Now, if I punch a small hole in the bag, water will gush out through it. And even though the hole is small, the total rate of water leaking out could, say, double. The free flow through that one little opening can be as much as the constrained flow through the rest of the bag. Likewise, if I put a hole in the wall of the house, heat will stream out, cutting the energy efficiency of the envelope sharply. Obviously, it will pay off handsomely just to plug that one hole.
With energy-efficient buildings we are soon going to start paying a lot more attention to plugging holes. Affordable technology now exists to build a tight, well-insulated set of walls. It’s pretty easy to add an energy-efficient roof, too, and those are becoming more commonplace as buyers and builders turn their attention to that former weak spot. And we hardly have to worry about doors. The standard door is now about R-10. Only a few specialty models, like those made of solid wood, are much lower than that.
But that still leaves a few significant holes—areas of the envelope that leak heat at a much faster rate than the rest. As the walls and roof become more and more energy-efficient, these holes will account for a growing share of the heating and cooling bill. Anyone looking to improve the energy efficiency of the building will come to realize that it won’t do much good trying to increase the R-value or cut the air infiltration of the main parts of the envelope—they already let little heat or air through, so any new measure won’t have much impact. Instead, they will look increasingly at the little things, which have come to account for much more of the load. If I could just improve those a few more R points or tighten them up a little, they will reason, I could save maybe 20 or 30 percent off my total bill.
One of the big culprits is the windows. Modern energy-efficient windows still have an R-value of only about 4. Fortunately new windows are very tight and add little to air infiltration. But the main walls of, say, an ICF house are closer to R-20. This means that the conduction heat losses through a square foot of windows is about five times as much as they are through the main wall area. Since windows average about 15 percent of the wall area, that means there would be about 75 units of heat traveling through the windows (5 x 15) for every 85 units of heat going through the rest of the walls (1 x 85). That means almost half the conduction through the wall area is actually through the windows. I’m oversimplifying here, but you get the idea.
Another problem area is the exposed edge of the foundation. In most energy-efficient homes with a basement or stem wall foundation, that is insulated to take care of the problem. But sometimes not. And the edge of a slab foundation is almost never insulated. That’s potentially a very, very big hole.
Then there are the little detail areas. These include places where connections are made to the envelope and insulation is cut out, such as floor-to-wall and deck-to-wall connections.
As the trek to energy efficiency moves ever on, look for more and more attention to go to plugging these holes. The pressure will be on them, in more ways than one.