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Tighter building envelopes require improved ventilation

Use of the bathroom fan may have been an adequate method of ensuring ventilation in homes in the past, but today's tighter building envelopes often call for a more effective strategy.

As building standards start to require improved energy efficiency and tighter building envelopes, builders must also consider ways to ensure adequate ventilation in their homes.

The primary challenge is venting moisture that builds up internally.

"There are three primary sources of moisture internally in the home," says Dave Ricketts, president of RDH Group, which specializes in building enclosures. "There's human breathing and evaporation from the skin, transpiration from plants, and the way the home is used – showers, cooking, laundry. You need to dilute that moisture or you may have problems with air quality, and you can create condensation that can lead to mold."

"Traditionally, the thinking was that natural ventilation caused by leaks was good enough to ventilate the home," says Doug Overholt, Power Smart New Home program representative. "But in reality, that only works well in the periods when you have cold temperatures or warm ones outdoors.

"In the broad shoulder seasons of spring and fall, when you don't have a large temperature differences between indoor and outdoor, the air does not actually move in a predictable manner. That's when you can run into problems."

With leaks sealed, air quality can suffer

With tighter envelopes, the natural leaks that occurred in older construction are sealed, so the home can become pressurized or depressurized, depending on relative temperatures and/or use of one-way ventilation systems such as exhaust fans.

This can lead to air being drawn into the living space from dirty areas such as the crawl space, compromising air quality, or it can cause moist air to be forced into wall assemblies. Tighter envelopes, therefore, require better approaches to ventilation.

Bathroom fan's not enough

Ricketts says the "lowest level of technology" that builders have typically relied on for ventilation is the bathroom exhaust fan. With tighter envelopes, however, simply drawing the stale air to the outside is insufficient – a supply of fresh air and distribution throughout the home is required too.

"Otherwise the air can smell, you can have elevated humidity and condensation, and you risk problems with mold, as well as back-drafting and carbon monoxide buildup," says Ricketts.

The solution: balance, control, heat recovery

In some homes, builders have added passive fresh air intakes that allow fresh air to enter whenever an exhaust system is turned on. Often, however, these stay open all the time, driving up operating costs.

"We're paying more and more for energy, so why should we have it just leaking out of the house?" Overholt comments.

The key to ventilation is balance – allowing adequate fresh air to enter to replace the stale air being vented, keeping the home at an even pressure. Ideally, says Overholt, this is achieved with ventilation controls.

"These systems have a motorized damper that can be controlled, so when an exhaust fan turns on, it sends a signal to the damper and the supply side opens, and when the exhaust turns off, it closes. Moisture can also trigger the exhaust, so when the interior air reaches a certain relative humidity, it turns on the fan and opens the supply."

The heat recovery ventilator option

Overholt says the "Cadillac" system is a heat recovery ventilator (HRV). These run the exhaust and supply air systems past each other (without mingling the actual air) so that heat is transferred from exhaust air to the incoming air, pre-warming it.

HRVs offer excellent efficiency by reducing energy waste, but they do add in cost. And since they include a filter and drain, the homeowner needs to know about maintenance requirements. Overholt says builders who use them should ensure they are featured as a selling point of the home.

"When I talk to builders, I say it is an opportunity to communicate to your customers about the benefits: 'You're getting your air filtered, your supply air warmed; we've put it in because it's more energy efficient, and it's going to lead to you having better indoor air quality.'

"It is up to the builder to mention this value, otherwise it's just a sunk cost."

Whether you opt for HRVs or simply focus on fresh air supply, as you tighten your envelopes, remember to plan for balanced, controlled ventilation to maintain the building quality you designed and built for.