Greening the Building Shell

In 2010, I’m taking a room-by-room approach to green building. My columns will examine specific rooms and how to make them as green as possible. To begin, I’m compelled to address the largest “room” in a home, and that’s the building shell itself.

Way beyond VOCs, recycled content and driving hybrids, by far the greenest thing you can do is design a high-integrity thermal envelope so your structure consumes as little fuel as possible for heating and cooling. This kind of “at-scale” application of green principles really makes a difference. All the recycled coffee mugs, reusable grocery bags and other recycling contribute positively to the green effort and I don’t want to diminish that. However, the benefits of someone recycling, say, a ton of old magazines is environmentally negated hundreds of times over if the structure is bleeding heat/cold, guzzling fuel and pumping out CO2.

Greening of the building shell should improve the thermal performance of the building using materials that: A) are nontoxic/low-toxicity; B) are harvested and/or manufactured in a sustainable way, and; C) don’t consume a great deal of fuel when shipped. Let’s take a category-by-category look at the principles driving your options, starting at the top.

Roof. Reflective or even light-colored roofs lower cooling loads, sometimes dramatically. (Black shingles reflect about 5 percent of the sun’s heat; gray shingles 20 percent, and white shingles 25 percent.) Look for an Energy Star roof, which meet baseline reflective qualities. Note: They are not all metal. Some three-tab products are Energy Star-rated, which feature reflective granules that turn back sunlight. MIT is experimenting with roof panels that change color with temperature to match reflectivity to conditions ( When sheathing, note that radiant barriers or insulated sheathing are key to any good system.

Certified wood. For framing and sheathing, use certified lumber. Frankly, it’s difficult to purchase lumber in North America that isn’t sustainably harvested. For political reasons, USGBC’s LEED standard accepts only FSC certified lumber, while ignoring other good standards. But NAHB’s National Green Building Standard recognizes both FSC and SFI. Nonetheless, look for a label and learn its background.

Air infiltration. Insulation is tested for R-value in an environment with no air infiltration. In the field, however, air infiltration will always rob R-value. (According to DOE, air that infiltrates from the outside can account for half the energy required to heat or cool a structure.) For truly energy-efficient walls, spec products that cut down the air infiltration and work in concert with your insulation package.

Weather barriers must be breathable, but sometimes code calls for a vapor barrier on the inside surface of walls in cold climates (and some mixed climates) where we want to prevent any warm, humid indoor air from seeping into walls. That’s a very green feature indeed. (Note that housewrap is always a sealing system, which usually includes tape and flashing. Not using the entire system can violate the warranty.)

Adhesives, caulks and sealants. When sealing, note that adhesives, caulks and sealants are three entirely different products, each with specific design applications. Each is important when sealing the building shell, but no matter what you use in the appropriate setting, it has to be a low- or no-VOC product. There are five general types or technology platforms and water-based products is the greenest. A second technology platform is polyurethane. Nothing flashes off but CO2, so they’re green choices.

A third platform is silicone, which offers great flexibility. The fumes you smell are from a solvent chemically identical to vinegar. The fourth platform is hybrids, which offers the best of silicone’s features with the best of polyurethane. Hybrids cure rather than dry; many are low-VOC or solvent-free. The last platform is solvent-based which contains derivatives like xylene and benzene. Avoid them.

Insulation. Cooling U.S. homes costs $40 billion yearly and consumes 15 percent of all U.S. power, 90 percent of which is created with coal and fossil fuels. Proper insulation can easily save 20 percent of fuel costs. There are intriguing choices out there that go way beyond standard batts, such as insulation with “smart paper” facing that changes its molecular configuration to allow moisture in or out. But for all insulation, consider formaldehyde, a known carcinogen that is widely used in binders and adhesives. Also, look for low-emitting or formaldehyde-free products. For filling gaps, use spot-application spray foams that are isocyanate-free and formaldehyde-free.