I need to install fiberglass batts under my (raised) house. I have read that the paper should generally face the warm side, so it depends on where one lives. I live in South Louisiana, so I plan to place the batts with the paper facing outside. But there are clear instructions on the cover of the insulation packs that say that the paper should not be left exposed, so it will need to be covered, but with what?
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What is your reasoning for paper down? The region you live in supports that choice I presume?– JackAug 12, 2019 at 2:35
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Yes, I live in New Orleans, where the weather is mostly hot and humid. It can get cold in the winter, but on average the hot days are more common than the cold ones.– ValerioAug 12, 2019 at 3:29
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1My experience is the paper goes on the side where you are standing when you install it, so you can staple it. If one side already has a surface installed, then that choice is made for you. E.G. Doing your crawlspace the paper goes down whether you live in Guyana or the Yukon. Of course there's no such thing as too many vapor barriers!– Harper - Reinstate MonicaAug 12, 2019 at 4:16
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1There are wire insulation supports for that purpose when the vapor barrier is truly needed to go to the heated side.– JackAug 12, 2019 at 5:28
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Check with others who have installed it in N.O. and elsewhere in South Louisiana and find out what the result has been. IMHO modern a/c units are so much more efficient than older units that the incentive to insulate the floor is less than formerly.– Jim StewartAug 12, 2019 at 10:22
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At the least i would cover it with 1/4" plywood. At the most I would cover it with 4X8 sheets of cement board. Depending on the moisture present under the home, would temper the choice further.
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Then I would not use the plywood unless it is pressure treated and that will most likely not be found 1/4" thick, only 1/2" and thicker. I would go with the cement board, unless the pressure treated plywood is cheaper– JackAug 12, 2019 at 5:25
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Thanks, I will look for treated plywood or cement board. There are a couple dozen R-30 batts that I had installed recently after removing the paper (because I was told it wasn't needed in hot climates). Should I re-do those, or is it OK to leave the fiberglass without the paper, and just cover it with plywood or cement board?– ValerioAug 12, 2019 at 17:09
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You should be ok to leave it as it is, the material you will use to cover the insulation will keep the moisture from blasting through and creating problems.– JackAug 13, 2019 at 4:26