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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!
 
Enrgxprt said:
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, it’s 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, something a Florida blown-in insulation contractor would handle professionally.

Good stuff!
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Hello. My partner and I are strongly leaning toward getting open-cell spray foam insulation installed in our attic, and we wanted to get some feedback from people who have experience with it. We’re having it professionally installed, and we’re comfortable with the price. One of the things we like most about it is that it would allow us to put decking in the attic for storage, which is a huge bonus for us. However, we’ve come across some concerns about moisture, mold, and mildew. Specifically, we’ve read that open-cell foam can potentially trap moisture, which could cause problems with the roof over time. We’ve also seen some claims that insurance companies might not cover homes with spray foam, and that some mortgage lenders even refuse to finance homes with this type of insulation.
 
There are multiple manufactures and formulations of spray foam both open and closed cell.

My understanding closed cell has a higher R value and cost per inch and is impermeable to moisture. Open cell is less cost and R value per inch and permeable to moisture. However I was provided with product data for an open cell foam that at 4 inches we tested to be a suitable vapor barrier and for an attic applications under specific conditions did not need a thermal barrier.

The failures I have seen for spray form are all application errors; bad mix, apply outside of temperature limits or on to cold surfaces, miss application of intumescent paints by chemical reaction or lack of cure time of the foam before application
 
neyox said:
Hello. My partner and I are strongly leaning toward getting open-cell spray foam insulation installed in our attic, and we wanted to get some feedback from people who have experience with it. We’re having it professionally installed, and we’re comfortable with the price. One of the things we like most about it is that it would allow us to put decking in the attic for storage, which is a huge bonus for us. However, we’ve come across some concerns about moisture, mold, and mildew. Specifically, we’ve read that open-cell foam can potentially trap moisture, which could cause problems with the roof over time. We’ve also seen some claims that insurance companies might not cover homes with spray foam, and that some mortgage lenders even refuse to finance homes with this type of insulation.
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So yes foam can trap moisture coming from the homes occupants..

Like any antibiotic that a doctor writes a script for, you need the full dose, and for the recommended time otherwise you get sicker. Foaming an attic needs a few more things to make it a "prescriptive" solution.

The humidity that is normally produced by occupants cant leave the home through the upper areas from stack effect now that you have a sealed attic.

In the winter, absolute humidity is low outside. Ideally you would have 40-45% RH indoors. If the inside surface temperature of your assembly's is above the dew point, your safe. We learned from the Building Performance Inc, Building analyst that 40-45% RH produce the lowest mold counts. There are types of mold that like higher RH and types that like lower RH and the sweet spot is the safest range.
Go figure, our lungs work best at these numbers.

Now, a tight house with poor ventilation can have problems as the RH can (and will) rise naturally. When I say ventilation I don't mean the traditional attic vents where the soffit and gable vents provided ATTIC ventilation. I mean whole house ventilation for you the occupant.

Absolutely provide the spray foamed attic with active heating and cooling (treat that space as any other habitable space) You'd be surprised how little most attics need. I regularly see attics that have 1800 sq ft of ceiling below it needing less than what one 4 or 5" supply run out provides.

Ok, now the attic is warm in the winter (just like any other part of your house) we need to talk about whole house ventilation.
Im unlike most, and in my climate zone believe one energy star labeled exhaust fan, with a 6" exhaust port necked up to 8" for a flex runout to a 6" exterior termination on a true ventilation timer (without any hard turns) is by FAR the best...

Its counterintuitive, why make the house leak when so much work went into making it tight...
But Infiltration is not controllable. Not the amount you loose, not when you need it. Nothing about it.
Most typical homes can leak 5x or 10x what they ideally should.
When exhaust only is set correctly (in a well sealed home) your able to remove excess indoor humidity (at the right amount) and its replacement air (coming from outside) has a drying effect.

Also, a foamed roof typically has a MUCH better R value, loosing less heat from conduction.

Now summertime conditions we would expect the cooling system to dehumidify the home.

SO you followed my directions, find your home is a tad on the dry or humid side.
Guess what, you can make the exhaust fan run longer or less to dial in your indoor humidity.

Now, Open cell is NOT a vapor barrier. In my location 2 coats of paint on the drywall is all we need. If your in zone 4, were you planning on painting over the open cell foam ? Id bet no. If it were my home, and in my climate, Id get 2" of closed cell on the sheathing and then fill the cavity (any beyond) with open cell.

Now you wont have to worry about the 40% RH wicking through the open cell and condensating on the roof inside surface.

Don't forget, foam in an attic needs an intumecent coating to meet code. Code talks about what standard needs to me met depending on the product.

I know there appears to be a lot of back and forth, but understand the benefits FAR outweigh the typical approach any by ALOT.
 
steveray said:
It depends on your climate, but if you vent the rafter bays it should mitigate most of the moisture .
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Steveray,
Just as an FYI, with a hot roof there should be NO ventilation in the rafter bays. There should be no gable vents, no soffit vents NOTHING.
There should ALWAYS be whole house ventiliaion. (on a timer, controllable etc.. see my prvious post)

Back before indoor plumbing was in our houses, we had very little insulation. With the advent of sinks, water closets etc, we now had to worry about freezing pipes and added insulation. What happened was (especially baloon framied houses) the moisture in the home from cooking, cleaning showering etc was allowed to exfiultrate into the attic (along with all of our warm air). What that did was allow warm, moist air to condensate on the first cold surface in the attic.
Now to fix it, we said Oh, lets make sure we allow enough cold dry air into the attic to pull out that moisture.
We kept doing silly things to solve problems in the wrong way.

We should have simply sealed holes and penetrations into the attic (So our warm air didn't carry moisture into that space).
We should have at the same time made sure our attic insulation was in contact and inline with that indoor air barrier (drywall, lath plaster etc)

Funny, that way the attic becomes a MUCH colder place and now attic ventilaiton is even more important.
Air sealing the attic flat is REALLY hard to do well. Its almost a guarntee (with a properly trained installer) that spray foam can do both the air sealing and be in contact....

The insulaiton being in contact is important. Next cold frigid day go outside with a nice warm jacket and pull the jaket off your skin by a few inches and tell me whats warmer, when the jacket is touching you or when its a few inches away... I know we all know how much of a difference that makes...


Spray foam can and does often guarantee the best if not perfect airseal.
If we do this on the underside of the roof, that space now should be treated just like any other indoor area...
 
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