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I was in an unconditioned ventilated crawl space where extruded foam panels were installed under the joists and the gaps were sealed with foam spray. Batt insulation was encased in the joist bays.

Is this not the same error implications as installing batt insulation with the vapor retarder facing the wrong way? IE: potential for moisture to be trapped in the batt insulation.

The owner said the installation was part of the cash for caulkers program.

What do you think of this practice and what do you think they were trying to achieve? The panels were 1/4 inch thick equaling R1 value.

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What do you think of this practice and what do you think they were trying to achieve?

I think it's well intended but I think the installer is unaware of the potential problems of sealing the floor joists up like that.

I don't understand why folks can't just put down a complete vapor barrier, add some nice 2" polystyrene, add unfaced batting at the perimeter and just close the vents for the really cold months in their region. It seems to be working great at my house. We don't have anything growing on our framing and the floor is plenty warm in the winter.

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There should be a vapor retarder between the sub-floor and finish floor so if it's indeed there then my guess is that it depends on the play between the two retarders especially if the R-value in the batts is high and the winters in your area are cold and damp enough. Dow high performance panels are between 2 and 3.5 perms, making it a class III retarder.

Of course, I'm a southerner so what do I friggin' know.[;)]


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It would have to get cold enough on the inside face of that foam board to cause moisture migrating out of the house to condense.

The floor joist bay might stay conditioned just enough to make condensation a non-issue. Most of your interior moisture will be driven through vapor diffusion upward into the attic space and outward into the exterior walls because those areas will be colder and drier than the floor joist cavities.

Most crawlspaces are relatively warm compared to outside, even in your climate. Unless the crawlspace gets cold enough to cause the inside face of that foam to cool moisture migrating into the joist bays to dewpoint, you aren't going to have any moisture issue and the vapor is going to diffuse gradually.

Check around with the energy gurus in your area to see what they've been experiencing.

Heck, since Dr. Joe L. an email and ask him to toss an opinion at it. The guy never misses a chance to castigate someone for dumb moisture-related issues. He might be happy with it or upset enough to mount a full-scale assault on the folks in the State of Maryland.



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John's climate is fairly warm, even compared to here, so I think the risk is low, but get the wrong occupancy going on and you could have trouble. A friend of mine rented his house to a family with a lot of kids, who never use the bath fans or range hood, have a couple of dogs, and cook a lot. After 20 years of no problems... now he has them.

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