# Spray foam between attic trusses?



## jacselee (Mar 5, 2014)

I live in MN, client wanted spray foam inbetween trusses in attic. I don't think that's a good idea, what do you think?


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## EricBrancard (Jun 8, 2012)

Are you saying he wants to spray foam the bottom side of the roof deck to bring the attic into the conditioned space envelope?


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## NJGC (Apr 5, 2014)

I did this when I built my house and it's the best thing I ever did! The attic is never more than 10* warmer than the rest of the house. There is next to 0 heat transfer from the hot roof deck into the house and with the air sealing properties of spray foam my heat and A/C stay in much longer. Cheap cheap cheap utility bills. I foamed the whole house but I'm most happy with the semi conditioned attics. Just take this space into account when sizing HVAC equipment


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## ScipioAfricanus (Sep 13, 2008)

It is a good idea if it is thought out in conjunction with entire heating/cooling system and design.

As Eric has mentioned about if it will be a part of the interior conditioned zone.

Andy.


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## NJGC (Apr 5, 2014)

jacselee said:


> I live in MN, client wanted spray foam inbetween trusses in attic. I don't think that's a good idea, what do you think?


What are your concerns?


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## jacselee (Mar 5, 2014)

EricBrancard said:


> Are you saying he wants to spray foam the bottom side of the roof deck to bring the attic into the conditioned space envelope?


No it is NOT A CONDITIONED SPACE.


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## SamM (Dec 13, 2009)

Spray foam on the underside of the roof deck can be problematic. If there's a roof leak, it won't leak through, it'll just rot the sheathing. Not a high likelihood but it's there nonetheless.

If the roof deck is instead sprayed with half pound foam i t might be better (that will allow water through).

I've seen it done on cathedral ceilings. That's not too bad of an application for it.


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## Morning Wood (Jan 12, 2008)

SamM said:


> Spray foam on the underside of the roof deck can be problematic. If there's a roof leak, it won't leak through, it'll just rot the sheathing. Not a high likelihood but it's there nonetheless.
> 
> If the roof deck is instead sprayed with half pound foam i t might be better (that will allow water through).
> 
> I've seen it done on cathedral ceilings. That's not too bad of an application for it.



That does not sound like advice I would follow.


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## SamM (Dec 13, 2009)

Morning Wood said:


> That does not sound like advice I would follow.


Why is that?

I'm only talking theory at this point, if you have a reason why that's a bad idea I'd like to know.


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## Morning Wood (Jan 12, 2008)

If we are in Minnesota you do not want to do a hot roof with open cell foam. Basing your insulation on the fact that your roof is going to leak is also poor practice. Do the roof right and make sure it is dry. Closed cell foam acts as a vapor barrier on a hot roof. You use enough closed cell to make sure winter time moisture laden warm air on the inside does not reach the dew point inside of the closed cell foam. 
Now, if you were to double sheath the roof (the best way to do it, and maybe the priciest too) you could do things differently.


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## EricBrancard (Jun 8, 2012)

jacselee said:


> No it is NOT A CONDITIONED SPACE.


Currently it isn't, but that would be the reasoning for foaming the roof deck. Is there HVAC equipment up there? What is the homeowners reasoning for wanting to do this?


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## mnld (Jun 4, 2013)

My entire house roof is spray foamed. Story and a half. Old part is eighty years old. New part is eight. All hand framed and gables ballooned. I have a friend that was one of the first spray foamers in the area and I went off of his recommendations. Another advantage to foaming right to the roof sheeting is the increased strength and rigidity. Walking on my roof is like walking on a sidewalk. Well, an 8/12 sidewalk at any rate.
I know they did some studies up in Duluth years ago on the advantages/disadvantages of 
hot roofs vs vented. Might want to check into that.

I have been very satisfied with mine but I have virtually no dead space in my rafters. Here's a couple pics, all the second floor space is usable space. Spray foam was the only feasible way of insulating most of this.

Edit. Definitely agree with Morning Wood about closed cell for hot roof.


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## mnld (Jun 4, 2013)

jacselee said:


> I live in MN, client wanted spray foam inbetween trusses in attic. I don't think that's a good idea, what do you think?


What part of Minnesota are you from ?


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## fjn (Aug 17, 2011)

Spray foam between rafters is doable.I would take several steps to insure the best outcome. I would apply a peel and stick to entire roof deck (Grace Ice and water) no doubt costly,also attach a rigid foam to bottom of rafters prior to drywall. That will reduce thermal transmission through framing,which could cause ghosting .


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## Joasis (Mar 28, 2006)

Last several new homes I have built are all spray foamed with open cell 6 inches or greater to underside of rood deck. No issues. Very good energy efficiency. I used to be opposed, but OI am on board now. 

But, this is Oklahoma, and metal roofing.


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## SLSTech (Sep 13, 2008)

Lots of good points above jacselee, especially by Eric * Morning Wood. For a quick primer you might want to start here http://thehtrc.com/2011/what-is-a-hot-roof

As for what part of MN - yes that is a factor especially in regards to snow & preventing ice dams. If your roof is designed for 50PSF of snow load, you can still go with a hot / encapsulated option BUT you should also vent above it (2 layers of sheathing / not baffles). As for OC vs CC - in your climate & many others closed cell is the best & truthfully only good option. Joasis should be more than fine with OC down in Oklahoma but he also isn't dealing with the humidity & snow issues 

One other quick point, you will need at least 6" of CC foam (8 would be better) & you need to make sure it fully covers the top chords by at least an inch or more - you can use the performance path option in the codes to get by with less roof foam


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## jaydee (Mar 20, 2014)

question is way to vague........


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## pappagor (Jan 29, 2008)

that it is.


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## jaydee (Mar 20, 2014)

pappagor said:


> that it is.


AGAIN... AND ALWAYS.:blink:


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