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We are building a house in Hanover, N.H.

Based on discussions from GBA, we are adding 2 1/2" XPS to the exterior of the sheathing. What is not clear is what (if any) treatment occurs near the ground level -- i.e., on the bottom edge of the 4x8' panel of XPS.

If left exposed, is it susceptible to rodents, bee nests, etc? The siding will be Hardiboard.

Asked by Frank Torti
Posted Apr 17, 2017 9:05 AM ET
Edited Apr 17, 2017 9:59 AM ET


4 Answers

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The bottom of your insulated wall needs metal flashing and insect screening. GBA has produced a video that shows how these materials are installed. Here is the link:
Video: How to Install Rigid Foam Insulation Outside a House.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 17, 2017 10:03 AM ET
Edited Apr 17, 2017 10:04 AM ET.


I can't expand on Martin's link, but howdy nearly neighbour, we're in Peterborough!

Answered by Ben Balcombe
Posted Apr 17, 2017 3:05 PM ET


If the 2.5" of XPS is to push past the IRC's R11.25 minimum for dew point control on a 2x6 framed house in zone 6, it's not really enough margin. As XPS loses it's HFC blowing agents over the decades it's performance drops to about R4.2/inch when measured at 75F mean temp through the foam.

Although it's performance increases with falling temp, counting on more than R4.5/inch wouldn't be prudent. At 2.5" that would leave the bare minimum R11.25 specified by the IRC.

A greener choice would be 3" of Type-II EPS, which tests at R12.6 @ 75F mean temp, and R13.5 @ 40F mean temp. It's blown with a much more innocuous blowing agent, pentane, most of which is released from the foam & recaptured during manufacturing.

Alternatively 2.5" of polyisocyanurate would get you there. Even though polyiso's performance drops with temperature, at 2.5" it would still be good for at least ~R12 during the coldest weeks, and much higher during most of the heating season. Polyiso is also blown with low-impact pentane.

The greenest foam though is reclaimed/reused foam of any type. Just be sure to derate re-used XPS to the fully depleted R4.2/inch.

These folks in Barre VT deal in reclaimed and factory-seconds foam, and are probably close enough to your site to be worth the drive (they might even deliver... or not- it's about an hour each way):


Answered by Dana Dorsett
Posted Apr 17, 2017 7:49 PM ET


Sorry to be the party pooper here, but I think it is becoming abundantly clear that XPS has no place in a "green" building (as I'm assuming are discussed at greenbuildingadvisor.com). I don't have the calculations on hand, but I think the energy (carbon emissions) saved will never make up for the global warming potential (GWP) of the XPS.

Answered by Ethan T ; Climate Zone 5A ; ~6000HDD
Posted Apr 17, 2017 9:31 PM ET

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