The zero net energy building is connected to the electricity grid and over the year produces as much energy as it consumes. While any building can be zero net energy simply by attaching enough solar panels to it, value engineering balances load, comfort delivery systems and renewable energy production systems.
In the typical building system we make the thermal envelope target R-20 or better for below grade walls, R-40 or better for above grade walls and R-50 or better for the roof. While many will say this is excessive, insulation is cheap compared to solar photo-voltaic. With zero net energy as the goal, insulation is to be lathered on.
The windows should have a u value below .25 if possible. This means triple pane, low-e, argon filled as a minimum and no metal anywhere. Once the buildings thermal and air barriers work, the windows will be the one significant load left and u-.33 windows can be 90% of the cooling load and more than 50% of the heating load. The right windows are less expensive than solar pv.
The air barrier never works in any building, the question is how bad is it? Every building has so many holes in it that it leaks like a sieve. The goal is to plug those holes up, giving particular attention to the small ones that seem insignificant. The pressure on the building envelope is the same, no matter how well sealed the building is so the small holes get effectively bigger the better sealed the air barrier is.
Many will say that a house has to breath and sealing it up too much is bad and will lead to a sick building. They are right about the building needing to breath, but it should not breath through the skin. Every building needs a set of lungs, taking out the bad air and bringing in the new air 24hrs a day. We do this using balanced ventilation systems, not uncontrollable leakage through the air barrier. A leaky building will not ventilate at the right amount, for most of the time it will be either too leaky or not leaky enough and it will be in the wrong places. By using a balanced ventilation system the building has controllable ventilation and thus the right amount of ventilation all of the time. We can then recover the heating and cooling energy in the exhaust air and transfer it to the incoming air using a heat recovery or energy recovery system.
The heating and cooling system will be a geothermal system designed to be as efficient in operation as possible. By reducing the load significantly with the superior thermal and air barriers, we can afford to use the most efficient geothermal equipment and an enhanced ground connection system. We are always being told our ground loop design is larger than needed, and this is true if the design is for the minimum needed rather than the optimum for operation. It is a better value to have optimum system operation than operating on the edge based on efficiency and life cycle cost.
Now just add solar. Solar thermal for hot water heating and solar pv for electricity to power everything else. Make sure the lighting is led or compact florescence, the appliances are Energy Star, and you eliminate phantom loads. The solar is now affordable as the load is, more or less, 70% lower than the typical new building. Mortgage the improvements for 30 years at 6% and pocket money from the day you move in, or do nothing and pay ever higher energy prices.
A zero energy building locks in energy prices at zero for the life of the building. A typical building will choke on the cost of energy over time.