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