Well, I now have exact figures, this time in metric, where it is easier to spot errors. Ponzini used 0.76 cubic meters of the B component, of which maximum 12% is HFC-245fa, or 0.0912 cubic meters. The density of HFC-245fa is 1320 kg/m**3, so the weight of HFC-245fa emitted is 120.38 kg. Since HFC-245fa is about 1000x as potent a GHG as CO2, that would be 120,380 kg CO2 equivalent, or 120.38 metric tons. Assuming again a 30% reduction in heating gas use due to the insulation, 1240 lbs of CO2 eliminated is around 0.562 metric tons, so it would take 214 years for carbon footprint payback. Double gulp!
Note that the 30% reduction in heating gas is just an estimate. It is based on a spreadsheet model of our house that I made a few years ago, which didn't take the floor into account, and probably has other errors. So it's possible that we might get better performance, or even maybe worse if I overestimated somewhere. We will see in the coming years.
A carbon footprint payback time of 214 years sounds like a long time, but fortunately, there's carbon offsets. You can buy tax deductible carbon offsets at Carbonfund.org at $10 per metric ton. They have a variety of programs in the categories of renewable energy, efficiency, and sequestration. I like the forestry projects because, in theory, they can be long-lasting, 80-100 years. Here's a link to one, the Nez Perce Reforestration Project in Idaho. Though HFCs stay in the atmosphere a much shorter time than CO2, as discussed in the previous post, the foam will outgas for a while until it stabilizes, and I would like my credits to last as long as the house or even longer. So I am on the hook for a donation to Carbonfund.org of
Now, you might ask, since carbon offsets are so cheap and the insulation was so expensive, why not simply buy carbon offsets. Actually we do that already for the gas we use for heating, and there were other reasons for using closed cell foam in this house, as discussed in the last post. But there is no guarantee that the next owner of the house will be so diligent. With the foam in place, the house should be good for another 30-100 years, long after I'm gone. And it will continue to save carbon.