Address the Basics First
The design of an energy-efficient house begins with a well-insulated, air-sealed shell and very efficient HVAC equipment, which means a minimum 90 percent AFUE (annual fuel utilization efficiency) furnace and 13 SEER (seasonal energy efficiency ratio) air conditioner.
Anyone intending to build an energy-efficient house needs to be sure these basic requirements are met before considering exotic (and expensive) components like photovoltaic modules.
Orient the House Properly
Passive-solar design does not need to be complicated; a few simple steps can save significant amounts of energy. Yet most new-home builders still pay almost no attention to orientation.
If the lot size permits, a house should always be oriented with its long axis aligned in an east-west direction. In most climates, about half the home's windows should be facing south. In hot climates, it's important to minimize the number and size of west-facing windows.
Install Basement Wall Insulation
According to the prescriptive requirements of the International Energy Conservation Code, basement walls should be insulated in climate zones 4 and higher.
Basement walls can be insulated from the exterior or the interior. Most builders find that installing interior basement insulation is easier and cheaper than installing exterior basement insulation; far too often, however, they get the details wrong.
Interior basement insulation is effective only if the work is properly detailed and meticulously installed. The rim-joist area must be air sealed (either with sprayed polyurethane foam or very careful caulking), and the rim-joist area and walls must be carefully insulated with rigid-foam sheets or sprayed polyurethane foam. Never use fiberglass batts to insulate basement walls.
Exterior basement insulation usually performs better than interior basement insulation. It locates the wall's thermal mass within the building's thermal envelope; if installed properly, it can be used to protect the rim-joist area. Also, by keeping the concrete warm, it prevents the condensation and moisture problems often associated with interior basement insulation.
Install Better Windows
Windows represent the weakest thermal link in most building envelopes. Unfortunately, the U.S. Department of Energy has chosen to set a very low bar for Energy Star windows, so Energy Star labels provide little guidance to builders. In most parts of the country, in fact, an Energy Star window is equal to a code-minimum window.
Specifying windows can be complicated, but a few general principles apply. Casement windows usually have less air leakage than double-hung windows. In heating climates, the best windows will have a lower U-factor than windows minimally complying with Energy Star standards (U-0.35). Consider investing in windows with argon-filled triple glazing and two low-e coatings; such windows are available with a whole-window U-factor as low as 0.17.
In south central and southern climate zones, Energy Star specifications call for windows to have a maximum solar heat-gain coefficient (SHGC) rating of 0.40. In these zones, consider purchasing windows that beat this standard — that is, windows with an SHGC below 0.40. Specifying glazing with a very low SHGC is especially important for west-facing windows, since these are the ones most likely to contribute to overheating.
Install Rigid Foam Wall Sheathing
Many cold-climate builders still cling to the belief that foam sheathing creates a wrong-side vapor retarder and therefore contributes to wall rot. In fact, the inside surface of foam sheathing will be much warmer than the inside surface of OSB or plywood sheathing, and will therefore be less likely to support condensation. Foam-sheathed walls, if built correctly, are less likely to have moisture problems than walls sheathed with OSB or plywood.
Foam sheathing wraps a home's walls in a warm jacket, keeping the framing warm and dry and greatly reducing thermal bridging through studs. Furthermore, if foam sheathing is held in place with vertical strapping, a rain screen is created behind the siding.
Builders making the switch to foam sheathing must choose one of three strategies for bracing walls against racking. They can install traditional 1x4 let-in braces, diagonal steel strapping (for example, Simpson TWB straps), or, at the corners, sheets of well-nailed 1/2-inch plywood. The plywood can then be covered with 1/2-inch rigid foam to match the thickness of the 1-inch foam installed everywhere else.
Of course, before settling on a bracing method you should make sure your local building inspector approves of your plan.
Install a Drain-Water Heat-Recovery System
One of the simplest and most cost-effective ways to reduce energy used for domestic hot water is to install a drain-water heat-recovery device.
The best-known such device is the GFX, which consists of a length of 3- or 4-inch copper drainpipe surrounded by a spiraling cocoon of 3/4-inch copper tubing (see Notebook, 3/97). Designed to be installed vertically in a plumbing waste line, a GFX unit transfers about 55 percent of the heat energy in the drain water to the incoming supply water. In homes where residents prefer showers to baths, a GFX can save 20 percent to 25 percent of the energy used for water heating.
The best thing about a GFX unit is its indestructibility: Having no moving parts, it is likely to last as long as the house in which it's installed. Model S3-60, the whole-house model (a 3-inch copper drain 60 inches long), costs $520.
Install a Solar Hot-Water System
Rising energy prices have made solar hot-water systems a good investment in most parts of the country. At sites beyond the reach of natural gas pipelines — where conventional water heaters must be fueled by either propane or electricity — an investment in a solar hot-water system will usually have a fairly quick payback.
A substantial fraction of the hot water needs of most families can be met by two 4-foot-by-8-foot collectors. It's almost always better to have an oversized storage tank than an undersized tank; if the budget permits, install a 120-gallon stainless-steel indirect water-heater tank from Amtrol, Bradford White, Burnham, Heat Transfer Products, Triangle Tube, or Viessmann. An instantaneous gas water heater can be used for backup.
Upgrade the Mechanical Ventilation System
Because an energy-efficient house has a well-defined air barrier and very low air-leakage rates, mechanical ventilation is essential.
Ventilation can be provided with a simple exhaust-only system (a timer-controlled bath exhaust fan, for example) or a passive supply system (such as a passive fresh-air duct, controlled by a motorized damper and connected to a furnace's return-air plenum).
But the most efficient way to provide fresh air to every room is with an HRV or an energy-recovery ventilator (ERV). Currently, the most energy-efficient ERV available is the RecoupAerator 200DX from Stirling.
Install Dedicated Ventilation Ductwork
Every HRV deserves dedicated ventilation ductwork. Ducts designed to distribute air for heating or cooling are not optimal for distributing ventilation air, so don't try to use the same ducts for both purposes.
A forced-air heating system usually draws its return air from a big grille in the hallway. An HRV, on the other hand, should draw its exhaust air from bathrooms, utility rooms, and the laundry room. Unlike forced-air heating ducts, ventilation ducts are sized for low airflow; usually they measure only 4 inches or 6 inches in diameter.
Install a Better Lighting Package
Installing compact fluorescent instead of incandescent bulbs is probably the most cost-effective energy upgrade in any home. Now that the quality of compact fluorescent bulbs has improved and prices have dropped, make sure all your houses are incandescent-free.
Arrange for Blower-Door Testing
Do you know how much air leaks under your rim joists or bottom plates? If you're still a blower-door virgin, you haven't yet earned the right to brag to customers about construction quality. Most blower-door contractors can recount stories of proud builders humbled by the revelations of a door-mounted fan.
Once you're familiar with the lessons taught by whole-house depressurization, you'll probably be more conscientious with gaskets and spray foam on your next house.
Tweaking the Recipe
It goes without saying that it's possible to build a high-performance house that deviates from these guidelines. The recommendations are based on logical principles, but they inevitably reflect my own biases. Furthermore, specifications for an energy-efficient house depend greatly upon local climate.
Before settling on any construction details, you should always investigate methods used by other energy-efficient builders in your region.
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