"What's in your walls? Have you dense packed them? You have R-10 on the sheathing? Nice! I'm jealous. How did you make R-10? Sounds like 2" of XPS. A 4,000 Sq. Ft. house could certainly be 40,000 btu/hr if the air sealing if handled properly. If it's passivehaus like it could probably be 1/2 that. People are nuts when it comes to building envelopes. I want to cry every day when I go to work and see these houses, many of which are less than 20 years old."
No, The original post was mine but I was trying to do math for what I thought would extrapolate with higher levels of insulation that would still work with the smallest condensing furnace out there with the 2x6 construction with 2" exterior foam, close attention sealing during the build, etc. My exterior foam is R-Max and is 3/4" inch thick, 2x4 walls with fiberglass all installed when it was built.
My top floor has a shared wall with the attic where the studs go straight down for the lower floors with 2x4 framing, but the attic rafters are separate 2x4 2 foot OC with only the interior 2x4s with insulation. I'm adding insulation to the outer 2x6s but I can't find the insulweb stuff, I don't need much, just enough to cover 11 feet wide by 4.5 feet tall. The other half of the upstairs I put fiberglass there but I can't get a roll of fiberglass across the 2 feet of headroom that I have when I need to traverse the vault that is between the two halves of the attic and I'm not about to cut 7 or 8 sections and drag them across that section unless I really need to.
The hard part to get cross, smaller than it looks.
Looking at the area that was completely open from the attic to an interior lower level wall. You can see the 2 foot OC framing if you look to the right of this image.
2 inch foam spray foamed into one of the openings shown above, this was the first one and I was a bit sloppy, the others look better but I didn't take a picture.
The opposite side of one of the upper level walls, this is part of the section that I haven't gotten the insulation into. Ideally I'd like to dense pack this with cellulose. You can see what damage that opening to the attic does from the second picture.
...I also have a skylight where the drywall surround is sheathed but not insulated yet and I imagine that is the biggest energy sieve of the attic. I need to get up there, planning to wrap that with 2" foam, should I put two layers and have 4"? I really actually want to remove that skylight but I don't know how much work that would be because I'd need to cut out the drywall surround for it, cut it out of the roof and patch the roof and the ceiling. Not sure how hard that would be but I think it is out of the cards for me to do that job, especially the roof part.