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How do I properly seal the underside of an addition built on concrete piers in New England?

palacedays01 | Posted in Energy Efficiency and Durability on

I have an existing conditioned space addition that was built on concrete piers in a zone 5, New England climate. The floor structure was insulated with fiberglass and covered with OSB.

The OSB is rotting and the fiberglass insulation is black. All of that material is being removed. What sheathing and insulation materials would you recommend as replacements in this totally exposed floor?

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  1. GBA Editor
    Martin Holladay | | #1

    You have several options:
    1. Spray polyurethane foam.

    2. Some type of fluffy insulation -- dense fiberglass batts, mineral wool, or dense-packed cellulose -- in the joist bays, with a layer of rigid foam to create a continuous layer of insulation under the joists. The seams of the rigid foam insulation should be taped with housewrap tape or another high-quality tape. Then the rigid foam should be protected with a layer of OSB or plywood.

  2. LucyF | | #2

    I am planning on building a home on piers in the humid south (upstate SC). I was wondering the same thing. Do you air seal the underside of the floor with the same attention to detail you do the ceiling? What about access to mechanical stuff, wiring, ducts, etc?

  3. GBA Editor
    Martin Holladay | | #3

    Doug makes a good point. If the soil level is high, you may need to lower the soil level. Just be sure that everything drains away from the house -- you don't want any puddles under the house.

  4. user-723121 | | #4

    My guess is the OSB rotted because of the close proximity to the soil under the addition. We saw a lot of this in the Twin Cities with wood windows in split entry homes. The lower level windows were rotting in about 10 years time and the upper level windows were still in fair shape. The higher humidity level near the ground and also spashing from rainfall caused premature window failure.

    I would second Martins's advice, blanket insulation, rigid foam insulation on the underside of the joists and some type of moisture resistant covering over that. Make sure to air seal. This type of room can be very comfortable providing the insulation and air sealing is top notch.

  5. palacedays01 | | #5

    Doug and Martin,
    Thanks for the help. The air space under the addition is approximately 36" and does slope away from the house foundation, however one mistake the previous builder did was to drain the rainwater from the main house to the area under this addition which was enclosed with a plywood backed lattice skirt. Both of these details will be changed to allow air circulation for drying.

    Martin, I am assuming you are recommending at least 2" of XPS applied to the joists and taped at the seams and perimeter before sheathing and skirt boards are applied?

  6. GBA Editor
    Martin Holladay | | #6

    Two inches sounds good; more never hurts. I would use polyisocyanurate, not XPS, because polyiso is more environmentally benign.

  7. palacedays01 | | #7

    Thanks Martin, I'll make a note of it.

  8. user-659915 | | #8

    I'd recommend fiber cement board rather than OSB/plywood. Better critter protection.

  9. wjrobinson | | #9

    Skip the OSB. Use pressure treated 1/2" plywood for final protective layer.

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