Received a quote for $2400 for spray foam. Open cell foam at 5" thick. Stated it would be R19.5. Is it me or isbthat alot of $$$ for a low R value. With the R38 batts it works out to $1300 and double the R value.
I forgot to ask the going rate for open cell for my past project. Without doing the math it seems like the price you were quoted is well under half of what people have been paying for closed cell.
The important thing is to get the R value ratio of foam to fiberglass correct for your climate zone.
With R 19.5 in foam and approximately half of your I-joists left open that really only means you have room for around R-19 in fiberglass a bit more if you use mineral wool.
That ratio is good for up to Climate Zone 6. I'm assuming you're climate zone 4 or 5 and I'm not sure how those ratios change for your area other than to say you can likely use less spray foam and more fiberglass (or whatever).
When doing a combination of spray foam and any sort of other insulation the general rule of thumb is to spray the minimum amount of foam to create and vapor and air barrier and meet that ratio for your area and the depth you have available. Once you spray anything more than that the thought is you're wasting money. I don't think a 50/50 ratio for your area is necessarily a bad idea as it only increase your safety when it comes to dew point control.
I had ~3.5" of closed cell spray between 12" I-joists for the roof and cantilevered wood floor of my cabin (a completely open crawl space). From what I read open cell was a poor choice for this and I assume it would be as well for a garage under a living area.
Did the contractor specifically recommend open cell?
