Designers who work with zero energy homes say "system engineering" is the key to making it happen. The term refers to designing all the components of the unit to work together smoothly. We know and expect this of our cars; the engine, the transmission, the brakes, the chassis are all matched to work as a system. To maximize efficiency, the same concept is applied to a home. The building shell, the windows, the HVAC, the hot water, the electrical are designed and constructed to optimize performance.
Here's a look at the components:
1) Design and Orientation: Roof overhangs, window size and placement, and overall home shape have a major impact. Consider the direction of prevailing winds and how to manage solar gain. If you will be using solar collectors, ensure that a portion of the roof faces true south. The placement of porches, garages, trees, and nearby buildings also have an effect.
2) Insulate and Seal the Building Envelope: Most of a home's energy is spent heating and cooling the building envelope. Minimizing heating and cooling requirements is the most important step in building a ZEH.
a) High R-value Insulation.; For a zero energy home, use high R-value insulation that provides a continuous, unbroken layer around the building envelope. Remember to insulate under the slab as well. Use a high-insulation, tightly-sealed roof system, such as SIPs or closed cell spray foam. Seal all holes, cracks, and penetrations through the floor, walls, and ceiling to unconditioned spaces.
b) Concrete Thermal Mass. Build exterior walls and floors with concrete. Concrete will moderate temperature swings, and can even dampen seasonal fluctuations by "spreading" the constant ground temperature from the footings throughout the home.
c) Airtight Construction. ICFs will ensure airtight walls. Ensure that all windows and doors are tight and meet the required air-leakage standards. The roof and/or ceiling needs special attention, as do the kitchen and bathrooms, to make sure they have adequate ventilation but stil maintain energy efficiency.
d) Windows and Doors: Use triple glazed windows and well insulated or double external doors. The passive gain of incoming solar heat through the windows will cover close to 40% of the heat losses if all guidelines are followed. Skylights can decrease artificial lighting requirements, but use a high-quality, double glazed product.
3) Increase Heating and Cooling Efficiency: The heating and cooling systems need to be carefully matched to the high-efficiency building envelope. Standard-sized equipment will "short-cycle," turning on and off so frequently that it will negate any energy savings. Buying properly sized equipment will ensure maximum efficiency. Another benefit: smaller equipment will cost less.
a) Buy as high-efficiency equipment as affordable.
b) Take Advantage of Earth Temperature: Install a geothermal pump or/and a ground-air heat exchanger where space and cost conditions permit.
c) Radiant Floor Heating: Radiant floor heating provides warmth in a pattern that mimics the body, improving comfort. It heats evenly without drafts and uses far less energy than conventional systems. It also allows reduces noise and allows room by room zoning.
d) Optimize Ducting: Design the supply and return ducts appropriately and seal tightly using approved tapes or mastic. Run ducting in conditioned space only.
e) Alternate Cooling Methods: Where appropriate, consider alternative cooling systems such as ventilation only or evaporative coolers.
4) Decrease Other Energy Requirements: Decrease energy needs as much as possible throughout the home.
a) Install efficient lighting: Use compact fluorescent bulbs where possible, and consider installing permanent fluorescent fixtures throughout the home
b) Install efficient appliances: Use the most energy efficient models available. Focus especially on the refrigerator, dishwasher, and laundry appliances, as they use the most energy. Look for the Energy Star label to help make decisions.
c) Tankless Water Heaters: Use tankless water heaters, especially if the home will not be constantly occupied. Low-flow fixtures in showers and faucets will also decrease hot water demands.
d) Turn off lights, computers, and appliances when not in use: The homeowner has the most significant impact on the actual performance of the ZEH. Programmable thermostats and photo-sensitive outdoor light fixtures will do some of this automatically. Simple things such as turning off lights when leaving a room or closing doors when performing even quick tasks outdoors can eliminate "wasted" energy.
5) Install Energy Generation Equipment
a) Evaluate tax and other incentives: State, federal, and local grants are available to offset the cost of renewable energy. Tax credits are also usually available.
b) Install properly sized electrical generation equipment: Solar panels (photovoltaic systems) are by far the most common. In some areas, wind generation may be effective as well.
c) Consider a solar hot water pre-heat system. Southern, sunny climates may find solar pre-heat systems cost effective.
Remember the concept of "system engineering." Often, low-efficiency components can be compensated for in other areas. For instance a house with many doors may require significant improvement in other components, While a well designed, moderately sized house may find standard doors sufficient.
Vern McKown, builder of the affordable ZEH in Okahoma, used this technique to keep costs down. "We looked at what would be the most cost-effective way to get it done," he says. "For instance, we could have done the ground source heat pump and integrated the hot water tank system, but we could install a tankless hot water system cheaper and get the same net result. Or we could have gotten the solar hot water tank, but again, it was significantly more than the tankless hot water system, so when it came to hot water, we went with the most economical option."
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