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Moisture questions in insulated shed floor

IsaiahC | Posted in Energy Efficiency and Durability on

I’m insulating a pre-existing shed about 200 Sq ft in Zone 6A (Central Maine) for a friend who wants to turn it into a 3-4 season guest house. He plans to heat with a kerosene space heater or the like. Since the building will only be used, and heated, intermittently, he wants to minimize material cost rather than achieve the highest performance possible. However, our goal is to do this durably and prevent future problems. Our plan is fiberglass bats in the walls (2×4), sloped roof (2×6) and between floor joists (2×8).

My question regards the floor, which is raised on concrete piers and has about 3-4 feet of space under it open on the sides (no problem to crawl in and install under). My tentative plan is to put rigid foam across the bottom face of the floor joists to add some R value and to prevent ground moisture from reaching the fiberglass. Casually reading some GBA articles on insulated floors (; ) seems to affirm this is a good approach. I’m wondering, though, whether moisture from inside reaching dew point is a concern in the floor as it would be in a wall assembly.

Is it important to follow the thickness calculations like in this article for the foam on the bottom face of the floor joists? This would mean at least three inches probably since the batts will be R30, adding to the expense. I had planned to use 1-2 inches of foil-faced polyiso.

Could I avoid the thickness requirement by using breathable EPS on the floor joists so moisture coming from inside can dry to the outside? Or will moisture coming in from below then be an issue? Would laying down a ground cover help? There will likely be snow drifting in so this seems unlikely to work.

Either way, I plan to create a complete air barrier to the degree possible by taping seams (as the shed is preexisting I won’t be able to insulate where the supports are, so there will be some thermal bridging).

I also plan to attach PT plywood on the out/bottom side of the rigid foam to prevent critters. Will this change the breathability issue?

I appreciate any advice.

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  1. Expert Member


    Your plan will work fine. EPS will make the floor more forgiving, but I wouldn't be too concerned about the permeability of whatever foam you use, or the thickness. The minimum amounts are generally only applicable to walls and roofs. The pt plywood also isn't an issue.

    In terms of rodent-proofing the floor. My experience is neither mice nor rats will chew through materials unless they have some indication there is something (food, shelter) on the other side. So they invariably start at cracks between the plywood and a beam, or post. Sealing the edges and joists, or backing them with mesh is good insurance against their attacks.

  2. IsaiahC | | #2

    This is helpful, Malcolm, thank you. What do you mean by more forgiving?

    And then for rodent protection, you're talking about putting wire mesh in behind the rigid foam where the pieces come together and then sealing these seams with tape?

    1. Expert Member
      MALCOLM TAYLOR | | #3


      More forgiving in that EPS will allow any moisture that did get into the floor to dry out - but I'm not sure that's very important.

      It's where the plywood meets the piers that rodents are most likely to attack. Backing them with mesh might be good insurance. The seams in the plywood should be alright as they have nowhere to stand to chew them.

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