posted 9 years ago
I'm an Energy Auditor. This is the kind of stuff I do, calculating like this.
What country are you in? I'm in the USA, and I'm used to working in feet, °F, and BTUs. I don't know if that will just add to the confusion or not. One other thing I'm used to using is Heating Degree Days. This is a rough measure of your climate, and is calculated from comparing the indoor temperature to the average daily outdoor temperature, and then adding all those daily numbers up for the year. In my area, with a 65°F indoor temperature, we have about a 7200 HDD climate. Looking up the climate for Missoula Montana, I see that with the same 65° base temperature, they had a 7259 HDD season last year, which I understand is warmer than normal. Washington, DC had 4117 HDD in the last year.
There are two ways a building loses heat. It can go out through a solid surface, or it can go out with the air that filters in and out of your building. Let's look at air transported heat loss first.
To figure out air transported heat loss, you need to know how much air moves through the building. I use a special fan and pressure gauge system. I find the air flow at 50 pascals of negative pressure, then do a bunch of math to turn that into air changes per hour. The goal for natural air exchange is .35 air changes per hour. Tighter than that is good, but will require mechanical ventilation to make sure you have enough air flow for your health and comfort. There is no good way to find your air flow without this tool. Anything else is a guess. With experience, I've gotten fair at guessing, but I'm often wrong.
So, say you have one air change per hour, which is very leaky. Take the volume of your house, and multiply that by 24 hours. That's how much air goes through your house each day. Multiply that by .0182 BTUs per cubic foot of air. Then multiply that by your heating degree days. For instance, let's say my house is 6000 cubic feet. 6000 x 24 X .0182 x 7000= 18,345,600 BTUs. That's heat lost by air movement. That was the easy calculation.
Now let's look at solid surface heat loss. This is heat lost through walls and ceilings and closed doors and windows, and downward through the ground. For each surface, you have to compute it separately. Find the area of the surface, the R-value, the HDDs, and factor in the 24 hour day. So, lets say I have a wall that's 20' long and 8' high. It has two windows, which are 10 square feet each. That means you have to compute the 140 square feet of the wall separately from the 20 square feet of the window.
Let's say the wall has an R-value of 15, which is nothing special. 30 would be twice as good. 30 would lose only half as much heat. But let's say R-15. 140 square feet x 24 hours x 7000 HDD =23,520,000 BTUs. Divide that by the R-value, and you get how much heat is going through the wall: 1,568,000 BTUs.
But here's the thing that isn't in the books: Heat moves by convection, conduction, and radiation. Air transported heat loss is convection. But in solid surface heat loss, we're dealing with conduction through the wall, and mostly radiation to take the heat away on the other side. Because of this, the radiant temperature of the background is very important. When I look out my windows, I see trees, which are mostly the same temperature as the air. But the clear sky is very cold. This is why we put more insulation in attics than walls. We find that you can multiply the BTUs going through the wall by 0.7 and get a fair number. Multiply the result from the same calculation for the roof by 0.8. Multiply the foundation number by 0.4, and when doing the foundation, count the wall area to 2 feet below the exterior soil line. Do each surface separately, and then add up all the BTUs for each surface. Use a spreadsheet program and be careful. The math is not hard, but there's lots of it, so it's easy to make small mistakes.
You'll need a reference table to pick an appropriate R-value for each wall and door and window and ceiling. If the insulation is fiberglass in good condition, assume the value is 75% of what the table says. If it's in bad condition, assume it's 20% of what the table says. Stone has an R-value of R-1 per foot of thickness. Concrete is about the same. So is a single pane of glass. Two panes of glass is R-2. You can see why movable insulation over windows at night is such a good thing. In my region, the recommended values for new construction are R-60 ceilings, R-35 walls, and R-15 basements. Windows should be at least R-2, but I've seen triple pane windows with special coatings and xenon gas between the layers that claimed R-8 performance. I'm not sure I'd spend that kind of money though. Windows are expensive, and cover only a small area. Basic windows and good insulation elsewhere is probably better use of your money.
The best return on investment is air sealing. Small holes are everywhere. The material to fill them in is usually cheap.
If you burn wood, you need combustion air for the fire. It's best if you have a closed stove and a tube that brings outside air directly to the fire, rather than pulling cold air through the heated room. A dedicated combustion air supply is safer too.
One other thing: Thermal mass means nothing. It will keep your building cold as well as warm. It will reduce temperature swings, but it will not reduce the amount of fuel you need. If you live in an all stone house in a cold climate, be prepared to burn huge amounts of fuel.
Good Luck.
.png)
1
