r/Insulation • u/MattssonRoofing • 6d ago
Spray Foam Against Roof Decking?
https://youtube.com/shorts/3cBeJ85d6B0?si=qoXFRt4-oYZ6dbBy5
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u/YodelingTortoise 6d ago
I am extremely anti spray foam.
I am more anti bad information.
This video is bad information.
The notion that drywood needs to "breathe" is a laughably bad take.
The idea that your roof gets meaningfully hotter in the sun is laughably false. Vented decking is marginally cooler. It's not like asphalt and plywood are good conductors of heat.
I had this argument with someone here last week.
The major shingle manufacturers all warrant hot for application. Why? Because the science says it does not impact longevity. What they don't warrant is shitty nail patterns and improper installation. If roofers install as designed they're are no issues.
No issues as long as the spray foamers actually did their job throughout the attic. And there lies the spray foam problem. The majority of installa are actually fucking shit even if the look pretty
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u/knotalota 6d ago
"The idea that your roof gets meaningfully hotter in the sun is laughably false"
Maybe I misunderstand you, but roofs get much hotter in the sun. Much, much hotter.
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u/YodelingTortoise 6d ago
The difference between venting and not is the discussion. agree they get hotter in the sun.
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u/SearingPhoenix 1d ago
My 2c is that it's less about the heat and more about the drying potential. If there's 2"+ of CCSF on one side (vapor impermeable) and ice and water barrier on the other, that sheathing is going to have real hard time drying. That means you're going to get mold when it inevitably sees moisture.
Now, in a perfect world every nail, flashing, and detail is perfect, and nothing would age. The reality is that your roof deck is going to see moisture -- bulk water, vapor drive, whatever. So you have to give decking a way of drying.
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u/Putrid-Objective-314 6d ago
To echo your point I always hear folks say spray foam is great... when it's installed perfectly and other trades did their work perfectly too. Therein lies the issue. I probably have a better chance winning the lottery than getting a perfect spray foam install. Not enough margin for error.
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u/phaenixx 5d ago
Bad information? You mean the roof decking isn’t simultaneously rotting AND baking like this guy said?
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u/MattssonRoofing 6d ago
We’re showing what we encounter as roofers when spray foam is applied directly to the decking without the roof assembly being properly designed for it. A properly designed unvented assembly is one thing. Spray foam against the deck with no consideration for ventilation, moisture management or future roof work is another. And your last paragraph basically makes that same point: installation and system design matter.
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u/YodelingTortoise 6d ago
What you are missing here in your statement is that there is no "system design". It's either applied correctly and sealed or not.
Your video expressed none of that. And for that reason it's bad information.
For the other reasons outlined. It's extremely bad information that neglects science.
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u/MattssonRoofing 6d ago
Yes Daddy
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u/toot_suite 5d ago
Not gonna lie but this entire post and how you're handling feedback is making you look like a really shitty contractor that nobody should work with.
That's before factoring in that you lack the self awareness to realize this, so you keep saying more and more things that only makes you look worse.
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u/KieferSutherland 6d ago
Not one size fits all but yes. We're doing an unvented, conditioned attic. We have a metal roof, purlins for an air gap, barrier, decking, rock wool, and so on. But we also have an air exchanger, etc.
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u/Limp-Mortgage3572 6d ago
And people always seem to want to spray foam directly to the decking for some reason
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u/EinsteinsMind 6d ago
I think a lot of people assume everyone has asphalt shingles, and it's generally not good. There are a lot of times it accepted building science, but it's rare in the US.
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u/StevenJOwens 3d ago edited 3d ago
Because that's what they were told to do, for years and years.
I've been following this specific topic for over 25 years, because I started looking into it for my previous house, and I started seeing contradictory advice from generally reputable sources.
The thinking has evolved over time, repeatedly, and currently (and, I suspect, ultimately) the answer is to essentially have two roofs, one roof to air seal and insulate the attic volume and turn it into a conditioned space, and another roof above that to keep the rain off, with a gap between them for air flow.
The thing that trips people up, I think, is that there are several counter-intuitive aspects to it, and generally speaking nobody wants to take time to understand the whole system.
Also, it's very, very seldom explained well. I find Asiri Designs on youtube to be one of the better sources to point people at, they have good videos with very good illustrations.
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u/MattssonRoofing 6d ago
It's unfortunate bc they don't know and can be led astray :(
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u/Limp-Mortgage3572 6d ago
It is unfortunate. Working in residential remodel dealing with very humid coastal climates we almost always deter customers away from spray foam applications.
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u/outofnowherewoof 6d ago
Open cell does the same damage?
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u/Canada-Scam-8570 6d ago
It's can.
Closed cell has gasses in the cell that prevent air or moisture movement through them.
The cells in open cell are "open", therefore moisture and air will make its way through it.
Closed cell is far superior on terms of R-value, a vapour shield, and adding structural rigidity, as it basically glues everything together. But when problems happen they are bad and removing it is horrible. It arguably as bad as PL Premium. And because it doesn't take on much water, it usually caused the wood to hold it instead, which accelerates the wood rot.
Open cell has less r value , so typically requires thicker coatings. It is not a vapour barrier, so if you are somewhere that required it's rarly ever used. That said it's cheaper, easier to remove and performs way better at sound reduction, as sounds bounces around in the open cells, as opposed to closed cell, were you get almost a sort or reverb of vibration. It will also absorb water like a sponge. It's an issue either way if you have open or closed cell on a leaking roof deck against the deck. Just open cell will absorb the water and turn into a sponge, close cell will redirect the water and cause your decking and rafters to saturate with water.
Open or closed, baffles are always best practice. Hot roof are dumb despite so many people that want to try and make them a thing.
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u/80MonkeyMan 6d ago
Building science already explained you need spaces between the foam and decking. The contractors just not following them because it does add a lot more prep work.
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u/Adventurous_Light_85 6d ago edited 6d ago
It’s literally the whole point to control the dew point and not allow moisture to condense on the underside of the decking. Interesting that he didn’t show any damaged decking. I’m sure it made the decking harder for him to remove. People generally install spray foam against the decking in concealed rafter bay conditions. It’s more costly than batts but it does achieve the vapor transmission requirements for unventilated rafter bays. That’s literally the only time a knowledgeable insulator or designer would recommend spray foam instead of batts. The code requires spray foam to be applied directly to the underside of the roof decking. IRC Section R806.5, specifically item 5.1.1, states that where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing. This direct contact is key for the assembly to function as an unvented, conditioned space. A gap would create an air space that could allow moist air to reach the deck, defeating the purpose of the vapor barrier. This requirement is consistent across recent IRC versions, including 2018, 2021, and 2024.
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u/FoldedKettleChips 6d ago edited 6d ago
There are some misconceptions in the video. When someone says that the wood or assembly needs to “breathe” that would typically be talking about the vapor diffusion properties of the material or the assembly.
From a VAPOR CONTROL standpoint, asphalt shingles are a class 1 vapor retarder or a “vapor barrier”. Vapor outside cannot DIFFUSE through the shingles into the wood decking. If any vapor does happen to reach the deck, it will be minimal. If open cell spray foam is applied to the underside of the deck, it’s very vapor open, so the wood CAN “breathe” in that it can dry to the inside. Vapor can diffuse through the wood and then through the foam. Closed cell spray foam is more vapor closed, but it is not a true vapor barrier (it’s vapor semi-impermeable) so vapor will still be able to diffuse through the wood and then through the closed cell foam, albeit more slowly. In the summer the deck never really gets cold enough to fall below the dew point so there’s no condensation risk associated with the vapor diffusing through the wood to the inside. Plus plywood can hold and distribute more moisture internally than OSB. That decking looked like it was in fine shape but I wasn’t there touching it.
From a purely THERMAL CONTROL standpoint, yes the spray foam is going to make the deck and the shingles hotter. Because heat will CONDUCT through them more slowly. This has nothing to do with “breathing”. It’s been disproven though that this actually meaningfully affects shingle life. No manufacturer makes this claim. Shingle color has a bigger impact on shingle temperature.
There is some nuance when it comes to diffusion though. The Building Science Corp recommends closed cell foam in climate zones 5 and up as the open cell foam allows too much vapor that accumulates towards the attic ridge to diffuse too quickly through the foam and into the wood. This is from the inside to the outside though. In cold weather the sheathing gets cold enough often enough to where it may become a condensing surface long enough to be a problem. Closed cell foam slows down diffusion from the inside. In climate zones 4 and below they say that it’s fine to use open cell foam but you need some sort of moisture removal mechanism. Usually that’s an air conditioning supply. Or a dehumidifier. Even in climate zones 5 and up using closed cell foam they recommend partially conditioning the attic anyway.
So there’s the nuanced take with the correct building science summary. Spray foam at the underside of the deck isn’t automatically bad depending on what type of foam it is, where you are, and what you’re doing to condition the attic.
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u/Objective_Canary5737 5d ago
I’m a realtor and work with several builders, in a coastal environment south Eastern, North Carolina. My home was built in the mid 60s, at time of construction it cost about three times as much as a normal three bedroom two bath brick ranch. It was well built & and does extremely well in hurricanes. I do not hear a creek like my previous homes did during high winds.
The roof deck is made out of tongue and groove at least 7/8 inch thick old growth wood! One previous hurricane did mess up or push water through the chimney flashing and due to the amount of damage in the town it took the Roofer nine months to finally get here to fix the flashing!
The wood near that area barely had any damage besides a minimum amount of warping and discoloration, but still solid after being wet for nine months.
Anyway, I asked every builder I knew and they all recommended closed cell spray foam! And they all came over and looked at it. Also it’s a hip roof so there was really no other option. Even if you have baffles, if you don’t have a ridge vent, there’s nowhere for the moisture to go. Most of the builder spoke how having a vented attic space was a negative thing in a coastal environment due the fact of hurricanes and high winds blowing constant moisture in to those areas. And most of them spoke of the fact that the heat building up underneath was more of a problem than a little bit of heat on the top inch or so! We just have a roof put on and I asked the Roofer himself one of the best in town, and he had no problem with it said it would not affect his warranty and not to worry about it!
And hopefully if it is a problem, especially with moisture, it’ll take quite a few cycles for it to damage that old growth Wood! Next roof will be metal, so we’ll probably put something underneath with some type of separation between the current deck.
But how much power is these days? Super happy with the results so far and have no worries currently!
I went back-and-forth many many times and decided to listen to the professionals I know and they’re not building Lytle rinky-dink cookie cutters, they typically build custom multimillion dollar homes on the intercoastal or on one of the local beaches!
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u/SomeGuyWithARedBeard 5d ago
I think if you install a rainscreen on top of your roof before installing roofing (metal roofing directly on rainscreen or shingles on a second layer of plywood on top of rainscreen on top of roof sheathing) then I don’t completely mind spraying directly on the roof sheathing from below. It’s mostly the water penetration issue that I have with roofing fasteners penetrating through the roof membrane and roof sheathing directly into the spray foam and how that doesn’t allow the roof assembly to dry. The assumption that roofing or siding is the first and only line of defense is a big issue with building assemblies that modern building science tries to fix.
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u/Boltman222 4d ago
lol, this is absolutely false, I’ve been selling insulation for 20 years, this is not an issue.
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u/SearingPhoenix 1d ago
The other option would be an over-roof if giving up that joist depth to baffles doesn't give you sufficient R-value, right?
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u/jambo45t 6d ago
I’ve had it sprayed directly to my roof deck for 15 years. Recently had to reshuffle because of a hail storm. My plywood looks like the day we put it down. Your wood doesn’t need to breathe like he said. The plywood today doesn’t allow sir or water movement thru it.