On Saturday, May 17, 2014 11:13:59 AM UTC-4, wrote:
On Fri, 16 May 2014 22:26:53 -0700, "Bob F"

wrote:



wrote:

Hi,

My attic has 2x6 joists. The (ancient) fiberglass insulation between

the joists appears to be matted down to about 2" thick.



Do I rip it out and replace with 6" thick batts and add more above

that?

Or do I leave it, add 4" batts, and add more above that?



All opinions appreciated.



I have no idea where you are, but up north, thay are using a lot more than 6"

these days. I filled the joist space, then did another thick layer over that

(R30??).



He DID say he was going to add more above.

I missed that. I guess enough insulation to fill it up
to the level of the top of the joists, then loose cellulose
gives the best energy solution. I wonder what the energy difference
is if you use just thick batts? There would be some gaps
between them, ie between joists, which would cut down the
performance. Topping it with blown-in would be better from
that standpoint. Only thing about blown-in I don't like is
that if you have to do some work, eg find an electric, phone,
CATV etc cable, it's going to be a pain in the ass. But I
guess you don't have to do that very often.



Matted down insulation has poor insulation value. I'd remove the old

first - and be tempted to have a spray insulation applied (about 2

inches) to act also as a vapour barrier and then fill with blown

insulation above that.

Sounds like unnecessary complication to me. I agree if it's matted,
it has low R value, but so what? It's still greater than zero, and
if you put what you intend to put on top of it, you have the new plus
whatever is existing. A little more insulation and you avoid the
work and disposal, etc of ripping the old stuff out.