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Insulation options for open post-and-pier foundation

QuestionI recently built a lake cabin that sits on a post-and-pier foundation. The floor joists are 12 inches, and the distance from the bottom of the floor joists to the ground varies from 2 feet to 7 feet. We have no plans to skirt the foundation's perimeter so the crawlspace will stay wide open to Mother Nature. What's the proper way to insulate the floor joists? Is a vapor barrier required? Assuming we installed some kind of insulation under the floor, what is the best way to rodent-proof the floor? We use the cabin spring, summer, and fall. Thank you.

AnswerI'm jealous; my wife and I would love to have a lake cabin! And I think you're on the right track in wanting to insulate the floor. It can get chilly in the spring and fall.

A good starting point would be to consult the local building department or electric company or cooperative for their guidelines. I do know you will want to use fiberglass batts with an asphalt-coated Kraft-paper or aluminum face to create a vapor barrier. Install the insulation with the vapor retarder facing the living space.

There are two schools of thought regarding the underside of the subfloor. You could insulate the floor joists using 9 1/2-inch thick R30-faced batt insulation held in place with insulation supports, i.e., galvanized chisel-pointed wires in various lengths for use between 16- or 24-inch on-center joists and available at your local home center. The air space above the 9 1/2-inch insulation in the 12-inch joist cavity will help provide a warm floor inside the cabin. Alternatively, you could fill the cavity completely using R-38 faced batt insulation. It will provide additional insulating value for not too much more than what would be invested in the R30 insulation. Again, install the insulation with the face up against the underside of the subfloor.

To help protect the floor assembly from moisture and rodents, recruit a volunteer to help you cover the underside of the joists with 4x8 sheets of 3/8-inch exterior soffit material. This is not the time to go macho—if you don't already own a pneumatic gun, rent one to install the soffit material. In the limited work space you describe, this tool will make your life a lot easier.

Then use an exterior caulk to seal all areas where the soffit butts against beams and against any siding or exterior finish surface that (hopefully) extends below the floor joists and covers the ends of the soffit material and leaves room to caulk. Proper sealing will help keep out any pesky critters!

If you are concerned that soffit material alone will not adequately protect against rodents, then consider attaching 1/4" x 1/4" square wire mesh to the underside of the floor joists before installing the soffit material. Rodents will have a heck of a time gnawing through the wire mesh unless they carry a pair of wire cutters with them or a reciprocating saw with a metal blade. If they do, they must be some big rodents.

With the underside of the cabin open to Mother Nature on all sides, it isn't necessary to create a moisture barrier. However, if you want to prevent the growth of weeds, cover the ground under the cabin with landscape fabric and then use river rock, pea gravel, or crushed gravel to hold it in place.

If (like me) you hate to install insulation, consider hiring someone for this phase of the project. Another option would be to consider a sprayed-in foam insulation product such as Icynene (www.icynene.com). Foam insulation provides both a high R-value and a thoroughness of coverage that greatly increases the airtightness of homes, although floors are not usually the culprit in heat leakage. You'd still want to install the solid soffit material to protect the installation and the cost would probably be twice as much as the cost of fiberglass batt insulation.

You might want to read an interesting article titled "Crawlspace Design & Construction" found on the Sounthern Pine Council's Raised Floor Living website and Nathan Yost, M.D.'s article "Before You Design, Build or Renovate" found on HUD's website.

In the interests of quality control, you might also want to hire an independent inspector to perform an inspection for this project or—at the very least—maintain a photo record of the work performed. Good luck, and as much as I wish we had a lake cabin, I'm glad it's you and not me doing this work!

Copyright © 2004, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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