How to Build a Net-Zero House · Mar 24, 05:00 AM

One way to think about a net-zero house is to understand that you are paying for everything up-front. All the energy costs that you would pay for on a monthly basis throughout the life of that house, you are paying when you build it. Your mortgage will be higher, certainly, but the question to ask is, will your total monthly costs for owning that house be higher as energy costs inevitably continue to rise? Achieving net-zero house does not take rocket-science. But it does take disciplined decision-making, a systems approach, and a larger front-end investment in energy-production equipment. Below is a general overview of the four steps involved.

Don’t build more space than you need.

You have probably heard the expression that the greenest building decision you make is in deciding what not to build. This is especially true in a net-zero house. What you don’t build, you don’t have to purchase very expensive solar panels or wind generators to heat and cool and light. So avoid duplication of spaces; build spaces that are multi-functional; and keep spaces small. The Not So Big House books are excellent in showing clever solutions to making small houses that feel right.

Build the most efficient building envelope you can afford.

Your heating and cooling system’s basic job is to modify the temperature within your house. If we can make the thermal barrier between inside and outside as formidable as possible, less heat will escape in the winter, less heat will get in the summer, and the system will have to do less work. Less work means smaller equipment and less energy. It is possible—and it has been done— to design a house with an envelope so efficient that the house can be heated all winter with the energy of one light bulb. That wall, however, was actually two walls and was about 18” thick. Not everyone can—or wants to—go that far. We can, however, using SIPS and/or foam insulations, make walls and roofs that go a long way in that direction at a reasonable cost.

Windows and doors are the other big component of your building envelope and here strategy in placement as well as high efficiency is important. The best building envelopes go beyond simply providing a good energy barrier; they are intelligent about it, and are designed to let energy pass though when it is beneficial. Good passive solar design maximizes windows with southern exposures, and minimizes windows facing east and west. The reason is simple. Window and overhangs can be placed on south-facing windows so that the sun can heat the interior in the winter and be kept out in the summer. This kind of control can only be accomplished on east and west facing windows by mechanical means.

Reduce energy demand.

With reduced size and a good energy envelope you have gone a long way toward reducing your home’s energy demand. Appliances, hot water heating, lighting and other electrical equipment are the other big generators of energy demand in your house. They all draw down energy and depending upon their efficiency, they all create additional heat within the building envelope—heat that your mechanical system then has to use energy to remove in summer months.

Just as you have reduced space down to what you really need, reduce the size and number of energy using fixtures and appliances down to what you really need. Then select only those of the highest efficiency. For example: use color-adjusted warm fluorescent lighting rather than incandescent; use only Energy Star certified appliances.

Most importantly, use the very highest-efficiency mechanical system and hot water heating equipment you can purchase. This part of the project takes the most careful design and coordination of all, if it is to create synergies with the other components in the building system.

One item often overlooked by those doing net-zero houses is “ghost loads”— power used by all those computers, printers, and TV’s and appliances that have LED lights glowing around the clock. In most homes today, ghost loads can account for around 25% of all electrical power use, and most people are not even aware they exist. In a net-zero house, these loads can make or break the project.

And, oh yeah, add solar.

At this point you have reduced your energy demand to a minimum, but you are still in the red as far as your energy use. Now you must balance the ledger, and generate the amount of energy to equal the demand. Solar panels are the most common answer for electrical power, although small wind turbine technology is making wind power equally attractive in some areas. In addition to photovoltaics, solar power can heat not only the water in your hot water tank, but also your whole house, if you use a radiant hot water heating system. All net-zero houses must have some combination of these energy-harvesting systems. But if you have done the proper planning and made disciplined decisions about where you “spend” your energy, the solar measures will not break the budget and you will arrive at Net Zero.

— David Peabody

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