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I'm currently working on finishing my basement. Right now I'm working finishing up the insulation and am getting ready to install a plastic vapor barrier. Since I live in Massachusetts which is a cooler climate, I plan to install the vapor barrier on the inside of the exterior walls.

Before I put up my framing I installed 2 inch R-10 rigid foam board insulation against the concrete walls. However this is not a continuous seal since the existing insulation (from when the house was built) in the ceiling floor joists is between the top of the foam board and the floor above.

The problem I am having is that the existing fiberglass insulation installed in the above floor joists will be in my way from creating a continuous seal across the insulated wall.

I could cut the ceiling insulation where it is in my way so that I could reach the barrier all the way up to the above floor boards, however this would be difficult, especially in areas with ductwork between the joists.

The other alternative (that I can think of) would be installing the vapor barrier across the ceiling as well as the walls. This would be a lot easier than trying to work around the existing insulation in the ceiling. Would this be too much of a vapor barrier, especially when added to the existing foam board? Would it cause any issues? Is there any reason I should avoid installing the vapor barrier across the ceiling?

Here is a picture of where the insulation in the ceiling would get in the way of me creating a continuous barrier across the wall to the ceiling: a picture of where the insulation in the ceiling would get in the way of me creating a continuous barrier across the wall to the ceiling

Here is what it looks like in one of the joist cavities. A picture of what looks like in the joist cavity

Note that there are still some gaps above the wall that I am planning to add insulation to before I install the vapor barrier.

Here are some other details that might be pertinent:

  • I am planning on installing a drop ceiling.
  • My main floor is above the basement. The main floor and basement floor have separate heating/cooling zones.
  • I am using R-13 mineral wool insulation for the walls.
  • My house was built in 2019
user2704238
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2 Answers2

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Install the vb/ab on the wall. For the joist cavities above the wall that have the fiberglass insulation inside them, get rigid foam insulation and cut it to friction fit. Tape the four sides of each piece of insulation to air seal the foam to the framing. Best tape would be passive house tape - I like the Tesca Vana tape - but you could use Tyvek tape or Tuck tape.

I assume that the ceiling insulation is mostly for sound and that the level above is interior.

isherwood
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Fresh Codemonger
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To insulate the rim joist, cut loosely fitting pieces of rigid XPS foam board for the joist space above the wall, and use expanding foam to hold and seal it in place. Use foam around ducting and other small & difficult cavities.

Looks like it's FG batt with facing, and the facing serves as vapour barrier. There's no rigid foam behind it, right? Sine you have foam board against the wall, you don't need, and shouldn't add further vapour barrier. Remember, by code you are not allowed to leave the rim joist cavity & wall unfinished, because it says so on the rim joist's foiled insulation.

For finishing, drywall is fine.

As for the faced FG batt, there shouldn't be XPS behind it, otherwise you risk trapping moisture.

In conclusion: you have XPS against the wall and foiled insulation in the rim joist cavity, so no further vapour barrier is needed in your situation.

XPS is extruded polystyrene, often coloured pink or blue. EPS is encapsulated polystyrene, often coloured white and more "crumbly". XPS doubles as a vapour barrier and has higher R-value than EPS.

Per building code in my region (West coast), tape is not allowed as a primary barrier and such a foam seal is required instead. Tape can be used to seal ship-lap XPS seams and overlapping vapour barrier sheet seams, but not the framing/board interface. Notice in the picture below how the foam board edge is slightly beveled to accommodate space for foam.

For fire rating, DuPont and Syntec XPS (and I'd think most others) do NOT require a finished wall, but it is brand dependent. EPS generally does require a finished wall.

enter image description here

Image: https://www.finehomebuilding.com/project-guides/insulation/insulating-rim-joists-2

P2000
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