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Thoughts on Open Cell Insulation on the Underside of the Roof Deck

Looks like closed cell. I am seeing more under roof deck and basement wall insulation here Charlton Ma (zone 5) with heat pumps as an strategy meet the HERS 42 score needed here to meet the energy code
 
I would not use open cell for a "hot roof" design in a heating or primarily heating climate because warm moist interior air would penetrate it and condense. Whether only closed cell or closed cell and other, I'd do everything to assure it could dry inward.

In a mostly cooling climate, Is want a vapor barrier as close to exterior as possible and again allow drying to interior. So probably appropriate in 1A. No comment on this particular photo, just the concept of a hot roof with open cell.

Generally I'd prefer no hot roof design. I believe they are more likely to fail as a result moisture problems then a properly vented attic with a good vapor barrier and lots of carefully installed insulation on the ceiling.
 
With proper installation this works just fine. I agree with @bill1952 but that's not always an option if the people paying for it want a certain thing. The best method I've seen was to spray ~3" of closed cell, then top it off with enough open cell to get to the required R value.

When I say "proper installation" what I'm really getting at is the roof construction. If everything is properly constructed, and properly maintained, then there shouldn't be any problems. I have seen re-roof disasters because of improper installation/maintenance. What would have been a minor repair in a ventilated attic was a major problem.
 
"Closed cell" offers better thermal resistance and moisture protection due to its denser structure, making it a more efficient insulator, while "open cell" is typically cheaper and better at filling irregular spaces in an attic, but has a lower R-value per inch and is less resistant to moisture, It is Moisture permeable .
 
bill1952 said:
I would not use open cell for a "hot roof" design in a heating or primarily heating climate because warm moist interior air would penetrate it and condense.
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So then once water vapor that migrates through the open cell foam hits the cold roof deck and condenses, the idea is that the foam prevents the liquid water from re-evaporating as readily as it would in the uninsulated condition?

Cheers, Wayne
 
wwhitney said:
So then once water vapor that migrates through the open cell foam hits the cold roof deck and condenses, the idea is that the foam prevents the liquid water from re-evaporating as readily as it would in the uninsulated condition?

Cheers, Wayne
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Correct...condenses and rots the deck.....I just don't see how it is really any different with closed cell with the vapor from the outside....
 
wwhitney said:
So then once water vapor that migrates through the open cell foam hits the cold roof deck and condenses, the idea is that the foam prevents the liquid water from re-evaporating as readily as it would in the uninsulated condition?

Cheers, Wayne
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I believe it would result in mold and rot. I may be wrong but don't think of open cell foam as drying out real well, that it will hold the water and ice longer than fiberous insulation.

I'm not a warm climate guy. Love sitting here watching first big snow of the year from my desk. I've only learned and understood 2" of close cell will stop warm interior moist air and it's inner surface will stay above the dew point, so no condensation. I don't think open cell will do that so it's going to get wet.
 
The devil is in the details. And in my experience few get the details. Lots of problems being reported for this, with reports that some insurers are reluctant to cover some of the problems. I think we may not see all the unintended consequences for some time, though we may be starting to see some already. I have seen some, usually the mold and rot issue. Also saw a building burn to the ground because of the chemical reaction involved, but that was over a decade ago.
 
To meet R806.5 for unvented attic, I believe the insulation needs to be air-impermeable, thus closed cell. On GreenBuildingAdvisor, there are frequent posts with home owners complaining about bad smells with incorrect installation, and the remedy seems extremely difficult if not impossible. Some people there also believe the use of spray foam insulation has significant Global Warming Potential (GWP) from the blowing agents (link). To address the GWP I had specified Heatlok Soya HFO on a project, but it was never built so I don't know how the cost & availability of installers would work out.
 
jar546 said:
So one method I have seen is open-cell up against the roof sheathing and then after that dries, closed-cell to finish the required thickness.
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Interesting, exactly the opposite as I've seen here. I assume that in your climate people are primarily cooling spaces? Here we are primarily (exclusively) heating.

We hit 82 this year, that was a record high. Like, ever.
 
The photo looks like open-cell foam sprayed with intumescent paint. This is done a lot in our CZ-4 for complex roof and ceiling shapes or on 1-1/2 storied homes. The code reference is Table R806.5.
1734817124594.png

Both closed cell and open cell foams are considered air impermeable. The difference is their vapor permeability. As long as the foam is applied directly to the roof deck and the minimum R-value in the chart above is followed, there will be no condensation and no venting is necessary. BTW, most foams today do not require the intumescent paint in an enclosed attic as they have fire retardant built-in. Most roofs are simple enough that it is more cost effective to apply insulation to the roof deck and vent the attic.
 
Energystar said:
Both closed cell and open cell foams are considered air impermeable. The difference is their vapor permeability.
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2021 IRC R806.5(4) says "In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation." If I understand correctly, open cell foam would not be a Class II vapor retarder on its own, and so would require such a coating in those climate zones.

Cheers, Wayne
 
Energystar said:
The photo looks like open-cell foam sprayed with intumescent paint. This is done a lot in our CZ-4 for complex roof and ceiling shapes or on 1-1/2 storied homes. The code reference is Table R806.5.
View attachment 14948

Both closed cell and open cell foams are considered air impermeable. The difference is their vapor permeability. As long as the foam is applied directly to the roof deck and the minimum R-value in the chart above is followed, there will be no condensation and no venting is necessary. BTW, most foams today do not require the intumescent paint in an enclosed attic as they have fire retardant built-in. Most roofs are simple enough that it is more cost effective to apply insulation to the roof deck and vent the attic.
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No venting is required by current code? Shouldn't conditioned air be supplied in that area for moisture control?
 
The building science is changing on this. There's a pretty decent study done in Canada on things like hot roofs. If a client references this, we'll accept the hot roof design.
 
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Gvandrew5 said:
No venting is required by current code? Shouldn't conditioned air be supplied in that area for moisture control?
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I think they were saying that the roof venting is not required, not that "new" space does not need conditioned. Logically, if you relocate the building envelope from the ceiling to the roof, the conditioned space increases.
 
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