living lightly

From the start, we intended to have an air-to-air heat exchanger in our new home. Regular exchange of inside (stale) air with outside (fresh) air is necessary in any building to maintain good air quality. Bringing in some fresh air and exhausting moisture, odors, and gasses that accumulate minimizes problems with moisture and maintains good air quality, without opening a window. This transfer of air between inside and outside causes a loss of heat in cool weather and an increase of temperature during warmer weather and affects the inside temperature.


Regardless of more stringent building codes, a standard, stick-built house has numerous cracks and crannies that allow exchange of inside air with outside air. An owner doesn't have much control over this. In the numerous vent holes and spaces around pipes and wires, there is an abundant transfer of air, regardless of the weather.

However, when done according to specifications (mastic between and tape over all seams), SIP construction greatly reduces air leaks and heat transfer. So in a home like ours, with SIP floor, walls and roof, seams sealed, we needed to balance the need for fresh air with loss of heat on cool days. After all, we are seeking efficiency in every way. Also, we have hydronic radiators, not a ducted, forced-air heating system.

We wanted and specified to our contractor a cross-flow, air-to-air heat exchanger , which is a heat recovery ventilator for the entire house. It would transfer heat from the outgoing indoor (stale) air to the incoming outside (fresh) air. This results in significant savings in the cost of heating or cooling a house and conditions the inside air. And this influx of fresh air is controlled: the fan is relatively low power but runs continuously, or on a timer, or other schedule.

Our general contractor's adamant refusal to install a heat recovery ventilator wasn't just strike one in our relationship with him. At that time, we rationalized that there wasn't all that much efficiency and cost saving of having one. But what we noticed later was that there was no provision for whole-house ventilation and that the construction of our house would not meet the Washington State Ventilation and Air Quality Code.

Too late by our judgment, but nevertheless, after attempting to gain compliance with our requirements, we terminated his contract (for numerous reasons) and with a house half complete we began making the best of a difficult situation.

Our own contractor now, we talked with a heating subcontractor about meeting the code, and most importantly, maintaining indoor air quality. We agreed on a relatively simple solution. We installed four vents at critical places in the outer walls. And we programmed the heating control system to cycle the two bathroom exhaust fans on and off on a regular schedule to push stale air out and pull in fresh air through those vents. We also measure indoor relative humidity and CO2 levels, and when either of these rises past s suitable level, the control system runs the fans until the level subsides.

Our results so far:

  • Overall RH stays below 60% in warm weather
  • Winter RH ranges from 42 to 50%
  • CO2 levels rise only when there are lots of people in the house, which is seldom

Given the values indicated by the indoor sensors, we feel our solution is working well. We are tuning the schedule for cycling the fans. Given there is no way to reduce indoor RH when outdoor RH is above 90%, we are considering a dehumidifier for the spring and fall times when the outside temperature is above 50 degrees.