I am in the process of building a new house. Studs are in and sheathing is on. It's a 2.5 story with (future) finished basement. Total of 6700 square feet with the finished basement. Heat is geothermal heat pump, two 4 ton units in zone 4-5 (right on border). We have a LOT of windows. I was originally going to do something very efficient on the walls but due to the number of windows I felt that there was a diminishing return. We put in 2x6 stud bays and want a tight house. I am thinking that foaming the corners and then filling with cellulose would get me close. I also need to do something in the attic. I would like to have a vented attic chase. What type of insulation systems are recommended here?

The number of windows doesn't change the heat loss (in btus) through the rest of the wall. So I'd stick with the originally planned wall insulation. Taped rigid foam + cellulose is hard to beat.

That's true, but it does change the balance point at which more insulation is just a diminishing return. Half my heat load was windows, and I bought very efficient windows. There are 49 of them though.

The original idea was foam, but the bid was an extra $6k for foam. I am questioning whether I get anything from the foam VS just cellulose in the walls. I know that gives me an average R15 or so due to the framing factor, but the studs are about R6 due to the thickness. Again, spending more money here isn't worth it to me if I just gain a few % on my bills every month.

My first house was a 1958 ranch with no insulation. We replaced the siding and blew cellulose in the walls and covered with 3/4" foam and new siding. I never did notice a difference in the heating bills in that house. That just killed me, but it also had a bunch of windows. Increasing the R value of the whole wall assembly didn't do anything for me, just cost money. It never was an expensive house though, $150 heat bill at most in winter.

Also I'm not a huge fan of foam in homes. I've become less so after starting the project. I saw the foam under my concrete smoking where the metal flashing reflected the sun onto it. I don't really want something that prone to fumes or fire on my home.

Are there other sheet insulation products that could do this?

The point of diminishing returns is independent for each element of the house. Whatever you do for insulation elsewhere doesn't effect what is the best wall insulation, btu or $ vs cost wise. OTOH, If you have a fixed budget, then you can correctly argue that you have other things to spend money on that have a better ROI.

People tend to notice percentages of the total heat bill, but that's not relevant to the best ROI for a specific element. One may spend $5K per year on heat, but it doesn't mean that it isn't worth $1 to plug a $5/year air leak.

ROI calculations are best done with software such as BEopt.

You could use Roxul COMFORTBOARD if you want melt proof. But be sure to do something to get a very good air seal. Maybe tape the sheathing.

Posted By Estrogen Hostage on 05 Aug 2013 12:15 PM
Also I'm not a huge fan of foam in homes. I've become less so after starting the project. I saw the foam under my concrete smoking where the metal flashing reflected the sun onto it. I don't really want something that prone to fumes or fire on my home.

Are there other sheet insulation products that could do this?

The high density rigid rock wool panels available are well suited to this application.  From a cost point of view it's a significant uptick from EPS or polyiso, but it's completely fireproof, and has very minimal outgassing from the binders used (the vast majority of which get cooked off in the process of manufacturing the panels.) The compressive strength isn't as high as the mid & high density foam, and the weight is 4-5x that of foam, but it works, and works well.