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Community and Q&A

Why do my clapboards have ice forming on them?

Fred Wildnauer | Posted in Energy Efficiency and Durability on

I recently resided part of my house because of moisture and insect damage. I took the opportunity to replace windows and add insulation. I sprayed foam in part of the house where I had to remove damaged sheathing and covered everything with 1″ foil covered rigid foam (all the seams were taped with foil tape). I then applied strapping over the foam as per as detailed in an article in September 2010 of Fine Homebuilding and then nailed the wood claps to the strapping figuring that the resulting space would allow any moisture to drain along the face of the rigid foam and out the bottom. We had our first hard frost last week and I noticed every piece of strapping was visible because frost had formed on the exterior surface of the claps everywhere except where the claps contacted the strapping. Is this good or bad?

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  1. GBA Editor
    Martin Holladay | | #1

    Neither good nor bad; it just is.

    The ventilated air gap cools the back of the siding, so the areas of siding directly over the air gap are the coldest. That's where the dew or frost is likely to collect on cold mornings.

    The wood strapping acts like insulation, keeping certain areas of the siding a little bit warmer. So the areas of the siding directly above the strapping aren't cold enough to allow the formation of dew or frost -- at least under certain conditions.

    Don't worry about the phenomenon. It doesn't last long.

  2. Riversong | | #2

    If, as it sounds from your description, you sandwiched new sheathing between impermeable spray foam and even more impermeable foil-face foam board, then you've created a double vapor barrier that may lead to future moisture problems and degradation of the sheathing (particularly if it's OSB).

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