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I need to insulate under a floor that has hydronic radiant heat. I am going to use fiberglass batts between the joists of the floor, with the kraft vapor barrier upward (toward the warmer area). The area under the joists is an unvented crawl space.

I have a good bit of R-30 batts, 10 1/4 x 16, left over from another project. I would like to use it in the spaces, which are a true 6 inches deep (combination of old full-sized lumber and engineered beams). I could simply strip off about 4 inches of the fiberglass, but I hate handling that stuff any more than necessary. If I compress the batts into the 6 inch space, am I significantly compromising the R value? I realize it won't be R-30, but will it be less than R-19, which is the common value for 6 inch insulation?

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Yes, and no. This table shows you some idea, although it doesn't appear to show an actual number for 10 1/4 compressed to 5 1/2. Extrapolating, I'd guess you'll end up somewhere around R-22, which is still higher than R-21 for typical 5 1/2" batts.

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  • I plotted the R-values from that table for R-30 insulation; compressed to 6" it's more like R-22 than R-24 but as you say, still better than R-21.
    – Niall C. ♦
    Commented Nov 20, 2015 at 16:50
  • Nice! Thanks for the plot, I edited to say 22 instead of 24.
    – DrewJordan
    Commented Nov 20, 2015 at 16:52
  • Feel free to incorporate it
    – Niall C. ♦
    Commented Nov 20, 2015 at 16:52
  • Did it. Two days in crawlspace hell. Let's hope the toasty feet carries over into the winter months.
    – bib
    Commented Nov 23, 2015 at 22:24

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