Tackling heat loss in the home
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.
In the winter, the energy we're paying so dearly for is lost mainly by two ways: air leaking in (and out), called infiltration; and conduction, through our walls and windows to the outside. 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 in a comparison to 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 in the wall 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 thicker is rapidly becoming, with the price of fuel, necessary. Someday, in I fear not too long, we'll look back and see that we were pretty dumb about this. I remember when fuel oil was 23 cents. Like, I'm a prophet?
We insulate over our heads in the attic to R40-plus 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 0.33 (R of 3). In terms of heat loss, windows are awful, and I think window manufacturers, knowing this, use 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 BTUs equals 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 here. It's 65 inside when your furnace starts; it's minus 30 degrees outside at the coldest. Moral of the story: Turn your furnace down.
Example: wall loss. An R19 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 (8x40) x 95. Total heat loss is 1,824 BTUs per hour.
Let's do a typical double insulated glass window that measures 4 feet by 4 feet. It has a U value of 0.33, so heat loss equals 0.33 x 16 x 95, or approximately 500 BTUs 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, can be 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 wherever 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 around the framing of 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 either the inside or 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 10 years. A good investment is one that pays back. Invest in your home now; you'll get the payback sooner and for longer.
My pet peeve: adding expensive geothermal solutions to homes. That's treating the symptom. Fix the house. Top of the list: Seal the attic and other air leaks. Get a blower door test and find the leaks.
This is all going to get worse, unfortunately.