That’s the subject of this week’s Q&A spotlight. A tight, well-insulated building envelope is fundamental to a high-performance house. So is a heating and cooling system that keeps it comfortable with a minimum input of energy. What happens when the construction budget can’t handle the added costs of high-quality windows and extra insulation as well as high-efficiency mechanicals?That basic question is what’s plaguing Dave W as he works to complete plans for his new home.“Which would you recommend for spending $$ on first: better windows and insulation to reduce heating/cooling requirements, or higher efficiency systems (including geothermal) to reduce costs for meeting the requirements?” he asks in a Q&A post at GreenBuildingAdvisor.“Perfect world would obviously mean doing both,” he adds, “but limited resources ($$) means we’re probably going to have to make [a] choice between options.” RELATED ARTICLES Concerns over minisplit performanceJesse Lizer wonders what happens to the performance in extremely cold temperatures. Moreover, in researching options for his own house (which he covers in a separate Q&A post, Lizer says that ground-source heat pump systems aren’t necessarily that expensive. One quote he’s gotten puts the cost of a ground-source system at $13,000 (after tax credits), which would be substantially less than the special cold-weather ductless minisplit system one manufacturer recommends.The key, Holladay points out, is that a superinsulated house loses heat very slowly, so anything short of a prolonged cold snap shouldn’t be a concern. “If you’re worried,” he write, “install a couple of inexpensive electric-resistance space heaters to get you through the occasional cold snap. That’s a lot cheater than a $25,000 ground-source heat pump.” Weighing the allure of a ground-source heat pumpA geothermal system (more accurately referred to as a ground-source heat pump) is especially intriguing for many homeowners, including Dave. The systems are electrically operated, so they don’t burn any fuel directly, and they can return several times more heat than the energy required to operate them.This energy output-to-input ratio, called the coefficient of performance or COP, is what makes ground-source heat pumps so attractive. The systems are also better suited to cold climates than conventional air-source heat pumps, some of which need a boost from electric resistance elements when temperatures fall into the 30s.The main reason for the high cost of a ground-source heat pump is the expense of installing the ground loop, whether installed horizontally in trenches or vertically in bore holes. And improvements in air-to-air heat pump technology, including the ductless minisplit systems now offered by several Asian manufacturers, could be game-changers.“Ductless minisplit heat pumps have an average COP that is nearly the same as most geo-source heat pumps,” Kevin Dickson tells Dave, “yet the installation cost is just a fraction of what a central geo-source heat pump would cost. More efficiency improvements are happening in the air-to-air heat pump arena,” he adds, “but the cost of drilling and/or laying geo-source coils is not going down. I’m betting on air-to-air technology to make geo-source technology obsolete very soon.” Spending where it countsDuctless minisplit systems begin to look very attractive in superinsulated houses where heating loads are very low and conventional heating equipment is overkill. And that’s at the heart of Dave’s dilemma.“The answer to your question is simple: skip the ground-source heat pump, for the reasons that Kevin mentioned, and focus your attention on improving the thermal envelope of your home,” writes GBA senior editor Martin Holladay. “Anyone who pays attention to these issues realize that for a single-family house, a ground-source heat pump makes no economic sense compared to the ductless minisplit heat pumps from Asia, which are capable of heating a home when the outdoor air temperature is -17 degrees F.”Armando Cobo put it this way: “Whether you have limited resources or not, every house should have: 1. Top envelope seal job and moisture management detailing. 2. Best insulation that you can afford. 3. Best windows that you can afford. Those three will reduce your heating and cooling loads to the max (within you budget), where you can reduce the size of the HVAC system. Hopefully, the HVAC system is designed, installed and commissioned properly.”James Morgan also signs on for the envelope-over-mechanicals priority list, summing up with this: “A top-of-the-line bilge pump is only needed if you have a very leaky boat.” Heating a Tight, Well-Insulated HouseEquipment Versus Envelope Q&A: Do geothermal systems live up to the hype?Green Basics: Heat PumpsGreen Basics: Insulation Overview Our expert’s opinionHere’s what Peter Yost, GBA’s technical director, had to say:It feels a bit like piling on, but optimizing the building envelope as much as you can afford over the space conditioning systems is the clear winner in my book.A couple of key points:Importance of design: Spending money on optimizing the building envelope means spending money on design first. Building configurations, window placement, and total square area of glazing have a lot of influence on building performance, even before you get to building assembly design and construction. And layout can become really important if you get the space conditioning loads low enough such that an open floor plan means minimized distribution.Do what we do best: As a builder or remodeler, optimizing the building envelope performance is largely in your control whereas optimizing the HVAC system is likely to place much of that control in someone else’s hands. Systems such as ground source heat pumps, in my experience, are very dependent on the quality of installation and the engineering of the system. Spending the money on your expertise and craftsmanship is a more certain return, in my book.Shift to importance of other loads: Interestingly, if you get your space conditioning loads low enough, then the performance of equipment related to other loads — such as hot water, appliances, and lighting — can become as important to your overall energy performance as your HVAC system and its performance.Long-term logic: Lastly, you can almost always change out mechanical equipment much more easily than you can radically change building envelope performance, particularly when the space conditioning loads are relatively small because of your killer envelope. Put your money in the envelope — initially and for the long run.