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Our kitchen floors (hardwood) are freezing, and we found that crawl space insulation below those floors appear to be poorly installed.

First, the insulation appears to be installed upside down, with the vapor barrier facing the crawlspace interior (the label on the insulation says that the paper should be facing up - towards the heated space). Some of the insulation is falling out of the joists too.

Second, the insulation is R-19 at 6 1/4" deep - but it is stapled to the bottom of the
9 1/2" deep joists which (so 2x10s). That means theres about a 3" gap between the insulation and the floor above which I think is probably the cause of the cold floors.

Now, I just want to know whether or not its a bad idea to use this R-30 insulation: https://www.homedepot.com/p/Johns-Manville-R-30-Kraft-Faced-Fiberglass-Insulation-Batt-16-in-x-48-in-K1242/206116298#overlay

Like, if I used R-19 and installed it correctly (so the batting is all the way up against the floor) would that be enough to solve my issue? Would R-30 be even better than R-19 or is there a diminishing level of returns after 19?

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  • What is the material on your kitchen floors?
    – Jack
    Feb 16, 2022 at 1:50
  • "Would R-30 be even better than R-19 or is there a diminishing level of returns after 19?" The answer to those is yes and yes. There are diminishing returns at every increase in R value (R2 is twice as good as R1, R4 is twice as good as R2, R8 is twice as good as R4, R16 is twice as good as R8 and there is better insulation at every increase in R-value - if it didn't cost money and take space, R100 would be common. But it costs money and takes space, so people stop at whatever point they feel is "good enough" or after they have filled the available framing space.
    – Ecnerwal
    Feb 16, 2022 at 1:52
  • Whereas builders who don't intend to live in the home tend to stop at "what's the absolute bare minimum we are required to install by local codes...operational cost means nothing and installed cost means everything if you are building it and letting someone else pay the utilities.
    – Ecnerwal
    Feb 16, 2022 at 2:01
  • @Jack hardwood, just added that the batting is also falling out of the joists.
    – rugbert
    Feb 16, 2022 at 2:22
  • 1
    if it's falling out that's probably the problem.
    – Jasen
    Feb 16, 2022 at 3:04

1 Answer 1

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R-30 is not a bad idea for your floors. I see, by your profile you are in DC, I used to work in that area for a loooonnng time. I think the most important thing is making sure the thermal barrier is intact, whether it be R-19 or R-30. If it were my place, since you are willing to redo all of it, I would save some time and expense and reuse what insulation is in good shape still. Less to drag out and take to the landfill.

The kraft paper facing is supposed to be installed towards the floor, it does not depend on if it is tight to the floor or not as long as the ends are sealed properly from air (drafts) infiltration. To hold the insulation in place you could use either wood lathe stapled or nailed to the underside of the joists or use metal insulation supports, sometimes called "lightning rods" or "tiger paws" depending on who makes them. At least when I was an insulator these were the brand names they were called.

Back at reusing the original. The insulation does not need to be pretty, it needs to fill the cavity from joist to joist, band board to band board. To insure the band board is filled properly, cut the height of the joists 1" longer than the height and stand the pieces vertically in place, with the run that is in the joists butting into that. That way if the insulation has some reason to sag at the ends, there will always be insulation covering all the way up to the underside of the first floor bottom wall plate. After those details are taken care of, you can add another layer of new material to get the thickness you need to get R-30 if that is the goal you want. After the original supply is exhausted, finish it off with all new R-30.

Or if you choose to do away will all the original, then simply install all new material.

If the cost becomes a factor, and you decide to go with R-19, the 3" air space is all it is, air space. When the other details mentioned are done properly. If there are drafts entering from somewhere, it wont matter if it is R-30 or R-19.

More about using R-19. If you use insulation supports, the insulation can easily be set tight to the underside of the floor. If you use wood lathe, the stuffing of the ends of the joists at the band boards will keep the potential problem of drafts at bay and the R-19 can lay on the wood lathe with the air space above the insulation. This has a benefit in some cases if there is plumbing in the floor system, it is simple to let the insulation pass under the pipes, keeping the pipes in the heated area.

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