[S-F]

I'm not sure about where you're from be here that kind of house is called a "Cape". They are IMO the most frequently mis insulated houses on the planet. With the way people try to "insulate" them I'd rather live in a yurt.

Here's the catch: People insulate behind the kneewall. I know, the damn auditors have me do it all the time. And it's more or less a wast of my time and good cellulose. That approach is like putting lipstick on a pig. It just doesn't work. You can't remove a portion of the house from the thermal envelope without making it as tight as a freezer. And no one can possibly do that with a kneewall. The proper way to do it is to insulate under the roof. If I had a cape I would make it a hot roof. At least until the top of the slope. I'd seal the soffit up entirely, screw/strap polyiso under the roof and blow cellulose behind it. Also don't forget to do the same on the crawl space gable ends. A house needs to have the entire thermal boundary consistently in the same place. You can either use the ADA (airtight drywall approach) or make the sheathing air tight. Most people use a combination of the two and fail miserably. So you spent all afternoon building, insulating and air sealing your fancy new door to your crawl space.... but what about the transition between the kneewall and the floor? How are you going to make that air tight? You can't. Your crawl space is cold and all that cold air comes through your baseboards. You can't even fix this with spray foam. And let's not forget the floor/ceiling joists that are there. Your crawl space is cold and all that cold air moves through your first floor ceiling and under your second floor feet. What to do about that? Get on the phone with the high density spray foam people and then sell a kidney. The auditors have me do all kinds of stupid things there. Like jam a wad of fiber glass in and then try to dense pack the cavity. Not air barrier there. I'd rather have a tight house and open a window than have my feet freezing all winter. Bring the crawl space into the thermal envelope, have a more efficient house and gain conditioned space. Bargain. Oh, and it's a LOT! easier to do than screwing with trying to insulate knee walls.

Sorry about the rant but I see this all the time and I want to cry every time I'm told I have to insulate a knee wall. I'm dieing when I see that they want me to blow am impossible to air seal wall that's only 3.5' deep when I'm sitting right next to a rafter bay that's almost 8". You can get R-27 in there no sweat and then have R 12 or so of polyiso on top of it. Frik'n R 40 roof. It's no R 60 but it's better than R 13.

Edit:

AC_Hacker, about the foam board loosing R value, it will happen to all of them. R 4 is about as good as you can get with air. Anything over that is due to fancy gasses trapped in plastic. This is why polyiso always has a facing. It only really matters though for the outer edge as the foam is closed cell. But all of them are slowly making their way back to R 4 / inch as time goes on.