How are attic slopes modeled for foam insulation. When ever I've added a foamed slope I get a very low SIR with virtually no savings. What am I doing wrong?
You are doing nothing wrong. Modelling software does not properly reflect savings and I suspect 2 reasons:
1 - Air sealing. Foam provides insulation AND air sealing. Once you play with this you start to see that the bigger direct energy benefit of foam is air sealing. (Like a furnace with an ECM blower, you need to combine savings from multiple benefits). So remember not to obsess on one improvement, step back and look at the whole impact.
Oh - and smaller furnaces tend to model higher consumption, THAT has proven over and over to be TOTALLY inaccurate. So be cautious relying on the software for design. It's A tool, not THE tool. The most important tool is above your shoulders.
2 - Conditioned Space vs Unintentionally Conditioned Space. When you redefine an attic the software sees the additional space as additional load. In practice this space is an insulating buffer in and of itself. Heat transfer from conditioned to unintentionally conditioned is very slow due to small delta. And heat lost from unintentionally conditioned to outdoors is slower because IT has a smaller delta.
I'm sorry this is not a very scientific explanation, but it's a conclusion arrived at by struggling with the same question you are asking. In NY there is a "work around" that moves existing insulation to the surface being improved, then adding the foam to the existing surface.
Remember, SIR is an absurdity. Net cost is what people are really concerned about. True up your model, use accurate fuel rates, and you can get pretty accurate net cost. We've experienced realization of between .9 and 1.3, which feels about as close to dead nuts as one could expect given unforeseeable variables of behavior and weather.
Ok , so put some of the cost for foam as an air sealing improvement, that I got. Unfortunately Home Performance w/ Energy Star requires an SIR of 1.01 and if the heating fuel is gas or electric it must pass a TRC of 1.
So I'll see how it all works out.
In March they changed the rules. Some improvements are "deemed" cost effective even if they are not cost effective. (love that one, this thing has turned into a Frankenstein monster.)
Then later they changed the rules again. I believe Natural Gas requires SIR of .8
For approval I think air sealing can't be more than 49% of the cost.
Combine machine gun "change notifications" that are then not followed up with updated Eligible Measures lists and everyone is confused. Because challenge and thus cost of getting jobs approved has gone through the roof, I believe very few are even trying to run work not "deemed" effective.
Last one I'm aware of is January 1, which it completely inaccurate. Good luck!
Paul, since you are in NY, call Earl Hicks or Terry Clough at CSG. They'll tell you the work around.