Replies

1. Expert Member
   Dana Dorsett | Nov 20, 2014 01:54pm | #1

   I'd need a better description to advise on the best remedy. When you say "uninsulated roof channels", are you referring to an ventilated insulated roof deck (insulation between rafters, with a small 1-2" ventilation gap between the insulation and the roof deck)?

   With fiberglass, in general the R per inch goes up with density, thin spots/ compressions and gap rob quite a bit of performance.

2. GBA Editor
   Martin Holladay | Nov 20, 2014 02:03pm | #2

   Robert,
   Like Dana, I'm confused by your description. Are you insulating between your rafters? What sort of access do you have?

   I'm not sure what "hand-packed fiberglass" is, but the R-value of well-installed fiberglass ranges from about R-3.0 to R-3.8 per inch.

3. 
   Scot69 | Nov 20, 2014 03:25pm | #3

   We had 3 foot opening in a 8 foot deep rafter bay, we ended up sliding a osb panel with 1.5 inch standouts down the opening in pieces, stuffing fb batts in to raise panels to sheathing. Is the r value still there after packing it in that hard. Thanks

4. GBA Editor
   Martin Holladay | Nov 20, 2014 04:02pm | #4

   Robert,
   I guess the answer depends on how hard you packed it.

5. Expert Member
   Dana Dorsett | Nov 20, 2014 06:28pm | #5

   As long as there are no voids from bunching & rolling, the compressed batt will have a higher R per inch than an uncompressed batt. If you packed it in with a hammer so that it was 3x as dense as the normal installed density it will be somewhat lower R.

   For instance an R19 batt installed at 5.5" thickness performs at about R18, or R3.27/inch.

   If you pack it into a 3.5" deep cavity it performs at R13, or R3.7/inch.

   If you pack it into a 2.5" cavity (a 2x3 nominal depth) it runs R10, or R4/inch. That's more than twice the density, but much higher R/inch.

   You have to really pack it pretty hard to give it a lower R per inch, but the total depth & density will determine how it performs as-installed.