Air-tightness and healthy homes should go hand-in-hand, however, certain old habits often get in the way. People need fresh air, but we don't want that air to be filtered through every crack and fungus-filled crevice in our home. The solution is to build a very tight building envelope, then provide the right amount and type of ventilation where it is needed. All of this is for naught if it is not operated correctly, so perhaps that is where you should start, if you are considering tightening up your home. If you are having a new home built, or considering tightening up your existing home, start by religiously using your bathroom and kitchen exhaust fans, every time you are in the room, or every time you are cooking something. Even though your existing fans are noisy, and you have to open a window to allow fresh air into the house instead of having smoke come back down the chimney, you will start developing the habit of operating your fans when and how they were intended to be used. You will also probably develop an even keener appreciation for the new quiet fans your new (or newly remodeled) home will have. You will also experience some un-intended consequences, like odors that seem to come out of the walls when the fans run without opening a window first. This should give you an extra appreciation for the quality air you will experience in your new, tighter home.
Air-tight walls are not just about keeping the heating (or cooling) energy in your house, they are also about keeping bad stuff out. Every time a fan runs, or the wind blows, pollens, mildews, fungus spores, and any measure of other nasty stuff is being filtered out inside your old leaky walls. Over time, this stuff builds up, and some of it finds the often-moist environment inside your walls very attractive. It can grow and multiply, and lead to a very un-healthy home. Tight walls do not allow anything to get in, it is all filtered out on the extreme outer or inner surfaces of the walls. The new tight house is fitted with newer quieter and more energy efficient fans, and it incorporates a supply of make-up air for each exhaust fan employed. Ideally each fan has it's own powered source of make-up air, as would be provided by a Heat Recovery Ventilator (or Energy Recovery Ventilator, depending on your climate zone), but there are a number of ways to provide the correct balance of air exchange.
ASHRAE 62.2 is the ventilation code specifying the correct amount of fresh air needed for the average home. It makes certain assumptions regarding occupancy, you can use those assumptions to adjust your air-flow accordingly. For example, 62.2 assumes that two people will occupy the master bedroom, and one person each for the other bedrooms. It specifies 7.5 cfm (cubic feet per minute) of air flow, continuous, for each occupant of the house. If you have more, or less people residing in your house, you can adjust the continuous flow accordingly. 62.2 also specifies 1 cfm for every hundred square feet of conditioned floor area, so a 2,000 square foot home should have 20 cfm in addition to the 7.5 cfm per person. A three-bedroom, 2,000 sf home, using this formula, would need 50 cfm of continuous ventilation. This may seem like a lot, but consider that the average house leaks about two or three times that much, and it really isn't much.
In mild climates, or during mild times of the year, there is nothing at all wrong with turning off the mechanical ventilation system, and simply opening windows. You may not get the same level of filtration supplied by your built-in HEPA filter, but there is nothing quite like feeling that first fresh, warm breeze of spring (or summer) blowing right through your house. When the air comes through planned openings in the house, like windows and doors, it is not creating the petri-dish situation inside your walls mentioned above. A tight house allows you to control the amount, and the source of all the air coming into your home.
There are a number of building methods, including SIPS (Structural Insulated Panel System) and ICFs (Insulated Concrete Forms) that are inherently tight. Conventionally framed homes can be made tighter by adding foam sheathing layers on the outside, or through the use of house wraps and tapes. Often lots of time is spent on tightening up the wall assembly, yet little thought goes into the under-floor area or the ceiling and attic. Every hole is a hole, and they are all important. You wouldn't go out in a boat if the captain told you it only had a few holes in the hull, would you? There are often some huge holes in floor assemblies, such as under a bathtub or shower. These should all be plugged, caulked, or foamed-in as appropriate for the size and configuration of the hole. For a retro-fit, the under-floor can be one of the most challenging areas to address, as it is often really wet and nasty in the crawl-space. If the floor was previously insulated, the old insulation will probably need to be removed in order to seal the floor diaphragm above it. Do not lament that the removed insulation will need to be discarded, it is probably full of molds, pollens, and rodent droppings. Once you have air-sealed the floor, and re-installed new insulation batts, consider adding a layer of foam sheathing underneath your floor joists. This will help to air-seal the cavity, and might prevent rodents and other pests from making their home in your nice warm floor insulation.
While you are in the crawl-space, make sure you have air-sealed and fully insulated any ducts what are down there. The energy code only requires ducts in crawl-spaces to be insulated to R-8, but when you think about it, the ducts are tasked with carrying the warmest air in the house from the furnace to all the other rooms; why would we not want the ducts to have the best and most insulation of anywhere in the house? If you can, create a foam insulation shell around your ducts, keeping them on the warm side of your floor insulation, so that the only heat loss is to the inside of the house, not down into the cold crawlspace.
Apply a similar thought process to your attic, sealing cracks around lighting fixtures, with special attention to recessed can lights, and other larger penetrations. Remember that the tops (and bottoms) of walls are full of holes where wires and pipes penetrate, and those are all open to the inside of the house where electrical receptacles or plumbing fixtures are located. If you work to seal up both ends, even on interior walls, you will have a better chance of at least getting one end sealed well.
Think of the shell of your house as being much like a human body: it should be air and water tight, and it only breathes through intended openings; in through the nose, out through the mouth. The nose has a built-in filter, and it is intended to be used on intake. Let your healthy house breath as intended, only through the correct openings!
Ted L. Clifton is a designer-builder from Coupeville, Wa., with over 45 years of hands-on experience in the construction industry. His two companies, Zero-Energy Plans LLC, and CVH Inc. have won five Energy Value Housing Awards, and two National Green Building Awards for Concept and Research. Ted has been closely involved with the development of both his local Built Green program and Built Green Washington.