Why caulk outside window?

If I caulk the inside why do I need to caulk the outside? Isn't the air sealed once I caulk all gaps on the inside?
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Does it ever rain where you live?
--
snipped-for-privacy@phred.org is Joshua Putnam
<http://www.phred.org/~josh/
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Research the way walls are built. Pay close attention to flashing. Sealants are used to exclude water, including wind driven water. TB
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You don't, except to prevent rainwater leaks. The vapor barrier should be on the inside in a cold climate, to prevent condensation inside walls...
It's also easier to find and fix air leaks from the inside, on a cold day, using a large exhaust fan and cheap siliconized latex caulk that dries clear.
Nick
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If you caulk both sides, you'll have a dead air space, and dead air space increases insulating value.
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In your case, you don't need to caulk inside or outside. The plastic sheeting over all the windows takes care of everything. And who cares if your trailer gets a little water in it anyway? It's probably the first time the floors have even come close to being washed.
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To keep water out of the walls. The caulk on the inside is for airflow, and looks.
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John Hines wrote:

I forgot to mention that the new windows I am installing are completely SEALED from any air/water leaks. I guess the caulking would be a BACKUP measure.
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One caulks the gap between the sealed window unit, and the wall it is installed in.
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Rick wrote:

Helps keep the heat in. Helps keep water and insects, like termites, out.
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