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Dense pack blown in fiberglass vs mineral wool

Salesi | Posted in General Questions on

My wife and I are building a new house and the topic of dense pack blown in fiberglass verses mineral wool insulation needs to be decided soon. We had the blown in fiberglass at our last house by a contractor and it performed well. However mineral wool, R23, could be a choice that we could install ourselves since we do not have the equipment for the blown-in. Having not lived in a house with mineral wool, I was wondering how it compares to the fiberglass. What I liked about the blown-in fiberglass, besides the insulation, is the sound proofing. Also I think it helped air seal the house since it was so dense. Every part of the stud bay was packed solid, around electrical boxes, pipes, etc. Any comments on how the two compare in real life.

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Replies

  1. Jon R | | #1

    You will often get answers like "DON'T use fiberglass, it isn't as resistant to fire" or "DON'T use mineral wool - it costs much more".

    But the right answer (for many things) is almost always "it depends". For a really correct answer, you would need to quantify the differences and then weight them for your use case. Not easy.

  2. Expert Member
    Akos | | #2

    If your house is already well air sealed (which should be a priority for new construction), that benefit of dense packing becomes irrelevant.

    There is a lot of miss information about the noise benefits of dense pack walls, the reality is dense packing provides slightly less sound attenuation than batts (dense pack "connects" the inside and outside sheathing increasing sound transmission). The difference is small enough that for all intents and purposes it doesn't matter. If you want truly quiet house, reasonably sealed drywall over resilient channel or a double stud is the way to go.

    With 2x6 24oc walls, the R value of the insulation doesn't matter all that much. The difference between an R19 and R23 batts in the overall insulation value (R19 vs R17) and actual energy use is not that much.

    I'm a big fan of cellulose. If you are dense packing, you can get a pretty high R value wall for relatively cheap by cross strapping your 2x6 with either 2x2 or 2x3 on edge before blowing the insulation. This gets an overall assembly of R27 with 2x3 on edge, with not much extra insulation cost.

    Overall, I would go with the easiest insulation that you can get a good install with. That matters more than the type of insulation.

  3. Expert Member
    Dana Dorsett | | #3

    >"Any comments on how the two compare in real life."

    How they compare "...in real life..." may depend on how carefully the batt installer sculpts and fits the batts around electrical boxes, plumbing, and wiring, etc., and the quality of the air sealing job prior to insulating.

    Air sealing with polyurethane caulk or other purpose-made sealant & tapes is necessary for either, but more important when using batt insulation. That type of air sealing is superior to counting on the dense-packed fiberglass to tighten up the cavities by clogging the exfiltration paths (which it does, but nowhere near as tightly as goop & tape.)

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