Indoor Air Quality
As stated earlier, SIP houses will normally be tighter than average homes. The question here is sometimes asked, "At what point should you stop tightening so the home isn't too tight?" The easy answer is: build as tightly as you can practically build, then add some type of mechanical ventilation system for background ventilation. Before describing those systems for your SIP house, a few words about a home's key air pollutant.
Carbon monoxide: In today's new homes, the most serious indoor air quality concern is carbon monoxide (CO). Concerns about virtually every other potential indoor air pollutant all tie for a distant second. Most properly functioning gas water heaters and furnaces don't generate CO during their burn cycle; however, if combustion gases are drawn down open flues into the house during a burn cycle, and those combustion gases are reburned, they can rapidly generate dangerous levels of CO. The easy (but not cheap) answer to this problem is to install sealed combustion appliances. (Also see Heating and Cooling section.)
CO can also enter a home through connections between the house and the garage. This can happen for 30-90 minutes after a car has started up and out of the garage. The strategy here is to seal the house off from the garage.
For the record, the materials in SIPs do not contribute significantly to the multitude of other pollutants of concern in new homes today. Those pollutants-formaldehyde from exposed particle board, volatile organic compounds from some paints and finishes, off-gassing from some carpet padding, etc.-are best dealt with by simply not installing the offending products in the first place. Where they are present, some type of background ventilation strategy will reduce the potential problem.
Ventilation strategies. First, provide spot ventilation in kitchens. Since gas stoves tend to generate CO during their first several minutes of operation, plus release other combustion gases that should be vented, any gas stove should have an exhaust fan installed that vents to the exterior.
Your lowest-cost whole-house ventilation system consists of an upgraded bath fan. Pay upwards of $100 to get a quiet unit-one that is rated for continuous use and that generates less than 1.5 sones of fan noise. Rated capacity varies, typically between 70 cubic feet per minute and 110 cfm. Mount this unit in the attic and connect it to an upper floor bath. Keep duct length short, and minimize restrictions in the ductwork; this may mean installing straight pipe instead of flex-duct. You can put a 24-hour timer on the fan such that it only provides you with the level of background ventilation that calculations indicate you need-perhaps 8 hours/day, perhaps more. Note that this unit puts the house under slight negative pressure-yet another reason why you should opt for sealed-combustion gas appliances.
Your other lower-cost whole-house ventilation system is a little more complicated. It relies on a special controller, called the AirCycler, plus an outside air duct hooked up to the return air on your centrally ducted heating and/or cooling system. Each time the blower delivers heated or cooled air, it draws in some outside air (putting slight positive pressure on the home) and circulates it throughout the home. During extended periods when the thermostat doesn't call for either heating or cooling, the AirCycler will turn on the HVAC blower for a limited period-say 5 minutes twice an hour. The duct to the exterior (typically 6 inches round) should have a damper for a one-time adjustment of the flow; some people also recommend a small in-line filter box/slot at the spot where the round duct joins the larger return-air plenum. This system may not work as well in extreme climates.
A more controlled and more expensive whole-house approach relies on a central exhaust-only ventilation system that vents from all baths, kitchen (but not above the stove) and elsewhere with just one fan (e.g., American Aldes). The fan is mounted in the attic, hooks up like an octopus (conceptually) to all the ventilation grills, runs very quietly, operates at variable speeds, is controlled by a 24-hour timer, and is equipped with a booster for extra flow from baths during showering. (The same issue about negative pressure and sealed combustion appliances applies, only slightly more so in this case.)
The most expensive whole-house system consists of a heat-recovery ventilator (HRV--sometimes known as an air-to-air heat exchanger) connected to full ductwork run throughout the house. Details about installation are readily available from the numerous manufacturers. When properly installed and operated, it offers homeowners the most control of their ventilation.
The caveat "when properly installed and operated" applies to all the systems described above. Since mechanical ventilation systems have been installed infrequently in most parts of the country, effective design and installation are critical to efficient provision of healthy air within a home.