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5.5″ Mineral Wool Batts vs 2″ Closed Cell Urethane Foam + 3.5″ of Cellulose

jvidamins | Posted in General Questions on

Building a new 2,600SF ranch. Got 2 quotes from my local insulation installer. I originally was planning on insulating the 2’OC 2×6 exterior walls with Rockwool (R-23). They told me they thought a cheaper and better insulating (R-26.4) option would be to spray 2″ of Closed Cell Urethane Foam, then 3.5″ of Cellulose over that. The bid they submitted for the mineral wool option was for mineral wool on the main level and the entire basement (2×4 walls 16″OC) was $30,135. Their bid for the spray foam/cellulose combo (but with R-15 kraft faced fiberglass batts in basement) was $23,003. 

What are your thoughts on which main level insulation strategy is better? I also have Zip R-9 (R-9.6) for my exterior sheathing, so inner wall insulation will be in addition to that.

Ceiling will be R-60 cellulose.

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  1. Expert Member


    No real thoughts on your main question, but noticed both the options show batt insulation in the basement. Unless there is also some impermeable insulation between it and the concrete walls, that's a problem.

    1. jvidamins | | #2

      Oh yes. I forgot to mention that I have R10 foam board insulation installed on the inside of all the basement walls.

  2. Expert Member
    Michael Maines | | #3

    What is your climate zone?

    Do they state what R-value they are assuming for closed cell spray foam and cellulose? I use aged values for spray foam of R-5.6/in, and R-3.6/in for cellulose, for a total of R-23.8, not R-26.4.

    Why not just insulate the walls with cellulose? It would be lower R-value but also cheaper, less complicated, less environmentally damaging, and lower risk of health issues. The ~15% drop in R-value compared to the spray foam or mineral wool options would result in roughly 2-3% higher annual fuel bills, probably on the order of $50/year.

    1. Expert Member
      Akos | | #4

      +1 on this. Go for full fill of cellulose. It should be WAY cheaper than the spray and fill and gets you similar assembly R value. Also since you have ZipR, it means you are taping the sheathing for the air barrier, so the air sealing aspect of the spray foam doesn't matter.

    2. jollygreenshortguy | | #5

      Yup. Zip-R and cellulose sounds like the best way to go. It will be cheap, provide plenty of insulation and use the most benign materials for the natural environment.

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