As part of my remodeling / energy retrofit project, the half-century old aluminum siding is to be removed from my home so exterior Roxul can be installed. It turns out that each piece of siding is backed with 1/4" EPS. The siding will go to the recyclers but I hate to see all that foam go to landfill. Any idea how I can repurpose it?

The foam is actually recyclable, but finding someplace to take it may be the challenge. What's going on in place of the siding?

LP Smartside, over 2 3/4" of Comfortboard IS (of which 1.5" will have a lot of thermal bridging due to the strapping).

Why is there thermal bridging from the strapping?

Normally you have a continuous layer of rigid board, and 1 x 4 strapping on the outside of the rigid board through-screwed to the studs 24" o.c. with pancake head timber screws. The only thermal bridging is the screws, which are limited in number & total cross section, with very LOW thermal bridging. See:

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/installing-roxul-mineral-wool-exterior-walls

Just the transportation related greenhouse gas emissions of recycling EPS at a remote side is more than the environmental damage of fresh EPS, unless it is first compressed to a much higher density, and the distances are small.

Re-purposing it as below grade foundation insulation on site is probably about as good as you're going to do. If you're going to be installing a bunch of cellulose in the attic, breaking it up and burying it in the cellulose is pretty reasonable too.

The lowest level of the house (it's a split level) is brick over 4" CMU so there are no studs to screw into. My understanding is that the recognized method to install rigid insulation over masonry is to install 2x4 strapping on the bricks, insulation between the strapping, a continuous layer of insulation on top of the strapping, and then attach the siding through the continuous insulation into the strapping. Comfortboard comes in 1 1/4" thickness, which the LP rep says should work for fastening through (even though their manual says 1" max).

My builder feels it's best to continue the same method all the way up even though the upper story is stick built. Without the strapping he'd have 2 3/4" of squishy Comfortboard to fasten the siding through. I don't think he's done this sort of outsilation before though; I don't live in a hotbed of greenness.

I like the ideas of using it below grade, or mixing it with cellulose in the attic. (Now I am wishing I'd saved all the EPS packaging I've thrown out over the past 5 years to mix in my attic cellulose!)

But you could also pick some of the upper walls, and use EPS in place of some of the layers of comfortboard, e.g. between the 2x4s.

Any plans for future insulated shutters? Or basement interior insulation? Those could be places to use it too.

Breaking it up and scattering over the fiberglass batts in the attic before the cellulose crew arrives may be my best bet. There's also a knee wall which I've sealed and covered with 1" polyiso; I was planning to cover that with some extra Comfortboard but I may put some EPS in between. As far as below ground use or to replace some of the Comfortboard, the builder is doing that work and I don't know that I want to ask him to start putting together scraps - his time is my money. i don't see myself insulating inside the basement - since we had the oil boiler removed the only heat generated down there is waste heat from the freezer, and the new heat pump water heater more than makes up for that. It's going to be interesting to see how cold it gets down there this winter and how well the water heater handles it. It could well be that instead of losing heat through the CMU walls we will be gaining it. We will be adding external insulation around the 2 accessible walls of the basement down to about 3' below grade - had planned on Drainboard but Roxul doesn't seem interested in selling it in less than truckload quantities, even on the same order as Comfortboard IS.