Air sealing a building reduces or eliminates air infiltration. An airtight building is more energy-efficient than a leaky one, but good ventilation is essential to maintaining a healthy, comfortable indoor environment.
Hearing the words “air pollution” usually brings to mind images of cars and factories spewing contaminants outside, but few people think about the quality of indoor air. Because buildings are enclosed, indoor pollutants have a tendency to build up to the point where the quality of indoor air is actually poorer than outdoor air! What produces these pollutants? Mold, chemicals from cleaning supplies, cosmetics, paints, air fresheners, smoke, VOCs from building materials, and particles shed by pets are common sources. Contaminants such as these are associated with asthma, headaches, fatigue, and allergies, which can be significant enough for a structure to be labeled with sick building syndrome. Aside from avoiding products and chemicals that can cause irritation, how can indoor air quality be improved?
The prevailing notion guiding past building projects was that they needed to “breathe” to protect occupant health. While it’s true that buildings without proper ventilation are susceptible to moisture problems, mold, and fume buildup, when ventilation happens accidentally by way of cracks and holes in a building’s shell, the building is also extremely inefficient and uncomfortable. According to ENERGY STAR, the holes and gaps in a typical home result in the same amount of air leakage as leaving one window open year-round! In today’s buildings, ventilation works in conjunction with air sealing and insulation to ensure comfort and efficiency.
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) offers standards (ASHRAE 62.1 and 62.2) for maintaining acceptable indoor air quality in new and existing homes.
Drafts, difficulties regulating temperature, and high energy bills are signs that a building has air leakages. A blower door test can determine exactly how much air is leaking from a building by sucking air out and measuring how much it takes to create a change in air pressure within the building. Buildings with tighter envelopes will require less air to be removed to be depressurized. Energy auditors also use infrared imaging to identify leaks and areas of insufficient insulation based on thermal differences.
It’s crucial that a tightly sealed building is ventilated properly.
Older buildings often have major leaks, which can be a source of large energy losses as conditioned air escapes through this uncontrolled form of ventilation. It’s no surprise that systems like Passive House and ENERGY STAR have air tightness requirements. When a blower door is used to create a pressure differential of 50 Pascal, a Passive House is allowed no more than 0.6 air changes per hour, while ENERGY STAR buildings in Western Pennsylvania’s climate zone must not exceed four air changes per hour, a less rigorous measure.
It is crucial that a tightly sealed building be ventilated properly. Ventilation flushes out contaminated air, draws in fresh air, or both. Some ventilation systems also exchange heat or humidity between incoming and outgoing air to save energy and increase comfort.
Methods of Insulation and Ventilation
It cannot be overstated that insulation batts do not stop air! For new construction, there has to be a continuous air barrier around the building envelope. Some types of insulation do serve as an air barrier, but otherwise this is often accomplished with housewrap. In either case, all seams should be sealed with tape and duct connections should be sealed with mastic. Trouble spots like dropped ceilings, recessed lights, and wall penetrations for wiring and plumbing require extra care.
Existing buildings can still benefit from air sealing. Add caulking or weatherstripping around windows, seal ducts with mastic or heat-resistant tape, place foam gaskets behind outlet and switch plates, install storm windows, and add door sweeps to reduce a significant portion of leaks. In addition, make sure flue dampers are tightly closed or buy a fireplace balloon to block air flow through the chimney. For large gaps (larger than one-quarter inch) around windows or doors, use expandable foam to fill the space.
A ventilation system can be set up in several ways. The simplest systems solely exhaust indoor air, typically using a bathroom or kitchen fan. Look for a quiet model, especially if the exhaust fan will be used continuously, as called for in ASHRAE 62.2. In less airtight buildings, low volumes of exhaust can be compensated for with air coming in through leaks. Tighter buildings can employ passive vents to allow air to enter, avoiding an undesirable pressure differential. The downside of exhaust-only configurations is that conditioned air is lost to the outdoors, which means that energy is lost as well.
A step above an exhaust-only system is supply ventilation. This is a method for controlling where a building’s air supply comes from by actively bringing air into the building, while allowing it to leave through leaks or passive vents. Unfortunately, it takes a fair amount of energy to condition incoming air, although dampers can be installed to reduce airflow in temperature or humidity extremes.
The most sophisticated ventilation systems are balanced, involving both supply and exhaust. A huge benefit of balanced systems is that heat – and even moisture – can be transferred between supply and exhaust air, turning what would have been wasted into energy savings. A heat-recovery ventilator (HRV) can capture and reuse up to 80% of the heat energy in exhaust air. Energy-recovery ventilators (ERV) are similar, but also exchange humidity between air streams, reducing the work that an air conditioner or dehumidifier has to do.
Some ventilation systems are designed to run continuously, but a more efficient approach is demand-controlled ventilation. Using a schedule, occupancy sensors (motion, sound, video, or infrared), or (best of all) CO2 sensors, this type of ventilation adjusts fresh air to the actual needs of building occupants. Sensors of any kind must be calibrated and strategically placed to ensure that their measurements accurately reflect demand so that ventilation is sufficient.
Basic air sealing materials (caulk, plastic coverings for windows, etc.) are inexpensive and are a great place to start. Depending on how leaky a building is to begin with, air sealing can provide substantial energy savings!
HRV, ERV, and other balanced systems require more maintenance and cost more to install than the relatively simple exhaust or supply systems – but exhaust, supply, and non-recovery balanced systems may add to HVAC expenses.
- Energy savings
- Improved indoor air quality
- Better health
- Increased comfort