Green Building for Beginners
Are you are embarking on a construction project, but still uncertain about how to make your home green? Here is some guidance.
Green building websites can be confusing. One site might tell you that a green home should include spray foam insulation, a tankless water heater, and a geothermal heating system. After you’ve absorbed this advice, you visit another website, where you learn that spray foam is a dangerous petrochemical, tankless water heaters are overpriced gadgets, and “geothermal” systems aren’t really geothermal.
Eventually someone tells you that you can usually trust advice from Green Building Advisor. After reading a few GBAGreenBuildingAdvisor.com articles, you start to feel more comfortable. But it only takes a few clicks to end up deep in a thicket, even here at GBA. In no time at all, you are stumbling again.
One GBA article explains that a good way to ventilate your house is with a central-fan-integrated supply ventilation system. Hunh?
Another article advises you to aim for 1.5 ach50. What’s that?
Still another article talks about a type of insulation that is air-impermeable but vapor-permeable. At this point, you’re probably tempted to go back to the website that told you to buy a tankless water heater. At least that advice was easy to understand.
Don't give up just yet
But still, you keep on studying. This type of reading is painful, like learning ancient Greek, but you know that there’s a prize at the end of the tunnel: someday, you’ll finally understand Homer’s wine-dark sea — or at least be able to talk to your builder intelligently.
So you practice pronouncing “polyisocyanurate,” and then you go to your local lumberyard and order some from the guy behind the counter. He answers, “Poly what? Never heard of it.” (This actually happened to me a few years ago.) At that point, you don't trust any information found on the web, and you're probably ready the throw in the towel. And I don’t blame you.
Let’s start at the beginning
In this article, I’m going to aim my advice at typical homeowners (or would-be homeowners) rather than designers or builders. I’m going to assume that the readers of this column are basically starting from scratch when it comes to construction knowledge.
I’m also going to assume that you want to build a new home or renovate your existing home. Where do you start?
If you are planning to build, and you want your home to be energy-efficient and green, here are some basic principles to get you started:
- Ideally, a green home is small. If you follow good design principles, you can make a small space attractive, durable, and eminently livable for your family.
- The most important way that you can improve the energy efficiency of a home is to reduce air leakage. You are unlikely to end up with a tight house unless you choose a designer who understands the need for airtight construction methods, and unless you choose a builder who has already built a tight home that has been tested with a blower door.
- The second most important way that you can improve the energy efficiency of your home is to specify insulation with a higher R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. than the minimum values required by your local building code.
- Almost any kind of insulation can be made to work, although some types of insulation — most notoriously, fiberglass batts — are difficult to install well and usually perform poorly. Among the most popular insulation materials specified by green builders are cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. and mineral wool insulation. For certain applications, it’s hard to beat rigid foam or spray foam, although some green builders try to minimize the use of foam insulation products.
- The third most important way that you can improve the energy efficiency of your home is to specify windows with the right type of glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill.. At the risk of getting too technical, that means that you want windows with a lower U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. than run-of-the-mill windows. In hot climates, you should choose windows with a low solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) for your east and west sides. (Hot-climates designers should try to keep west-facing windows to a minimum, or include a wide porch on the west side of the house.) In cold climates, you should choose windows with a high SHGC, especially for your south side. These numbers — U-factor and SHGC — can be found on most window labels.
- If you have followed the above principles, your heating and cooling needs will be minimal, so you won’t need an expensive heating or cooling system. That’s why you probably won’t need a ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. (sometimes called a geothermal system) or a radiant floor distribution system.
- If your home includes forced-air ductwork — most homes do — the ducts must be located inside your home’s thermal envelope. Do not allow your designer or builder to locate ducts in a vented crawl space or a vented attic.
- Tight homes need a mechanical ventilation system. There are three basic choices: an exhaust-only system consisting of one or two high-quality bath fans; a supply ventilation system that introduces a little fresh air into your furnace ductwork; or a balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). system that includes a heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) or an energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.). Any one of these systems can work well. At this point, you don’t have to understand all the ins-and-outs of these three ventilation systems, but eventually you may want to read up on these different ventilation strategies.
- When it comes to domestic hot water, designing a compact piping arrangement — with a layout that places your bathrooms as close as possible to your kitchen — matters more than the type of water heater you choose. Installing a solar water heater rarely makes financial sense.
- At least two aspects of your home are hard to change: the height of your ceilings and the width of your roof overhangs. Make sure that your ceilings are high enough (in my opinion, a 9-ft. ceiling is almost always preferable to a 7-ft. ceiling) and that your roof overhangs are adequate (in my opinion, wide overhangs are usually better than stingy overhangs).
- You can choose trendy interior finish materials like bamboo flooring or earth plaster if you want, but the decisions you make on interior finishes are unlikely to matter very much from an environmental perspective — especially compared to energy efficiency decisions.
Questions for your designer or architect
Before you choose a designer or architect, ask the candidates a few questions:
- What makes a house green? (A designer who mentions things like “keeping the house small” and “keeping energy bills low” is on the right track. A designer whose first answer is “choosing green materials” may not be the one you want to hire.)
- What design elements contribute to energy savings? (Good answers include “paying attention to airtightness” and “making sure that the walls and ceilings are well insulated.” You don’t want to hear, “Choosing a furnace or air conditioner with a high efficiency rating.”)
- When you draw plans, do you indicate the location of a home’s air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.? (If the designer says “yes,” that’s a very good sign. A designer who says “no” may be a good designer, but he or she will probably require some hand-holding and education.)
Questions for your builder
Of course, you want to choose a builder with a good reputation and many satisfied clients. But you want more than that: you want a builder who understands house-as-a-system principles and who has paid some attention to building science issues. Here are some questions for your builder:
- Have you ever built a home with an airtightness goal included in the specifications?
- How many homes have you built that were tested with a blower door? What were the results? (One way that builders report blower-door results is in “air changes per hour at 50 pascals.” This metric is often written as “ach50.” Results of 1.5 ach50 or lower are quite good. Results of 1.5 to 2.5 ach50 aren’t bad.)
- What’s a good way to address thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the studs? (Good answers include “we can build a double-stud wall” or “with exterior rigid foam.” A not-so-good answer would be, “What’s thermal bridging?”)
- Do you install a polyethylene vapor barrier on the interior side of your walls? (In locations that are more than 200 miles from the Canadian border, the answer you want to hear is “no.”)
If your designer or builder suggests any of the following features, consider the suggestion a red flag:
- Forced-air ductwork installed in an unconditioned vented attic
- Fiberglass batts to insulate the interior of a basement wall
- Spray foam insulation that doesn't meet minimum code requirements (because “R-20 spray foam performs better than R-38 fiberglass”)
- Adhered “manufactured stone” veneer over OSB wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen.
- Recessed can lights in an insulated ceiling
- Pull-down attic stairs in an insulated ceiling
- Tongue-and-groove boards as the finish material on a cathedral ceiling insulated with fiberglass batts (unless gypsum wallboard has been installed first)
- A powered attic ventilator (that is, a fan to keep your attic cool).
It’s easy to get sidetracked by opinionated designers and builders. But everybody has opinions, including me. Here are a few basic principles — yes, these are just opinions, too — that I advise you to adhere to if you want to build an energy-efficient home:
- Aim for a compact shape with few ells, bays, and bump-outs.
- If possible, orient the long axis of the house in an east-west direction.
- In most U.S. locations, it makes sense to include more windows on the south side than the north side. (This advice doesn't apply in hot climates or the Southern hemisphere, however).
- Spend some time researching window specifications; choose glazing that is appropriate for your climate.
- Hire an engineer or a certified energy rater (rather than an HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor) to determine the size of your heating and cooling equipment.
- Locate all forced-air ductwork inside the home’s thermal envelope.
- Include a mechanical ventilation system.
- Do not install any recessed can lights in an insulated ceiling.
- Insist on a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas.. If possible, include an airtightness goal in the job specifications.
The purpose of this article is to get you oriented to a few basic topics, and to prevent any major mistakes that will be expensive to fix later. Of course, there is a lot more to learn on the topics mentioned here; the more you learn, the more you may develop strong opinions on topics like insulation materials, window specifications, ventilation equipment, or heating systems.
If you take these topics in small bites, using the resources on the GBA website to research one topic at a time, you may be able to make the transition from a confused homeowner to a building science expert. Even if you abandon your self-education efforts halfway through your project, at least you’ll have enough knowledge to negotiate intelligently with your designer and builder.
Martin Holladay’s previous blog: “Insulating Low-Slope Residential Roofs.”
- Latin American Technical Education Foundation
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