A blog reader recently asked me to about the performance of the house now that it's a couple years post-occupancy so rather than answering him directly, I figured it's about time (OK, way past time) to post an update.
The short answer is that I'm very pleased with the performance of the house. I can walk around the place in jeans and a t-shirt on even the coldest winter day. There's never a draft, even when the winds come howling off the lake and hit the house straight on. And the snow lingers on my roof long after it's melted off every other roof in the neighborhood.
I'm using about 1000 kWh of electricity per month which might seem high for a energy efficient 1500 sq. ft. house, but around 250 kWh is used just to run the dehumidifier in the unfinished basement. Without that the humidity in the basement would quickly rise to levels where mildew would start to grow. It's a bit galling that so much of my energy budget goes toward this one function.
My other energy hog is a large saltwater aquarium system that consumes about 300 kWh of electricity per month. We all have our vices.
My heating calculations seemed to have been accurate enough as the small Fujitsu heat-pump I selected has been able to keep the house warm even on the coldest nights. The first winter I was in the house we had some long stretches of brutally cold weather with nights going down to -10 °F and days only going up to 0. During those nights the house would go down to about 60 °F - perfectly acceptable.
During the summer the heat-pump has more than enough power to keep the house cool, but I did make a mistake in not including a ducting system of some sort to bring the cool air up to the upstairs bedroom. This is not a problem in the winter as warm air naturally rises but in the summer it gets quite warm upstairs. I had hoped that the air-to-air heat exchange system would suffice for that but it doesn't come close.
Woulda', shoulda', coulda'
The main thing I would differently is that I wouldn't worry so much about energy efficiency! I got so caught up in energy efficiency that I completely dismissed the idea of a woodstove - a decision I now seriously regret. I'm trying to figure out how I might retrofit one in, but it's tough when you don't design it in in the first place. In the same vein, I wish I had more operable windows on the lake side of the house to let in the sounds of the outdoors during nice weather.
Construction-wise I would strongly consider cellulose-filled double-wall construction rather than covering the house with polyisocyanurate panels, primarily because of the cost. And stacking the roof with polyiso panels means lots of very long screws being driven through the roof sandwich searching for rafters, with the potential that many may miss their target. Plus it definitely complicates putting solar panels on a roof as most solar panel systems are designed to be bolted to the rafters.
I now wish I had gone with a cold roof system where there is an outside air gap between the roof itself and the insulated exterior of the building envelope. If there's a problem with a hot roof and water somehow gets into the insulation, it's difficult for the insulation to dry out. With a cold roof, any water that gets past the shingles winds up in the air gap where it can easily evaporate come dryer weather.
Another thing I would like to fix is the damper system on the exhaust hood for the stovetop. Sometimes when it's windy I can hear it opening and closing just from the wind. It's a straight open path for air to escape the house. Plus the hood itself is metal attached to a metal duct that extends to the outdoors so it's a pretty bad thermal short circuit. Because it does need to be fireproof the whole way there aren't a lot of solutions.