Does insulation lose it's insulating properties over time?
Is it better to remove old fiberglass insulation and then lay new, or just lay new over old?
Your best approach to increase the insulation value in the attic would be to overlay the existing insulation with additional. Even if the existing insulation has lost some R value due to compaction or compression, the amount of loss would be minimal. You'd not make up the cost in time and money to remove and replace with new.
Thanks! One follow up question
I wasn't sure which way to go, and certainly adding new over old is a little cheaper and a lot less costly in terms of time, dust in my lungs, removal and disposal.
Lastly, if r-11, r-13 and r-15 are all 3.5" thick, why would anyone use anything other than the r-15?
Cost and availability
I'd guess the deciding factor would be cost and availability. However, you're right that all being equal, the smarter choice would be the higher R value.
Your best bet would be to blow in cellulose. You can buy it at HD or Lowes (but can sometimes find it cheaper at local hardware stores). It's also much cheaper than batts.
Originally Posted by jirocpa
When you are laying fiberglass batts, there are going to be gaps between the batts. In addition, fiberglass "loses" some of it's thermal efficiency as the temperature drops.
I've got blown fiberglass on bottom, covered by blown cellulose.
I plan to avoid cellulose just in case I wanted to add or remove ceiling fixtures, I didn't want to deal with the loose stuff.
So, sticking with batts or rolls, I have another question for you experts...
should I add enough to fill the joists, then run another layer across the joists? Or, just add r-30 or something thick right on top in the joists?
Is there a big difference in the efficiency either way? Will I lose heat through the wood joists themselves?
Thanks again, everyone!