Definitely get a vapor barrier between you and the down. You don’t want to create condensation inside the bag. After the down gets a little wet it’s useless.
This could be misleading . A vapor barrier helps when the temperature does not reach freezing and there are several days out . Additionally a vapor barrier can be warmer for some , but clammy ...however it can feel warmer .
However if temps in your tent get above freezing very often it is less of a concern . Even if it’s 20 degrees out if the sun hits the tent the tent will be warmer .
Also a little wet and useless would seem to be a bit of an exaggeration . Yes it’s useless in a washing machine but in most cases there may be a small degradation with a some moisture , not entirely useless .
Really in real cold a thin synthetic blanket over a down bag is a pretty solid combo as it just helps move the moisture .
It’s also important to note that moisture on the shell does not always mean that there is moisture inside . For instance The shells on WM bags have a higher waterproof rating than many tents .
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