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merlanvp 07-30-2010 09:59 AM

rigid insulation under an attic storage floor?
Hi folks,

I live in Pittsburgh PA, house built in 1969, and have a 16 x 16 area in my unfinished attic with plywood flooring. I use the folding ladder access to store a bunch-o-stuff in the attic (no issues with the heat/humidty cycles, no other storage choices, and unwilling to toss the stuff up there). Overall house dimensions are 30'x'40'

I am going to pull out all the current insulation (2 layers of faced batts), vacuum up all the old critter-nest remains, and replace with blown insulation to R38 (either replace or box 5 non-IC rated recessed lights). I have been thinking that since the 6" of batt insulation under the floor is no where near the R38, I could pull the flooring up, place rigid insulation under it, then replace the flooring. I would replace the old batts as well with R19/6" batts (the attic joists are 2x6, 16OC).

I am contemplating using either "SUPER TUFF-R 2" x 4' x 8' Polyisocyanurate Rigid Foam Insulation" with R-13 or the "F150 Board 2 In. x 48 In. x 96 In" with R-10. I figure with the compressive load of stored stuff spread out by the flooring, the rigid board should not collapse much, if any. Not sure if it would be necessary to place 1/4" plywood under the rigid board to spread the support loads from only the joists to the entire board.

Any comments/thoughts/suggestions? Should I speak with the local building code folks and see if there are any issues?

rjniles 07-30-2010 01:14 PM

I would pull up the floor and sister 2 by 12's to the ceiling joists in the storage area. Blow in 10" of cellulose (R-38) and install plywood over the sistered joist in the storage area. Build a small rail around the area to keep stuff from getting pushed off the plywood.

Since you access is through pull down stairs you might have to cut the new joists and plywood into smaller pieces to get them up there.

Not sure I would pull out the old Fiberglas as it will be a real mess. You could just blow over top of it all. Mark the locations of electrical junction boxes for future access.

Proby 07-30-2010 01:40 PM

I would just put 6" thick R-19 fiberglass insulation underneath the floor.

After R-20 or so the energy savings decrease considerably.

Gary in WA 07-30-2010 02:18 PM

Is there a poly vapor barrier on the ceiling material, attic side?

Has the attic been air-sealed?

Be safe, Gary

merlanvp 07-30-2010 02:27 PM

no vapor barrier will exist after I pull up old and critter-worn insulation. I intend on air-sealing the attic after I get all the old stuff out, and before blowing new stuff in. Given the 3-6 layers of paint that have been applied in the house's lifetime, I was presuming a vapor barrier was a bit moot at this point. Are you suggesting I install a vapor barrier before blowing in?

Gary in WA 07-30-2010 07:32 PM

"Are you suggesting I install a vapor barrier before blowing in?" ------No, I was checking because the plywood has a low permeability rating to trap any moisture coming up from below. As you do not have one, only paint, it could still rise with the hot air from below. A true vapor barrier primer would stop it, depends on how much solids are in the existing paint: Using the foil faced foam board could cause problems as it is a true v.b., not a retarder as ply. Leave gaps between the 12" ply to allow water vapor to vent upward. Good to remove the two layers, each with a vapor retarder. Also good to remove the batts without blow cellulose over as it would be about 1-1/2# per square foot and compress the batts, negating their value. Remember to add 20% extra blow for settling to reach the intended R-value.

“Because of these characteristics, foams used for construction require a covering as a fire barrier. One half-inch thick (1.27 cm) gypsum wallboard is one of the most common fire barriers. Some building codes, however, do not require an additional fire barrier for certain metal-faced, laminated foam products. Check with your local building code/fire officials and insurers for specific information on what is permitted in your area.” From:

The blue foam board has a compressive strength of 25psi, so does the pink. The blue has a flexural strength of 50#psi for every 1” thick, the pink has 75#psi per in.

Be safe, Gary

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