Ask Focus on Energy

There are a couple of ways to answer this, read on for both

I am not sure if you are asking about condensation inside the chimney or if it causes condensation to occur/form on other surfaces in your home such as windows, ceilings, walls, inside closets, etc. I’ll respond to both perspectives.

Inside the chimney - A chimney balloon typically reduces the temperature of the inside surface of the chimney (during the heating season) because cool/cold air is more dense (heavier) and subsequently “falls” down the chimney. If moisture laden air (from inside the home) is allowed to come in contact with those cool/cold surfaces, condensation will occur when the surface temperature is below the dew point temperature. Often the chimney balloon doesn’t completely seal the migration of air and associated moisture in vapor form, making the likelihood of condensation common. The question becomes how significant is the condensation on the inside of the chimney and does it represent a problem? Because the condensation is typically not caustic, it typically does not cause significant damage to the chimney.

Inside the home – A chimney balloon reduces the amount of a home’s conditioned air (and associated moisture) to be exhausted to the exterior. It is the equivalent of turning off a mechanical exhaust fan because chimneys affect air pressures and air exfiltration/ infiltration in homes in the same way as mechanical exhaust fans. It can be said that chimneys are passive exhaust fans unless they are overcome by mechanical exhaust equipment like exhaust fans, clothes dryers, vented vacuum systems, etc. When chimneys are heated, the effects become exacerbated.

With every cubic foot of air being exhausted (exfiltration) from a home, by any means, another cubic foot of outside air is “pulled” in (infiltration) to take the place of the exhausted one as a result of the pressure difference that is created. During the winter, the infiltrating air is much drier in actual, moisture content and through dilution causes the relative humidity inside the home to be reduced. If there is too much air exchange, the relative humidity can cause human discomfort (dry skin), static electricity, and shrinking of building materials and furnishings. Too little air exchange drives the relative humidity up as we produce moisture inside the home as a result of using moisture generating devices, respiration/perspiration of humans and animals, cooking, bathing, aquariums to name a few sources, as well as bulk water entry through the building shell (unintentional leaks and penetrations).

Relative humidity has a direct bearing on dew point temperatures. If any surface is below the dew point temperature, condensation will begin to form. It is usually first noticed on windows because windows are typically the coldest visible surfaces. But please know that condensation will form on any surface, visible or not, whose temperature is below the dew point.

As you can probably see, chimney balloons don’t cause condensation, but their use can affect how a house operates with regards to pressure differences, exfiltration/infiltration rates, and relative humidity. It is wise to make concerted efforts to reduce air leakage (in and out). The more effective the air sealing measures, the more control you have over the actual ventilation rate, the relative humidity inside your home, and the indoor air quality of your home. Homes that are effectively “tightened up” use less energy and are generally more comfortable year round. That is why there is such emphasis placed on the importance and value of air sealing homes. It is especially important to do prior to insulating or re-insulating attics and basement rim joist/foundation intersects.
 

Published Monday, March 15, 2010