Why Spray Foam Under Asphalt Shingles is a Bad Idea

In this episode of Smith House, Jordan Smith, a design builder with Smith House Company, dives deep into the controversial topic of using spray foam on the backside of asphalt shingle roofs. With a focus on building houses that are beautiful, functional, resilient, and healthy, Jordan explains why he believes this common practice may not be the best approach.

⚠️ Key Points Covered:

Warranty Concerns: Some shingle manufacturers don't allow spray foam directly against the decking, potentially voiding warranties.
Replacement Challenges: If decking needs replacement, the spray foam must also be replaced, leading to added costs and complexity.
Exploring Alternatives: Jordan explores the drawbacks of traditional ventilated attics, the benefits and limitations of spray foam, and proposes a third approach for optimal performance and longevity.

🔍 Video Breakdown:
Introduction: Jordan introduces the topic and outlines his concerns.
Ventilated Attic: Discusses the benefits and challenges of traditional ventilated attic systems.
Spray Foam Solution: Explores how spray foam addresses some attic ventilation issues but introduces new challenges.
Optimal Roof Assembly: Introduces a continuous insulation house as a potential solution, highlighting its benefits over traditional methods.

🔧 Building for the Future: Jordan emphasizes the importance of thoughtful building practices that prioritize durability, energy efficiency, and ease of maintenance.

💬 Engage with Us: Share your thoughts in the comments below. Agree or disagree? Let us know! Have suggestions for future topics? We're all ears!

👍 Don't Forget to Like, Subscribe, and Follow! Stay updated with our latest content on Instagram and TikTok.

🔗 Connect with Us:

Thanks for watching, and see you in the next episode of Smith House!

[timmmahhhh]

Thanks for the great video Jordan. I'm an architect and 20 years ago the builder I do most of my work with came to me with the approach of doing spray foam to the underside of the roof deck. I was reluctant at first due to lack of ventilation but was sold on the conditioned attic. I have probably about eight houses with this, most with that builder. I also renovated a church and did the same method to help eliminate ice damming. The damming didn't go away not so much because of the new insulation, but probably because the existing masonry walls had minimal insulation that was allowing heat to get to the underside of the soffits and roof. It was also on a West wall with a tree creating shade and Shadow throughout the day, which screwed up any continuous daylight on the roof. I did a little more research and found a Lstiburek video where he said you still have to ventilate these roofs, particularly in Northern climates. Your solution with the battens and an additional sheathing system is what I concluded we should had done and I advised the church that when they re-roof they should add this as well. I also came across a system called the DCI SmartBaffle, a PVC panel like corrugated cardboard that creates ventilation chutes between the joist underneath the sheathing. I honestly don't know which approach would be cheaper but I plan to look at it the next house I do.

[JaneSpencer-ow]

I've been a journey carpenter for 35 years, and I love your building envelope system. I live in Canada, and built R2000 homes for years. Your method is very well founded. 👌

[shaharazad.]

So after 100 years when you've replaced your roof 2 or 3 times and grandchildren inherit your home after you're dead, then the spray foam becomes an issue....

[cmm]

Definitely interested! Thank you for taking the time and sharing this.

[sjhanksaz]

Why not secure the old deck the best you can then add a second layer on top of it?

[hkgonra]

I’ve been wondering specifically about this and I’m very glad to see a builder who I respect going straight at it with an explanation.

[andygapsky]

Replacing the roof wood is more of a pain in the ass than calling a guy out to spray the repaired areas. There’s houses from the 20’s with original wood on the roof, your first two concerns about spray foam are not even worth stressing over.

[mattofferdahl]

Can foam to roof deck with no problems. Both can pose issues and challenges when repairing. Enough R-value and humidity control and you're good inside. Good roof outside. Water will eventually make it through the foam. Can cut out areas and repair. Also can seal out leaks and prevent costlier damage.

[rickymcgrath]

Spray foam does not void a shingle warranty. The Magnusson-Moss Warranty Act prevents this. What it does do is give the manufacturer the easiest out to be able to deny a warranty claim because the deck is unventilated and the manufacturer will inevitably claim the inadequate ventilate caused the failure and it’s not a manufacturer defect. All manufacturers have issued TB’s on this.

[mikem]

I really like the idea but what if it’s a more complex roof with valleys, no one really builds simple gable houses anymore.

[chriswelles]

Looks great functionally. I'm not quite sure how you could build it though without putting down a deck before the rigid foam. I don't believe the rigid foam would support anyone's weight.

That being said, I've seen you do some really impressive work, so I imagine you'd pull it off somehow.

[Nairaville]

That's a pretty nice design! Very hard to find subs that want to try new things, but hope to find people I can get creative with. I might have to do a lot of the extra work myself on my next house.

[ScottyDMcom]

I've got a small cabin in a heating microclimate, so zero worries about hot days. I'm tired of the insane price of delivered propane, and I need to replace the wood-fiber siding and asphalt shingles anyway, so I thought I'd super insulate, cancel the propane, and go all electric.

I've puzzled over insulating the roof. I considered closed-cell spray foam on the underside of the deck, but the building department requires peel-n-stick roof membrane on the lower half of the roof deck to avoid damage from ice dams. I pictured my roof sheathing turning to rotting mush from trapped water. Also, I've got the impression that getting a decent R-value with only spray foam could get pricey.

I came up with exactly the same idea you show. I'll remove the shingles and roof sheathing, place multiple layers of rigid foam board insulation directly on the rafters, tape all joints, line up wood furring strips with the rafters, put plywood or OSB decking on the furring strips, finally install the peel-n-stick and shingles.

Polyiso is brilliant for Texas, but not so great in Colorado. They've changed the blowing agent over the years to address pollution issues, and have currently settled on an agent that loses R-value as the temperature drops. However, the R-value for XPS goes up with dropping temperature. Maybe I would like a bit of closed cell spray foam between rafters, so any type of polystyrene mounted directly on the rafters is out (EPS or XPS). So foil-faced polyiso directly on the rafters, then a layer or two of XPS above that.

I recently picked up a 250-count box of 10-inch TimberLOK screws from my local big-box store at a really good price. Assuming 1.5 inches to penetrate the furring strips, and at least 1.5 inches to anchor into the rafters. This leaves 7 inches for rigid foam board insulation. 1-inch polyiso and three layers of 2-inch XPS sounds about right. This should give about R40 at 25° F. Of course it'll be water-tight, air-tight, and moisture-tight, so by comparison it'll destroy the performance of any type of fluffy insulation. Even so, I might need to add a bit of insulation between rafters as my building department requires at least R49 on top (they seem to be counting on people using fluffy insulation).

I think it could work.

[PZ-ABCTZ]

I have seen a 50% drop in my electric bills: I had an energy audit done on a 33 year old home that had R38 on the ceiling joist(attic) and no insulation in the roof rafters. New Energy Start standard in Baltimore is R60 in the attic -- local power company is pushing R47 -- local code is R38. My basement and crawl spaces were vented to the outside and there was no insulation in the basement ceiling. Energy auditor wanted to spay foam the rafter and crawl space walls. My neighbor has his own roofing business and he said I should not/not put spay foam in the rafters -- I agreed. I had no problem with space foam on the crawl space walls. I decided to install Rafter Baffles, R38 fiberglass insulation in the rafters, and Perforated RadiantGUARD under the rafters—the house already had ridge vents. The attic temperature now stays about half way between the interior house temperature and outside temperature. The attic can dry out like a vented attic via the Perforated RadiantGUARD though the fiberglass insulation (Rockwool would have been better, but $$$$). Rafters sheathing can dry out via the air space between the sheathing and Rafter Baffles. My effective attic insulation is now a little less than R72 (R38 in rafters plus R38 in ceiling joist) because of the R13 insulation in the attic side walls. The attic HVAC system temperature now stays near the half way point between inside and outside temperatures -- better than setting in 17F in the winter and 140F in the summer. I also added foam boards to crawl spaces and basement walls. I also installed an ERV In lieu of Energy auditor 's suggestion to use a bathroom vent set to vent the house at 50cfm - 50cfm would have cost me $$$ in the winter and summer. by the way, I only turn on the ERV when the outside temperature is +/- 3 degrees of my inside temperature. (might have been more cost effective in using the bathroom fan but no one sells a controller connected to the house inside and outside temperatures to control a bathroom fan.) I still have some insulation work to do behind some knee walls and when the HVAC system is upgraded, I'll add R8 to the attic duct work.

[bstrac]

I spray foamed my roof several years back prior to finishing the attic. I'm currently contemplating a roof replacement and am very concerned about what I might have to do if some of the decking needs to be replaced. While I like how well the product performs, part of me wishes I never did the spray foam in the first place.

[karlsapp]

How do you get your shear values on this assembly? I assume a structural screw through the flat 2x?

[kdd]

Hi Jordan, this might be a silly question, but is there a risk of condensation on the inside face of the 2" of polyiso? Or is it considered permeable enough that any moisture would condense on the roof deck and be dried out by the venting? In the case of the former, should the attic be conditioned appropriately to remove moisture to in the interior?

[mattrandolph]

I agree with your exterior insulation and air gap. Would a radiant barrier be effective within that gap or do you not need to worry about it with the foam board? I disagree with using shingles. They are a terrible product: They don't last, they damage easily, greater chance of leaking, and they absorb heat and run way hotter. I just had my standing seam metal roof installed on our 1, 500 sq/ft new build a few weeks ago for only $7, 200, so the price isn't that bad either. We are also spraying foam under the roof deck, which is zipped and taped.

[zackdreamcast]

If you have an issue it’s a roof installation, not the foam.

[TriDaddy]

Defiantly lose some shear strength when you put 2" foam board between the rafters and furring strip/ventilation gap… the labor costs and additional materials for this complex.. I don’t see the advantage when you look at studies of spray foam on the roof deck only increasing temperatures 2-3 degrees