You know that during a cold day, standing outside by a bonfire will help you stay warm-but all the heat in the world won't keep you toasty for long if you're in shirtsleeves in January. The same is true in the summer, when you can drape yourself over a window air conditioner—but if your house isn't well-insulated, you're going to be looking for more relief a few minutes after you shut it off.
Houses work in much the same way. Heating and cooling are responsible for more than half of the energy use in most homes. Without a good, airtight enclosure to keep warm and cool air where you need it—your "coat"—you can pour a fortune into heating and cooling bills and still feel uncomfortable all season long.
So when we built our Energy Efficiency Lab Home, we wanted to make sure heat, cold and damp would have a hard time getting in, and that properly conditioned air wouldn't leak out. The choices we made will show us just how airtight we can build a home but still keep it comfortable and inviting for families.
Most homes don't have insulation under their concrete foundation slab, but under the lab home's foundation we put two inches of XPS, or extruded polystyrene, foam insulation-enough so that the lab home project manager, Kevin Brozyna, said he hopes families will be able "to walk around the basement without shoes on" and not feel a chill.
For the walls, we staggered the wooden studs, then installed spray foam around key areas like band joists and windows and doors, and filled the increased inside space with a spray-in fiberglass insulation that is highly efficient and repels damp. The windows are triple-paned and insulated to keep sunlight and extreme temperatures outside, and the doors are multilayered and tightly fitted to cut down on drafts and heat transference. Outside, the house is covered with another two inches of XPS insulation with an outer housewrap layer that is virtually airtight and watertight. Vinyl siding is installed as the last layer.
The roof clearance was raised several inches to accommodate 17 inches of fiberglass insulation, and the housewrap extends into the attic to prevent moisture or wind from getting in under the eaves. None of the lights in the second floor of the house are set in the ceiling, to eliminate possible sources of drafts. The roof is then finished with reflective shingles, which look the same as regular ones but reflect some of the sun's heat to keep the house cooler in summer.
During construction, we also put in hundreds of sensors, which you find out more about in Testing and Monitoring. These little devices will tell us how airtight we've made our lab home's enclosure, how well it insulates in winter and summer, and will monitor other important factors, like condensation in the walls. We've learned the more airtight a home, the more you need to keep an eye on moisture buildup and ventilate to ensure that the air stays fresh and comfortable—but the tradeoff can be a heating and cooling system that doesn't need to work nearly as hard to make that happen.
R-what? The better and more insulation you put into a home's walls and ceiling, the better R-value they'll achieve. The R-value is how much heat a barrier will transfer-the lower the value, the more heat gets through. A well-built and well-insulated U.S. home has walls with an R-value of about 17. On the Zero Energy Lab Home, the walls are R-40, and the roof is R-60. Those high ratings mean we can use a ground source heat pump for the entire house's heating and cooling needs.
