Insulate attic by adding fiberglass to top of plywood floor?

locust78January 31, 2011

I have a 20 year old duplex that has its attic floor insulated with fiberglass between the 6 inch joists.

Some years back, I dropped in a number of half-inch 4x8 plywood sheets in order to use the space for storage. Most of the attic floor area is now covered with plywood, except for several feet on each side where it gets close to the eaves.

I was wondering if I could do anything at this point to increase the R-value of my attic floor without pulling up the plywood. Would it make sense to roll out some more fiberglass on top of the plywood (in the places where I'm not storing stuff)?

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Blueboard styrofoam gives more R value per inch than fiberglass, plus, you can walk on it. That's what I would use. Then I'd put 1/4" plywood over top and screw everything to the joists.

They just showed this option recently on This Old House.

    Bookmark   February 2, 2011 at 7:55AM
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I'm having cellulose blown into my attic on Monday. They are taking up the plywood we put down 35 years ago. They're keeping the plywood in one spot that I want for storage. They'll blow cellulose under the floor there. The cellulose is going over some ancient fiberglass batts.

I saw the TOH episode about insulating an attic. The blue styrofoam made sense. Maybe you can watch it on their site or even catch the rerun because it was so recent.

Good luck!

    Bookmark   February 3, 2011 at 10:32AM
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Thank you both for your posts.

I did get to see that episode of TOH. That blueboard styrofoam thing's got me interested. Can it really hold up underneath a plywood floor, with all the walking & storage that might go on? I always picture styrofoam as smashing down under pressure.

    Bookmark   February 3, 2011 at 10:44AM
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It's not because it's blue. Let's all learn more: read wikipedia and other sources.

There are an infinite number of types of styrofoam. E.g. look at Wedi, a styrofoam onto which you can lay floor tile, and outdoors too! There are dozens of competitors to Wedi also, and they all buy the same source material, or similar. With a "styrofoam" substrate to tile, one trade off is the hollow noise it makes when you knock on it. Not a problem with most shoes and with bare feet.

Yes, you can walk on a plywood-on-styrofoam floor, if you have bought the right material and if you have installed it right.

They even use styrofoam now as a substrate underneath concrete foundations of entire buildings weighing tons.

    Bookmark   February 3, 2011 at 1:54PM
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You can attach a grid of furring strips on top of the existing plywood, and cut the foamboard to fit between the strips, then lay down a sheet of thin plywood on top of that. That way, the furring strips support the plywood overlay and it won't smash the foamboard.

    Bookmark   February 3, 2011 at 9:30PM
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I also have a plywood floor over most of my attic & want to add more insulation. I was told I would need to remove the plywood floor, add 2X6's to give me ~10-12" of depth for batt insulation to achieve a R-38, & replace the plywood (I need the storage).

However, since I have some black mold spots on the ceiling of 3 of the 4 upstairs rms, I think there may be a problem with the vapor seal. I think I should pull up the plywood, take out the old insulation, wash down the attic floor with bleach, & make sure there's a proper vapor seal, before I put down new insulation.

I also thought that if I used rigid insulation, I could achieve the desired R-38 or higher with the existing 5-6" depth. Does anyone know approximately how much more expensive would it be if I covered ~70% of my ~900 ft2 attic with rigid insulation, rather than using batt insulation? Does anyone have any experience installing rigid insulation? I couldn't find the episode of TOH which showed it, but from what I've read, it seemed as if I could do it myself.

What did you do? Anyone else have any advice? Initial cost is an issue, but so is difficulty & time to recoup the cost. Although rigid insulation is more expensive, I think I could save money by installing it myself. On the other hand, since I would have to raise the existing plywood floor by ~6", in addition the cost of the 2X6's, I would need to hire someone to do the work.

BTW, I have a 45 yr old colonial in CT near LI Sound. That is why I want to increase the insulation to R-38. Thanks.

    Bookmark   March 20, 2011 at 5:21AM
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I always had in my mind that rigid foam was 3x the cost of batt. It is definitely a diy project. For what you describe based on hiring the labor, rigid foam may be cheaper.

Rigid foam around me is $13 for a 4x8 or $.40 a sq ft for r-4. So you are looking at $240 for r-4. So final cost depends on what your baseline is. If you only have r-18 then you are looking at $1200.

Now - rigid will outperform batts in every way. You could probably add r-12 and it would be comparable to adding r-20 in batts. But that is really a guess and if you wind up sealing significant air leaks with foam, then you will make even more of a difference.

    Bookmark   March 20, 2011 at 6:28AM
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