Replies

1. 
   frenchy | Jan 10, 2009 08:59pm | #1

   first fiberglas is a lousy insulation media..

      second R value and fiberglas is a moving target at best..

    But stuff it in there and hope for the best..

2. 
   DanH | Jan 10, 2009 11:38pm | #2

   I suspect that the thinner stuff is higher density.

   God is REAL, unless explicitly declared INTEGER
3. 
   Clewless1 | Jan 12, 2009 03:30pm | #3

   Manufactured density will give you varied values of rated/tested R-values. I can buy R-19 or R-21 for my 2x6 walls. I think I can even get R-23.

   Higher density is usually also more expensive. Theoretically there would be no advantage to the 8 1/4" batts. HOWEVER ... in an attic where air flows free, some might argue that a higher density is doing you more good, relatively speaking.

   I would tend to blow an attic w/ cellulose, though. Or just make sure it is installed neatly to limit the typical gaposis that occurs w/ batts in your attic (the wingless kind). 

1. 
   rick12 | Jan 13, 2009 08:54pm | #4

   Thanks for your input.
   I'm learning here.
   If your dealing with fiberglass and wishing to stay within the 2x6 floors in an attic, is more better? In other words, are you better off packing in and compressing more fiberglass between the joists or should you keep within the manufacturer's thickness printed on the package? Can too much fiberglass mean less R value?

1. 
   DanH | Jan 13, 2009 09:12pm | #5

   You'd have to compress it 3-5x to get less, in terms of R per inch. But compressing much beyond the normal density of your better-rated fiberglass doesn't improve things much either.You do want to fill up the cavity, though, and not leave substantial open volume.
   God is REAL, unless explicitly declared INTEGER

2. 
   frenchy | Jan 13, 2009 09:32pm | #6

   somes good more is better and too much is just starting to be fun!. 

     shoot for a R50 or better

3. 
   Clewless1 | Jan 14, 2009 05:07am | #7

   I don't think any of us are experts, here (with the compression thing). We do know when you compress the insulation into a smaller space you lose R-value, but your R per inch generally increases. The more fiberglass you put into a cavity, the more R-value you get.

   There is probably some limits, though. If you are thinking of putting in a floor in the attic ... the downside of to much insulation is pressure on the ceiling that you may not care to have. If you are intentionally compressing it, I wouldn't particularly recommend it ... i.e. if you have the space to put in two full R-19 batts, that would be more economical than compressing e.g. 4 R-19 batts into the same space for greater R-value.

   Like the other guy said, though ... focus on filling up cavities ... around ceiling joists, boxes, wiring, piping, etc.  Seal air leaks BEFORE installing any insulation (e.g. everywhere a wire or pipe penetrates the top of a wall, J boxes, etc.). Put effort into the air sealing ... it can be your most important effort. Insulation is relatively easy. Sealing air leaks after the fact is NOT a good idea no matter how you slice it.