Insulation and Moisture

Dear NH,
I need to insulate a metal pole barn. I am going to have a wood burner for heat and want to make it fire resistant, but was told that fiberglass insulation would cause a moisture problem and rust out my metal siding. Need your viewpoint. Thanks.

Dear AP,
Because metal surfaces do not 'breathe', moisture can get trapped within the insulation when room moisture condenses on the cold metal. And the more weather-tight the building is, the greater the problem since the moisture can't escape. If the barn is galvanized metal, the rust process will be undeniably slowed but not indefinitely. Add to this the fact that wet fiberglass insulation is not much more useful than no insulation, all your work will be wasted.

The only way to avoid this is to either 1) install a moisture barrier over the insulation such as plastic sheeting or 2) use a type of insulation that has vapor barrier qualities. The only moisture resistant insulation that also acts as a vapor barrier is spray foam.

Spray foam will deaden sound, add to the strength of the structure and keep moisture away from the metal. Spray foam is not fire resistant, so it is important to keep fire away from it. I can only suggest that you consult with your local building inspector on this matter. Generally, these products should be covered with a more fire resistant material such as wallboard.

For my readers, if you don't know what a metal pole barn is, there is a barn builder online at with many photos of complete barn and some construction detail.


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