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#5792
alexking
Participant

Hi Jo,

As you know, secondary glazing doesn’t prevent condensation, but it normally reduces it in comparison to a window without single glazing.

In this situation, I find the dewpoint concept helpful. If you have a 17 degree temperature inside and 70% relative humidity, your dewpoint will be about 11.5 degrees. That means if any indoor surface is 11.5 degrees or less, condensation will form on the surface.

As Ian identified, your triple-layer curtains are great at keeping heat in and for stopping the bulk of the air movement against the windows, but they don’t stop all air movement. Air (and moisture) will still diffuse slowly through the curtain material and around the edges. Since the curtains are keeping the heat inside, the window (and secondary glazing) temperate will fall further when it’s really cold outside. As soon as the inside of the secondary glazing falls below the dewpoint, condensation will form.

This is pretty common here in the South Island, I get this daily.

The points I would take from this:

1. Secondary glazing will always reduce the condensation on windows, because it will raise the temperature (on the inside surface of the film) relative to just a single pane of glass. Keep recommending secondary glazing, it still helps.

2. Good curtains, by contrast, always increase condensation on the windows, because they lower the temperature on the glazing. Mostly this is a good thing, it means you aren’t loosing as much heat out the windows. It does not mean you have a damp house, although people mistake condensation in these circumstances for that. It just means you have a colder window surface. It will not be harmful as long as you don’t let moisture pool and grow mouldy.

3. Do you have a moisture problem? Not necessarily. 70% humidity (RH) is high, but not hugely high. Humidity is produced by people breathing, pot plants, kitchen and bathroom, and the air will hold heaps of moisture at high temperatures. If you’ve had the house at 24 degrees for a long period it will suck up moisture from the sources available and when the temperature drops it can’t dump it immediately so the RH will go up as the temperature goes down. Perhaps don’t heat the house too hot for long periods, or look at ventilation practices again?

One technical “fix” for this (if you see it as a problem, I don’t necessarily) is better performance glazing. E.g. “proper” double glazing, low-e etc. Or another (3rd) layer of secondary glazing. Or shutters on the outside.

Another would be a balanced mechanical ventilation system.

All of which are either expensive or geeky and awkward. So most likely you just need to wipe off the condensation a few times a year.