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Community and Q&A

Cupping floorboards

Roger Steinbrink | Posted in General Questions on

I’ve been working on a house in New Orleans off and on for 5 yrs., and just recently my client informed me that all the floor boards from the front of the house to roughly 10 ‘ back are cupping across the whole front of the house.

It’ s a raised (4′) home built in 1830.There is batt insulation underneath held in by chicken wire, with about a 1″ gap between the insulation & the floor. No subfloor. There is no water intrusion from any other source. The batts are bone dry.

This seems to have occurred over a 3 month period, which coincides with the duct work being re-done in the attic. I’ve had the HVAC guys come out with moisture meters, and they’re telling me that the relative humidity in the house is 60%. We checked all the joints, and everything seems to be tight.
They have no idea what’s causing this, nor do I. Any thoughts, or ideas?

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  1. Kevin Dickson, MSME | | #1

    The house is at 60% RH, at an average indoor temperature of say, 75F.
    During the last three months, the outdoor RH averaged 75% at a temperature of 80F (I'm just guessing). That may be enough of a difference in absolute humidity to cause the bottom of the floorboards to swell relative to the top.

    The next step - measure the water content of the wood on the top and on the bottom. These gauges are cheap:

    The cupping should lessen over the winter because the outdoor absolute humidity goes down. But once the boards cup, they usually don't go back to flat when the they dry out.

  2. GBA Editor
    Martin Holladay | | #2

    To keep the flooring from cupping, you need to keep the flooring indoors, in a stable, climate-controlled environment. Right now, the bottom of the flooring is outdoors, exposed to high outdoor humidity during the summer, while the top of the flooring is indoors, exposed to dryer conditions. Of course the flooring is stressed and cupping.

    To bring the flooring entirely indoors, you need an air barrier under the flooring. And you need to install a layer of insulation that is not air-permeable, to keep the bottom of the flooring at the same temperature and humidity conditions as the rest of the indoor environment.

    There is no subfloor. (Plywood subflooring would be good, because plywood is a vapor retarder.) There is no air barrier whatsoever under the flooring -- just fiberglass batts (which, like a furnace filter, are air-permeable) and chicken wire.

    The usual method of insulating this type of floor is to install a continuous horizontal layer of foil-faced polyisocyanurate insulation under the bottom of the floor joists. The seams of the polyiso should be taped, and the perimeter of the polyiso should be sealed with caulk, canned spray foam, or high-quality tape. Two layers of polyiso with staggered seams is even better.

  3. Kevin Dickson, MSME | | #3

    Definitely make sure that you have solved the problem before you sand the floor, or the boards will crown when they dry out.

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