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Beyond R30 in an almost perfect climate?

carlhaber | Posted in General Questions on

The climate here in Oakland Ca. is very temperate. As I’m trying to wrap up a personal building project on my own home I’m stuck on how much attic insulation will help control attic solar gain during those few hot summer days. R30 would be the minimum I’d install as I’ve got 2×4 ceiling joists presently filled with Rockwool. I could either layer another course of r15 or spend a $150 more on r30 fiberglass rolls. (R30 is the only available unfaced FG option available locally.)

So layer with r15 Rockwool or r30 fiberglass?

Thanks all!

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  1. Expert Member
    Dana Dorsett | | #1

    IRC code minimum for zone 3C (Alameda County) is R38, and R50 is still financially rational on a lifecycle basis (even higher if done on the cheap.)

    Adding 10" of blown cellulose wouldn't be insane, delivering a better fit, higher air-retardency, and better infra-red opacity than R30 fiberglass.

    1. Deleted | | #2


      1. Expert Member
        BILL WICHERS | | #3

        The box stores usually will include free rental of a blower with some minimum purchase of cellulose. You might find that the cellulose ends up being cheaper overall than the rockwool you're looking at. Cellulose will certainly be easier to install.


        1. Expert Member
          Dana Dorsett | | #5

          >"You might find that the cellulose ends up being cheaper overall than the rockwool you're looking at. Cellulose will certainly be easier to install."

          That's right. R for R a cellulose solution would be quite a bit cheaper than a rock wool solution, even if you had to hire somebody for the afternoon to keep the hopper on the blower full. It would probably be comparable to or cheaper than doing it with low-density fiberglass, and would deliver a more satisfactory result.

          Even at the big orange box store you can get 1000 lbs of all-borate cellulose (36 bags @ 30lbs/ea) for about $460, which is good for 1000 square feet of R38, and anything over 20 bags includes 24 hours of "free" blower rental.

          How many square feet of attic?

          1. Deleted | | #6


  2. kenorakq | | #4

    I blew 20" of cellulose into the attic of my workshop (720 sq/ft) ..cost me about $650 for material...the blower was free with the cellulose purchase (mind you I had to do some minor repairs..but free is free)... cost about $50 for beer and BBQ goodies for my work crew.

    It took 4 hours with 3 peeps, me in the attic, 1 guy feeding the bags to the machine and another toting and opening bags. I'd give it a solid 5 out of 10 on the difficulty and PIA easy peasy..

  3. Expert Member
    Dana Dorsett | | #7

    >"Back to my original post: My 1000 sq' 2x4 joist attic has just been installed with R15 Rockwool. The climate is almost perfect save for a handful of warm summer days.

    One more more course of R15 Rockwool or R30 fiberglass to deal with those 10 hot days?"

    R30 fiberglass, no question about it. You'd at least make it to IRC 2018 code-min that way, and thermal performance WOULD be measurably higher year-round, not just on those "...10 hot days...".

    Note that going with just the additional R15 rock wool would indeed meet code minimum (= the lousiest performance that's legal to build) max U0.043 under California Title 24 2016, Section 150 (a) with a modest amount of margin:

    Mind you, U0.043 (R23 "whole assembly") isn't anywhere near the limits of what's financially rational for a zone 3C location. See the zone 3 row of Table 2, p 10 of this document:

    The suggested R50 "whole assembly" would be U0.025, more than twice the performance (less than half the heat rate) of a Title 24 2016 code minimum attic. With R30 fiberglass on top of the R15 batts it won't quite make the BA-1005 table's levels, but it'll be well above Title 24, and beat the IRC 2018's U0.030 (R33 whole-assembly) max U-factor by a bit. With R15 rock wool it would still be over U0.030, despite the R15 thermal break over the rafters.

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