Air Leakage
Buildings have air entry and exit — air leakage — that may be far beyond what is needed for ventilation. Ventilation means many things. We ventilate to bring air (and oxygen) inside buildings so we can breathe. We also ventilate the indoors to remove air contaminants, even if we are just clearing the bathroom of moist air after showering.
Air leakage is not under our control and excess or inadequate ventilation may occur in a residence. On a windy day, a home may be excessively ventilated (especially if no one is around), while on a quiet day it may not be adequately ventilated when filled with people. Four people in a 2000 sq ft house would call for 50 CFM (cubic feet per minute) of ventilation by health and industry standards*. This is equal to the capacity of a typical bathroom fan. Some people ventilate their house reliably by leaving the bath fan on.
Using the Air Seal and Ventilation Guide
The Air Seal and Ventilation guide (“Cost of Air Leakage vs. Multiple of Ventilation Standard”) shows air leakage cost as the multiple of the cost for fan-forced exhaust that would meet a ventilation standard. This means it’s OK to seal the house and install a fan that has the capacity to exhaust air to meet industry standards*. In this scenario, the air leaks that remain now become the ventilation supply air opening. Air leak savings is the difference between having several times as much air (on average) as you need for ventilation, versus having ventilation to an industry standard, which will require tightening so that air movement can be placed under control of a fan.
For other guides, you might have some idea what the present cost of energy use might be. In reality, house air leakage changes moment by moment as the wind blows, and each house on the block is different. As a rule, older houses are leakier than new ones, houses with a fireplace leak more than those without, and windows, even leaky ones, usually don’t add up to more than 10% of the total house air leakage.
Rather than guess where the whole house leakage might be, it may make more sense to establish your ventilation need. Then work toward tightening the house so that a ventilation fan can “take over” and move the amount of air recommended for ventilation regardless of the direction of the wind.
For Example: Assume your house is similar to a four-square and has three times more air loss than the ventilation standard would call for, meaning it has a multiple of 3 on the horizontal axis. Find the 3 and go straight up to the red line, where is says $341. Find the cost of the ventilation standard, or “1.” For the four-square, which is $149.. Subtract the one from the other and you have the expected heating cost reduction of controlling your air leakage by having ventilation instead. In this case it is $192.
Note that the Air Seal and Ventilation guide shows cost of ventilation including the cost of natural gas to heat the air exhausted, plus the cost of electricity to operate a quiet Energy Star certified fan.
Heat Recovery
It is possible to “grab” heat from exhaust air and transfer it to supply air without mixing the dirty exhaust air with the incoming air. In fact, incoming air can be cleaned as it is sucked into the house by a “Heat Recover Ventilator”(HRV). A line below the basic exhaust ventilation cost shows the 75% cost reduction possible by using an HRV.
Also notice the cost associated with controlling the HRV according to occupancy, and additional tightening. There is up to 50% savings, although it is important to realize that, by setting the exhaust fan off when no-one is home, air infiltration will occur using half of the leaks as entry points and half as escape paths. Eliminating air leaks will produce savings with ventilation fan turned off, however, in older housing, eliminating air leaks is not realistic. At least you can control the amount and direction of air leakage by converting air leakage to planned ventilation.
*The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) ventilation standard (62.2) calls for 7.5 cubic feet of air per minute for each person, and 1 cubic foot per minute for every 100 sq ft of conditioned floor area.
