12-18-2013 How to Build a Net Zero House
Interview with David Peabody
By Douglas Elbinger, Energy Policy Analyst, GreenLancer.com
My search on the web for net zero home experts kept leading me to Mr. David Peabody, who stands out, according to Google, as a leading authority on the subject ….so I asked him to consent to an interview. I have filled in some holes in my notes with material from his website (see link below). David has led Peabody Architects, in Alexandria, Virginia, since 1992, designing over 100 homes and additions. At the turn of the millennium, he committed the practice to sustainable residential design, becoming a Passive House Certified Consultant in 2009. He articulately describes what it takes to build a Net Zero House. –Douglas Elbinger
The bottom line is always the first question.
Can Net Zero Energy Homes be Cost Effective and Affordable?
One way to think about a net-zero house is to understand that you are paying for all ‘energy cost of ownership’ up-front. All the energy ‘expenses’ that you would normally pay for on a monthly basis throughout the life of that house, you are paying when you build it. Imagine that you are converting an energy expense into an energy investment. Your mortgage will be higher, certainly, but the question to ask is, will your total monthly costs of ownership be lower as energy costs inevitably continue to rise? Achieving a 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 conservation, use, and generation.
Net Zero Energy homes, unlike many so-called ‘green’ homes do cost little more to build than conventional homes. Simple, cost-effective, standard green building designs and techniques, as well as readily available materials, equipment, and appliances can be used to build Net Zero Energy homes. In conjunction with state, federal, and some utility incentives or tax credits for solar PV panels, solar hot water, energy efficient heating systems and appliances, these techniques can bring the sales price of a Net Zero Energy home to as little as 5% to 10% above that of a conventional home built to code, depending on the programs available in each state. The additional expense for a Net Zero Energy home can usually be paid off within about five to fifteen years at current energy prices. Or the expense can be wrapped into the cost of the mortgage, and amortized over 30 years at current interest rates. The advantage of wrapping the cost of your utilities into your mortgage is that if energy prices go up the mortgage payment is fixed, and the cost of your “utility bill,” which is now the cost of your energy efficient upgrades, would then be tax deductible under the mortgage interest tax deduction. Without the tax credits, which not everyone is able to take advantage of, the additional costs for a Zero Energy Home compared to a conventional one may come to about 10 – 15 % more.
Another reason that Zero Energy Homes are affordable is that they are a positive investment: they usually have a higher market value than similar homes built to code. Increasingly buyers are looking for homes that are airtight and well insulated with low energy bills, and increasingly realtors are finding that energy efficiency features positively affect the re-sale value of a home
A house we are building now will be about 8% more expensive in terms of bricks and mortar cost. That ends up being about 4-5% more in terms of total project costs. Projected monthly energy savings ($520), when compared to a conventional home, more than offset the increased monthly mortgage cost. And those monthly energy savings are only going to go up as energy costs rise.
In designing a NZE home what are you first priorities?
First priority is; 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 solar panels or wind generators to heat, cool and light. So avoid duplication of spaces; build spaces that are multi-functional; and keep spaces small.
Can you tell me more about the importance of building envelope?
Essentially, 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.
Explain how you reduce ‘energy demand’ in the design stage?
If you are to achieve a NZE home you must plan to reduce energy demand in the planning stage. Using a multitude of incremental energy efficiency measures, with reduced size, and a good energy envelope you have gone a long way toward reducing your home’s total 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. 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 ‘ready’ 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.
Now that you’ve reduced demand to a minimum, how do you balance the energy equation?
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. In addition to photovoltaics, solar thermal 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.