Home investments pay off

By Alan Linda | The Daily Journal

Published Tuesday, July 29, 2008

This is about heat loss in your home. With the growing cost of heating fuel driving us nuts, we’re all facing—or going to be facing_decisions about what to do about it.

We can only improve our heating and cooling systems so much; after that, it becomes a matter of improving what the heating and cooling system is heating and cooling_your house.

Heat travels in three ways: conduction, like if you were holding the handle of a hot cast iron frying pan; convection, which involves air molecules being heated and sent around the home by a fan; and radiation, which is how the sun transmits heat through the vacuum of space, and why it’s cooler under shade, away from the sun.

Next, energy travels from warmer to cooler. Not up. Not down. From warmer to cooler. Hot air rises, which confuses a lot of folks, but heat travels from higher energy states—think atoms jumping around on that hot frying pan—to lower ones—think your hand hanging onto that hot frying pan handle.

So, in the winter, the energy we’re paying so dearly for is lost mainly by two ways: conduction through our walls to the outside; and infiltration—air leaking in and out. Some energy—a little—travels in the winter to the outside by radiation, because any warm surface also radiates. Stop conduction, you’ve stopped radiation.

Now, here comes “R” value. “R” stands for resistance to heat travel, and is based in fact if not purpose on wood one inch thick having an R value of 1. You hear this term used when discussing insulation value. For example, a 6-inch-thick “batt,” or hunk, of spun fiberglass insulation has an R value of 19. In the world of R values, this is pretty good, and all homes being built now have 6-inch thick walls. Building them any thicker and insulating them any better becomes a matter of payback, and the math for doing that says it isn’t worth it. Someday, maybe.

We insulate over our heads in the attic to R 40 right now, because hot air rises, and puts more energy pressure up there. That’s because the warmer something is, in this case air, the more those atoms are jumping around trying to get to someplace cooler.

Engineering tables don’t talk about R values, they talk about U values. Where R is the resistance to energy loss, U is the acceptance of a material to let energy travel through it. A “U” value is the reciprocal of R, or 1 divided by R. Conversely, R is the reciprocal of U, or 1 divided by U. So, for example, something with an R value of 10 has a U value of 0.10, or 1 divided by 10. We need to know U values because windows are measured in them. A reasonably good window right now has a U value around 0.35 (R of 3). In terms of heat loss, windows are awful, and I think window manufacturers, knowing this, used U values to keep customers buying bigger and more windows. If more folks knew how much heat is lost through a window, maybe they’d buy fewer and smaller.

One last thing. The larger the temperature difference is from inside to outside your home, the larger the heat loss. That formula looks like this: Heat loss in Btu’s = square area (like one square foot) times temperature difference (inside to outside) times U value. We use worst case temp differences inside to outside of 95 degrees. It’s 65 inside when your furnace starts; it’s minus 30 degrees outside at the coldest.

Example wall loss. An R 19 wall 8 feet tall and 40 feet long has a U value of 0.06. (Remember, that wall isn’t all insulation. Some of it is wood framing structure, which isn’t as good.) Multiply 0.06 x 320 (8 x 40) x 95. Total heat loss is 1,824 Btu’s per hour.

Let’s do a typical double insulated glass window that measures 4 ft. by 4 ft. It has a U value of 0.33, so heat loss equals 0.33 x 16 x 95, or approximately 500 Btu’s per hour heat loss.

Put four of these windows in that wall, and they account for more of the heat loss than does the wall. Windows, it turns out, are the biggest losers in your home.

Now comes the really big part of the heat lost in an older home, and that happens by convection, which is either the cold air leaking into your home or the warm air leaking out—take your choice. This is called infiltration, and is the sum total of all the cracks in your house, which are around the windows and doors—where you can’t see them under the walls—and where the house meets the basement, plus where ever holes for wires and pipes are drilled through into attics and basements.

Up until just a few years ago, carpenters took fiberglass insulation and, using a putty knife, stuffed it into the cracks around your doors and windows. Compacted fiberglass insulation isn’t insulation when it’s packed like that; it’s now a conductor. Add to that the fact that fiberglass doesn’t stop air leakage, and now you know why adding replacement windows to your older home by just replacing the glass part of the window—called the sash_ and leaving the old frame in the wall isn’t going to help your main problem, which is the infiltration through the cracks beneath your wall, around that old window.

Gutting an old house and bringing infiltration down and R values up is not fun. Or cheap. Totally replacing old windows and getting them properly sealed against infiltration is really difficult because of issues inside the wall around the window and because of issues outside with siding.

In new houses, we install an air barrier inside, called vapor barrier and made of thin plastic sheeting, and an air barrier outside, which will let moisture escape if it gets trapped in your wall, but won’t let wind in. All this is to restrict infiltration and exfiltration losses. In new construction, this is easy. In an older home, it requires complete removal of both the inside and outside wall covering, which is a labor intensive mess, although this does allow one to do the window replacements correctly.

You don’t have to be a rocket scientist right now to guess where fuel prices will be in ten years. A good investment is one that pays back. Invest in your home now, you’ll get the payback sooner and for longer.

This is all going to get worse, unfortunately.

Alan Linda writes from his New York Mills home.