# Spray foam and ventilation



## Erwin (Nov 30, 2021)

Hi, Erwin- new to this forum.
have decades of construction knowledge, but I have tried everything for 10 years on my house. Garage, new construction, I used open cell spray foam on underside of Zip sheathing- been great
house was fiberglas- Cape with shingle roof. Ice backup many times. Ripped entire roof clean to 9/16 plywood- installed brand new metal after ice and water shielding. 5 years ago, I spray foamed- closed cell sprayed entire under side after I proper vented
just found another leak and saw water on underside of roof while renovating my upstairs bathroom 
WHAT AM I DOING WRONG- THANKS


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## roofermann (Jul 7, 2012)

Can't speak to your situation exactly not having seen it firsthand. But in the 30 yrs I've been doing residential re-roofs every single home that had foam (panels or spray) tight to the underside of the roof sheathing had issues with sweating (condensation), form just a bit of dampness to full blown rotted sheathing and rafters. Those homes the had good postive ventilation of the sheathing underside had little or no issues.


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## Erwin (Nov 30, 2021)

roofermann said:


> Can't speak to your situation exactly not having seen it firsthand. But in the 30 yrs I've been doing residential re-roofs every single home that had foam (panels or spray) tight to the underside of the roof sheathing had issues with sweating (condensation), form just a bit of dampness to full blown rotted sheathing and rafters. Those homes the had good postive ventilation of the sheathing underside had little or no issues.


thanks- keep hearing fine to sandwich sheeting with ice/water out and closed cell in, but….


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## roofermann (Jul 7, 2012)

From what I've gathered by reading about this on here and other forums, it has something to do with the amount of r value. Not enough and the temp differential between outside and in gets you condensation. Take this with a grain of salt, I'm not an expert.


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## Erwin (Nov 30, 2021)

roofermann said:


> From what I've gathered by reading about this on here and other forums, it has something to do with the amount of r value. Not enough and the temp differential between outside and in gets you condensation. Take this with a grain of salt, I'm not an expert.


Got it, thanks


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## anonymous (Jan 6, 2022)

roofermann said:


> Can't speak to your situation exactly not having seen it firsthand. But in the 30 yrs I've been doing residential re-roofs every single home that had foam (panels or spray) tight to the underside of the roof sheathing had issues with sweating (condensation), form just a bit of dampness to full blown rotted sheathing and rafters. Those homes the had good postive ventilation of the sheathing underside had little or no issues.


Hello-We're in Minneapolis MN and had a 'hot roof' polyfoam install done directly onto deck in the attic & knee walls. No one told us there should be ventilation somewhere. Where would the ventilation have been? We were told no ventilation needed-& to remove the 'bird' vents that are now there-they fill those holes w/ plywood, then roof felt, then shingles w/ ice & water shield around perforationns & up 6' at eaves & valley. But no one talked about ventilatiion for foam job. Help. We need a new roof now & not sure how to deal with roof rot at tear-off time, & how to repair damaged foam if we need to. We'll be using class 4 GAF shingles. Also not sure how to write this into the contract wo we're protected-any thoughts here?


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## RooferE (Jan 10, 2022)

It's to my understanding that when a roof is insulated at the actual roof rafters and not the ceiling joists you have a closed system and do not want ventilation.


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## anonymous (Jan 6, 2022)

RooferE said:


> It's to my understanding that when a roof is insulated at the actual roof rafters and not the ceiling joists you have a closed system and do not want ventilation.


That's correct per my research, & manufacturers. I wonder about underlayment before tthe singles-maybe a good idea to putt down ice & water shield over the whole roof just to be on the safe side, instead of only 6' up the eaves, & around penetrations; just cover the entire roof, then shingle it?


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## rexroof (11 mo ago)

Erwin said:


> Hi, Erwin- new to this forum.
> have decades of construction knowledge, but I have tried everything for 10 years on my house. Garage, new construction, I used open cell spray foam on underside of Zip sheathing- been great
> house was fiberglas- Cape with shingle roof. Ice backup many times. Ripped entire roof clean to 9/16 plywood- installed brand new metal after ice and water shielding. 5 years ago, I spray foamed- closed cell sprayed entire under side after I proper vented
> just found another leak and saw water on underside of roof while renovating my upstairs bathroom
> WHAT AM I DOING WRONG- THANKS


Ventilation is required by roof manufacturers (1 square foot per 150 square feet of attic floor space, or 1-300 if there is balanced ventilation between the eaves and the ridge. It makes no sense to foam rafters and roof decks. The idea is to keep heat in the living quarters and not in the attic. New construction is different though.


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## rexroof (11 mo ago)

RooferE said:


> It's to my understanding that when a roof is insulated at the actual roof rafters and not the ceiling joists you have a closed system and do not want ventilation.


I bring this up every year at GAF Expo. Answer is always the same. If shingles fail from that the warranty is voided. Terrible answer. Closed systems as stated above don't need the ventilation and new construction homes get that on interior walls and attic walls, ceilings, and rafters. As long as the roof shingles don't fail from over heating then it works.


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## rexroof (11 mo ago)

Most roof shingle manufacturers refuse to take a position. Atlas Roofing says it must be ventilated regardless of foam on the underside which of course defeats the foaming. Insulation belongs on the floor of attics keeping heat in the living quarters. Foam insulated roof decks over indoor pools have been know to have damage. Eaves can be ventilated with Smart Vent of similar under the eave roof shingles, ridges with Ridge Vents or powered fans with heat & temperature control.


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