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Attic loose-fill fiberglass vs. cellulose in Zone 3

matlockjeffries | Posted in Energy Efficiency and Durability on

I’m a DIYer who knows just enough to get myself into trouble, but am obsessed with GBA. Here’s my question:

Just purchased a new-to-me (1950s) house in OKC (Zone 3) with no attic insulation. My plan is to thoroughly air-seal and spray in loose-fill insulation to ~R60. It is a ventilated attic space with HVAC ducts already there; thankfully, though, the ducts run along the attic floor and will be fully covered with insulation.

The big question arose, however, when I went to the local box store and found that loose-fill fiberglass insulation is now priced about 25% lower than cellulose. I have read the problems with convection, loss of R value as temperature falls (granted, temps below 20F are rare here). My biggest concern with fiberglass is its inferior air sealing qualities.

So, the question is: in my climate zone, would I be better off with R60 cellulose or R~75 fiberglass, since the cost would be roughly equivalent?

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  1. davidmeiland | | #1

    IMO you would be better off with cellulose because (a) it's greener, (b) it's a lot less itchy/sneezy, and (c) rats, mice, and other critters seem a lot less likely to want to move in.

  2. GBA Editor
    Martin Holladay | | #2

    You will save hardly any energy by switching from R-60 to R-75, so the difference in R-value -- even if one were to assume that the two insulation materials performed as rated -- is trivial.

    In a loose-fill application on an attic floor, I think cellulose performs better than fiberglass. So I would use cellulose. But at the depths you are planning, either product will work.

  3. user-945061 | | #3

    As much as I love cellulose, sealing ducts in unconditioned space is a really big deal. If at all possible, have someone with a duct blaster test and seal the system (ideally in a single day so you don't have to incur the cost of duct blaster testing x2). You might consider using the practices described in this report:

    Employ the same principle with air sealing your attic. Measure the performance improvement with a blower door. Neither insulation will air seal when blown fluffy in an attic, air-sealing is an entirely separate job. Scaling back the insulation thickness to code compliant for your region and spending more on duct and air sealing will probably yield better results. You can always blow more insulation later. From personal experience I can tell you that it's really hard to seal ducts when they're buried in 16" of cellulose.

  4. user-945061 | | #4

    Deleted repeat post.

  5. matlockjeffries | | #5

    Sounds good, thanks folks!

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