Basement insulation with drainage pipe on concrete wall

frank_ca2020

6 years ago

I framed my basement walls about 5 inch away from the concrete wall, since there is a drainage pipe in the middle. I am planning to install XPS boards, that ideally need to be cut in half and added on top and bottom of the pipe. Afterwards I am planning to add fiberglass insulation between the studs and close with drywall, without a vapor barrier. However, I have a few concerns, hoping someone can help me to find the best approach.

1) By cutting the XPS in half to skip the pipe will interrupt the protection of the XPS. How should I fill that, to make it closed from top to bottom?

2) After adding a 2 inch XPS behind the framing I still have a decent gap to fill. Fiberglass will be added within the studs but there is space between the fiberglass and the XPS, about 3 inch or so. Is it fine to leave that space? Will air circulation create any issues?

3) As some contractors suggested me, should I give up on XPS and install unfaced fiberglass within the studs and close with drywall. The fiberglass will not touch the concrete wall and will use the 5 inch space between the wall and fiberglass as a way to dry any possible moisture. Does it sound decent as an approach?

(See attached picture to have an idea of the setup. Note: this is taken from a youtube video, but it's exactly as my basement).

Thanks a lot for helping.

Featured Answer

formulaross20

6 years ago

1. Fill the horizontal gap between the XPS and the drain pipe with foam in a can.

2. Don't leave an open 3" plane between fiberglass and the XPS; use thicker fiberglass bats so that the bat contacts the foam. Yes, this will cost a little more. Fiberglass bats essentially break the "air" up into lots of small convection loops in series, those multiple loops in series are not as good a heat conductor as one single loop, that's how bats add insulating value. Leaving one large 3" loop allows decent horizontal heat transfer (poor insulating value), plus adds a heat transfer means in the vertical direction. Not good.

3. You should be using un-faced fiberglass, basement walls need to be able to dry to the interior and a vapor barrier on the inside surface would interfere with this (see Building Science Corporation web site for more info). Keep your design approach of foam against the concrete walls and fiberglass on the interior.

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Comments (8)

frank_ca2020
Original Author
6 years ago
@formulaross20,

Thank you very much for your feedback. Really helpful.

Going back to item #2, could I just go with:

- foam against the concrete wall;

- 3" gap from foam to stud-wall (due to the pipe running on the concrete wall);

- drywall on the stud-wall;

- no fiberglass between foam and drywall.

I understand I would have a lower R value but, since I am mostly willing to preventing mold, if this approach is suitable, I would proceed as described.

As a last question, I would need to install a fire-stop top plate between the stud-wall and the concrete wall. Can I use pressure-treated lumber (2x10 in this case), since it will lay on the top of concrete wall? If not, what should I use?
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seosmp
6 years ago
last modified: 6 years ago
We went with Rockwool rather than fiberglass (XPS foam boards on cement). Not sure what the general consensus is on that approach (particularly in terms of moisture/mold).
wacokid
6 years ago
There is no perfect wall system, not even the “BSC” has one. I like your first concept with the air gap. The only thing on that is holding the unfaced fiberglass batts in place on the backside.
frank_ca2020
Original Author
6 years ago
I inserted the XPS behind the existing studs. I am really tempted to just close with drywall, without adding fiberglass within the cavities. There will be about 3” 1/2 + 1” 1/2 airgap Between the XPS and the drywall. can that be a problem?
mononhemeter
6 years ago
I don’t know, but if you are concerned about mold, why not put rockwool in? R-value is 3 per inch.
frank_ca2020
Original Author
6 years ago
You mean XPS, rockwool and drywall or just rockwool in the cavities without XPS?
worthy
6 years ago
last modified: 6 years ago
The safest play is no moisture sensitive insulation in the basement wall cavity at all.

As for just XPS alone, that would depend on your climate zone. At the least, meet IECC minimums even if they are not required in your jurisdiction. I've used ccspf on the interior basement walls, then just framed without further insulation. Incidentally, I always put the baseplate on top of 1" strips of XPS, as basements are often subject to minor flooding at one time or another.

As explained by formulaross20, the airgap is counterproductive. Indeed, research has shown that even the slight airgaps between foam board and the concrete wall should be short-circuited with a closed grid glue application rather than just dabbing glue on the foam boards.