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

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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It looks to me like Climate Zone 4 and colder, you can have an unvented attic with an air impermeable spray foamed roof deck, and no conditioned air is required for that space.

I think it's a horrible idea, but that is how R806.5 seems to read.
 
BN4537 said:
It looks to me like Climate Zone 4 and colder, you can have an unvented attic with an air impermeable spray foamed roof deck, and no conditioned air is required for that space.

I think it's a horrible idea, but that is how R806.5 seems to read.
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It does apply to the ceiling being attached to the rafters and requires "The unvented attic space is completely within the building thermal envelope." Seems to me if it's within the thermal envelope it's somewhat conditioned. I guess you could have an insulated attic floor, and the attic would get colder (in my zone 6 climate) or warmer, but why add an extra layer of insulation and if the attic floor is insulated, is the attic really within the thermal envelope?
 
bill1952 said:
It does apply to the ceiling being attached to the rafters and requires "The unvented attic space is completely within the building thermal envelope." Seems to me if it's within the thermal envelope it's somewhat conditioned. I guess you could have an insulated attic floor, and the attic would get colder (in my zone 6 climate) or warmer, but why add an extra layer of insulation and if the attic floor is insulated, is the attic really within the thermal envelope?
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Yeah, the part that scares me is the lack of airflow.
 
BN4537 said:
Yeah, the part that scares me is the lack of airflow.
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I'm not crazy about "hot roofs" but they can work. And much commercial construction is same thing - metal deck with foam on top and a ceiling under it - and that seems to work. If you can prevent condensing surfaces, it should be ok. Joseph Lstiburek has some good thoughts on this.
 
All my experience is in climate zone 1A and 2A and I try to avoid spray foaming the underside of decks if possible. But if the client is pushy then I go open cell with a small supply up there to (try to) control moisture.

I wish vented nailboards were more price competitive.
 
I had a builder-developer who was going all-in on spray foam on the underside of the roof sheathing. Then they had 2 workers hospitalized who had inhaled the fumes during the application. They have long term damage. For them, the safety risks were too high (even though the workers should have known better).
 
Im in climate zone 4, we have almost as much vapor pressure in the summer as we do in the winter. 2 coats of paint on drywall is all that's required for unfaced fiberglass batts. Get alot of hot roofs here. I have always recommended 2" closed cell on the sheathing and then open after... most insulation contractors say, well the skin of the open cell is a vapor barrier. I say show me the manufacturers data. I get nothing.
Regardless of open/closed I have always recommended that the hot roof be conditioned. BSC and Joe put out a video on how moisture moves up to the ridge in a ping pong movement. Glad I paid attention to the fundamentals of building science 19 years ago!
 
The Alaska Cold Climate Housing Research Institute in Fairbanks has quite a lot of data and design guidelines on insulation in cold climates. They have several examples of placing all the insulation on the outside (roofs and walls) where they insulate the exterior to bring the dew point outside of the structure and leave the framing cavities empty.
 
I feel like Im hogging up a few threads here and there... Its tough for me to not talk about this stuff,
as I have been passionate about it since 2006.. So for that I apologize!


Absolutely the best way to manage insulation in real cold climates, Id guess building science wise, its probably the best way in any climate.
If anyone is truly interested in architectural details, a resource worth its weight in gold is Joes book Building for cold climates and his website,
buildingscience .com

I have been involved with a few Passive houses here on long Island and one where that type of hot roof was done...
Got tricky at the fascia and soffit though. They ran continuous insulation down the fascia, back under the soffit to meet the exterior insulation under the siding.

Going even further than that, there is something called a cold roof where a typical hot roof with exterior continuous insulation is built but over that is another roof with an air space between the two. If I remember correctly, the idea is that with heavy snow loads, and the associated R value of snow, the melting point of snow could be reached at the roof surface, so adding an air space prevents any heat from touching the surface that the snow sits on. Exterior vented roof deck ??

If you think about it, its kind of like a wall with framing, exterior insulation, 1x2 or something similar for the air gap, and siding but in this case you rotate the wall to the pitch of a roof and swap cladding for roofing.

Good stuff!
 
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