give you an R-value of about R-22. One inch of polyiso (the thinnest layer of your above-sheathing insulation) will give you about R-6. If you add the two together, that's about R-28. That is less than the minimum code requirement everywhere in the country.

However, it's close to the minimum (R-30) in Climate Zones 1, 2, and 3.

Assuming you are able to hit the minimum code requirement for ceiling R-value, there is nothing wrong with having an air gap between your ceiling drywall and the underside of your open-cell spray foam.

For more information on these issues, see Insulating Low-Slope Residential Roofs.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 02/09/2014 - 06:58
Edited Sun, 02/09/2014 - 06:59.

Helpful? 0

"Thanks Martin! We will definitely increase to at least 8" of open cell and possibly increase the minimum thickness of the Polyiso to 2" - if possible at the drains with low parapet concerns."

Answered by Norman Luke
Posted Sun, 02/09/2014 - 07:34
Edited Sun, 02/09/2014 - 07:35.

Helpful? 0

Norman – I’ve taken a different approach to flat roof drainage in the SW for many years. I do not design drains because they can freeze and get plugged-up with debris so the maintenance issue is high. I rather do “canales” or scuppers with the roof slopped to one side. If the truss is designed with ¼” slope, you don’t need to increase the slope with insulation; it’s too expensive. I would rather increase the slope in the trusses to ½” and just install the “flat” rigid foam over the decking. The framer needs to build crickets to the scuppers, and make sure they are not draining to a door or window.

Answered by Armando Cobo
Posted Sun, 02/09/2014 - 11:57

Helpful? 0

Norman, I fully agree with Armando here. First, increase the slope to 1/2", because when you spec 1/4" you are asking for standing water on the roof. Look around at rooftops and you'll see plenty of low slope roofs with standing water after rainfall. Those roofs were spec'd at 1/4" slope but Murphy's law and/or entropy got the better of them. Second, drain to one side through the parapet rather than to an interior drain. Third, slope the OSB deck, and then your polyiso can be a constant thickness.

Answered by TJ Elder
Posted Sun, 02/09/2014 - 13:49

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