insulation

Single pane windows are terrible insulators.   So much so, that covering up my windows with clear plastic wrap cut my electric bill from $50 in january to $25 in february.   The plastic wrap blocks convection.   The windows are the weakest link by far -- there's no point in improving wall insulation when you lose 90% of the heat through the windows anyway.   You can approximate the relative amount of energy loss through a surface by measuring its interior surface temperature.  For example, the wall surface is only 1 degree below room temperature, while the window surface is 40-50 degrees below room temperature, which basically proves that the windows are hemorrhaging energy.   The relationship between convective heat transfer and temperature difference is probably between linear and quadratic in this case, because larger temperature differences cause faster flows of air over the surface.  (bouyancy would be linear but drag would be roughly quadratic so I'd expect the speed of the air flow to increase as the square root of delta T)   Black body radiation is neglegible compared to convection at these temperatures.  So in this example the windows must have *at least* 50 times the conductivity of the walls, but the figure is probably more like 300 times.

 100% of my electricity consumption goes to heat the apartment, because all electricity consumed by electrical appliances or the heater is converted into heat, by the law of conservation of energy (ignoring the negligible amount of EM that might be produced by these devices and escape the apartment).    So as long as it's really cold outside, my electric consumption is determined by only two things:

1. The conductivity of the apartment.

2. The temperature difference that I want to maintain between the apartment and the outside. 

Asking which of these is more important is like asking whether the length or the width contributes more to the area of a rectangle.      Improving the wall insulation without doing something about the windows would be like putting a fifth lock on the front door while leaving the back door wide open.

gorgonheap says...

When I was in school they taught us how to calculate the amount of heat loss from our structures in relation to the types of window (E-rating), the area of windows. It's unfortunate when I hear of home or business owners who go for the cheapest single pane windows they can find. They end up paying a little over double what their electric bill would be otherwise because of the poor insulation.

Another place to look is the door. I lived in an apartment that had a hollow core interior door as our exterior door. Not only were the elements harsh on it, but over 80% of our heat loss was through it. The one thing you don't want to cheap out on with your home is windows and doors.

Also in many houses. (especially those built before 1985) Do not have proper insulation in the attic. Heat moves a lot like water, only up instead of following gravity. If you think of your house as a upside down tub that's holing heat the roof should be one of your strongest retention systems.

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