Double glazing, dampness and condensation

Double glazing is big business these days. The building code now effectively requires all new homes to be fitted with double glazing, and the market for retro-fitting double glazing to existing windows is booming.

I'm a big fan of double glazing. It's an excellent way to insulate the window system.

However, there's one common misconception being promoted by the majority of double glazing companies that I'd like to dis-spell.

Reduced condensation does not mean reduced dampness and moisture

Condensation is a common problem for homes in New Zealand, and double glazing does help to prevent condensation from forming on the inside glass surface. It does this by making the inside glass surface warmer, which means the moisture in the air around the glass doesn't condense.

However, this doesn't mean the moisture has magically gone away. It's still there, in your home.

In fact, the newer the windows, the more air tight they'll be, so installing double glazing, or retro fitting double glazing to existing windows actually tends to increase the moisture levels in the home. It's just that you don't notice it as much because you don't see it on the windows.

So why is excessive moisture a problem? You may ask, if you can't see the moisture on the windows, is it really a problem? Yes it is. Excessive moisture or dampness leads to mould and mildew growth. It's also linked to asthma and other respiratory health problems. Dampness in the home also damages furnishings and fittings over time, leading to costly repairs and maintenance.

Just because the moisture doesn't condense on the windows anymore, it doesn't mean it goes away. It lingers in the home and when the temperature of any surface drops below the dew point, it condenses on that surface. This could be your bedding and furniture, or your carpet or your clothes.

What about indoor air quality? Indoor air quality is a measure of the level of CO2 and other pollutants in the air. Double glazing does nothing for improving indoor air quality. In fact, because the home often ends up more air tight after it's installed, the indoor air quality is usually worse.

So how do you properly address moisture and indoor air quality? The answer is Ventilation.  Ventilation allows the damp, polluted indoor air to be gently replaced by drier, fresher outside air.

And yes, the outside air on a cold wet winter's day is still far drier than the air inside your home - in fact the colder the air, the less moisture it can hold. This is why condensation forms in the first place. The air temperature around the surface drops, the colder air cant hold the moisture any more, so it releases it as condensation.


In the majority of homes, natural or passive ventilation is more than sufficient to meet your ventilation needs. Natural ventilation can be achieved by leaving your windows slightly open all day and night, although this is not really an option for most households these days. There are security issues, insects can come inside, and it also tends to over-ventilate, causing excessive heat loss.

A far better solution is to install secure passive vents into the windows. These can be left in the open position 24/7, providing secure, low-level continuous ventilation without any noticeable heat-loss. The market leader in retro-fit passive vents is Easy Air. These are also ideally suited to being installed at the same time as double glazing, whether it's new windows or if you're double glazing existing windows.

1 comment:

Anonymous said...

Liked this article a lot, highlights the awareness of moisture you need to have even if you have double glazing installed.