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Q&A Spotlight

Do All Houses Need Mechanical Ventilation?

With a limited budget, is it acceptable to allow some airflow through the building and skip the fans?

Finding air leaks: A blower door test can identify leaks in the building envelope that increase heating and cooling costs.
Image Credit: Green Building Advisor/The Energy Conservatory

A central aim of energy-efficient building is to eliminate air leaks through the roof and exterior walls. A leaky building envelope not only makes it harder to heat and cool a house but also increases the risk of condensation and moisture damage.

Builders are getting the message about air sealing. But the tighter the house, the greater the need for some type of mechanical ventilation — and that raises construction costs. Is it possible to build a house with just enough air leakage to satisfy fresh air requirements without a ventilation system while still reaping some energy rewards?

This is basically the question Karen Leu grapples with in a recent post — and the subject of this week’s Q&A Spotlight.

Leu works for an Arkansas-based energy-retrofit program that aids mainly low-income families. She wonders what’s the best use of a limited budget: building to ASHRAE’s Minimum Building Airflow Standard and skipping the ventilation, or aiming for more rigorous air sealing and accepting the extra expense of ventilation?

What’s tight enough?

One camp argues there are no exceptions to the mechanical ventilation rule. “All new homes should have a mechanical ventilation system — even those that are trying to include ‘just enough air leakage’ to supply fresh air accidentally, through the effects of wind and the stack effect,” says GBA senior editor Martin Holladay. “Depending on random air leaks to supply fresh air is nuts.”

Inadvertent air leaks are greater on windy days, or in winter when the stack effect is strongest. But, Holladay adds, what about windless days in spring and summer? It’s not enough to hope that homeowners will crack their windows at the right time.

Holladay, of course, isn’t alone here. But under the circumstances — a retrofit rather than new…

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  1. Gareth Jones | | #1
    This is 2 step process the process as under -

    Crush the bag flat and hold the opening tightly over the exhaust hood. The air flowing out of the hood will inflate the bag. Time the inflation. If the bag inflates in eight seconds or more, go to Step 2. If the bag inflates in less than eight seconds, turn the HRV to a lower speed, and repeat the test. Then go to Step 2.

    • Step 2:
    Swing the bag to inflate it and hold the opening against the wall around the HRV supply hood. The air going into the HRV will now deflate the bag. Time the deflation. If your HRV is balanced, air going into the HRV will balance the air coming out of the HRV. The inflation and deflation times should be roughly equal. If you find that the bag inflates twice as fast as it deflates, for instance, your HRV is unbalanced. If you can see no problem with the filters that would cause such an imbalance, you should call a service person to test and adjust your HRV.

  2. Forrest Fielder | | #2

    mechanical ventilation
    This balancing effort between the energy conservation and indoor air quality camps is still unsettled. Though California is moving to ASHRAE, the 2012 IECC (residential) still allows natural ventilation. This much is clear - there is an energy penalty to bringing in unconditioned outside air, and the routine opening and closing of doors and windows is how most homes are actually lived in. At a minimum, I'd prefer that the ventilation decision be the owner's (exhaust fans/windows/doors), rather than a one-size-fits-all requirement.

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