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.

>> 1) Has anyone ever done this type of work? >> 2) Does the method explained by my electrician make sense? >> 3) If so, what would you suggest for a backing before the foam is sprayed on? >> 4) What type of foam would you recommend? >> 5) Or is there a totally different way of doing this? Consider this concept: push crumpled newspaper into the wall cavity as a backing for the foam. Squirt spray foam in next to form a barrier (that sticks to the paper and everything else inside the wall cavity) to prevent the next stage's mud from sliding down inside the wall. Some foams dry very fast. Everything the electrical guy uses after the foam will be dry mix plaster. The pre-mixed stuff is for production work and homeowners, not someone who wants to finish today and go home for supper on time. If he uses painter's mud it will set beyond the working stage in five minutes and to the now-we-can-cram-more-mud-in-the-hole stage in maybe ten minutes. You have to work fast with it. The rest is just details anyone would be able to deal with. Some people would pull the baseboard and others would make holes instead of a long "slit". This guy would seem to be comfortable enough with his plastering skills to do it this way which admittedly LOOKS horrendous. I'm impressed with this guy's idea. Let the guy run with it. This post was edited by cold_weather_is_evil on Mon, May 5, 14 at 17:27

felchy, I'm in Calgary. We used spray foam in a few key areas that would be hard to seal well with bat and plastic sheets. But to do the whole house was just astronomically more expensive. About 4 times the cost. One company was really insistent on doing all spray foam. They kept telling us that it would pay for itself in 5 years. BS. It was an extra $30k for the spray foam. So in order for it to pay for itself in 5 years, we'd have to save $6k in heating bills each year. Even in our old, inefficient house, we don't pay $6k/year, so there's no way we could save $6k/year. Our plumbing and electrical were done before the insulation, so that wouldn't have affected the price. But it's common to leave the basement unfinished, so there's no plumbing and limited electrical run. And they do the insulation there when the house is built. Several years later, if you try to run electrical to finish the basement, it's a huge PITA to get through the rock hard insulation. Not like just running the cable along the squishy bat. So I can see if there is a need to do electrical and plumbing after your insulation, you'd encounter an extra cost there. The thing with the builder is that anything unusual is going to be more expensive. If his crew isn't familiar with it, they are going to take time figuring it out. Even if it's a great, easy to work with product, if he doesn't know it well, there's a risk. And he's going to need to build that risk into his price. One of the "crazy" things we've done in our build is 2" of exterior rigid foam. The exact R values is DH's territory, so I don't know what we've ended up with. But we have the code required 20(?) with bat, and then another 10? in the exterior. In addition to the extra R value, it gives us insulation over the studs and joists, which no amount of bat or spray foam is ever going to give you. It's also unusual, so we've had to fight for it. (The foundation guy didn't want to do it at all, so DH and I did it ourselves. And then the siding guys were okay with it, but the building inspectors lost their mind and needed an engineer's report on it.) But it's significantly cheaper than spray foam.

The moisture problem on the roof deck may occur when the deck is colder than the air rising into the attic. How is a whole house dehumidifier going to solve this problem in the winter. During the winter houses in SE PA are struggling to raise their humidity level to 40%. Many are running humidifiers in the winter which will further aggravate the problem. I have looked at a lot of new houses in the NJ and PA area. I can't recall any having HVAC equipment in the attic. I have never seen a house with a spray foamed roof deck either. I am skeptical you will find an insulation contractor who has experience doing this in a new residential home. If you do find one, you know the questions to ask. See if his answers make sense.