CONDENSATION

 There is a form of phase change that occurs all around us all of the time, the condensation of water vapor on cooling surfaces.   It can be a plague on our houses. Unwelcome condensation often means, among other things, wasted heat energy.

In nature, as air warms, its ability to absorb and hold water vapor increases.  It follows that, as air cools, the amount of water vapor that it can hold is reduced.

In winter, the cooler outdoor air is relatively dry, too cold to hold much moisture.   When that dry air comes indoors, it is warmed, and is now able to soak up a lot more water from showers, cooking, and washing machines. That dry air does come indoors, a lot of it!  In a modern house, the inside air is replaced by outside air about once every two hours. In older houses, which means most houses, complete turnover takes as little as one hour.

So, in a 1,600 square-foot house with the usual joint leakage, at least 300,000 cubic feet of dry outside air comes in each winter day. That air warms up, soaks up house moisture, carries it around until it leaks out of the house through a crack or a penetration, or until it meets a cold surface, cools to its dew point, and deposits the extra moisture there.

You will see the results of condensation on your windows, but you won't see it on sill plates, or behind clapboards. If you see stains on your ceiling, it may be because warm moist air is leaking around ceiling fixtures, or through skimpy insulation, condensing on some cold object in the attic, and dripping onto the ceiling.  If clapboards bubble within a few years of painting, moisture-laden air may be seeping thru' the wall and condensing behind the paint.
 
In addition to the moisture damage itself, there is another expense, the heat lost when moisture inside the heated space evaporates and leaks out of the heated space as water vapor.

Earlier, we pointed out that it takes only one BTU of heat energy to raise the temperature of one pound of water by one degree Fahrenheit. That is not a big deal. However, if we kept heating 'til the water turns to steam, we would expend up to 1,000 BTUs per pound of water, just to drive the water through Phase Change, from liquid water to water vapor!

Now, the air carrying that water vapor is going to move. If it hits a cold window pane, the vapor will condense there, and most of the BTUs released by the change in phase will be conducted outside. If the warmed air, with its water vapor, migrates through the walls, or cracks around doors and windows, it will take out, not just the few BTUs that it took to warm the air, but the thousands of BTUs that it took to convert the water to water vapor.