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I have a vapor barrier on the dirt floor already. The insulation will go in the floor joists above, paper side down.

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"When fiberglass batts are installed in the ceiling of a crawlspace, gravity always wins in the end" source

Faced or unfaced, it is almost always a bad idea to insulate the floor above a crawlspace, especially with fiberglass batts. Insulate the crawlspace walls and seal the vents instead. The one exception is if you are in a flood zone.

A sealed crawlspace will be drier, healthier, and usually more energy efficient than one insulated at the floor.

crawlspace with falling batt insulation[1]

  • That's nasty. Never had this particular problem. It's all about (well, mostly about) the cost. Fiberglass, rock wool and even sprayed in foam insulation are less expensive then a concrete rat pad. – DavidC Aug 31 '15 at 21:04
  • @littleturtle- I've insulated floor joists in crawlspaces on 2 separate jobs and neither ended failing. What the reference source you posted didn't account for was to fill the joists completely and meticulously. No gaps nor compressions. If warm air doesn't leak from the thermal boundry into a cold crawl space no condensate is produced. The insulation remains dry and intact. I admit that for the two crawl space jobs we added 2" polystyrene to cap the joists after filling them with R-19 batts. Also, it's not always economically possible to insulate the walls under the house. – ojait Sep 1 '15 at 0:10
  • @littleturtle-But I do agree with your answer in regards to making the crawl space as insulated as possible. – ojait Sep 1 '15 at 0:12
  • @DavidC, I'm not talking about a concrete rat pad. I'm talking sealing the vents and insulating the crawlspace walls instead of the crawlspace ceiling. You could use fiberglass, rock wool, rigid foam, or spray foam (in order of increasing cost), but regardless, an unvented crawlspace will be drier and have fewer moisture issues – littleturtle Sep 1 '15 at 19:15
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    @ojait, yes, putting the rigid foam below the fiberglass is a good solution if you really need a vented crawlspace. The foam is a much better vapor retarder than the paper facing. Nice diagrams and discussion of that and other solutions here: buildingscience.com/documents/insights/… – littleturtle Sep 1 '15 at 19:18
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The problem often overlooked is that the paper vapor barrier on the insulation should be oriented to face towards the room. Also, the paper must be in complete contact with the floor bottom to function correctly. So now the only way to secure the insulation in each joist bay is with metal push rods. Try to fill each joist bay fully and completely.

  • Paper side up. Is this incorrect? I was thinking paper side down and then tacking the paper on each side of the insulation to hold the insulation up against the floor. I have 2x10 joists and want r30 insulation. – DavidC Aug 29 '15 at 21:22
  • Using the what, David? – ojait Aug 29 '15 at 21:24
  • Whoops. Typing on my phone. Hit enter by accident. Check the edited version out now. Thx! – DavidC Aug 29 '15 at 21:28
  • No problem. The object of the paper face on insulation is to stop moisture from entering the wall cavity (thus "vapor barrier"). Insulation with paper face is always positioned towards the warm (in winter) side of the room. But that may not be the case if you live in a warm and humid location. Fill each bay to its fullest and cap with polystyrene panels screwed to floor joists. – ojait Aug 29 '15 at 21:53

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