To read more about indoor air quality from Pretty Good House: A Guide to Creating Better Homes, go to the author's website.

The planet is heating up, faster than we thought it would. As this chapter was taking shape in 2021, the Pacific Northwest in the U.S. was not just breaking but destroying heat records. Portland, Oregon, hit 113°F. Temperatures in Portland, Maine, were well into the 90s in June, a rare occurrence. Historically in Maine, the ocean breezes, the mountains, the tree cover, and, of course, the northern latitude help keep summers mild. Even in August, average highs in Portland rarely hit 90°F, and average lows are often below 60°F. But summer heat advisories are becoming increasingly common.

This obviously points to the need for immediate action on global warming, but it also means that many of us are including or retrofitting air conditioning (AC) in our projects, often for the first time.

As with heating, the best first response for a PGH is diminishing the load (see Chapter 4 for more info). Overhangs that block summer sun but let in lower-angle winter sun, deciduous trees or vines that only block the sun when there are leaves, thoughtful use of windows, shades, and low-e coatings, and, of course, insulation and air sealing will all keep your cooling load down. Simple things help: opening the windows at night to flush the house with cooler air, then closing them during the day to keep the hotter air out; choosing a lighter roof color in hot climates to reflect heat; and using fans to cool off individuals rather than the whole house.

If it’s really hot for days (and nights) straight, even   a PGH will sooner or later heat up uncomfortably.  If you’re heating with air- or ground-source heat pumps, you already have a source of air conditioning. Essentially you are just reversing the operation of the system, capturing heat from inside and releasing  it outside. As with heating, a cooling system needs to be designed to match the load, and the design criteria may not be the same.

All AC units are, in fact, heat pumps. The big outdoor units you are used to seeing are doing essentially the same work as the outdoor units of a mini-split. Their efficiency has improved significantly in the past decade. They pair well with forced-hot air systems, using the same ductwork. The systems that aren’t heat pumps are typically called evaporative (or swamp) coolers, and they use the cooling effect of evaporating water instead of refrigeration. They are simpler, cheaper, and more efficient than heat pumps in dry climates (and are fairly easy to make yourself), but much less effective and can worsen humidity.

If you need to cool only one room (like a home office or bedroom), a window unit can make sense. As with other systems, make sure you air-seal well and change or clean the filters. EPA’s Energy Star is a good guide to units’ efficiencies.

However you cool your PGH, be  aware   of  humidity. As we know from the classic example of summer condensation on a soda can, warm, moist air   will condense on a cold surface. The magic number here is the dew point. A function of relative humidity and temperature, the dew point is the temperature at which water vapor will condense into liquid on a surface. And we know that condensation inside your house can cause all sorts of problems, mold and rot chief among them.

To stay above the dew point, you can either keep the house hot enough (above the dew point temperature) or drop the humidity (thus lowering the dew point). In a hot, humid climate, the summer dew point can be above 70°F, and at 90°F and 72% relative humidity (which can happen in humid parts of the U.S. in summer), the dew point temperature is 80°F.

Additionally, at the warmer end of the range that humans like, humid air is much less comfortable than dry air at the same temperature. We cool our body by sweating: it is much easier for our bodies to get rid of that sweat through evaporation in drier air. “It’s not the heat, it’s the humidity” is, in fact, exactly right.

Designing, Commissioning the Mechanical System

For the most part, a Pretty Good House seeks to simplify high-performance building by providing rules of thumb and guidelines. When it comes to heating and cooling systems, however, rules of thumb don’t work very well. Some form of energy modeling is necessary. Building codes usually require ASHRAE Manual J calculations to determine room-by-room heat loss. The results can be accurate, but the calculations often are fudged to make things easier for the supplier or contractor. Oversizing equipment is often not desirable, as heat pumps work most efficiently near their maximum capacity and their efficiency drops off dramatically if oversized. Oversized air conditioners can’t dehumidify the air effectively.

No matter which system you choose to heat, cool, or ventilate, designing the system carefully is critical. If you have enough heat capacity to  meet  the   absolute  coldest temperature you’ll ever hit, which might be only 15 hours per year, your system will be oversized the other 364 days and 9 hours. Instead, we use what’s called a design temperature. It is typically a temperature linked to  a percentage of the year on which it will occur.

For example, the ASHRAE manual lists Portland, Maine’s design temperatures for heating as -1.7°F (-18.7°C) / 99.6%, and 3.2°F (-16°C) / 99°. This means that, on average, the temperature is above -1.7°F 99.6%  of the year, and above 3.2°F 99% of the year. Some charts will also include the number for 98%. For cooling, our numbers are 86.7°F (30.4°C) / 0.4% and 83.3°F (28.5°C) / 1%, meaning, again, that temperatures are above those temperatures for those percentages of the year.

At the opposite end of design is making sure your systems are performing as designed. Are the heat pumps running at the expected rate? Is the ventilation moving the expected amount of air? It is a step often overlooked but critical, especially in a high-performing home, where efficiencies can easily be lost due to poorly functioning equipment. We cover what is involved in detail in Chapter 10 on verification and client education.

Verifying that equipment is operating as designed is called “commissioning,” which can be done by the con- tractors who installed the equipment, the manufacturer, or a third-party auditor. But it is critical that it be done, and the results recorded. It is a good time, as well, to make sure that the homeowner understands things like maintenance schedules, filter replacement or cleaning, and who to call for servicing or repairs. Make sure you have a plan for commissioning before contracting for the work to be done. If the installer isn’t planning on doing it, tell them you are planning on it and they will need to make corrections if the equipment isn’t performing to specs.


Dehumidification is an essential complement to cooling in humid climates. Most air conditioners, including heat pumps, can handle dehumidification. They simply blow the warm, humid air over the cold coils (this is what produces the drips that invariably fall on your head as you walk under a unit). Central AC systems may have add-on dehumidifier units (and, for that matter, humidification units, for arid climates or for heating season in cold climates).

As with short-cycling in a heating system, a poten-   tial problem with a PGH is that your cooling system won’t run long enough to  dehumidify   the  air. This can kick the unit into “dry” mode, which steeply   drops the temperature of the coils to condense more water vapor from the conditioned air, using significant energy in the process. You can also use a stand- alone dehumidifier, but these need to be emptied or plumbed to a drain, and typically do a good job of dehumidifying the immediate area but not the whole house. Many older homes in Maine have damp base- ments, and it’s not uncommon to have a dehumidifier running in them much of the year. Dehumidifiers can be significant energy users, so research carefully if you go that route. If you will need to dehumidify the house frequently, a separate system with its own duct- work will be much more effective and efficient.

Humidity, of course, can be a problem year-round. Other than water leaking in from outside, indoor humidity is the most destructive source of moisture and must be managed. Much of it comes from simply occupying the house—cooking, bathing, cleaning, watering plants, pets, and simply breathing. Inexpensive digital hygrometers (devices that measure relative humidity) can help you monitor humidity levels. Typically, houses are best kept between   30%  and 50% relative humidity. In winter, falling below  30% can lead to all the problems of dry air (cracked skin, bloody noses, etc.). In summer, rising above 60% can lead to condensation and comfort problems. An excessively dry house is not dangerous to the structure in the way an excessively humid one is.


Ventilation is the final piece of the HVAC equation. We want to get excess humidity out of our house ASAP, in any season, before it can cause problems. In a kitchen or bathroom, it is relatively simple and common to install a range hood and a bath fan. These units should be vented to the exterior—recirculating, ventless range hoods or bath fans vented into the attic are ineffective at best, destructive to your house or health at worst. Fans should have effective dampers to prevent outdoor air from blowing in when the fans aren’t running.

Research into indoor air quality (IAQ) has been a huge help for builders and designers in the past decade or two. The most commonly used standard for ventilation is called ASHRAE 62.2. ASHRAE is the American Society of Heating, Refrigerating and Air Conditioning Engineers, and Standard 62.2 is titled Ventilation and Acceptable Indoor Air Quality in Residential Buildings. It is the basis for most building code ventilation requirements.

The foreword states that “The standard describes the minimum requirements to achieve acceptable IAQ via dwelling-unit ventilation ... Dwelling-unit ventilation is intended to dilute the unavoidable contaminant emissions from people, materials, and back- ground processes.”

The kitchen is a major contributor to poor IAQ. If you have a gas stove, the combustion exhaust (including carbon monoxide and formaldehyde) is an issue before you even begin cooking. We often recommend induction stoves, since they are all electric, very safe, and energy efficient.

Even without combustion, cooking produces fine particulate (objects small enough to pass through your lungs into your bloodstream), airborne hydrocarbons (some of which are carcinogenic), and other pollutants. There are three effective strategies we’ve found: ducted range hoods, salads, and take-out.

Assuming you’ll tire of the latter two eventually, let’s focus on range hoods. There are two parts to gauging how well they exhaust: air movement and capture area. The first is measured in cubic feet per minute (cfm). There are various rules of thumb for sizing these fans. In a PGH, we recommend a fan with a capacity of 300 cfm, strong enough to ventilate but not overpowering.

In a reasonably airtight home, there is another complication. Unless there is a hole somewhere letting in as much air as you’re trying to exhaust, your range hood or bath fan isn’t going to do much. In a drafty house, there are plenty of places for that makeup air to come in, but in a PGH you need to plan for it.

There are various ways to provide makeup air, but increasingly we are using a relatively simple method that does a good job sealing itself when not in use. It is a duct from the exterior with an electric damper. It is wired so that it opens when the range hood turns on and closes when turned off. Where to bring this  air in is, naturally, the subject of some debate. Some argue for bringing the air in close to the range hood, so you are not cooling down the whole room. Others argue for bringing it in under the refrigerator, to cool down the coils and help it run better. ASHRAE currently recommends bringing 60% in through the kitchen, the rest from elsewhere in the house. This is another area of research; hopefully we will have more definitive information someday soon.

More and more range-hood manufacturers are now selling makeup air kits, which makes planning and installation much simpler. (Note that the building  code doesn’t require makeup air unless the fan moves more than 400 cfm—which rules out most bath  fans).

Another consideration is how effectively a  range hood captures airborne pollutants. If pollutants from cooking don’t end up in the hood, the fan is just exhausting fresh conditioned air. The effective capture area is a function of fan speed, shape of the hood, distance from the range surface, and size of the stove.

The best hood would be a foot wider than the range in all directions and about 10 in. above the burners. This would make it a little tough to cook, though, so we make compromises. The smaller the hood and the farther away from the stove, the more ineffective it is.

This is especially true for island hoods, which have to draw air from all four sides and don’t have the back- splash or flanking upper cabinets to help direct air into the hood.

Bath fans are simpler but still require some thought.  In a cold climate especially, the warm, humid exhaust air from your shower can easily condense in the exhaust ductwork, and you want to make sure that cold condensate doesn’t drip back on your freshly shampooed hair or pool in the ductwork and grow mold or mildew. With a unit that vents out through a wall, make sure that the ductwork pitches slightly toward the exterior, so condensate can escape. Also, assemble the ducts so that the connections funnel water out of the house.

It is trickier if your only option is vertical, either through the roof or with a significant rise before turning horizontal. Your best bet is to make sure that the ductwork stays warm (and thus above the dew point) by careful insulation and air sealing. A more powerful motor, or an additional in-line fan closer to the exterior, can help ensure that the exhaust keeps moving and has less chance of condensing.

Whole-house Ventilation

Now that we’ve taken care of the major sources of airborne pollutants and moisture, what about the rest of the house? We’ve all heard that a house needs to breathe, right? Well, wrong. A house is not a living organism and thus has no need to breathe. What it needs to do is stay dry. Occupants, on the other hand, do need to breathe. A whole-house ventilation system helps with both.

In most older homes, the ventilation system is the air moving into and out of the house via drafts. You don’t really know where the intake air is coming from, or  the exhaust air is going to, so you may have good or bad IAQ depending on which way the wind is blowing and how hard. Is it pulling air from the meadow next door or from the moldy crawlspace? Is it pushing to the exterior or into a wall or ceiling cavity?

Once you start doing a better job air sealing, whether in a PGH or a Pretty Good Reno, you need  to  be more deliberate. In bedrooms, CO2 from respiration can easily build up to unhealthy levels overnight. During the many hours of the day when neither the bath fan nor the range hood is running, there are still indoor pollutants that need to be diluted and moisture that needs to be exhausted. The way to address this is through a well-designed ventilation system. The essential elements are that the system is:

  • Balanced (the same amount of air that is being exhausted also is entering).
  • Comprehensive (all areas of the house are getting adequate ventilation).
  • Room-specific (it’s better to supply some rooms and exhaust others; especially true with supplying bedrooms and exhausting bathrooms and kitchens).
  • Sized right.
  • Carefully ducted:
    • Run as short and as straight as possible. Excessive length and bends significantly reduce airflow.
    • Rigid metal or plastic. If you need flexible ductwork to make a difficult-to-access connection, like at a dryer, keep it as short as possible.
    • Within the conditioned space. Going from warm interior to cold attic and back again (or the opposite in summer) wastes considerable energy.
    • Airtight. All ductwork should be air-sealed at all connections with either a duct mastic or mastic tape.

There’s a catch: there is an energy penalty. In pleasant weather, opening windows may provide enough ventilation. But once the weather is hot, cold, or humid enough that you close the windows, any ventilation  by definition involves an exchange of conditioned air (cooled, heated, or dehumidified) with unconditioned air. Once conditioned air is exhausted, the incoming air must be heated, cooled, or dehumidified. So the goal should be sufficient ventilation but not over- ventilation. ASHRAE 62.2 or whatever your building code requires is a good place to start. A simple air- quality or CO2 monitor can help adjust the system  once it’s in place.

The most common systems for providing balanced ventilation are heat- and energy-recovery ventilators. Heat- recovery ventilators (HRVs) have heat-exchanging cores that capture as much as 90% of the heat in the airstream, effective in both winter and summer. With an energy-recovery ventilator (ERV), moisture also is transferred, making dry outdoor air more humid in the winter and moist outdoor air less humid in the summer. A couple of systems are available that can switch between the two functions.

Most systems are ducted from a central unit to the main spaces in the house, exhausting from areas that create moisture and odors and supplying to living and sleeping areas. Others are point-source ventilators, meaning they ventilate single rooms. Which system is best depends on many factors, including climate, occupancy, and whether bathrooms are on the same system as the rest of the house.

Some homes use exhaust-only ventilation, essentially leaving a bath fan running on low speed continuous- ly. This usually saves money initially, but the energy saved using balanced ventilation eventually pays for itself, and because the incoming air is filtered instead of coming in through random gaps in the structure, the indoor air quality should be higher as well.

Much (virtual) ink has been spilled on whether and where to use an HRV or an ERV, but in the end the quality of the unit itself, along with the installation and the maintenance of the unit and ductwork, out- weighs any advantage of one system over the other.