Insulate Attic floor using rgid or EPS foam sheets?

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Old 03-18-02, 11:47 AM
gubs18
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Insulate Attic floor using rgid or EPS foam sheets?

Currently I have an unfinished attic that has 3.5“ of FG in the joists (no floor). I want to add insulation but don’t want to build a 2nd floor (or platform) so I can store items over the new insulation(which will extend about 5” over the 2 x 6 joists).

Can I insulate the attic floor using foam sheets(rigid or ESP), glued to plywood sheets? My thinking is I’ll get almost the same insulation value I need and the solid floor for storage; A 2” rigid sheet is suppose to have a R14 rating (plus the 4 or 5R from the existing 3.5"). I understand it is more costly to use foam sheets than the other insulation, but contractors want $1k to roll or blow it in, which can buy a lot of 2” foam sheets ($22 ea) and plywood and do it myself (I have a walk up attic so it’s not of a hassle to carry up sheets). If the contractor did it, I’d still have to buy & carry the sheets for the platform.

Is this thinking too simple? Is rigid or ESP sheets prone to mice/insect problems? Fire or misture hazard? What am I missing?

I do understand also, that if I don’t add insulation between the existing FG and new floor, I’ll leave open a channel for air to move around (which I’ll deal with after I get some answers to today’s question).

I'm sure most would say not to put anything in attic, but I have no basement, making the attic a must for certain stoarge.
 
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Old 03-18-02, 06:48 PM
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You can go to http://www.eren.doe.gov/consumerinfo/refbriefs/bd4.html this is a U.S. Dept. of Energy brief on vapor barriers. The topic that concerns your application is "PERMS". It will explain to you why you can't put the rigid insulation over the fiber glass insulation.

Though this site deal primarily with vapor barriers, the principle that applies to reduction of moisture in heat and thermal effect of insulation are the same, which is diffusion.

If the rigid insulation went first then the fiberglass, you would not have a problem. That's because the one which is more dense went first. If you were to reverse it, as in your application, moisture would form where the two met. This is also why I don't recommend mixing different types of insulation. Even though it is less likely that condensation would occur if you mixed fiber glass with cellulose, the possibility is still there. FG, cellulose and foam may use diffusion as the dominant heat transfer mechanism, they do it differently and they all absorb and expel moisture differently too.
 
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Old 03-19-02, 07:57 AM
gubs18
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thank you Resercon for your reply.

few followup questions:
1. so even if I leave the 3" air channel open between the existing FG & rigid panel, I would still have a moisture problem?

2. i guess i would have the same problem even if put plywood down 1st (over the 3" channel), then the rigid panel - correct?

3. how about just the plywood (no rigid panel) over the 3" channel? don't many attics have floors?
 
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Old 03-19-02, 11:34 AM
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Yes you can floor the attic because wood has a higher absorbancy for moisture than the insulation and a slower expulsion rate. You can't sandwich the wood floor between insulation for the same reason.

Allow me to make a suggestion. Build the floor up and lay fiber glass insulation, either unfaced batts or loose fill fiber glass. The way to build the floor up is very similar to building a wall on the floor before standing it up. The only difference is that you don't stand the framing up and you lay it perpendicular to the floor joists. Do it in sections of 4 feet by 8 feet and slide the section where you want it on the floor joists, then tack it down. Do another section and lay it adjacent to the other sections until you have the desried floor area. You tie the sections together by laying down 3/4th inch plywood (subflooring grade, tongue and groove). And you'll have a floor that you could put a piano on if you want. And no moisture problems too.

Good Luck with your project. Oh, by the way, it's a little more expensive to do it my way than your way, but no where near $1,000. A good size area, material cost should be around $200.
 
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Old 03-19-02, 02:55 PM
gubs18
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Thanks again Resercon! I'll take your advise (and great suggestion) & follow it to the letter.

i'll let you know how things went in a few weeks.

Appreciate your time!
-Lou
 
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