Ventilation for Modular Home Crawlspace

What should I recommend for a modular home that has the crawl space closed in. It has 6" poly covering the dirt but there are no vents. Should it be 1 foot of vent for every 150 square feet? Or because of the poly can it be 1 foot of venting for every 1500 square feet?

2018 IRC

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Is it a true “Modular” home or a “Manufactured/ Mobile” home. Could be a big difference for your answers.


It’s a mobile home. It’s on wooden piers and wooden footings. It basically has a plywood skirt going around it.

A mobile home typically has the vapor barrier attached to the underside of the manufactured unit, installed at the factory. This protects the insulation and floor structure from humidity under the home.
You may find that many mobile homes might also have a vapor barrier on the ground. It will typically be only 0.006" thick, not 6".
I would be interested in seeing the wooden piers you mentioned in your second post.
Are you a student or a working inspector?

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Here are the wooden piers under the modular home. It’s not a permanent foundation. You can see lots of surface rust on the I-beams of the chassis. There is a rodent barrier covering the floor joists but the home owner did some diy and cut many holes through it. He did try to tape it up the best he could. There is 6 mil poly covering the ground but it did not extend up the side walls the recommended 6 inches. I’m a long time student ( took two home Inspections certified courses) and just recently a home inspector. This was my first modular home so it was a different experience from regular bungalows with basements.


Your paragraph describing what you see really has you on the right track.

Do you have questions about what is acceptable? What to recommend?


Sean, I agree with Brian. You described it well. Good job.

Here are some suggestions based on your pictures.

  1. The wood piers are a problem. You don’t want to have wood in contact with the ground. Piers should be concrete and no more than 2" of wood shims at the top. Wood in contact with the ground can absorb water.
  2. The skirting appears to be OSB, which also should not be in contact with the dirt because it can wick moisture and rot or support mold growth.
  3. The TPR discharge pipe from the water heater appears to drain to the crawl space. This is not allowed.
  4. I do not see any anchorage in your pictures. Anchorage may be in the corners only and out of view but I can’t see any in the pictures. You may want to check what types of anchorage are present (if any)
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Bert, good points. As for the wooden footings they are pressure treated and are on top of the 6 mil poly. There is definitely to many shims, but the home is stable. The anchors are in the corners out of view. The TPR drain is a good catch, I missed that one.

Thanks, I recommended to the owners to get some ventilation for the crawl space and to attach the 6 mil poly to the side wall skirting.