Thread: Exposed vapor barrier

Can this type of vapor barrier be exposed in the crawl space. House was built in 89, no instructions on the material.

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IIRC metalized reflective barrier "mylar" is both flamable (smoke and flame index(ies)) and conductive (as in mylar balloons causing power outages, etc.).

Might be another manufacturer's metalized Polyester Film or Plastic Sheet

"Mylar" is a trademark name of Dupont's products, which are Polyethylene Terephthalate (PET) film.

HTH,
H.G.

P.S.

Or were you asking about the un-covered, un-weighted exposed clear (apparent) "poly" on the "floor"?

There was nothing written on it. Thanks

whoops, I was editing and adding a post-script while you were posting M.S., re-check prior post, if you get a chance please.

Nope the walls.

In my area vapor barriers on walls are problematic. I recommend that they be removed because they're often installed in a manner that can cause problems different from those that they're intended to correct. Unfortunately some municipalities are requiring the barriers on the walls without realizing the potential problems.

Originally Posted by Eric Barker

In my area vapor barriers on walls are problematic. I recommend that they be removed because they're often installed in a manner that can cause problems different from those that they're intended to correct. Unfortunately some municipalities are requiring the barriers on the walls without realizing the potential problems.

Do you mind expanding on that Eric? Installed in what way that causes problems?
Thanks.

Bert,

When vapor barriers (plastic sheeting) is used on exterior walls moisture will build up and condense on the exterior side of the barrier in the cold weather. On walkout basements for example, builders in our area will usually put the barrier over (on the inward facing side) of the insulation between the wall studs. Time after time I have found condensation, or evidence of its previous presence, trapped in the insulation, even in brand new homes that have not yet been occupied. The result is mold on the barrier, insulation and wall substrate. Now a question may come up: how does mold thrive on things like plastic, glass, fiberglass and concrete? Actually the food source is the dirt, dust and other debris that can be found on such surfaces.

It's also important to differentiate between a barrier (foil or plastic) and a retarder (kraft paper), one stops vapor transmission and the other only slows it down. Because a building will be subject to stack effect (warm air rising) and varying pressure differentials, the moisture that builds up behind the barrier can transfer into the first floor walls above.

If a barrier were to be used on the foundation it would be best to terminate it under the foundation sill/mud plate to restrict the moisture migration into the wall above. I've never seen that done, at least that I can remember. Try to explain any of this to the run of the mill contractor and you'll probably loose whatever inkling of credibility that you may have had with him. I hope that this helps.

Thanks Eric. Would you say that you see it mostly in new homes or both in new and old. The reason I'm asking; In new homes it makes sense that there would be considerable moisture in the foundation wall and in the framing lumber yet. This could cause condensation.

Bert,
I really don't see the wall/foundation barriers in the older homes. It would be my guess that due to the new homes still having damp lumber and really damp concrete that the condensation issue would be more pronounced. How this practice came into being I don't know but it was definitely from someone who has little experience and understanding of moisture and the importance of having a structure breath.

Bert,
I was digging around for a photo for another thread and came across this one. It exemplifies the problem with vapor barriers on perimeter walls.