Rigid foam insulation over attic subfloor


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Old 02-28-14, 10:22 AM
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Rigid foam insulation over attic subfloor

Hello all,

I am trying to bring my ceiling/attic floor insulation up to r40-r50 but I only have 2x4 joists with access from below (I am trying not to rip up the tongue and groove subfloor). The 2x4's are the old true 2x4's and they have another 7/8" furring strip between the joists and the attic subfloor giving me a total cavity height of almost 5".

I wanted to spray foam the entire cavity with open cell polyurethane foam and put rigid insulation board on top of the subfloor and then put another 5/8" mdf board on top of that. The idea is to be able to use the space for storage.

The 2x4's run a maximum of 10' unsupported. The more I read about vapor barrier and such, the more I think this might not be a good idea. Could anyone help with my situation? Any suggestions would really be appreciated!
 
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Old 02-28-14, 01:09 PM
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Joist that small are not rated for storage.
Your area calls for that R rating for a reason, 5" would be about 1/2 of what you should have.
 
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Old 02-28-14, 04:16 PM
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You won't need R40-50 using foam insulation. They call for that much because typical insulation like fiberglass allows air to pass through it which decreases the effective R-value. Adding 6" of rigid foam would be more than enough. Stager and seal all seams. MDF is way too heavy. Use plywood. With 2x4 joists, I too would be wary about storing anything up there.
 
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Old 03-03-14, 09:56 AM
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Thank you for your replies, I will definitely consider sistering my 2x4's with 2x8's in order to gain more space for insulation and to stabilize the floor for storage, which is important for us to have.
 
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Old 03-03-14, 10:07 AM
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Sistering the 2x4's may not help as a truss is more than just a bottom chord. If you have any supporting walls in the middle it may help. Remember, you may know not to overload the storage, but time passes and people change and that is why codes and engineering need to be followed.

Bud
 
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Old 03-04-14, 05:13 PM
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Bud, it doesn't sound like the OP has trusses; just dimensional 2x4 joists.

Wcawley, I would make the furring strips 1x4s 24" o.c. and install either spray foam or mineral wool batts to the top of those. I would install 3/4" T&G plywood for the flooring. That full assembly will give you both a decent R-value and decent support for most storage.
 
 

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