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Q.I’m building a tight house with above-average attention to air sealing, and I plan to ventilate with a Panasonic exhaust fan running continuously. Will the cracks around windows and doors admit adequate makeup air, or do I need to provide wall vents?

A.Bill Rock Smith, building consultant and former contractor, responds: Studies have shown that even a tight home usually has enough openings in the building shell to provide makeup air for the base ventilation rate of most homes (45 to 90 cfm). Dedicated passive makeup air inlets have been shown to be ineffective, since the fans used for ventilation typically do not generate the high level of negative pressure (10 to 20 pascals) needed to draw outside air through the inlets.

The main concern for your proposed system is not whether the house has enough cracks for makeup air; it is whether the makeup air will be drawn from the wrong locations. Potentially, an exhaust-only ventilation system can cause backdrafting of open combustion systems (fireplaces, water heaters, furnaces, or boilers), or the entry of soil gases into the home. If you plan to use exhaust-only ventilation, it’s important to install a pre-radon mitigation system and to use only sealed-combustion appliances. Before using any open-combustion systems, a worst-case depressurization test of the house should be performed.

For more information on residential ventilation, see Judy Roberson’s article "Choosing a Whole-House Ventilation System" (9/00).