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Home Energy Experiment #2: How well does plastic insulate?

9 April 2009 485 views No Comment
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This question is important to many homeowners and renters because we believe that placing plastic over our windows will halep us save energy on cold winter days and nights. Some plastic makers claim that plastic adds at least 1R value to the window.

I doubt this to be true. But using the information presented in the home energy audit kit, we can easily see heat loss with and without the plastic to see how much of a difference it makes.

We can do ad hoc thermography as talked about in the energy audits guide, and use the energy calculators included with the home energy audit kit to get a better idea of what’s going on with our hallway window.

On this particular day, the sun was shining. This first measurement was taken at 6:30 pm.

Energy measurements of the hallway at 18:30

Energy measurements of the hallway at 18:30

We see that there is a small area in the bottom left that is not reciving direct sunlight, and thus is colder than the rest of the window. Keep in mind that plastic is over this window – yet these readings are still being taken.

Next, we look at the same window, with the plastic still over it, at 11:30pm:

same hallway at 11:30pm

same hallway at 11:30pm

Notice the huge change! As the air cooled outside, but the temperature remained relatively constant inside, the window became much much cooler – even with plastic insulating the glass and pane! At worst, we’re going from 68F to 51F in about 2 feet.

Let’s remove the plastic from part of the window and take another image:

same hallway, plastic removed

same hallway, plastic removed

Notice in some areas, there is about a 1 degree difference from the plastic being removed. Using this information, we can deduce that plastic in this instance is only marginally effective. This window definitely needs to be replaced.

So…how much energy is this window losing? Using the heat loss calculator from the energy audit kit, the window is losing about 27 watts of power, or in terms of heat, about 93 btu.

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