Installing a bathroom exhaust fan and the proper way to vent it...

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Unlikely. You would actually have to chance the laws of thermodynamics to make a difference. One area you might not be considering is that it's not only against code, it's also more expensive to heat the home. Bath (and kitchen) fans carry a tremendous amount of moisture. Dump this into the attic in cool/cold weather will cause this moisture to settle into the insulation; degrading the performance and moving the dew point closer to the heated side. It's not out-of-the question to consider a R-value loss of 50% or more (one test showed much more) on cold days.

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Dennis wrote:

You entirely missed what I wrote. The whole idea is NOT to dump moisture into the attic by NOT running the fan during cool outside conditions.
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Your right, I didn't get that from what you originally wrote. But that defeats the whole purpose of ventilation, doesn't it? The purpose it to ensure use year round. But I have to admit, you are correct. If you don't use the fan, not much moisture will get trapped in the attic.

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It occurs to me that this isn't necessarily true of half-baths, though.
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Less to be certain. But just the air by itself carries a lot more water vapor than can dissipate in cold air, and a fan will draw air from outside the half-bath. It still will be moisture-laden air and that moisture will collect on the insulation. Not as much or as fast, but it will affect the R value to some extent. Best it goes outside in the winter (whenever possible).

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