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What’s the best way to control moisture in a cottage?

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I am in the process of building a cottage for my family, but I do not intend on installing a mechanical ventilation system. I'm considering not installing a vapor barrier on the ceiling as a means to reduce moisture inside. My concern is that when the warm moist air contacts the cold air in the attic it will form condensation in the fiberglass insulation, which may eventually lead to mould problems. What are your thoughts?
—Sterling Parsons, Newfoundland

Your concern is a very valid one. You are correct: No vapor barrier will result in moisture damage and mould in the ceilings. In short, you must have a vapor barrier installed on the “warm in winter” side of any wall or ceiling separating a heated space from an un-heated space.

Now that that is out of the way, let’s look at your root concern of moisture.

There are two kinds of moisture to deal with in a cottage. First, introduced moisture from showering, cooking, etc, and second latent humidity. The former is seldom a problem with cottages that have a proper bath fan and range hood installed and properly vented outside, and even ones that don’t are usually okay, as people going in and out of the cottage will swap out the air.

The latent humidity, which is the moisture that is naturally in the air, is what makes some cottages smell musty after being sealed up for long periods. A common approach is to either crack a couple of windows in the non-heating months or install closeable passive vents in the basement and upper floors to allow cross-ventilation of the space. In winter months you shouldn’t have a problem because the relative humidity in the air is very low. Also, if you are planning on using the cottage in the winter, simply leaving your heat set on low, which will also keep pipes from freezing, will keep things even drier.

Ryan Shervill

Ryan Shervill is a field editor and contributor to Outdoor Canada, as well as an avid hunter and shooter. He is also Canadian Home Workshop’s online Ask a Pro and contributing editor.

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Jul. 16, 2012

10:48 am

When I built my cottage many years ago, I followed a contractor friend’s advice an installed a 4” x 10” vent low in the structure. I incorporated a heavy ¼”x ¼” galvanized hardware cloth screen plus a bug screen to keep out varmints and insects. I covered it on the inside with a standard furnace floor grate. I also build a frame to hold a screen for the attic access panel. When I close up the place for winter, I make sure the low vet is open and replace the attic access pane with the screen. Convective heat flow created by the sun heating the roof and through the windows is enough to keep the building nice and dry.

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