How should we insulate the attic?

August 9, 2016 7:50 AM

Our attic needs a thorough cleaning and insulation job and we are exploring our options. I think we are deciding between blown-in cellulose (R38) or blown-in fiberglass. The contractor we spoke with did not recommend batts as he said it would not fill all the gaps in the attic. We also do not have recessed lighting so there isn't concern about the loose stuff falling out of there. We'd sure appreciate any thoughts though!
posted by madonna of the unloved to Home & Garden (8 answers total)

Our house had blown in cellulose about 30 years ago. I am convinced that the house is extra dusty because that stuff is breaking down over time. Having said that, perhaps the technology as advanced over the old ground up newspaper, mixed with boric acid. Be sure to ask.
posted by Midnight Skulker at 8:30 AM on August 9, 2016 [1 favorite]

I insulated my attic with unfaced fiberglass rolls. Just unroll between the joists, then put a layer perpendicular to the joist on top. Any gaps can be filled with scraps. I also like it because if I need to work up there, the top layer can be rolled up. No shoveling necessary.
It's easy to keep a gap between the insulation and the roof, to avoid ice dams.
posted by H21 at 9:59 AM on August 9, 2016 [1 favorite]

Batts are harder to fit into gaps and irregular spaces, whereas blown materials especially cellulose will fill those gaps. But realize none of these things and especially fiberglass bats will stop airflow, which is an issue just as important or more important than putting a thermal blanket over the whole attic. I'd recommend blown cellulose, but more importantly they need to block up places where air leaks through (called "bypasses" in the trade) with materials like rigid foam insulation (for big holes) and caulk/spray foam (small holes).
posted by werkzeuger at 10:07 AM on August 9, 2016

BTW that air sealing I'm talking about will address the dust issue, because you're sealing leaks from the conditioned side i.e. house interior to unconditioned spaces like the attic.
posted by werkzeuger at 10:09 AM on August 9, 2016

Finally if you'd like to read up on this stuff, you can't go wrong with Taunton, and for a more technical understanding check out the Building Science Corporation.
posted by werkzeuger at 10:15 AM on August 9, 2016

Air sealing, air sealing, air sealing! You want to clean it out well and then seal all the gaps so that you stop air leakage (which is a very significant source of heat loss in many cases).

Blown in insulation is good idea over batts for the gap filling reason (and because it is significantly cheaper than batts plus labour). Cellulose versus fiberglass is not a big deal, but cellulose is a little better at slowing air movement as it is denser. Fiberglass is also kind of nasty stuff (irritates the airways, etc.) and cellulose is less unpleasant and takes less energy and raw material to manufacture. I'd choose cellulose, even at a higher cost.

R38 is not very much insulation. What is your climate? Do you have ducting in the attic?
posted by ssg at 10:52 AM on August 9, 2016 [1 favorite]

Response by poster: We are in Northern California, so pretty mild climate. We do have ducting in the attic.
posted by madonna of the unloved at 11:38 PM on August 9, 2016

Make sure you seal every single joint of the attic ducting before you insulate up there! Energy loss from leaky attic duct can be very significant and is very common.
posted by ssg at 7:43 AM on August 10, 2016 [1 favorite]