Ask SAIL: Cabin Insulation

I would like to add some insulation to the cabin sidewalls, because we sail in Maine, where it often gets chilly. Is there a way to do this without developing a future mold problem?
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CABIN INSULATION

Q: I am restoring a 1963 Bermuda 40. The interior has all been removed and refinished. Before I reinstall it, I would like to add some insulation to the cabin sidewalls, because we sail in Maine, where it often gets chilly. Is there a way to do this without developing a future mold problem? This boat had a severe mold problem, which I have cleaned up, and I do not want to have to repeat the job in the future.

George Clark, Raymond, ME

DON CASEY REPLIES

In theory, all you have to do to prevent condensation from forming on the interior surfaces of the hull and deck in cold weather is to keep your cabin heat away from those surfaces. You can do this by lining the hull and deck with an adequate thickness of fiber, foam or bubble insulation so as to isolate them from the interior space. Unfortunately, it is not easy to perfect this separation. Anywhere warmth finds it way behind the insulation, condensation will form, particularly on a solid fiberglass boat like the Bermuda 40. Over time, mold is likely to follow.

A more reliable solution, particularly as you now have the interior surfaces exposed, is to use spray-on urethane foam, like they do on steel boats, where condensation can lead to disaster, not just discomfort. This type of insulation bonds to the hull and deck surfaces, perfecting the separation required to prevent condensation. Spray foam is messy and toxic, so it is best applied by a professional. If you elect to do it yourself, be sure you have the proper safety equipment.

Another relatively easy and reliable option is to use a self-adhering closed-cell elastomeric foam, such as Armaflex. This type of product comes in sheets and rolls, and goes on like thick tape, making complete coverage mostly a matter of determination. Note that even if you do a perfect job insulating the interior fiberglass surfaces, condensation will still form inside portlights and on metal fittings. Here you need to catch the drips or improvise some temporary insulation.

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