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The Problem

Water vapour is always present in the air. The amount of water vapour that air can hold depends upon its temperature. Wood always contains water. It constantly exchanges water vapour with the air, picking it up when atmospheric relative humidity is high, and giving it off when relative humidity is low. When the amount of moisture fluctuates, wood will expand or shrink.This natural behaviour of wood is responsible for some of the problems sometimes encountered when wood dries and is what causes gaps in wood flooring, uplift on roof trusses, doors to close improperly, floors to squeak, miter joints to separate on casings and baseboards to come away from the walls (this last item is generally a result of the wall studs shrinking not the mouldings).

Older houses built before the 70's are "naturally leaky" - enough air finds its way into the house during the winter months through various cracks that the air is fresh but is also affected directly by the humidity of the outside air. Today, houses are sealed so well that mechanical ventilation is required. They are now controlled versions of the "naturally leaky" house. During the winter, mechanical ventilation (HRV) removes stale air and introduces fresh, dry outside air. In the winter, this air is extremely dry - and can drop the relative humidity in your house to the point where it can do damage to the finishes of your house and furniture and under extreme conditions even do structural damage. Radiant floor heating is becoming increasingly popular and is creating another menace to baseboard mouldings, creating a hot dry climate at floor level.

Indoor relative humidity under these conditions can drop to a desert like 15% during cold periods and rise above 75% during humid summer months. Moisture content of wood casings swing from 4% to 16% and 6" wide casings can grow more than 5/32".

Recommendations

Install high quality mouldings that have no more than 8-12% moisture content. At Mountain Moulding Ltd. we use only lumber that has been kiln-dried to 6-8%. It will pick up a bit of moisture again after that but we monitor the wood to ensure that it does not exceed the 8 -12 % moisture content before it leaves our plant. Use products like finger - joint pine that have a mix of grain orientations within a single piece so one part constrains the movement of the other.

In a new house there are many sources of humidity - curing concrete, curing drywall cement and drying paint just to mention a few. The longer you can let your house dry before installing trim the better. When you do take delivery of your wood mouldings, allow them to acclimatize inside your house for 2-3 days before application. Do NOT store mouldings directly on a concrete floor, not even on blocks as concrete holds a large amount of moisture and cures for several weeks.

Don't run your house too dry in the winter. Your millwork and wood flooring will suffer. Maintain a relative humidity of 35-50%. This range is healthy and will help keep your wood looking good. We highly recommend that you invest in a central humidification system or a standalone humidifier if the first is not possible. Also a digital hygrometer, an inexpensive device available at most hardware or building supply stores that measures humidity and temperature. The hygrometer will show the actual relative humidity in your house, enabling you to fine tune. Remember that the investments in a hygrometer, humidifier, and dehumidifier are relatively small compared to the investment in your home and furnishings.

There is one small snag - moisture on your windows. As the temperature outside drops to -20 C, the inside pane of glass in a window becomes cold enough to cause condensation to form on the inside pane & start running down the window. Similarly, the colder it gets outside, the worse the problem becomes. We suggest that you simply put up with the inconvenience of occasionally "mopping up" your windows - it's all part of the responsibilities of home ownership. Don't run your HRV more often in order to reduce the inside relative humidity of your house. Even over the short period of a week ( about the maximum cold spell in Eastern Ontario), reducing the inside relative humidity can have adverse effects.

Keep indoor plants - they will improve air quality, especially during the winter.

Read the manufacturer's manuals for your heating & ventilation systems & become familiar with the routine maintenance procedures, such as air filter cleaning &/or replacement.

Understanding Your Heat Recovery Ventilator (HRV)

The primary function of your HRV is to provide fresh air to the home during the heating months when windows are generally closed. Warm, moist, stale air is taken from the home at exhaust points, - typically in kitchens, bathrooms and laundry areas - passed through the heat exchanger in the HRV & exhausted outside the house. At the same time cold, fresh air from outside is drawn through the HRV's heat exchanger where it is partially warmed. This air is then directed into the cold air return on your heating system where it is further warmed & distributed throughout the home. This air can be extremely dry.

During the heating season, turn the dial on the HRV to the lowest setting continuous speed setting available. The fan on the HRV will now run full time, ensuring a continuous supply of fresh air in the house. Running at a higher speed may lead to humidity conditions that are hard to control. Watch your hygrometer carefully during the winter - maintain 45% RH.

Timers found in the bathrooms & kitchens override the fan speed on the HRV to exhaust humid air & offensive odors quickly.

Turn your HRV to the off position (do not unplug) during the summer. The timers will still activate the HRV, causing it to run for the specified time. Open windows will supply all the fresh air required.

The primary automatic controller for the HRV is a dehumidistat usually found on the wall beside the heating systems thermostat. If the relative humidity in the home rises above the specified setting (minimum 35%RH), the fan on the HRV will run at high until the level drops to the appropriate level. The primary automatic controller for the heating system's humidifier is a humidistat, also found beside the thermostat.

If you have both a humidistat & a dehumidistat as controllers, use these settings so that the HRV & humidifier are not in conflict with one another:

Humidistat (humidifier) - 45%RH (ideal - 35% min.)
Dehumidistat (HRV) - 5% higher or 50 %