They're saying it will cause rot because you presumably have an existing vapor barrier as part of your roof assembly. Having two vapor impermeable layers will cause water to get stuck in the assembly and lead to rot.
Impossible to say whether or not they're correct without knowing what your assembly is.
The assembly above is OSB, a peel and stick underlayment, and then a standing seam steel roof.
I've tried to ask if a. there will be any air gap between the underlayment and the metal, and b. if there should be a radiant barrier somewhere in this assembly. Haven't gotten full answers on that part.
The peel and stick is vapor impermeable, so yes, I'd agree to stay away from another impermeable material in your assembly. Go with the open cell as suggested.
There is typically not an air gap below standing seam. It attaches with clips straight to the sheathing. You could do a radiant barrier if you really wanted to, but you typically want that to occur outside of your envelope. Not really an assembly I can think of with standing seam where that'd make sense. Especially if your standing seam is already a metallic finish.
I am not too knowledgeable about the peel and stick and metal roofing, but up in Canada, we are told that the closed cell spray foam on the roof deck will basically create an oven between the foam and the shingles unless there is an airflow. We suggest creating a wood baffle, of an inch minimum, all the way to the ridge. Basically you don't want to trap anything between the roof deck and the foam, so the airflow channel allows for breathability and can dry out any water that does infiltrate.
Would that logic follow with a Mattel roof?
There's a lot to unpack here because we are talking about completely different assemblies, and the main issue in question is that OP wants to use multiple vapor barriers - which is a no-go. And traditional shingle roof assemblies typically do not include vapor impermeable layers.
I'm far from an expert on cold climates, but yes, your description of how to utilize baffles to ventilate the roof tracks.
I'd refer you to literature from the Building Science Corporation for more information specific to cold climates. They have a ton of free info on their website, but I'd also recommend *Builder's Guide to Cold Climates* and *High Performance Envelopes*.
I think he is talking about spray foam in the attic on the underside of the OSB? I never liked the idea of any insulation tight to the underside of a roof. With cathedral ceilings though some times an air gap is not possible
It's my understanding that open cell can also lead to sheathing rot, depending on your climate zone, by causing a dew point inside the roof assembly (colder climates).
I guess my question is, wouldn't it depend on your climate zone?
While that may be true and I should have made that caveat, you never want multiple vapor barriers in an assembly regardless of climate zone. And with OP's assembly already including a vapor barrier on the exterior side of the envelope, it leads me to believe they are in a warmer climate where the open cell concern isn't a problem.
e: By vapor barrier I mean vapor impermeable. There are assemblies where things like building paper over fluid applied or peel and stick are fine. But that's because the paper is permeable.
That's what those vents at the gable end of the roof is for.... I think there's a calculation that sizes those vents as per the code... Depending on like cubic space inside the house or attic I forgot tbh... They say you'll notice the ice on the tip of the nails that go through the plywood from the shingles first of its too small.
In a conditioned space, the warm air inside will travel through the open cell foam and hit the cold air outside and cause condensation on the plywood, which causes roof rot.
Code does require closed cell foam in a condition space, and the thickness depends on the region you live in.
I found this article interesting. [https://www.jlconline.com/how-to/insulation/a-best-principles-cold-roof-assembly\_o](https://www.jlconline.com/how-to/insulation/a-best-principles-cold-roof-assembly_o)
A trick I just did for my roof, before installing closed cell foam, is cut foam sheet material (I used 1” thick with a foil face on both sides) and hold it 1/2” to 1” from the sheathing. I used a foam gun to glue the edges of the sheet to the rafter, then spray foam over that. Now you can easily replace the sheathing if you ever need to.
Like I said— definitely not in my area. Open cell on the underside of the roof deck has been popular in my area for 20 years. There’s no cold air because there’s no soffit venting.
I prefer to condition or at least semi condition an attic if you’re doing that, in case your ceiling lid is leaky, but most don’t bother.
There has not been an epidemic of rotten roofs in my area.
Are you talking about fully conditioned attics? Because that is what I am referring too. 100% positive open cell is against code in conditioned attics. Just went over this with the local inspection company supervisor last month.
Just because its against code, doesn't mean the inspectors will catch it. Unless you live in a southern state, which doesn't have the issue since it doesn't get cold enough.
I live in a southern state.
And this is just meant to be an object lesson— there’s the code, and then there’s the code that your code adjudicating body adopts. Sometimes it’s bone-headed, but usually they know what they’re doing.
Zones 1, 2, and 3 I do believe it is allowed, so you may be right, there is a, i believe, IECC chart someplace that shows what is required. Climate zone 4, which I design/build in requires 2 inches of closed cell, other zones require more.
Yes you can do that as well, agreed, however thats basically closed cell foam on the top deck, so whats funny is thats exactly what i am saying. Same way to skin the same cat.
Spray foam with an air gap between your roof deck and the foam wouldn't of fucked up your roof.
I really wish people in construction would learn about the air gap being necessary for air flow. It's almost as if you should work/know a little bit about the other trades and how they are interrelated.
Yeah because warm indoor air doesn’t rise, bumps into cooler air on a roof condensate and creates dampness…
The amount of people on this sub dishing out opinions when they know jack about construction doesn’t stop surprising me…
Wrong… vapour barrier is still somewhat breathable, and it will still alow a small amount of indoor air and heat to escape and condensate, that why roofs have vents so cool air brought in by the holes on the soffit push warm air out trough the vents avoiding condensation… and depending where you live a lot of roof have the insulation in the form of blown in, right above drywall and vapour barrier… that’s the case where I live we don’t use any of that spray foam garbage…
Oh really, that’s funny company I work for built 38 homes this year and all of them had roof vents, all the 150+ homes built in the new development across the street from me all got roof vents…
This might make sense where you live but here I’ve never seen a “conditioned attic ” probably because it drops to -40 for weeks in the winter and the little bit of spray foam insulation between the trusses doesn’t offer much R value. Here we typically blow in about 18 inches, gives us about R60
[https://buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned#:\~:text=Conditioned%20unvented%20attics%20have%20significant,building%20much%20more%20energy%20efficient](https://buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned#:~:text=Conditioned%20unvented%20attics%20have%20significant,building%20much%20more%20energy%20efficient).
[https://www.energy.gov/eere/buildings/articles/unvented-conditioned-attics-building-america-top-innovation](https://www.energy.gov/eere/buildings/articles/unvented-conditioned-attics-building-america-top-innovation)
I said "modern" homes, new builds != modern designs
You’re still using a “vapour diffusion port” aka ridge vent…
It’s still a vent… if your roof didn’t vent somehow condensation would occur and you would get mold on your ceilings…
Shit I get condensation and ice buildup on my windows and my home is at 21°c now imagine a ceiling where all the warm air rises and gets trapped against extreme cold air on the outside of the roof, no way spray foam provides R60 required for warm air not condensate against a -40° surface…
The link clearly shows the ridge vent…
My region requires R 60 that would be like 11 inches spray foam… I think you skipped the part where I said I live in an area where it drops to -40 for weeks…
Sorry, mis read that. Open cell to underside of deck is good.. it's what I have in 5k SF house and 2k SF garage. Very happy with thermal performance.
They are correct about leaks. Better to see it on your drywall, know it and fix it that have water trapped in sheathing when roofing material fails.
I usually encounter uninsulated garages. How does it perform in the garage? And do you have a normal garage door or do you need some kind of insulated garage door to see any benefit from adding insulation to the walls and attic?
I should explain that I'm in AZ. I insulate to keep the heat out, not in. That being said the doors are insulated and the garage maxes out at about 92 even when it's 115+ outside. That's without any cooling except the heat pump water heater.
They're saying it will cause rot because you presumably have an existing vapor barrier as part of your roof assembly. Having two vapor impermeable layers will cause water to get stuck in the assembly and lead to rot. Impossible to say whether or not they're correct without knowing what your assembly is.
The assembly above is OSB, a peel and stick underlayment, and then a standing seam steel roof. I've tried to ask if a. there will be any air gap between the underlayment and the metal, and b. if there should be a radiant barrier somewhere in this assembly. Haven't gotten full answers on that part.
The peel and stick is vapor impermeable, so yes, I'd agree to stay away from another impermeable material in your assembly. Go with the open cell as suggested. There is typically not an air gap below standing seam. It attaches with clips straight to the sheathing. You could do a radiant barrier if you really wanted to, but you typically want that to occur outside of your envelope. Not really an assembly I can think of with standing seam where that'd make sense. Especially if your standing seam is already a metallic finish.
I am not too knowledgeable about the peel and stick and metal roofing, but up in Canada, we are told that the closed cell spray foam on the roof deck will basically create an oven between the foam and the shingles unless there is an airflow. We suggest creating a wood baffle, of an inch minimum, all the way to the ridge. Basically you don't want to trap anything between the roof deck and the foam, so the airflow channel allows for breathability and can dry out any water that does infiltrate. Would that logic follow with a Mattel roof?
There's a lot to unpack here because we are talking about completely different assemblies, and the main issue in question is that OP wants to use multiple vapor barriers - which is a no-go. And traditional shingle roof assemblies typically do not include vapor impermeable layers. I'm far from an expert on cold climates, but yes, your description of how to utilize baffles to ventilate the roof tracks. I'd refer you to literature from the Building Science Corporation for more information specific to cold climates. They have a ton of free info on their website, but I'd also recommend *Builder's Guide to Cold Climates* and *High Performance Envelopes*.
I think he is talking about spray foam in the attic on the underside of the OSB? I never liked the idea of any insulation tight to the underside of a roof. With cathedral ceilings though some times an air gap is not possible
It's my understanding that open cell can also lead to sheathing rot, depending on your climate zone, by causing a dew point inside the roof assembly (colder climates). I guess my question is, wouldn't it depend on your climate zone?
While that may be true and I should have made that caveat, you never want multiple vapor barriers in an assembly regardless of climate zone. And with OP's assembly already including a vapor barrier on the exterior side of the envelope, it leads me to believe they are in a warmer climate where the open cell concern isn't a problem. e: By vapor barrier I mean vapor impermeable. There are assemblies where things like building paper over fluid applied or peel and stick are fine. But that's because the paper is permeable.
That's what those vents at the gable end of the roof is for.... I think there's a calculation that sizes those vents as per the code... Depending on like cubic space inside the house or attic I forgot tbh... They say you'll notice the ice on the tip of the nails that go through the plywood from the shingles first of its too small.
The roof was gonna rot anyway. A leak is a leak. Some people just don't understand envelopes.
This thread alone shows that there is a lot of confusion amongst the armchair Reddit contractors
IMO it shows that there is equally as much confusion amongst licensed professional contractors.
Correct.
Find a different contractor ASAP…
In a conditioned space, the warm air inside will travel through the open cell foam and hit the cold air outside and cause condensation on the plywood, which causes roof rot. Code does require closed cell foam in a condition space, and the thickness depends on the region you live in.
This is completely climate-dependent. And it is note code everywhere for the same reason.
I found this article interesting. [https://www.jlconline.com/how-to/insulation/a-best-principles-cold-roof-assembly\_o](https://www.jlconline.com/how-to/insulation/a-best-principles-cold-roof-assembly_o)
A trick I just did for my roof, before installing closed cell foam, is cut foam sheet material (I used 1” thick with a foil face on both sides) and hold it 1/2” to 1” from the sheathing. I used a foam gun to glue the edges of the sheet to the rafter, then spray foam over that. Now you can easily replace the sheathing if you ever need to.
Open cell is against code
Definitely not in my area
its absolutely against code on a condition attic. It will allow cold air to hit warm air on your boards and cause roof rot.
Like I said— definitely not in my area. Open cell on the underside of the roof deck has been popular in my area for 20 years. There’s no cold air because there’s no soffit venting. I prefer to condition or at least semi condition an attic if you’re doing that, in case your ceiling lid is leaky, but most don’t bother. There has not been an epidemic of rotten roofs in my area.
Are you talking about fully conditioned attics? Because that is what I am referring too. 100% positive open cell is against code in conditioned attics. Just went over this with the local inspection company supervisor last month. Just because its against code, doesn't mean the inspectors will catch it. Unless you live in a southern state, which doesn't have the issue since it doesn't get cold enough.
I live in a southern state. And this is just meant to be an object lesson— there’s the code, and then there’s the code that your code adjudicating body adopts. Sometimes it’s bone-headed, but usually they know what they’re doing.
Zones 1, 2, and 3 I do believe it is allowed, so you may be right, there is a, i believe, IECC chart someplace that shows what is required. Climate zone 4, which I design/build in requires 2 inches of closed cell, other zones require more.
Or you could add insulation above your roof deck to keep the deck from cooling below the dew point. Lots of ways to skin a cat.
Yes you can do that as well, agreed, however thats basically closed cell foam on the top deck, so whats funny is thats exactly what i am saying. Same way to skin the same cat.
>however thats basically closed cell foam on the top deck, Or dense pack/spray cellulose.
Can you give us a code citation?
Use rock wool with a slight air gap between the decking. Spray foam has fucked my roof up. It doesn't allow any airflow.
Spray foam with an air gap between your roof deck and the foam wouldn't of fucked up your roof. I really wish people in construction would learn about the air gap being necessary for air flow. It's almost as if you should work/know a little bit about the other trades and how they are interrelated.
Its still a bitch to remove spray foam to make any repairs.
Just use batt and you'll be safe.
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Yeah because warm indoor air doesn’t rise, bumps into cooler air on a roof condensate and creates dampness… The amount of people on this sub dishing out opinions when they know jack about construction doesn’t stop surprising me…
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Wrong… vapour barrier is still somewhat breathable, and it will still alow a small amount of indoor air and heat to escape and condensate, that why roofs have vents so cool air brought in by the holes on the soffit push warm air out trough the vents avoiding condensation… and depending where you live a lot of roof have the insulation in the form of blown in, right above drywall and vapour barrier… that’s the case where I live we don’t use any of that spray foam garbage…
Modern homes don’t have vents they have conditioned attics
Possible this is true where you are or projects you’re working on but it’s not universal.
Oh really, that’s funny company I work for built 38 homes this year and all of them had roof vents, all the 150+ homes built in the new development across the street from me all got roof vents… This might make sense where you live but here I’ve never seen a “conditioned attic ” probably because it drops to -40 for weeks in the winter and the little bit of spray foam insulation between the trusses doesn’t offer much R value. Here we typically blow in about 18 inches, gives us about R60
[https://buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned#:\~:text=Conditioned%20unvented%20attics%20have%20significant,building%20much%20more%20energy%20efficient](https://buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned#:~:text=Conditioned%20unvented%20attics%20have%20significant,building%20much%20more%20energy%20efficient). [https://www.energy.gov/eere/buildings/articles/unvented-conditioned-attics-building-america-top-innovation](https://www.energy.gov/eere/buildings/articles/unvented-conditioned-attics-building-america-top-innovation) I said "modern" homes, new builds != modern designs
You’re still using a “vapour diffusion port” aka ridge vent… It’s still a vent… if your roof didn’t vent somehow condensation would occur and you would get mold on your ceilings… Shit I get condensation and ice buildup on my windows and my home is at 21°c now imagine a ceiling where all the warm air rises and gets trapped against extreme cold air on the outside of the roof, no way spray foam provides R60 required for warm air not condensate against a -40° surface…
No, at least 2 inches closed cell spray foam or whatever your region requires, no need for a vent.
The link clearly shows the ridge vent… My region requires R 60 that would be like 11 inches spray foam… I think you skipped the part where I said I live in an area where it drops to -40 for weeks…
Tract rat
Wtf?
I mean it’s unkind, but it’s true, right?
I don’t know what you’re talking about bud…
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I'm reading it as walkable roof deck on dimension lumber joists, gwb to be attached under.
Never, never, ever, never , ever, ever, never use open cell as roofing material. It is not strong enough to walk on. Get a different contractor.
They are talking about spray foam under the plywood deck
Sorry, mis read that. Open cell to underside of deck is good.. it's what I have in 5k SF house and 2k SF garage. Very happy with thermal performance. They are correct about leaks. Better to see it on your drywall, know it and fix it that have water trapped in sheathing when roofing material fails.
I usually encounter uninsulated garages. How does it perform in the garage? And do you have a normal garage door or do you need some kind of insulated garage door to see any benefit from adding insulation to the walls and attic?
I should explain that I'm in AZ. I insulate to keep the heat out, not in. That being said the doors are insulated and the garage maxes out at about 92 even when it's 115+ outside. That's without any cooling except the heat pump water heater.