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Is Natural Ventilation Enough to Overcome Moisture Problems?

In the winter, does moisture form in the bottom corners of your windows? Photo:

In the winter, does moisture form in the bottom corners of your windows? Photo:

By Bob Tortorice

Moisture is the silent killer of houses. Just as with cancer, you may not know it’s a problem until it’s too late. But, in a way that’s similar to many cancers, if detected early, a moisture problem in your home is almost always curable. And luckily, with a lot of money your house can always be repaired no matter when the problem is discovered.

Fifty-plus years ago, houses were built so leaky that moisture was never an issue. Even though a family of four will produce 40 gallons of moisture per month, the house was able to dissipate the moisture through the air leaks in the walls. Other than the frost that formed on the windows, no other problem was created by this potential build-up of moisture. Of course, what was known but not addressed was the fact that heat escaped through these same “holes” in the house. At less than one dollar per gallon the “pain” of paying for oil was tolerable. But, since the oil crisis of the ‘70s, oil and gas have quadrupled in cost, giving rise to modern, energy-efficient homes that are built much tighter than those older homes. Unfortunately, the building codes that require builders to build tighter also say, “If the bathroom has a window, then a fan is not required.” The problem with this approach is that nobody opens this window in the winter, when ventilation is at its critical point.

Window condensation is a red flag that there is a problem. Photo:

Window condensation is a red flag that there is a
problem. Photo:

Moisture follows the air through the “holes” in the walls, but when it hits a cold surface like the back side of your sheathing in the wall or attic it condenses, turns to liquid and runs down the wall where it is absorbed into the wood and with the help of fungi and insects, the wood starts to rot.

A simple two-ply test can reveal how efficient your ventilation is:

Take one or two sheets of two-ply toilet paper and, with the bathroom fan running, place the paper against the fan grill. It should stick to the grill; if not, clean or replace the fan ASAP. You’re paying for electricity that’s not doing anything and, worse yet, you think that you’re venting the moisture from your shower, but you’re not.

Other tests you can use to check for ventilation:

  1. During the winter months does moisture form in the bottom corners of your windows? Look for water stains on the windowsill.
  2. Look at the nails in the attic roof, the points protruding through the roof’s decking. Are they rusted? If so, then moisture has condensed on them.
  3. Newer homes are more susceptible to moisture problems than older homes because we are building more energy-efficient homes, but not adding more mechanical ventilation. Don’t believe the builder who says, “I build houses that breathe.” Well-vented attics, bath fans or energy recovery ventilators are the only way to properly vent your home.

Bob Tortorice, Vice Chair of Build Green NH, of the Home Builders & Remodelers Association of New Hampshire and President of the North Country Home Builders and Remodelers Association of New Hampshire, has over 30 years of green building experience. He is the owner of Building Alternatives, Inc. and Alternative Energy Audits in Franconia. To learn more, call 823-5100 or go to

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