Used XPS or new EPS for outer layer of over roof insulation
I can get reclaimed 3″ XPS and 4″ ISO insulation locally for less than half the cost of new material. I am planning to reroof and put 4″ of ISO over the roof deck then a second layer of foam above that. I am in zone 5B and need at least R-20 above the deck to control the dewpoint (quick thank you to Martin for providing a place and tools to learn about these things).
Although XPS has a high GWP this is used so should I care, or should I get new nailbase with EPS?
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First, a note to readers: ISO is a brand name for a type of polyisocyanurate.
This is a moral or ethical question, not a building science question. It is similar to the dilemma faced by an animal rights activist who doesn't like the idea of mink farming, but who buys a mink coat in a used clothing store, "because the animals killed to make the coat are already dead."
Here's my conclusion: since the rigid foam was reclaimed from a demolition site or a reroofing job, it's good to recycle it and use it, regardless of the global warming potential of the blowing agent used to manufacture it.
But your own ethical compass may be different from mine. If you want, you can submit this question to "The Ethicist" column that appears every week in the New York Times magazine.
Two ways to think about it:
1) If you don't use the XPS it will get landfilled, and break down faster and release its HFCs or HCFCs faster. So you are helping responsible stewardship of it by using it.
2) If you don't use the XPS, someone else will buy it. In that other job, it might get used instead of new XPS. In that application, it will have a big benefit in avoiding new HFC emissions, whereas if you use it, substituting for EPS, it won't have that benefit. So you should let it go to that other job where it will have a bigger benefit.
Hmm, those argue for opposite decisions. I think #2 seems more likely to me.
Charlie, I'd say there's a looming option #3: if you use this salvaged XPS, you are only encouraging the salvage market for the material, which will enable a builder somewhere to tear out yet another perfectly good installation of rigid insulation and replace it with new, dangling the incentive of lowered cost by selling the old stuff.
These insulation "recyclers" are all over. Where does all this "nearly new" "once-used" insulation come from?
My rationale for using recycled xps:
It is primarily generated by commercial roof replacement. Reusable insulation is a byproduct. No one is removing it just so it can be sold. Before recyclers, it was thrown out.
Manufacturers are starting to develop less harmful blowing agents, so eventually even new xps will be more benign, but still a huge disposal will exist unless it gets recycled. Eventually, a better subslab insulation will be developed, but until then, why not recycle?
Nonsense Andy. Use recycle, by far the best for the environment.
Eric the problem will come in ten years when you have rot in your sheathing if you build a cold roof. Vent above, close off existing venting.
With any reclaimed XPS/polyiso/EPS the lifecycel environmental hit from blowing agent & polymer manufacturing have already been taken, and even used EPS or polyiso may well have been made using CFCs instead of the non-ozone-eating HFCs. Extending the lifecycle time period improves the cost/benefit of those already-taken hits by increasing the benefit.
Installing virgin stock EPS involves a new environmental hit, and though the pentane blowing agent is comparatively benign compared to whatever the used XPS was blown with, it's a fresh hit that arguably didn't need to be taken, and that goes with the environmental hits from the polystyrene as well. Re-use is (almost) always better for the environment than recycling or manufacturing new goods.
When using reclaimed XPS, presume that it's performance is it's fully-depleted R4.2/inch, not the labeled R5/inch, since you have no way of knowing just how long it has been in service (or how long it will continue to be in service.)