Vapor Barrier in Crawlspace Insulation

By Paul Bianchina

Q: I'm originally from Norway and I've seen several Norwegian Web sites that say you should cover the underside of the insulation in your crawlspace with exterior-grade plywood. However, the American sites say to leave the bottom of the insulation open or to cover it with a non-tight covering. I tend to trust the Norwegian way of doing things because they have to deal with such crazy weather, but I want to make sure I'm not missing something here if I don't do it the "American" way.
--Rune I.

A: Air will naturally move from a warm area to a cold area. The warm air in your house wants to move toward the colder crawlspace, and as it does it will take any moisture vapor it contains along with it. For that reason, when installing floor insulation, the paper or foil face of the insulation is installed facing up, against the underside of the subfloor, to create a vapor barrier that prevents most of that moisture from ever reaching the insulation.

However, that floor insulation vapor barrier still has gaps in it, so some moisture vapor will still get into the insulation. Having the insulation open below - facing the crawlspace - allows that moisture to escape. On the other hand, if you have added exterior-grade plywood below the insulation, which has several moisture-impermeable glue layers in it, you have now created a second vapor barrier below the insulation. Any moisture that gets into the insulation is now essentially trapped there, and if it builds up in sufficient quantities it can cause potentially serious structural and mold problems.

One last thing - when you insulate your floor, make sure that you add a vapor barrier over the ground, and that any water pipes in the crawlspace get insulated as well.

Copyright 2006 Inman News