The retrofit passivhaus (aka EnerPHit) stuff is well beyond me. I can't come up with how the ROI would pay off to tear down lots of drywall, add studs, insulation, and redrywall, replace windows, etc would pay off, even over an extended period but the insulation that I've added where I can do far and air sealing has done tons. It's clear to me though that I've got a long way to go for air sealing because I can watch the humidity rise on a high dew point day at a fairly fast rate even though I've got no latent loads in the house(time between showers, cooking, etc) and on a day with a matching indoor/outdoor dew point the relative humidity is solid. To me that's the universal indicator of air leakage. I'll do what I can to seal the place but outside of tearing apart the half of my basement to get at the sill plates that I can't reach, I can only go so far. I suppose I can seal off the finished area gaps from the unfinished part of the basement the best I can and hope for the best.
If I was going to tear down all the drywall and basically rebuild the envelope from the inside, I'd rather sell this house and build one that better fits my needs and go as super house as I feel fits my needs. For me I don't mind a little air leakage as S-F suggests and would like to be sure insulation cavities are sealed the best they can be and use well performing windows and thick offset stud walls with cellulose and do the best with air sealing as the house is built and hope I can track down a reputable contractor that can give me some sort of guarantee on airtightness.
There's that weird point that you reach where installing a solar heat system with lots of storage or going with baseboard heaters and using ductless mini-splits is cheaper than going the conventional ductwork natural gas HVAC route. I'd include a flex-ducted Recovery Ventilator as the only ducting in the house. If I get to that point, I'd be plenty satisfied and would like the ductless mini-splits for their dry function and variable capacity, not to mention heating without baseboard almost 100% of the time. I'd probably just skip the baseboard and use the little space heater I already have if I need the backup heat.
I won't get to that point retrofitting my house, even though I could keep it to temp with the smallest variable speed motor natural gas furnace that is sold today for homes and I suppose if natural gas gets expensive enough I'll toss in a mini-split as my current furnace is a 76% efficient clunker and my AC is 8.5 SEER, a 95% efficient furnace doesn't have the ROI for me when my heating bill has never been over $70 last winter and was less this winter. Electric bill has never hit $100 and only once went over $70. I think I'm doing pretty well already considering I have R19 walls and quite a bit of leak. I don't mind temp swings as much though, so usage and bills will change when someone else moves in.
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