A quick review why indoor air quality stinks: Buildings often get their so-called “fresh” air by infiltration. Drafty old homes exchange air through their building shells, picking up generations of collected dust such as pollen, dander, lead-paint and asbestos. Fortunately, they tend to leak so much the dust is diluted; however, the air becomes very dry with relative humidity (RH) often below 40 percent. RH between 40 and 60 percent is ideal for human health. Below 40 percent, viruses like influenza and measles thrive and our mucus membranes dry up making us more susceptible to respiratory infections. The flip side is more efficient homes tend to trap dust and chemical off-gassing. Moisture is trapped, too, and RH can climb above 60 percent allowing other viruses, fungi, bacteria, dust mites and molds to infect our air.
So how do I mimic a healthy living body and improve my home’s indoor air quality? In a perfect world I would make my vintage home air-tight and continuously provide outdoor air through a filter and exhaust the moist, stale, polluted air. For Portland and Seattle this air control is ideally done with a heat recovery ventilator (HRV). Like a lung, an HRV is a whole house ventilation system that both filters and conditions incoming air. The best practice is to install the system to provide continuous air to the bedrooms and extract air from the bathrooms. Efficient HRVs use less energy than a 20w light bulb and lose less than a degree or two of air temperature even on the coldest days.
Additionally, I will reduce leakage in our home, which currently leaks its entire air volume 25 times per day. The tighter the better, but from experience a reasonable target for an old home would be to tighten to 6 times per day. This will be accomplished by air-sealing accessible areas of the home, primarily the floor system and attic. We also plan to clean out, air-seal and convert our fireplace into a new home for plants. Our leaky old windows are in major disrepair, so we plan to replace them as well. To minimize adding chemicals into the home, I plan to use a combination of wool, cellulose, advanced tapes and low VOC sealants to insulate and air seal.
Finally we get to remove the monster lurking in our basement: a 30-year-old gas furnace with an octopus of asbestos wrapped ducting. The plan is to upgrade to minimal duct work in our conditioned attic that our new Mitsubishi heat pump and HRV will share. The heat pump now replaces the last combustion appliance in our home eliminating any concern of combustion air, or natural gas leaking into our air.
A couple simple tricks will help ensure best results for your air quality: I plan to balance the HRV to have slightly positive indoor air pressure to help ensure our fresh air comes from our HRVs filter and is not sucked in through the walls. This also helps to keep Radon from seeping in. The smart control for the HRV will respond to interior RH and change ventilation rates to help balance RH in the healthy zone.
Source: ASHRAE Transactions 1985 Vol. 91 Part 1