Heard of Performance based vs prescriptive based insulation standards for a foam roof job?
I received 4 bids for a spray foam insulation job in our 3rd floor renovation (a bedroom and bathroom and attic area). Closed cell foam is to be applied to the gable roof surfaces and vertical walls. I think code is R30 roof and R19 for the walls. Some of the rafter joists are 2 x 6 so I thought foam would be a much better solution than a vented roof assembly with fiberglass.
One bid came in with 2.5″ of CC foam on the roof surfaces. The other bids specify 4 inches of CC foam. on the roof. The rationale for the bid with shallower fill was that one can use a “performance based standard” rather than a “prescriptive based standard”. As CC foam is ~R7 / inch a 4″ fill satisfies the R30 code requirement. The “performance based standard” is based on the claim that foam at 2.5″ provides vapor barrier, complete sealing, and sufficient insulation to make the roof perform at R30. The claim is that in the real world the 2.5″ foam would make the roof perform as good as a roof insulated to R30 with fiberglass batts (which is probably not R30 in practice, I suppose). The shallower fill is ~20% less expensive than some other bids.
I’m not familiar with performance based standards. How would one really document this, by measuring heat flux through real world roofs using thermal imaging? Can anyone explain that the shallow fill foam solution is a sound idea? It seems logical that thicker foam will have greater performance. At some point more foam becomes a poor use of resources. I’m inclined to have 4 inches put in.
Curiously, the bids vary in cost by 100%, even though the work is clearly specified. No one is recommending additional work that would in any way explain the difference. I don’t think I’ve ever seen bids that vary this much.
Replies
The 2-1/2" guy
is trying to give you a justification to go along with the lower price he gave you, which got your attention. While I wouldn't buy what he's selling, he is pointing out one thing that you already know, which is that in a real-world situation, his foam might outperform much thicker fiberglass batts in a vented roof, where wind-washing could reduce the value of the FG to a very low level. But, his 2-1/2" will max out at R17 or thereabouts, and I wouldn't be be able to sell that to the inspector here as R30 no matter what. At 4" you are probably close to R30 but not quite there. Why not have them fill the 5-1/2" rafters full and do similar in other areas? It'll never be as cheap as it is right now to get a well-insulated job.
It's B.S.
"Performance based" codes typically apply to new construction or substantial renovation.
With that said, 2 1/2" wouldn't even pass in a "performance" code. Because a performance code is a holistic and takes into account the heat sources and all the other building components. In some locales, you may be able to meet performance codes with R-17 as part of an entire envelope package, but it is incorrectly applied here.
And while it may work better than garbage, it will not work better than 4" of foam.
-Rob
Codes and particularly energy codes often offer different methods of compliance. Prescriptive, component performance, and whole building performance. Prescriptive simply spells out e.g. R-values for this and that (walls, ceilings, windows, etc.). You simply install the prescribed value ... done and easy ... but often more stringent than another option. Component performance allows you to put e.g. less in the walls and more in the ceiling if the resulting heat loss will be equal or less ... requiring a fairly simple calculation. To say a certain R-value is 'required' is a bit of a misnomer under this option; technically no specific minimum R-value would be required. Whole building allows tradeoffs between systems ... e.g. walls and furnace efficiency.
I've been working w/ energy codes for 30 years, now. Your contractors suggestion that his performance based standard is plausible. However, we've seen in the energy industry many claims for 'equivalent' R-values through a variety of strategies ... radiant barriers, thermal mass, etc. While these may work under some circumstances, they often fall short of the claims of "equivalent R-value" and are often backed by unsubstantiated or shaky tests.
Your contractor is saying that the install will substantially reduce e.g. air leakage ... and therefore increase the 'effective' R-value of the construction. A little right and a little misleading. Right ... substantiall reduce air leakage. But other insulations [properly installed] would not necessarily promote high air leakage. It is improper to combine the affect of [reduced] air leakage with the R-value. He is making claims unsubstantiated for your house. While I respect that his product will provide a lot of additional energy benefit, it will NOT be a higher R-value for his 2.5" of insulation. That could be proven by a standard hot box test used to determine R-values of any insulation.
If you specified a specific R-value or if the prescriptive part of the code applies to this install, he needs to install a full thickness to achieve the specified/required R-value. He's playing games with the concepts here and he shouldn't be doing that. Shame on him. It is generally considered pretty sleazy to make this kind of a claim. Given 2 bids for the same material, he shouldn't be claiming equivalent performance. Make him drop the hype and correct his bid if he wants to compete.