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I have an older home (60's era) with rather cold floors. I was thinking of adding fiberglass batting to help with this, but when I went to measure the joist spacing, it seems MUCH larger than I expected.

Spacing between the joists in the photo is approximately 45.5 inches, which seems too large for fiberglass.

Example spacing

James C.
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  • Are you sure about the 45" spacing? That is huge? A picture looking lengthwise along the joists, with a tape measure across the joists would help. – SteveSh Nov 29 '22 at 02:16
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    And no matter what you decide to do, DO NOT use fiberglass insulation in a crawl space. – SteveSh Nov 29 '22 at 02:18
  • While you didn't ask about this, I would start by putting down 6 mil plastic on your dirt floor as a vapor barrier. – SteveSh Nov 29 '22 at 02:19
  • Not disagreeing, but if I'm the one installing all of this, I would prefer to get the insulation up first to reduce the amount of crawling I do on the vapor barrier. – James C. Nov 29 '22 at 02:30
  • especially do not use foil backed fibreglass, – Jasen Nov 29 '22 at 02:55
  • Having learned from (bad) experience, do _NOT_ use fiberglass batts! – FreeMan Nov 29 '22 at 14:52
  • @FreeMan My recent thought was to use fibreglass batts with chicken wire stapled to the beams as support and to reduce/prevent (hypothetical) rodent access. Could you elaborate on why fibreglass is a bad choice here? – James C. Dec 06 '22 at 06:05
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    Fiberglass in damp environments will wick and hold moisture. Whenever the air temperature in your crawlspace (or any surface in your crawlspace) drops below the dew point, moisture will form on any surface, including fiberglass batts. This moisture then wicks up into the bats and doesn't dry out. – SteveSh Dec 06 '22 at 12:33
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    One of the first things I had to do when I moved into my current house (rancher built over a crawlspace) was to have the fiberglass insulation removed from the ceiling of the crawlspace as it had clumped up & was falling down due to moisture accumulation. – SteveSh Dec 06 '22 at 12:35
  • What @SteveSh said. I did that in my place when we were young kids and had just moved in (before the internet knew everything). It was a nasty job pulling all the soggy fiberglass back out... – FreeMan Dec 06 '22 at 12:50

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When the floors are insulated out this way, (west coast) the insulation is held up by wood strips. The strips are typically called "wood lath" like the type used in old homes with plaster walls. It is tacked or nailed or staple in place, starting about a foot from a wall or other starting point and spaced about 2 ft apart. It can be closer to get around obstacles, I would not suggest going any farther apart. The material is 3/8" thick, 1 1/4" long fasteners will hold very well. I used my 1/4" crown stapler powered by air to set the ones I did. Put them up first, then thread the insulation in place.

I would also suggest laying 6mil vapor barrier down first. It is tough stuff, it may get shuffled around a bit, but can be straightened out. You will need to wear a Tyek suit in any case, it is miserable work, and DUSTY..... wear a respirator, a good one, cartridge type.

Jack
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Batts are typically 47" long. Trim them to 45" (friction fit with some compression).

You'll probably need to use the bag the insulation comes in to help hold them up, cut it such that the plastic will span and just staple it. I'd probably also use a 1x4 in the mid span of the 45" to help keep the insulation in contact with the floor. I like rock wool over fiberglass since it is fire proof, mold proof and insulates even when wet.

Your floor assembly looks like post and beam (not joist) and you have 2"+ subfloor boards spanning the beams? 45" seems like a lot for a subfloor to span interested to know how thick those subfloor boards are.

Fresh Codemonger
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  • +1 for "floor assembly looks like post and beam (not joist)", as this not only seems to be the case, but gave me the words to search for to enlighten myself. – James C. Dec 06 '22 at 06:50
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Forget batt insulation. It harbors rodents and this is the usual end result:

enter image description here

You need to:

  • Insulate your rim and band joists with spray foam
  • Attach rigid foam board to the foundation walls
  • Seal air gaps with silicone
  • Apply a vapor barrier over the dirt

The Earth is a natural source of heat. Your crawl space is cold because it's drafty.

MonkeyZeus
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