Combat damp with polythene

We had three loads of washing on the line as the sun set and the rain began. We brought it all inside and discovered the next morning why it is recommended never to dry laundry indoors - we had more than twice as much water on our windows than I've seen before.

Any amount of moisture in a home will seek the coldest surface on which to condense. Let the science lesson commence ...

Dew point is the temperature at which di-hydrogen oxide (water) turns from vapour to liquid. The good news is that this process releases energy; the bad news is that it makes your windows wet.

In most homes, a single-paned window on a long winter's night is the coldest surface - it is colder than, for example, a wall, a ceiling, a floor, a cat, or your forehead, but not colder than a bottle of beer fresh from the fridge. Yeah, right.

Actually, I'm serious. A cold bottle of beer "sweats" because the warm, moist air around it condenses (dew point reached) on the cold exterior glass surface.

Ever notice that once you've consumed the top half of beer that only the bottom half of the bottle continues to sweat? If not, tell your spouse you urgently need to conduct a scientific experiment, find a friend and peer review your results (hurray for science!).

Dew point depends on two components: humidity and temperature. In our home, the humidity is usually so low that no matter the temperature of our single-glazed windows, we won't get much condensation. You can't get weeping windows from low relative humidity.

High humidity in a cold home is bad for human health. Additionally, our bodies feel more comfortable at low humidity than high humidity at any given temperature, so the aim is to reduce the total amount of indoor water vapour.

There are a handful of ways that moist air can enter a home - showering, cooking, breathing, airing laundry, houseplants and rising damp.

Most of us will agree that we won't cut back on breathing, but we can avoid drying washing indoors. There are also ways to cut down on moisture while cooking and showering, which I'll describe another day.

I rarely ponder houseplants because we have none but, if you do, just be aware that they, too, "breath" out water.

Preventing rising damp, however, can be inexpensive and highly effective for certain homes in certain suburbs.

Because Wanganui has a variety of soil types, some areas suffer more from rising damp than others. For example, we are on sand in Castlecliff and experience no rising damp. On the other hand, we have a number of friends on Durie Hill clay that used to complain about cold, damp homes.

I used the past tense in that last sentence because all three of those families have installed polythene on the ground below their homes.

If properly installed, polythene can make a major difference in the "feel" of a home because of reduced relative humidity. It acts as a physical barrier to water vapour that wants to arise from the earth like an army of zombies waking from their graves to wander about your home causing mild discomfort, if not chaos and disarray.

Proper installation includes: 1) cutting the polythene around the piles; and, 2) taping the seams.

At about $1 per square metre, polythene is one of the cheapest and most effective investments householders can make if they know their home suffers from rising damp. Ours doesn't, but yours might.