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Will I have humidity issues with a closed cell retro?

I am considering removing all of my batting insulation and having my attic spray foamed. My second story is unfinished, meaning it has no drywall yet. So, that is a plus for this installation. My question is, if I have 6" of open cell foam put at the roof line and SEAL the attic, will I end up with moisture issues in the attic??? I live near Houston, so its humid. The installers I have spoken with insist that there is enough air transfer through drywall and the relatively few can lights to alleviate the potential issue. Also, my AC probably has a little leakage into the attic. However, I don't like the way that sounds...I don't think drywall has enough air permeability to solve the problem and I don't want to depend on an AC system that is insulated and covered with mastic at all the joints to 'leak' enough to solve the problem.

Is what they told me correct? If not, what is the solution to keeping the attic humidity down?

Asked by Jason Duke
Posted Aug 13, 2012 1:43 PM ET


11 Answers

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In Houston, the outdoor air is more humid than the indoor air. If you install an air barrier like closed-cell spray foam under your roof sheathing, you are keeping the hot, humid outdoor air where it belongs -- outside.

By the way, air transfer through can lights is a bad thing, not a good thing. If you have air transfer through your can lights, that's just another reason why it makes sense to install spray foam under your roof sheathing. What you have now is worse that what you propose, that's for sure.

Answered by Martin Holladay
Posted Aug 13, 2012 2:50 PM ET


Jason, hire the best. Talk to builders. Go with their advice. Go to present and past jobsites. Then you can make the decision with confidence. The best are safety first and highly experienced.

Martin, in his area open cell has been used extensively. They need to get their best advice IMO from experienced locals. Open cell costs less and outgases less. Open cell is water blown. Icynene has techs ready to help and may be best for aiding any of us with spray foam specifying.

Northeast Sprayfoam in NY is the best near me.

Jason said closed, then open later... Jason, use a good local company and go with their choice. Call their insurance agent too.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 13, 2012 2:53 PM ET
Edited Aug 13, 2012 3:14 PM ET.


Open cell is what my research and my locals use...my question is, if I go this route, will I have moisture issues? My installers say 'no' since the air can transfer through the drywall cieling.

EDIT: Oops...I just realized I wrote 'closed' cell in my original post title....I meant open cell. All of my quotes were for OPEN cell foam.

Answered by Jason Duke
Posted Aug 13, 2012 3:25 PM ET
Edited Aug 13, 2012 3:29 PM ET.


I don't think you will get significant vapor diffusion from the exterior to the interior through 6 inches of open-cell foam. Is that what your are worried about?

Most moisture travels through air leaks, not diffusion.

Indoors is dry; outdoors is damp. You want a good air seal and a thick layer of insulation to separate your indoors from your outdoors. Six inches of open-cell foam should be an adequate air barrier. It's not much insulation, though -- only R-22. That's less than the minimum code requirement.

In your climate zone (climate zone 2) the code minimum requirement for ceiling insulation is R-30.

Answered by Martin Holladay
Posted Aug 13, 2012 3:50 PM ET


What I am concerned with is will my attic stay at the same or similar humidity as the rest of the inside of my house if I use open cell foam to insulate my attic and seal it. In other words, if I go this route, will it require any mechanical means of humidity removal for the attic, or will the air transfer from the rooms to the attic be sufficient?

Answered by Jason Duke
Posted Aug 13, 2012 4:02 PM ET


No, you don't need a supply air register in your attic, nor do you need a dehumidifier up there. Researchers have looked into the issue. You'll be fine. (Still, see if you can afford to install at least R-30 insulation -- about 8 inches. That way you will barely meet code, which is better than having less insulation than required by code.)

More information here: Creating a Conditioned Attic.

Answered by Martin Holladay
Posted Aug 13, 2012 4:10 PM ET
Edited Aug 13, 2012 4:12 PM ET.


Thank you for your advice and help.

Personally, I do not think code requirement designed around standard insulations can effectively be applied to a spray foam setup. I think that with spray foam, the mere fact you have a sealed setup removes the necessity of an R30 rating.

What do you think? My installers will apply as much foam as I like, but insist there is a diminishing return to be had after 5.5-6" of foam. I have 10" roof rafters so I can accomodate much more....but is it necessary? Should I go for more R-value?

Answered by Jason Duke
Posted Aug 13, 2012 4:19 PM ET


Jason, use more and upgrade something like the floor or counter top later if applicable. It's probably only 1-2% added to your total cost.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 13, 2012 4:41 PM ET


The minimum insulation values specified in the code are the bare minimum you should aim for, in my opinion. More is better.

Answered by Martin Holladay
Posted Aug 13, 2012 5:30 PM ET


Don't buy into the misleading sales tactics of some open cell foam vendors- you very much DO need to meet the ASTM C 518 measured code R, despite the low air permeance of open cell foam. R22 under a 130F+ roof deck is simply not enough, and you WILL notice the difference, despite the diminishing return with thickness.

Don't sweat the moisture issue though- with an air-tight attic the attic's dew point will track that of the conditioned space, and the roof deck can still dry at a reasonable rate through even 10" of open cell foam. And the 10" is about what you should be looking at, even if it's above code-min. Covering it up with drywall with 4-5" of air space creates potential thermal bypass and fire-spread issues. Fill that cavity up!

Answered by Dana Dorsett
Posted Aug 15, 2012 4:10 PM ET


I have reviewed these answers, and while I find comfort in it, I have decided that the spray foam is going to cost more money than its worth. I have an entire insulated attic right now, which will have to be removed. On top of that, to insulate to 8 inches or more will cost upwards of 8,000 dollars. I can just add to my existing insulation for less than 1000 I think and use the left over money to replace energy sinks like leaky french doors. What do you guys think?

Answered by Jason Duke
Posted Jan 18, 2013 1:37 PM ET

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