Homes (particularly older homes) leak. The recommended ventilation for good air quality is about 0.35 air changes per hour (one complete air change every 3 hours). Most older homes and many newer ones suffer from air infiltration resulting in 3 to 4 times the required ventilation. In most cases, the drafts you notice are nothing compared to the ones you don’t. You won’t notice cold air leaking into your house in non-living areas such as crawl spaces. You won’t notice warm air leaking out of your house at all, but the volume of air in your house stays constant so if you notice air leaking into your house in one place, then air must be leaking out in another.
How much do leaks cost?
Lets look at an example close to home: my home. I live in Vancouver, BC, Canada. My home has about 230 m^2 of heated area with 2.2 m ceilings. As such the volume of air in my home is about 500 m^3. The density of air at 20°C at sea level is about 1.275 kg/m^3 so my home contains about 640kg of air. The specific heat of air is about 1kJ/kg/°C. That means to raise the temperature of 1kg of air by 1°C requires 1kJ (kiloJoule) of energy. The average outside temperature in Vancouver measured over an entire year is about 10°C. Therefore, to raise the temperature of the air in my home from 10°C to 20°C requires 6.4 MJ (megaJoules) of energy. If air infiltration is resulting in 3 times the recommended ventilation (ie 1 air change per hour instead of the recommended 0.35) then the air in my home is being changed an additional 15 times per day or about 5500 times per year more than necessary. At 6.4 MJ of energy per air change, that amounts to 35 GJ (gigaJoules) of wasted heat per year.
But that’s not all. My natural gas furnace is over 15 years old and has an efficiency of only about 60%, so it requires 58 GJ of natural gas to produce 35 GJ of heat in the living space of the home. This ammounts to about half the natural gas I consume in a year. It costs me about $14.50 per GJ of natural gas, so my potential savings are about $840/year just by sealing my home better without adding any additional insulation, or replacing my furnace. Adding insulation and replacing my furnace are worthwhile too, but they require a much greater capital investment for probably about the same return.
How can leaks be detected and sealed?
The easiest way to detect air leaks (either into or out of your home) is to probe around likely leak locations with a stick of smoldering insense. Leaks will become immediately visible by the movement of the smoke. This is best done on a windy day, but can also be done on a calm day by using your fireplace to induce low pressure inside your home. Light a fire and open the flu wide open to draw as much air up the chimney as possible. This will draw air into your home to replace the air going up the chimney, allowing you to detect the leaks. If the leak is around a window or door, then weatherstripping may be the solution. There are also temporary caulking products on the market that can be applied over the winter and pealed off in the spring. Clearly this is only useful if you don’t need to open the window or door during the winter. If the leak is around permanent fixtures such as lights, pipes, wall intersections, etc, then spray foam or caulking is the solution. Two good sources of additional information on sealing air leaks are:
How can I seal around my fireplace?
Fireplaces are usually a major source of air infiltration even if they are equipped with a damper or glass doors. Caulking and spray foam products cannot be used to seal inside fireplaces because the temperatures are too high. Instead use Furnace Cement which you should be able to find at any home supply store. It is important to clean and dampen surfaces (a spray bottle filled with water works well) before applying the furnace cement or it will not adhere well.
Should I seal my crawlspace vents?
Your crawlspace vents exist to remove moisture from your crawlspace. Conventional wisdom says that your crawlspace, being built directly over the ground, is a source of moisture, and ventilation is required to remove the moisture. However, exactly how much moisture your crawlspace produces may vary from a lot to hardly any at all. Also, your crawl space vents may be a source of moisture rather than a means of eliminating it. In winter, the warm air in your home rises, leaks out through various points in your attic or top floor, and cold, damp air is drawn in through your crawl space vents or any other openings at the base of your home. My home, built in the late ’70s, was designed with the crawlspace inside the building envelope (ie I pay money to heat my crawlspace). The crawlspace was also permanently vented to the outside. I have investigated my crawlspace at various times throughout the year and have never noticed any indication of moisture. Therefore, I sealed my crawlspace vents completely. Since then I have noticed no increase in the moisture level in my crawlspace or home. I recommend that you investigate your own crawlspace, and if you see no signs of moisture, seal your vents. You could reduce your heating costs by 10-15% by this one simple act. I did.
What if I seal my home too well?
The likelihood that you could “overseal” your home is slim for an older home. The best way to determine how much ventilation your home is actually getting is to have a professional heating, cooling and ventilation contractor perform a blower door test. Aside from that you can watch for physical signs. Long before you experience any negative effects from increased carbon dioxide levels, or lack of oxygen, you’ll likely notice water condensation, or dampness around windows. The primary purpose of ventilation, after all, is not to provide fresh air for you to breath, but to remove moisture and prevent growth of mold and mildew.