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Building Envelope


Build or renovate for efficiency

Whether you’re planning to build a new home or renovating or repairing what you’ve got, there are “smart” building materials that will save energy and may earn a federal tax credit.

Start with your building shell or building envelope, which is the barrier between air conditioned or heated space and unconditioned space, including the outdoors. Tighten with efficient windows and storm windows, extra insulation in the attic and walls, light-colored roofs or metal roofs and building materials such as insulating concrete forms or structurally insulated panels.

If adding a room or building all new, take advantage of passive solar design. That means designing with south-facing windows that provide good daylighting. Adding awnings, porches, shades and trees allow solar gain in the winter but block light and heat in the summer. Passive solar also may incorporate large masses of stone, concrete, brick or tile, known as “heat sinks,” that can absorb heat during the day, then radiant it at night.

Solar water heaters are more affordable these days as the price of photovoltaic panels has dropped. You’ll see an immediate reduction in electricity use. Traditional tank-type water heaters typically use 12 percent of a home’s electricity, according to Michael Bluejay, energy consultant.

For a cooler roof in summer, use light-colored shingles or metal sheets to improve efficiency by reflecting radiant heat. Darker colors absorb heat. The U.S. Department of Energy (DOE) says changing to a light-colored roof can reduce electricity use by up to 15 percent. Metal roofs have been shown to absorb 34 percent less heat than asphalt shingles and can reduce energy use by 20 percent.

Another cooling option, reflective coatings, are best for areas with hot, sunny weather much of the year. The Lawrence Berkeley Laboratory in California estimates coatings can save at least 25 percent in energy usage.

This table compares the reflectance of various roofing materials and shows that a bright white coating has 80 percent solar reflectance with roof temperatures of 15 degrees above the air temperature compared to black shingles that have 5 percent solar reflectance, with roof temperatures of 90 degrees above the air temperature. Go light!

Check out ENERGY STAR’s Roof Products Program and comparisons of roofing products.

For snug walls, add dense-packed cellulose or fiberglass or inject water-based, low-expansion foam insulation into existing walls. Spray polyurethane foams are best applied by professionals but do a better job of eliminating air leaks than other types of insulation. DOE reports that air leaks are responsible for as much as 40 percent of electricity use.

For additions, new wall options include aerated concrete, insulating concrete forms or structural insulated panels. Also consider 2x6 stud construction instead of the traditional 2x4 and 2x5.

According to the Energy Efficient Network, 2x6 uses fewer studs on the exterior, thus conducting less heat through the wall.

If you’re building from scratch, consider advanced house framing techniques, which according to DOE, replaces some lumber with additional insulation, thereby reducing waste and tightening the thermal efficiency of the house. Look for framing diagrams.

For warm floors in winter, insulating floors above a crawl space, slab or unconditioned basement to an R-25 is a good place to start.

Radiant floor heating is another option that if properly sized and installed can be 25 to 40 percent more efficient than forced air, according to experts.

Electric resistance mats are one type of radiant flooring, ideal for placing under a tiled floor in a bathroom or kitchen. The heat is electric, though, so your utility bill will reflect that. Another type is hydronic tubing, which can be installed in floors above unfinished areas like basements and crawl spaces. Keep in mind, though, that Hydronic tubing doesn’t replace the ducts you’ll still need for air conditioning.

Finally, inefficient windows lose as much as 25 percent of a home’s heat, according to DOE. For tight windows, replace single-pane windows with more efficient ones. Look for low AL (air leakage) ratings on new windows; 0.1 to 0.3 is the typical range. Also important are the “U-value” (1/R) and the shading co-efficient. Buy windows whose frames include thermal breaks that interrupt heat flow and loss.

Add window overhangs, awnings and deciduous trees outside and insulating drapes and blinds inside. According to the University of Missouri Extension, window coverings such as these with an R-4 value are cost effective because they block about 75 percent of heat loss.

Storm windows, either interior or exterior, can reduce heat loss by a whopping 25 to 50 percent, according to DOE, with payback as short as 5 years, according to the Missouri Department of Natural Resources. Plastic sheeting works if you can’t afford storm windows, and simple caulking around windows will save energy.

If you’re building a new house, keep the glass area at 10 to 12 percent of the floor area.  



The alphabet soup of building efficiently: AACs, ICFs and SIPs

If you’re adding a room, retrofitting an entire house or building from scratch, consider using autoclaved aerated concrete (AAC), insulating concrete forms (ICFs) and structural insulated panels (SIPs).

AAC, a foam-like material made of lime, cement, gypsum, water, sand and aluminum powder poured into panel molds, was developed in Sweden. It is more efficient than conventional concrete blocks and porous concrete but does not perform as well as SIPs, ICFs or even well-insulated wood-framed walls, according to House Energy.

If you’re planning to build a storm room for protection during high winds and tornadoes, heavy, ICF-built walls are extremely strong. ICFs are stackable foam concrete forms with a steel-reinforced concrete center sandwiched between two extruded foamboards. They typically have R-values from R-18 to R-35, according to the thickness of the wall, according to University of Missouri Extension, making them an excellent energy-efficient material. In contrast, 2 x 4 wood-framed walls are usually R-13 to R-18, and 2 x 6 wood-framed walls are around R-24.

ICFs also create a large concrete mass that can store and release energy, greatly adding to the thermal efficiency of the building.

SIPs are a third building option. They consist of plywood sheets laminated to a core of foamboard (up to 8 inches thick) that can be used for walls, ceilings, floors and roofs, offering energy savings of 12 to 14 percent, according to DOE. Construction is fast and may be so tight that fresh-air ventilation may be required.




How to evaluate efficient building materials

How can you evaluate product claims? How can you be assured that the insulation or insulated concrete forms you may buy for their efficiency really do perform?

While there are always product exceptions, the occasional lemon, you can buy smart by following these suggestions:
 

  1. Read consumer reviews – You can search for reviews online, but one source used by many is Consumer Review where you can read about product comparisons and reviews by actual users.
  2. Check ENERGY STAR – Many appliances and home improvement products are rated by ENERGY STAR. Before you buy, see if the products you are considering are ENERGY STAR rated.
  3. Talk to local buyers and users – The vendor should be happy to supply customer references whom you can contact for recommendations and personal experiences.
  4. Look for certifications – The product may have a third-party certification, sort of a Good Housekeeping Seal of Approval — such as ENERGY STAR – that gives you confidence in the product. Also find out if your vendor and installer are certified or have other credentials to qualify them to sell or install the product.
  5. Ask for a demo – Perhaps the vendor or installer can actually demonstrate the product’s efficiency or reference a study or test that documents results.