Appearing in The Chicago Tribune
DEAR TIM: I’m replacing my bath exhaust fan. The old fan was vented through the soffit. I don’t believe I have an attic, as there seems to be no access to the space. My roof is sloped but my bath ceiling is flat. What’s the best way to exhaust the fan out through the soffit? If I close off some of the continuous vent strip to prevent exhaust from entering the vents, how much should I do? I’m worried about damaging my house and I live in northern New Hampshire, where it gets very cold. What would you do if you were me? –Bob D., Dixville Notch, N.H.
DEAR BOB: You should be very concerned about damaging your house. I feel I can speak to your concerns because I also live in New Hampshire and know all about cold weather. Some of the answers to your questions go back to your high school physics class, if you can recall some of those memories.
Let’s talk about the science of what’s going on and allow me to relate to you some of my own observations. Here are three things we know are facts:
–Hot air rises.
–Water vapor will condense on cool or cold surfaces.
–Water will fuel wood rot by providing wood-eating fungi with much-needed moisture.
It’s hard to see air, as it’s invisible. But in cold weather, we get to see the foggy water vapor that turns to miniature clouds when we exhale our breath or we see water vapor escaping from a chimney, dryer vent, soffit vent or auto exhaust pipe. You never notice the white water vapor going downward. You may try to force it down, but it always starts to float up much like air bubbles rise up in water.
During the past few cold winters, my propane boiler belched out clouds of water vapor through a side wall vent near the ground. Ten feet above this vent is a roof overhang that gets coated with frost as the water vapor condenses on the cold painted wood soffit and fascia boards.
Source: Tim Carter, Tribune Content AgencyAsk The Builder
Article URL: http://www.chicagotribune.com/sns-201506152000–tms–askbildrctnab-a20150626-20150626-story.html