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R38C in 9 1/2" Joist Cavity


zoom38's Avatar
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NY

03-04-10, 08:23 PM   #1  
R38C in 9 1/2" Joist Cavity

I am going to start insulating a 2nd floor addition. I have 9 1/2" WI ceiling joists. My question is can I put R38C batt insulation in these joist bays? I understand that R38C is 10 1/4" so there would be some compression. I have a chart by Owens Corning which lists R-values for insulation compressed in a joist cavity that states that R38C would produce R35 in a 9 1/4" (dimensional lumber) joist cavity. My joists are 9 1/2" (less compression). My plans call for R30 so would I be wrong in putting in R38C?

Thanks
Gary

 
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03-05-10, 02:09 AM   #2  
Hi Gary, too early in the am for me to think, so what are WI ceiling joists? If this is a ceiling cavity that you want to fill completely, is there any venting involved. R-30 is right in between wall and ceiling recommendations, so I don't know where you are, other than Suffolk Co.

I'll be back when I'm awake .

Bud

 
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03-05-10, 07:08 AM   #3  
I used WI for Wood-I joists (9 1/2" tall) 16" oc. I'm looking to insulate the entire ceiling and there is soffit venting. I already installed provents to allow venting of the attic through the soffits. I'm in Brookhaven Town, Suffolk County NY.

thanks
Gary

 
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03-05-10, 07:23 AM   #4  
R38C is 10 1/4" thick, and is designed for 2x12 joists, so as to leave 1" of ventilation above the insulation. It is too thick for your 9 1/4" space.

 
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03-05-10, 10:55 AM   #5  
There are a couple of options. They make high density fiberglass, but I don't know what thicknesses it comes in other than 3 1/2" r=15. That's about r=4.3 per inch so would come close. Then there are the mineral batts, they tend to be a higher r-value per inch.

The solution I like best is a 1" layer of polyisocyanurate over the inside surface before the sheetrock. It adds the extra r-value and covers the wood surfaces for a better overall r-value. Although I-joists have much less thermal bridging.

But here is a caution. Stuffing insulation between I-joists is a poor fit. Batts are designed for 16" or 24" on center with 1 1/2" joists. Yours are 1/2", so your gap between joists is 15 1/2" or 23 1/2". That extra inch may not sound like a lot, but it makes for a loose fit and that encourages air circulation. And it gets worse. The step at top and bottom on each side created small voids where the natural shape of the insulation doesn't properly conform to the shape of your cavity. I don't have a number on this, but from experience I would estimate a 10% to 20% loss in r-value and it could be more. Voids are very bad for insulation.

The one job where we tried to come up with a solution, we decided that 1/2" rigid foam, cut to fit on each side, would bring everything back to the expected dimensions.

If you think about how fiberglass insulation is often poorly installed behind electrical boxes, that is essentially the same poor fit you will be getting, top, bottom, and the full length of every I-joist.

Bud

 
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03-05-10, 11:43 AM   #6  
Is there an attic space above these joists? If so, blown in cellulose may be appropriate.

 
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