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Attic insulation: Mineral wool batts or blown-in cellulose?

Jay | Posted in General Questions on

I’m in NJ, right on the border of Climate Zone 4 and 5. 1600 sq ft attic currently R-13 batts with lots of voids. I’m planning to airseal and reinsulate over the next month or two and need some help deciding between mineral wool or cellulose. In an ideal world, I’d take out all the existing insulation, clean up, install baffles, air seal, and then blow in insulation.

However, since I am DIYing in my spare time, mineral wool will let me go a few joist bays per night. In the next 3-5 years, I plan on gutting and rebuilding two bathrooms and a kitchen that are below the attic. Moving mineral wool seems a lot easier than moving cellulose and then reblowing. I also might be installing a ducted mini-split in the attic at some point. Comfort and current energy costs (oil heat) make this worth doing before future work.

Ceiling joists are 2×4. If I go mineral wool, I plan on doing a R-15 layer to get to the top of the 2×4 and then running R-23 or R-30 perpendicular to joists to minimize thermal breaks.

Should I just bust my ass and do blown-in or go with the mineral wool batts? Also, based on my climate zone, would I need vapor barrier or am I okay with just a (drywall) air barrier and good airseal work?

-Jay

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jay,
    You don't need a vapor barrier.

    Cellulose would be the preferred way to go, except for the fact that you want to be able to do the work a little bit at a time. So feel free to use mineral wool in light of your circumstances. Careful installation, with as few voids as possible, is the key to good performance.

    At the very end of the job, if you still have a few bucks in your bank account, you can blow 4 inches of cellulose (or more) on top of the mineral wool as a cap if you want. But you wouldn't have to.

  2. Expert Member
    Dana Dorsett | | #2

    Does the utility or NJ state subsidize weatherization these days (other than for low-income under federal programs)?

    I seem to recall a time when a DIY blown cellulose attic job in NJ was more expensive than contracting it out and cashing in the state & local rebates (that's still true in my state, MA), but that may have been under a particular utility or township.

    The material cost of cellulose is cheaper than rock wool batts, and if you have a buddy willing to wear a dust mask for a weekend day a 2-person crew can usually knock out 1600' of R50 in a long afternoon with a rental blower, assuming the chutes etc were already set up . Box stores typically throw in 1-2 days of free blower rental with a minimum purchase. The laborious part is really all the air sealing and setting up chutes and depth gauges.

    With 2x4s for attic floor joists you may have max loading capacity problems before adding ANYTHING up there! It may or may not be possible to convert the rafters & joists into an adequate truss structure, but you can probably sister on another set of 2x4s, or even 2x6s or a even deeper webbed-truss floor joist structure using OSB gussets with 2x4 top chord.

  3. Brian P | | #3

    I'm not a professional, but a couple points about your post:

    1. That's great you're going to do air sealing work on the ceiling drywall, it's just as important as the insulation that will go on top. If you're going to remodel rooms after this attic work, try to remodel without compromising all that air sealing work. Keep ceiling lights and other penetrations to a minimum (or just use wall lighting). Or if you have the height, consider a small cavity space below the drywall for the remodel electrical or other work.

    2. With a vented unconditioned attic, you don't want to install duct work up there. If you go with a ducted air source heat pump, try to keep the duct work inside the conditioned space....or try to make it work with ductless mini splits.

  4. Robert Opaluch | | #4

    Advantages of mineral wool batts (in addition to easy for DIY and phased installation you mention) includes fire resistance and sound attenuation. (Do you need those advantages?)

    Mineral wool batts and blown cellulose are heavier than fiberglass batts and blown fiberglass. Tradeoff considering the cost and labor of beefing up your ceiling joists. Mineral wool batts are not cheap, either (tho I like them for the initial layer or for walls where R-value/inch matters).

    I would reuse the fiberglass batts you remove. Just put them on top. Even if not in good shape, provides some insulation value for free (except your labor).

  5. Jay | | #5

    Thanks for the replies (and sorry it took so long for this response). My ceiling joists are actually 2x8, so that helps with the weight situation.

    Dana,
    I checked into NJ programs last year. Planned to convert to Natural Gas and weatherize. The rebate amount differs, but they also provide a 10 year, $10,000 loan at 0% interest (http://www.njcleanenergy.com/residential/programs/home-performance-energy-star/benefits-and-incentives).

    I may still go that route for the insualtion, but I didn't get a great feeling from the companies i spoke with. For example, the rep that came to inspect the house spec'd a Navien combi boiler (I forget what size) to replace my existing oil fired boiler. He never measured the total length of hydronic baseboard that was installed in my house and told me that short cycling was never an issue (this was right after I had read a bunch of GBA threads where you specifically talked about proper sizing of Navien combi boilers).

    I'll keep everyone posted on where this project ends up and will try to log my time spent doing this.

  6. Expert Member
    Dana Dorsett | | #6

    The random recommendations for boilers & other mechanical systems is sadly par for the course for HVAC companies. Since you have a heating history on the place by which to establish an upper bound for the heat load and can probably afford a tape measure to add up the baseboard lengths, you are in a better position to be able to specify the equipment than an HVAC company's drive-by sales-droid. Figure out what it's going to take, then either solicit bids for exactly what you want, or push back and make your own recommendations on equipment with the contractors, and tell them why you believe it's the right fit.

    Contractors who will listen to you will get your business, and you don't really want any part of those who don't listen.

    So, run your own napkin-math numbers ahead of any discussions with contractors:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/sizing-modulating-condensing-boiler

    Even though air sealing and insulating will lower the total heat load number a bit, it won't change the amount of radiation the boiler is dealing with. Unless yours is the leakiest house in NJ a somewhat lower "after upgrades" whole-house load isn't likely to change the range of boiler choices.

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