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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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    Check the recommended insulation in Massachusetts – Ruskes Feb 22 '23 at 04:35
  • Are those basement walls above or below ground? – SteveSh Feb 22 '23 at 12:35
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    what is the purpose of the ceiling insulation? additional thermal insulation for the house? – P2000 Feb 22 '23 at 18:24
  • @SteveSh The walls from the picture, and in the entire room from the picture are mostly below ground with only a couple feet above ground. There is a room that I will have to do that transitions from mostly below ground to a walk out. – user2704238 Feb 23 '23 at 01:02
  • @P2000 The ceiling insulation was already installed when I bought the house as a new construction in 2019. I assume it was installed as additional thermal insulation for the house since the basement wasn't being finished at that time. Now that I have a separate heating/cooling zone for the basement and the main floor I still like that thermal separation to maintain efficiency when I am not heating/cooling the basement. – user2704238 Feb 23 '23 at 01:07

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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  • The level above is interior. The ceiling insulation was added by the builder when the house was built in 2019. I think it is for thermal purposes since the basement was previously unfinished, but I still like it since my house has a heating/cooling zone for each floor including the basement. For the foam board are you saying to place it vertically back against the wall inside the cavity to protect moistier from coming through that way, or horizontally to help complete the vapor seal up top from the vapor barrier on the wall? – user2704238 Feb 23 '23 at 01:28
  • vertically but inline with your wall insulation plane. you can think of that piece of the joist bay as part of the wall just a portion that consists of more insulation than the lower portion. – Fresh Codemonger Feb 23 '23 at 08:58
  • So does that mean that the foam board would be pressing up against the fiberglass insulation in the ceiling? Wouldn't that still leave a gap for moisture to get through in the ceiling fiberglass insulation? Or are you saying I should remove the insulation from the ceiling? – user2704238 Feb 23 '23 at 13:26
  • I'd cut the fiberglass insulation to allow the rigid foam to seal the wall side of the joist bay. The portion of the joist bay you seal becomes effectively a little piece of wall. You don't want moisture to get to the cold side of the joist bay and that is what this rigid piece does. I'd still leave behind the foam filled with the fiberglass (though you don't want the paper still on it in that cavity. – Fresh Codemonger Feb 23 '23 at 22:08
  • Since I was getting slightly conflicting advice I decided to call my building inspector and see what they preferred. They told me that since I already have a vapor barrier in the joists, and with the foam board, that I should not add an additional vapor barrier since doing so could trap moisture and cause more issues. Thank you for your feedback though. If I didn't have an existing barrier, your suggestion would have probably been the way to go – user2704238 Feb 24 '23 at 17:17
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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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  • I agree about using XPS, But I believe XPS is considered a vapor retardant, and not a barrier. That's actually an advantage in many areas. – SteveSh Feb 22 '23 at 18:57
  • @SteveSh yes, correct, and in our region sufficient & allowed as VB. Also important not to wrap the fibre glass insulation in a non-breathing sandwich of foil and XPS. – P2000 Feb 22 '23 at 19:07
  • My house was built in 2019. I think the joist cavities were already done as you described. I edited my post to include a picture of the cavity. Does it look to you like it is done as you described? – user2704238 Feb 23 '23 at 03:06
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    @user2704238 thank you for the update. 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, and in the wall's XPS tech sheet. (Flammable hazard, and the insurance cares if there's a claim) – P2000 Feb 23 '23 at 06:39
  • where do you see this flammable hazard on xps? https://www.carlislesyntec.com/Document-Viewer/owens-corning-foamular-extruded-polystyrene-xps-insulation-product-data-sheet/05fde6b0-224c-9221-cba5-039b55d5b586 – Fresh Codemonger Feb 23 '23 at 09:05
  • Yes, it looks like a faced FG batt to me. Should I check behind it to see if there is foam board, or should it not matter since the facing is already acting as a vapor barrier? What do you mean when you say I am not allowed to leave the rim joist cavity and wall unfinished? I plan to add drywall to the wall, and a drop ceiling above. Does that qualify as finishing the rim joist cavity and wall? – user2704238 Feb 23 '23 at 13:18
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    @user2704238 as for finishing, yes drywall is fine. As for the faced FG batt, there shouldn't be XPS behind it, otherwise you risk trapping moisture, so check. – P2000 Feb 23 '23 at 14:19
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    @FreshCodemonger my mistake, 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. – P2000 Feb 23 '23 at 14:25
  • Okay, so I checked behind and there is no foam board, so I think I'm good. No vapor barrier needed will save me some time and effort – user2704238 Feb 24 '23 at 03:01
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    @user2704238 good luck with the project! – P2000 Feb 24 '23 at 04:49
  • @P2000 Thank you! Since I was also getting conflicting advice on adding the vapor barrier and cutting the ceiling insulation to complete the seal I decided to call my inspector and see what he prefers since that could vary based on climate. He confirmed what you are saying. Even though your answer does not answer my question directly it led me to the correct solution which was that I should not add vapor barrier since what I have is already a vapor barrier, so I'll mark it as the accepted answer. – user2704238 Feb 24 '23 at 17:13
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    @user2704238 excellent feedback, this is useful to other readers too. I updated my answer with the relevant parts in my comments to better highlight that in the end indeed you need no further VB. – P2000 Feb 24 '23 at 17:39