Home>Discussions>INSULATION & HVAC>Open Cell Foam Spray-up insulation under roof decking
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pjdaoust
Open Cell Foam Spray-up insulation under roof decking

On a recent episode of Ask TOH you were recommending spray-up open cell foam insulation as a remedy to ice damming. This is probably an effective remedy in the snow bound north, but my question is what happens if this approach was tried in the south where we have blistering heat all summer ? It is known that an attic that is not properly ventilated will cause the shingles on the roof to curl up and shorten the life of the roof. With the open cell spray up insulation the heat is trapped between the insulation and shingles wouldn't that affect the life of the roofing material ? Is this open cell spray up even recommended where the summers are hot ?

Paul J. Daoust
Raleigh, NC

motoguy128
Re: Open Cell Foam Spray-up insulation under roof decking
pjdaoust wrote:

It is known that an attic that is not properly ventilated will cause the shingles on the roof to curl up and shorten the life of the roof. With the open cell spray up insulation the heat is trapped between the insulation and shingles wouldn't that affect the life of the roofing material ? Is this open cell spray up even recommended where the summers are hot ?

Paul J. Daoust
Raleigh, NC

Is that truly a fact? My attic was completely unvented for 86 years. The current roof is 8 years old and the shingles look fine to me.

Venting attics didn't become popular until after WWII. Actually it's hard to pinpoint when the practice started. It probably began at the same time we started insulating attics.

So are modern shingles just inferior to shingles from before 1945??? Possibly.

You also have to consider how hot even a vented attic is and how little actual airflow there is compared to the solar heat gain on a roof. Plus you have wood sheathing under the shingles. Hardly a good heat transfer material. The numbers I've seen say that the cooling effect is around 5F.

Shingles still curl on well vented attics. I think it has more to do with quality of materials. That's my take.

pjdaoust
Re: Open Cell Foam Spray-up insulation under roof decking
motoguy128 wrote:

Is that truly a fact? My attic was completely unvented for 86 years. The current roof is 8 years old and the shingles look fine to me.

Venting attics didn't become popular until after WWII. Actually it's hard to pinpoint when the practice started. It probably began at the same time we started insulating attics.

So are modern shingles just inferior to shingles from before 1945??? Possibly.

You also have to consider how hot even a vented attic is and how little actual airflow there is compared to the solar heat gain on a roof. Plus you have wood sheathing under the shingles. Hardly a good heat transfer material. The numbers I've seen say that the cooling effect is around 5F.

Shingles still curl on well vented attics. I think it has more to do with quality of materials. That's my take.

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Any roofer will tell you that the attic must be properly ventilated to avoid shortening the life of your roof shingles.

As far as ventilating becoming popular after WWII, not sure why that would be the case except that it is also when the practice of insulating ceilings and walls became more wide spread. I have heard of cases where there is so much moisture up in a poorly ventilated attic space that it was condensing on the underside of the roof deck. That can't be good.

As far as plywood being an insulator between the shingles and the inside of the attic, while it may be an insulator to some degree it is certainly not as much as a foot of fiberglas. All you'd have to do is to touch the underside of that sheathing to find out how much heat is really coming through that sheathing.

Michael
Re: Open Cell Foam Spray-up insulation under roof decking

I keep hearing mixed things about this topic and there are a lot of factors involved. However, if heat rises, then it would stand to reason that the underside of the decking would actually be cooler if it is insulated than if it was not since the heat would only be coming from solar.

I live in the north and we plan on doing this same thing in a few years.

motoguy128
Re: Open Cell Foam Spray-up insulation under roof decking
Mclark wrote:

I keep hearing mixed things about this topic and there are a lot of factors involved. However, if heat rises, then it would stand to reason that the underside of the decking would actually be cooler if it is insulated than if it was not since the heat would only be coming from solar.

I live in the north and we plan on doing this same thing in a few years.

Not exactly. An attic gets hot because of the heat transferred form the roof deck. It's not coming from inside the house. However, the amount of airflow you get form passive ventilation only cools the attic to within 10-20F of ambient. probably about 90% of the roofs cooling comes from air cooling from the outside. The same convection currents inside a vented attic occur on the roof surface.

Now you will still get some cooing through the foam. If it's R20... and 80F inside the attic and 140F on the shingle surface, that's still 6000BTU's hour of cooling occurring on a 2000 sqft roof. You'll need 150CFM from 100F outdoor air to get that same amount of cooling. For powered venting, they recommend 10 air changes per house. That's around 500CFM. Yes, that's 3 times the cooling, but we're still only talking about 1.5tons of cooling. That's the same as large window AC unit. When the roof is 140F... how much good do you think that would do on a 140F 2000sqft surface??? Yup... probably about 5F at most.

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