Is exterior insulation a bad idea for us?
We are doing a renovation where we have sprayed closed-cell polyurethane insulation into 4×4 stud cavities and sheathed the house with plywood. Now we’re about to put 2″ of rigid XPS insulation on top of the plywood. Is this a bad idea, or perhaps useless?
I have looked at the 2009 IECC code for our area, Climate Zone 6, which says that R7.5 is required on the exterior to avoid condensation on the sheathing. With 2″ of exterior insulation, we will have R10. However, after looking at some literature on this website, the whole idea of exterior insulation used in combination with closed cell foam on the interior is making me nervous. Is it possible that we will end up with condensation and rot on the sheathing? Or is the extra insulation value here just not that worthwhile? Advice appreciated…
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With 3.5-4" of closed cell polyurethane on the interior you have a powerful 0.25-0.35 perm Class-II vapor retarder protecting against interior moisture drives.
With 2" of XPS on the exterior you would have a 0.5-0.6 perm Class-II vapor retarder on the exterior.
That's not "dries never" stackup, but it's close.
As long as the sheathing is dry (under 15% moisture content) when you install the exterior foam it should be fine, provided the window & door flashing is done correctly. It would be better to use rigid rock wool though, which at ~30 perms would provide an excellent drying path toward the exterior. R10 unfaced Type-II EPS would have twice the drying rate of R10 XPS, and is still in the Class-III vapor retardency range, not Class-II.
The strong benefit of exterior insulation is the thermal break over the ~R1.2/inch framing fraction. The high center-cavity R of the R6/inch closed cell foam is cut off at the knees by the R4-R5 studs, and barely higher performance on a whole-wall basis than filling it with cellulose or fiberglass. R10 sheathing over that would cut the heat loss of the framing fraction by about 2/3. Either would be better than XPS given your cavity insulation, but rock wool MUCH better.
Closed cell polyurethane and XPS are two of the least environmentally friendly insulation products due to the blowing agents used. XPS is blown with HFC134a, with about 1400x the global warming potential of CO2, and as it loses that blowing agent over a handful of decades it's performance falls to that of EPS of equal density. Closed cell polyurethane is blown with HFC245a (with but a handful of exceptions in the industry), with a gwp of about 1000x CO2.
Thanks very much for the response. If I had known the global warming footprint of these materials, I would have done it differently. And I think that we will not be putting the XPS on the exterior, rather, on a steel-framed wall in the cellar.
As Dana says, you can always use rigid mineral wool instead of XPS. It will allow the plywood to dry just fine should it ever get wet.
Assuming you already bought the XPS...
In the cellar you would need R15 continous insulation to meet code min for climate zone 6. With R10 XPS trapped against the foundation with steel studs you can still get to that performance level using 3.5" thick unfaced sound abatement batts designed for steel framed partition walls. The batts must be a full 16" wide, not the 15-1/4" batts designed for wood framing.
Alternatively you can add 1" polyiso or 1.5" EPS to the interior side of your 2" XPS layer.
If you don't already have the XPS, you can get there with 3" foil faced polyiso, or 4" of EPS, both of which are blown with pentane (at about 7x CO2) and no batts. If polyiso, keep the bottom edge elevated off the slab as a capillary break or it can wick moisture if the slab is ever damp. (Putting an inch of EPS under the cut edge of the polyiso would be a sufficient capillary break if you wanted insulation all the way to the slab.)
You've gotten good advice so far. For more information on the principles behind this advice, see this article: How to Design a Wall.