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Vapor Retarder and Insulation Between Unfinished Basement and First Floor

kaxixi | Posted in Energy Efficiency and Durability on


My mom just moved into a new house in New England.  It’s mostly well insulated, except for one glaring flaw:  the floor to the house site above a basement that isn’t conditioned, and there is no insulation between basement and the floor.

I’m considering fixing the issue by installing rockwool between the joists, as in this video:

I am wondering whether a vapor barrier should be installed before installing the rockwool?  I suspect the answer is yes, but am not sure, and want to make sure I do things properly.

Mom would keep the upstairs at around 70F in the winter.  The basement, by my estimate, will be in the 40s to 50s most of the winter.  It tends to be a bit moist (another problem I am gradually trying to solve).

Thank you,


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  1. walta100 | | #1

    Generally the smart move is to insulate the wall the much smaller surface area of the basement walls.

    It is almost impossible to seal the air leaks of the floor to basement spaces and insulation is useless when air moves thru and around it.


    1. Jon_R | | #2

      I agree with the conclusion - conditioned is better than an always damp basement.

      > the much smaller surface area of the basement walls.

      Say a 30x50 = 1500 sqft house. 2×8×30+2×8×50 = a not much smaller 1280 sqft. Then add the basement floor. And account for more expensive foam insulation.

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