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How does one prevent metal floor joists from transferring moisture/cold through a floor?

I'm trying to find any articles regarding how to prevent metal floor joists from transferring moisture/cold through the floor. Vapor barrier? Insulation? Please help! Thanks.

This topic is regarding a tiny house. The plywood floor will sit on top of a trailer. Most of the floor framing will be set inside the bed of the trailer, but at least two beams will be in direct contact with the floor/deck plywood. I need to make sure those 2-4 beams will be insulated to prevent any transference of moisture/temperature into the house through the floor. The trick is making sure that the plywood decking is able to be securely fastened to the beams in many places while still maintaining the integrity of whatever barrier is sandwiched in between. The insulation in the floor framing set into the trailer bed will be loose wool. Help? Suggestions?

Asked by Lara Mitchell
Posted Sat, 02/08/2014 - 17:54
Edited Sun, 02/09/2014 - 06:25


1 Answer

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It's usually not a good idea to put insulation between your framing members, because framing members act as thermal bridges.

It sounds as if you plan to use steel beams to support your floor, and will also have (wood or steel) joists in addition to the beams. On top of this you will probably have plywood subflooring.

The best place to install your floor insulation is above your subflooring. I suggest that you install EPS or XPS rigid foam insulation on top of your subflooring. Then you can install a second layer of plywood on top of the rigid foam, and you can fasten the second layer of plywood through the rigid foam to the underlying (lower level) of plywood with long screws.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 02/09/2014 - 06:22

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