I used great stuff to fill up a couple of difficult rim joist bays the other day. Here's a tip I'll add: do it in 'lifts', not all at once like I did. I turned around after I thought I was through and there was a cow plop of foam on the floor! It doesn't go far. When I trimmed off the masses of excess foam I had from putting too much in at other spots, I noticed some voids, but it seemed fairly consistent to me, and I am sure it would be functional. Expensive (I got some on sale for 4 bucks a can).

The board is a good idea, I saw one place I was prewiring being insulated, my co-worked said I'd love this guy's work, he's like a zen master. He was using cardboard-faced foam board, and sealing the edges with acoustic sealant. You have never seen anything so neatly done. He did all the headers with it, plus the garage ceiling. I guess that would be the way to go in the accessible areas; I was thinking of the tight fits for the spray stuff.

Good idea doing it in bits, it's horribly messy when it falls/gets carried away.

I dreaded this process but decided to use 1.5" XPS. Actually its not that bad. Just measure your joist, cut the XPS and shove it in. It friction fits nicely. Gaps can be caulked. Far quicker than foaming and less expensive.

I've been working on my mine for a while.
I does get slower when you put in several layers.
Throw in the working around the wires and pipes and it gets even slower. It might be more expensive than getting it foamed by a contractor too-I'm using a fair amount of foam.

I hope to insulate the rim joist of my 1910 home this summer. You can buy kits of the same faom some of the pros use over the internet here is one site. I think it has to be cheaper than buying hte cans.
Here is one of the sites I found. I just googles spray foam insulation.
http://www.sprayfoamdirect.com/

I used 2" and cut the pieces out of a 4x8 sheet and cut them 1/4" or so shy of the average bay sizes to give room for the foam to go in. I had a few that didn't want to stay against the rim joist as they were foamed and I didn't have any washer nails to hold them in tight. The few that gave me trouble got marked with a marker and I drilled a few holes in the foam board and shot foam in the back side and let it sneak back out the holes and shaved the excess off.

it is a night and day difference between the foam board and just having batting stuffed up in the spots. I don't have a heated basement and can tell a world of difference.
I think I got around 30 plugs out of one 2" 4x8 sheet of foam board with a few pieces left over for odd sized spots. One can of great stuff foam did between 8-10 plugs with a price around 6 bucks a can and the sheet price of $22.50. Once you get set up and going it doesn't take too awful long to do. I got the ones with obstructions fitted with a hand saw and in place first and then stuck 10-12 in a row so I just had to concentrate on foaming. I noticed that if you spend too much time putting the foam blocks in after the can has been partially used it wouldn't go near as far as if you started foaming and went from one to the next.

Thanks, that's some helpful info there!

Just a little FYI on basement and duct insulating and sealing. I haven't insulated my ductwork in the basement yet but I did seal them with mastic on both the returns and supplies and did the rim joist insulating/sealing with 2" foam board and spray foam.

All electric house usage:

Feb 2008 : 3600KW
Feb 2009 : 2979KW
Jan 2009 : 4600KW

The rim joist foam project and duct sealing was completed late January. Huge difference in comfort already. With the rate increases here I didn't save much money wise over last year but I sure would have felt the rate increase if I hadn't done it.

i like turdles!