Basement insulation question

4 years ago

Contractor is finishing a walk-out basement for me, so some exterior walls are partial concrete and then above ground exterior framing. The unfinished basement had the fiberglass insulation on the framed exterior walls covered with a white plastic, and insulation of the concrete part held up with some fabric.

Finished basement has furred walls floor to ceiling in front of those exterior walls. My question is; when re-insulating, should the plastic holding the insulation in the exterior framing be removed, or can it stay? does it matter? if left as is, there will be the insulation in the exterior wall, that plastic, possibly a small air gap, and the insulation in the furred wall.

Is the plastic any sort of vapor barrier or is it just there to hold the insulation in place?

The home is in the northwest if that matters, so relatively cold winters and dry, hot summers. It was built in 2020

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Comments (3)

worthy
4 years ago
last modified: 4 years ago
Hard to tell without photos. But double vapour barriers are to be avoided. (Of course, a moisture sensitive insulation is a Code legal mistake in any basement.)

If it were my house, I'd pull out the fg and install XPS or EPS boards, then frame and fill the space with the fg that came with the house.

At the least, make a few razor cuts in the interior vapor barrier so it doesn't trap moisture moving from inside to outside.
Frank Marsh thanked worthy
Frank Marsh
Original Author
4 years ago

Above is the before picture. I'm discussing e.g. the wall in the background in the upper left. The top part of the wall is above grade, framed, insulated, and has plastic sheet over the insulation. The bottom part of the wall is below grade and concrete below the framed wall. The insulation is held up against the concrete with some sort of breatheable fabric, there is no furred wall in front of the concrete.

Above is an after picture taken of that same wall, but standing in that room (same angle as first picture would be blocked). A furred wall has been framed an inch or two in front of the concrete and goes floor-to ceiling. In some places of the basement the existing insulation that was hanging against the concrete was saved and reused. In this particular area new insulation was put between the studs. I think it is R-19. And it is the same thickness all the way up, so there also isn't any additional insulation for the inch or two lip where the concrete is wider than the above ground framing. I can't get a picture of it now without removing the new vapor barrier, but when I posted the question, the old vapor barrier was still on the top framed part of the wall and they just put the new insulation in front of it. They might have scored it, but I don't think so.
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worthy
4 years ago
last modified: 4 years ago
My answer is the same. There should only be one vapour barrier. Even if the plastic that you're covering with more insulation was not supposed to be a vapour barrier, it is a vapour barrier. Incidentally, I would replace that mouldy piece of insulation on the right upper corner in the second photo. That it was mouldy from being against the concrete wall is telling.

Code minimum isn't the best way to go.
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