Would insulating a basement wall with XPS and reflective insulation create a moisture problem?

Question

I'm in Massachusetts (ICC 2015 w/ mods) and want to finish my basement walls. I'm thinking about using a combination of 2" XPS, 1x2s horizontal 24" OC, Reflectix (double reflective insulation, here), 1x2s vertical 16" OC and drywall. This should give me R-17 with the XPS (R-10) and R-7 given a 3/4 inch air gap both under and over the Reflectix. Would this be legit? Am I asking for moisture problem with furring between two vapor barriers (XPS and Reflectix)?

Follow-up details: Basement walls are all below ground and over ten years without water after doing some exterior drainage. There is efflorescence so I assume at least some moisture.

I'm not sure where the frost line is, so certainly higher up the wall the ground behind it is below freezing. Not sure of the wall surface temp itself. The basement is generally quite cold.

Answer 1

Putting this into an answer instead of an extended commment.

At first glance having a vapor barrier (the Reflectix) where you showed it makes some sense. Your wall is probably always going to be cooler than your basement. This means that you're more likely to have moisture condensing on the inside of the concrete wall from the room than the other way around (in the absence of any vapor barrier). If that's the case, you want a vapor on the room side of the interface, which is right behind the drywall.

But, you indicate you may be getting some moisture migrating through the concrete wall from the ground - the efflorescence you mentioned. If this is what's happening, that would indicate you want a vapor barrier right against the concrete wall.

So you can see the conundrum here (at least in my mind). You cannot say with certainty which location for an impermeable vapor barrier is correct.

So you might want to consider no vapor barrier or, at most, a semi-permeable one right behind the drywall. The XPS foam board is semi-permeable, so any moisture that migrates through the concrete wall can escape into the room. And the semi-permeable vapor barrier behind the drywall would reduce moisture filtration into the wall cavity.

Finally, my daughter's house in the mid-Atlantic region is similar to yours with a below ground unfinished basement and concrete block walls. She is looking into refinishing the basement, and is facing the same issues you are, so I hoping to be able reach some sort of consensus on this.

Answer 2

A fast and accurate method is to open a wall example on the site www.ubakus.com and to add/change layers. Switching from U to R values can be done by clicking on the spanner icon next to the result. Here is a screenshot from a similar question for a house in Europe.

Answer 3

Reflectix relies on air-entrapment to provide the required insulation.

Are you using "Reflectix® Radiant Barrier (RB4812550)" ? It's perforated precisely for the purpose of venting, so you'll likely have no moisture issues.

But if the permeance is low (i.e. a good barrier) I wouldn't create a moisture trap between the XPS and foil. Venting with air-passages is not an option of course, as it would undo the insulation effect.

You could consider breathable insulation, like fiber glass panels. Or start with thicker XPS and skip the foil.

Also, your local building code would specify the required XPS thickness, which is determined by typical moisture and temperatures, above and below grade.

The point is to ensure any condensation happens on the warm and open side, where it can breathe into the room.

You also don't want moisture from the outside "Earth", entering through the concrete (and XPS), to be trapped and accumulate without the possibility of airing and drying. This would be very little, of course, because XPS has low permeance. However, where XPS is applied as a vapour barrier it still permeates enough to require venting on the inside.

Then there is the risk of inside moisture permeating through the foil and condensating between the Reflectix and the XPS, but this depends again on the permeance of Reflectix, as well as the outside temperature (and of the "Earth") and the thickness of the XPS.