Spray Foam & Air Circulation Under Roof
Last Post 11 Apr 2009 08:43 PM by mac31313. 13 Replies.
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AltonUser is Offline
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24 Mar 2009 08:31 AM

At a home show in Atlanta last Saturday, I saw a new way to insulate under the roof deck and still maintain air flow. This was done by stapling a permeable cloth to the underside of the rafters and then spraying 3" of open cell foam to the underside of the cloth thus insulating the bottom of the rafters and leaving the space between the rafters open for air circulation above the foam.

For tall, steep roofs the permeable cloth was installed up the rafters for 10' and then 2x4's were installed horizontally like additional collar ties thus eliminating many square feet of foam.  A manhole with an insulated cover allowed access to the remaining attic above the 10' level.

This new system should allow air to circulate quite well for a gabled roof that has continuous soffit and ridge vents.  Hip roofs with less ridge vents might need some holes through the rafters at the high point to allow air to reach the ridge vents.

This new way should satisfy code inspectors who demand air flow under the roof deck to preserve the shingles.

Anyone want to comment on this method.  Pros and cons.

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Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
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wesUser is Offline
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24 Mar 2009 03:05 PM
Alton,
I can't think of any pros for this system.
With the preponderance of hip roofs and pre-engineered trusses, it seems likely that this system will become very labor intensive and very difficult to seal properly.
Also, do you deal with cape style homes that use the bottom of the rafters for ceilings?
This one will need to do some serious selling of its positives before I would consider it.
Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected]
buck3647User is Offline
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24 Mar 2009 03:49 PM
For me ALL structures built from this day forward would produce what they use. This should be the goal of every structure built. My structures will not only solve the rebuilding fiasco that happens after every disaster but it will solve the energy situation locally and as each local neighborhood becomes off the grid so does the community and as each community becomes independent so does the county and as each county goes green so does the State and so on.
Dream we must fore "Imagination is more important than knowledge," Albert Einstein

Someone has said that posting information and in this case my personal business info is not allowed?
Stifling the flow of ideas and contacts in todays world sort of makes no sense?

Anyway building an off the grid dome indestructible and needs no outside energy.
AltonUser is Offline
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24 Mar 2009 05:10 PM
Wes,  I think you are right.  Anything other than a simple stick-built ranch might be more trouble than it is worth.

Buck, you lost me on that one.

Do you have any suggestions on how to properly insulate a stick-built home with rafters.  Do you think this new method will work or will it create more problems?
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Bruce FreyUser is Offline
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25 Mar 2009 05:46 PM
Aside from the complexity issue of this procedure, one of the touted benefits of spraying foam on the underside of the roof is some improved structural integrity.  If that is true (which I think it is), then this method would likely eliminate that benefit.

With open cell foam, it seems there could be more of a condensation issue as well.

Code issues aside, I think the benefits of spraying the underside of the roof outweigh the negaitive of shorter shingle life.  I am not totally sure where I come down on the open cell/closed cell isue, although I lean slightly toward the closed cell camp.

Bruce
mac31313User is Offline
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31 Mar 2009 10:11 PM
here is a way,
spray foam under deck,
add strips (1x4) above deck and install metal roof
air flow from sofit to ridge vent

you get air tight attic, with reflective roof over vented space

mac
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10 Apr 2009 01:51 AM
Posted By mac31313 on 03/31/2009 10:11 PM
here is a way,
spray foam under deck,
add strips (1x4) above deck and install metal roof
air flow from sofit to ridge vent

you get air tight attic, with reflective roof over vented space

mac
We have a winner.

How about not installing a POS asphalt roof if you are spending on the money on insulation to do the job right and do just like Mac says.  That is the only way I go about it.

Bruce FreyUser is Offline
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10 Apr 2009 10:15 AM
Posted By mac31313 on 03/31/2009 10:11 PM
here is a way,
spray foam under deck,
add strips (1x4) above deck and install metal roof
air flow from sofit to ridge vent

you get air tight attic, with reflective roof over vented space

mac
.....yes, but what happens when you walk on this?  With no support, won't the metal roofing be damaged?

To get ventilation, you would neet to run the strips vertically, or leave frequent gaps with horizontal strips.

Bruce

wesUser is Offline
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10 Apr 2009 10:32 AM
Bruce,
Your point is well taken. Horizontal stripping has to accommodate air movement, which may or may not be a factor, depending on the profile of the metal roofing. But more of a concern to me is, with horizontal stripping, how do you account for condensation that may occur. With continuous stripping this moisture could be trapped, which could lead to rotting of the strips. On the few times we have used stripping, I have used 2x material with a large saw kerfs on the underside to allow this moisture to drain. However, I am not sure that this will be enough to eliminate the potential problems.
I prefer to apply the metal roofing directly to the roof sheathing over a good underlayment, to protect the sheathing.
Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected]
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10 Apr 2009 12:47 PM
In the case of the metal profile that I am thinking of, the batten are not butted directly up to each other and the metal roof only contacts them at selected points so airflow is permitted between the panel and the batten.

As far as walkability goes, quite easy to walk on as long as you step on the correct spots on the panel where they are reinforced and supported by the battens.

No need to really worry about condensation.  The roof is entirely impermeable to water absorption, unlike an asphalt shingle, and if you are spray foaming the underside of the roof sheathing, there should not be any humidity migration through the roof. 

Any windblown rain or moisture that makes it between, not much at all, will be directed off the roof by the WRB underlayment.  There should not be any condensation issues because the roof will certainly warm during sun exposure and burn off anything on the underside.

AltonUser is Offline
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10 Apr 2009 08:21 PM
By the way, there are synthetic horizontal furring strips for the roof that will not trap water and they will not rot.  I have seen them in Florida but I do not remember the brand name.
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11 Apr 2009 02:59 AM
That or you can just rip Azec down.
wesUser is Offline
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11 Apr 2009 07:45 AM
Alton,
If you remember the info on those synthetic furring strips, please post it or PM me.
I would be very interested in learning more about them.
Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected]
mac31313User is Offline
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11 Apr 2009 08:43 PM
horizontal strips on 2' -3' oc will not restrict air flow..the profile of the metal (pbr type) has a raised section every 9-12"
as to walking on it....once screwed 26 ga metal panels will span up to 5'(done on metal building all the time)
any moisture that could get in would be removed by air movement
mac
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