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Community and Q&A

Confirm or deny my thinking on this vented “crawl space”

user-1091831 | Posted in Energy Efficiency and Durability on

I recently took a quick look at a guy’s mobile home. He has vinyl skirting around the base of the home to conceal the underside along with 1″ of rigid foam behind that skirting. During the summer he removes a couple panels of skirting on each gable end to ventilate the underside of the home. Upon poking my head under the home, I noticed condensation on the structural beams, confirming my suspicions that this ventilation would cause a problem and may also be attributed to the rust forming. I explained to him how the cooler space was causing warm humid air to condensate on the steel. I suggested that he re-install these panels to minimize the amount of humid air to this space. However, he argued the following:

“It’s only been one year with the blue-board insulation inside the skirting. Without the venting, mold would be a possibility even though it’s on a slab. The rust is 24 years of not having any insulation, [and] not one year of venting during the summer. The condensation is more a result of continuous rain and abnormally high humidity. Condensation was not a problem last summer after we completed the set up as it is now. A constant air flow will dry it out.

My thinking is that the recently added insulation is now keeping this space below the home cooler than before. Now I’m just a young guy, but after having read several articles on GBA about the pitfalls of a vented crawlspace, I stand by my thinking when I say he should close those openings off, figuring that this type of enclosed space below the mobile home would act similarly. Being in climate zone 6, I realize the humidity is not as big a concern as it is down South on the east cost. However, I wonder if leaving it ventilated as is poses more or less of a long term risk to moisture accumulation than by closing it back up since it still wouldn’t be air sealed tightly. My question is, who is more right here? Thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    The answer depends on site conditions. Does the site have good drainage away from the slab in all directions? Or is surface runoff directed toward the slab? Is there any poly under the slab?

    In general, it's best if the area under the mobile home is either entirely open to the breezes -- with no skirting at all -- or sealed up tight and insulated. If the latter option is chosen, you want good drainage and poly under (or over) the slab.

  2. user-1091831 | | #2

    Thanks Martin. It sounds like one extreme or the other is the best choice, where as halfway measures don't work, as you point out in your article. I had also recommended that the skirting be sealed up tightly to improve the effectiveness of the insulation, but I hadn't put my two recommendations together to make complete sense of it. Thanks again.

  3. Expert Member
    Dana Dorsett | | #3

    There may be code issues with the XPS being applied without an interior side ignition barrier, or at least some intumescent paint sprayed on it to inhibit fires from starting or spreading. A mobile can go up pretty fast if the foam ever got going, since polystyrene melts and spread in a sticky puddle while burning. (Light some off with a torch on a concrete slab sometime!)

    Polyiso (particularly fire-rated Thermax) would be a safer material for that app, with a higher kindling temp, and a char rather than melt characteristic while burning. (Polyurethane is comparable to polyisocyanurate by those measures.)

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