After years of planning and months of work, we finally took occupancy of our new SIP/modular hybrid home in early 2009. After living in the home for several weeks we have had the opportunity to put the house through its paces. Now I can say with certitude, "Why wouldn't everyone want to build this way?"
Despite only being in the house for a few weeks, we've had the opportunity to experience living in the house through rain, snow, and outside temperatures ranging from ~0ºF to ~50ºF. Here are a few observations:
Indoor air temperature has been incredibly constant during even the coldest periods, ranging less than 5ºF from the floor above the unconditioned garage to the interior peak of the roof (in the conditioned storage space). The inside surfaces of the exterior walls are essentially "room temperature" except for areas with less insulation (i.e. exterior corners, door and window openings). As expected, these areas are a few degrees cooler.
On bright sunny days, the interior of the house heats to approximately 72ºF using only heat from sunlight coming through the South facing front windows. On these clear days, the heating system never comes on, even when it is quite cold outside. The conditioned basement does tend to get cool on these days, but that is expected. The foundation is (by far) the least insulated part of the house, we only provided enough BTUs to keep it "conditioned," and we never intend to make it living space. There is an important "catch-22" here - When the heat pumps are not running, they are not heating hot water.
This is one area where I would have done things a little differently. I would have gone with solar hot water rather than adding desuperheaters to the ground source heat pumps. With all the effort focused on reducing heating and cooling loads, I could have done a better job with the hot water. If you recall, I originally specified a GSHP manufacturer that could provide 100% of our domestic hot water on-demand, and do so at the efficiency of the heap pump (300-500%) (see: http://www.hydrodelta.com/product/megatek.htm). Unfortunately, their shipment delays meant we had to choose another supplier (Climate Master) whose heat pumps are just as efficient, but can not provide 100% hot water on-demand. The decision to switch heat pump manufacturers was made quickly, and we hastily had to find a way to heat our water. We ended up with a hybrid system that works, but is less than my ideal.
Both ground source heat pump units have desuperheaters (which use waste heat to heat hot water). When the units are running, they heat water stored in an insulated electric hot water tank (never plugged in). The water from this tank then goes to the inlet of the Noritz on-demand hot water heater. The on-demand heater is fueled by propane, heats according to demand, and is 94% efficient (see: http://www.noritz.com/homeowners/products/view/0841_series_condensing_tankless_water_heater/). The Noritz unit has enough heating capacity to heat all the hot water we may need without any help. Any "boost" provided by the heat pump pre-heat system simply means we use less propane to bring water up to final temperature of 130ºF.
Evacuated tube solar collectors are quite efficient, even on overcast days, and their cost can usually be justified - especially with available rebates and incentives. Originally, I had planned to install solar hot water, but put it aside after finding a single unit that could heat, cool, and heat 100% of our hot water. I was aiming for simple, and ended up with slightly complicated. Perhaps I'll add solar collectors in the future and give the storage tank some additional boost.
I love to see the faces of people who ask about our windows. They are triple-pane krypton filled and were manufactured by Alpen in Colorado. While the windows look and perform great (and have a nice geek factor) I am not sure they are worth the additional cost and effort. Since Penn Lyon refused to install my choice of windows in their factory, we had to install them onsite. This left a lot of work undone as all 32 windows had to be installed and trimmed out. This work could have been done both quicker and easier, and therefore less expensive, if done in the factory.
This house scored 38 on the HERS index. To find out what this means check out: http://www.natresnet.org/.