Last updated on March 16th, 2024 at 02:10 pm

So the last experiment was disappointing, because it appeared that plastic did very little to prevent heat loss for your leaky hallway window. Things aren’t looking so good for the thin plastic as insulation.

Luckily, we have another scenario. Plastic over part of a window has come “undone” giving us the perfect side by side comparison of surface temperatures over a window with and without plastic – without interfering sunshine and the heat from solar convection.

Below is an image with energy / temperature readings for the upstairs window, from the techniques in the Home Energy Audit Kit:

The line marks where the plastic has come undone.

So what does it mean? Its actually colder where the plastic is in place, meaning heat is being lost. But where the plastic isn’t, we find temperatures closer to IAT. This “reverses” our thinking and logic. We know the window is in bad shape, but according to this data, the readings are best around where there is no plastic.

The reason for this is the heated air is forced around the room through normal circulation. Heat also naturally rises. So warmer air is always near the top – notice the 73F reading near the ceiling. Since the heated air can’t be forced into the bottom portion of the window, the readings there can be taken as relatively accurate.

In other words, throw away the anomoly of the break in the plastic, and we see that the plastic insulation, on the shady side of the house, fails to keep at most 7F of heat in the room. This represents heat loss.

There is also heat loss on the upper portion of the window where the heated air is coming in contact with the window directly. It is mitigated only by the fact that it is near the top. Had the tear been at the bottom, and we would see the whole window “heat up” beyond IAT. All this energy would then be lost through the window.