Well I am finally going to place an order to finish my double studded wall assembly with open cell foam my wall is a staggered double studd of a 2x4 outer and 2x3 inner wall. This puts a total thickness of 6 inches not counting the exterior osb and siding. Sadly we didn't get any rigid foam on the exterior. In fact the builder said it doesn't even get cold enough for double studds. I also wish I had used 2x4s on the interior instead of the 2x3s.

I applied 1 to inches of closed cell foam completely coating the outer wall, the outer studs, and up on to the roof deck. The foam I chose was a 1.5lb foam which isn't completely closed cell but still adds structural strength. I described the entire spraying process in another thread which was for some reason deleted. Anyway, the stuff is supposed to be green as far as the blowing agent and takes a small r value hit. It is something like R5.8 after curing. So I am filling the rest out with the open cell foam since it will actually be cheaper than getting all the netting and a decent cellulose blower to dense pack. So at this point I will be looking at about an R24 wall without thermal bridging.

Having some concerns about the future I am considering another step. I know that the cost benefit of foam almost goes away after about 5 to 6 inches of foam. It also doesn't make sense to dense pack anything. So I am considering a third inner wall framed out of 2x4 to act as a service cavity. I would fill the wall with old stile low density baths.The idea would be if I ever needed to do any wiring that the only thing that would get messed up is a little fiberglass. I wouldn't be boaring thru foam or cellulos. I would also get another r13 on top of the r24. It would also be very cheap and add strength over 2x3s. I have several questions though.

1) in a central va cclimate would the extra fiberglass even make that big of a difference? Should I just sure up the wiring before hand and say the check with it r24?

2) the extra weight. Engineered 24foot beams over a 25x25 garage with 2x4 walls. Would it put too much stress on those walls with the extra weight?

3) would the time and money be better spent waiting and finding a contractor down the road that will retrofit the entire house with rigid foam under the siding?

Thanks.