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Foam insulation/Texas

Home in central Texas (Austin)
1 story on slab
Built in 2009
Approximately 2800 sf house, 1 story
1 hvac system
Ductwork is in attic
Blow in insulation is in attic
Garage is not insulated

What would be your recommendation if we wanted
to put spray foam in the attic and close it up?
Our build out is common practice here but I think it's
terrible from an energy efficiency standpoint as our
attic is over 100 degrees all summer

Asked by tdavis3
Posted Aug 12, 2017 8:35 PM ET

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4 Answers

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1.

T. Davis,
Converting your vented unconditioned attic to an unvented conditioned attic makes a lot of sense. There are lots of articles on GBA about this work. Here is a link to one of them: Creating a Conditioned Attic.

Answered by Martin Holladay
Posted Aug 13, 2017 5:34 AM ET

2.

Martin, nice little addition! I was speaking to my HVAC man yesterday about adding registers in the attic of our new house. Closed cell is going to be well OUT OF SIGHT cost wise. The next best option open cell, do as you mentioned in your revised article and install a vapor barrier on the underside of the roof prior to insulating. Any thoughts on a SPECIFIC vapor barrier? OP, sorry to derail your thread a bit but I've been reading everything I can get my hands on for the past two weeks on this EXACT subject. We are up I10 a stretch in Pensacola, FL and the same humidity as y'all have.

Answered by Thomas Roberts
Posted Aug 13, 2017 6:06 PM ET

3.

Thomas,
Q. "The next best option open cell, do as you mentioned in your revised article and install a vapor barrier on the underside of the roof prior to insulating. Any thoughts on a SPECIFIC vapor barrier?"

A. I don't recommend the use of open-cell spray foam on the underside of roof sheathing. Closed-cell spray foam is much safer. To learn why, read these two articles:

Open-Cell Spray Foam and Damp Roof Sheathing

High Humidity in Unvented Conditioned Attics

If you ignore my advice and install open-cell spray foam under your roof sheathing, the best way to install an interior vapor barrier is (a) to install a layer of interior drywall (for fire safety) and (b) to install a layer of vapor-retarder paint on the drywall.

You could also install a layer of some type of "smart" vapor retarder on the interior side of the open-cell spray foam -- for example, MemBrain -- but you would still need the drywall for fire safety. (As far as I know, MemBrain can't be left exposed. It needs to be covered by drywall.)

Answered by Martin Holladay
Posted Aug 14, 2017 4:43 AM ET
Edited Aug 14, 2017 5:10 AM ET.

4.

One option on spray foam. There is a place that rents their spray foam rig for 850 a day. The closed cell covers 4000 lbf, the open covered 16000. It was $2600 a set. So just throwing that out there, incase that puts foam within your reach. The only thing I didn't like was that the foams they spray have to be covered or painted, are not fire rated. If I can afford it I will put an inch of closed, and do the rest open myself, but I have allllllot about 7800 of roof to do in my case at 7.5 or so thick.

Answered by SLEaton
Posted Aug 15, 2017 4:03 PM ET

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