Basement Vapor Barrier and Insulation
We’ve written several articles about basement insulation and a cost effective approach to basement insulation using foam board and fiberglass insulation. Since writing those articles we’ve received quite a few questions about when to use a basement vapor barrier and when not to. So we thought it might be a good idea to clear up some of the confusion.
Understanding Vapor Movement
First off you need to think of your concrete (or block) walls as a huge sponge for moisture (water vapor). Over time and throughout seasonal changes in temperature concrete will “dry” out releasing a tremendous amount of water vapor. The adjacent sketch shows an unfinished, un-insulated, un-heated basement wall. We’ve shown arrows that indicate where the water vapor goes as the wall “dries” out.
Depending on the time of year it’s possible that all that humidity in the air will turn around and condensate on the cool concrete surface if the dew point is correct. The point here though is how moisture in the form of water vapor leaves the foundation walls and migrates into the basement space or outside above grade.