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Uninsulated cottage floor cupping

Nick_rudnicki | Posted in General Questions on

Clients cottage located in rural Nova Scotia. Built on posts. Floor not insulated. Building not heated when not occupied (most of the time) We installed solid birch flooring, and sanded and clear coated as a finished surface.

Client had asked about the need for vapour barrier between subfloor and flooring. Based on my research at the time, it seemed like this was not necessary.

Now, however, the floor is showing signs of cupping. 🙁

This leads me to wonder, was the vapour barrier the culprit on this one? Or the thermal stress of the floor not being insulated, while the indoor space was?


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  1. GBA Editor
    Martin Holladay | | #1

    Hardwood floors exhibit cupping when the underside of the floor assembly is at a higher moisture content than the air above.

    Installing a layer of foil-faced polyiso on the underside of the floor joists (installed in an airtight manner) will tend to isolate the floor assembly from outdoor humidity swings.

    Then you have to wait for dry weather -- and perhaps operate a heating system or air conditioner to help everything dry out. Hopefully, the flooring will settle down.

  2. Expert Member

    If it's any consolation I've seen severely cupped floors flatten out once the moisture problems below were solved. I'd bet Martin's advice will solve your problems.

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