Why doesn't secondary glazing steam up? (usually)

This window has secondary glazing
fitted with inconspicuous clips

Double glazing units mist up when the seal is broken. Secondary glazing systems aren't sealed. So why don't they steam up? Or when they do, how can it be fixed? If I seem to be obsessed by windows recently it is related to planning for the Transition Cambridge forum on Thermal Comfort in Older Houses in June. Today I have been looking at advice on secondary glazing options. I've found some excellent videos on how to install them too.


The ideal secondary glazing system would be cheap, invisible, easy to install, allows for window opening and lasts for ever. I haven't found one quite that good. The most invisible ones I have found are the plastic film sort where you stick a special film over the window with transparent tape on the frame and then shrink the film by heating it with a hair dryer to get rid of the wrinkles. This video makes it look easy - though judging by the reviews I have found it is quite possible to mess it up. For example if you don't get a good seal around the edge then the tape will come away when the film shrinks and pulls on it.

Another option I have seen which is almost invisible is a clear acetate sheet fixed over the window with screw-in clips. You can turn the clips to take the sheet out in the spring when you want to be able to open the window and refit it in the autumn. The only visible bit is the clips which are quite small - only 2-3 mm. The picture shows a window with these fitted. This video shows a range of options for DIY secondary glazing including clips.

However, if the windows steam up then they are not invisible and it can be much worse than that: I was once asked to advise a lady who got terrible condensation on the inside of the glass which rotted the wooden frame.

What causes condensation?

You get condensation when warm air holding moisture cools to the point where it can't hold that amount of moisture any more. All air has some level of moisture - with the possible exception of very dry air in deserts where there are no plants or animals. The air in our homes has moisture generated by people breathing, cooking, doing the laundry and so on. You will often get condensation in winter on the inside of a single glazed window because the glass is cold: if it is cold enough then when the warm, moist air inside the room hits the glass and cools it condenses. The way to stop this happening is to make sure the room is sufficiently well ventilated that the moisture in the air doesn't build up. This is why you can get condensation problems after fixing draughts.


What causes condensation in secondary glazing systems?

There are two kinds of condensation problems you can get, depending on where the condensation is. If the condensation is on the inside surface, then you have the same situation as the single glazed system. This means that your window is so draughty that it might as well not be there. The secondary glazing is effectively the only glazing so the inside surface is cold. The solution is to fix the draughts in your original window.

If the condensation is on the inside surface of the original window, then it is because there is too much moisture in the air between the panes. The outer glass is cold and the moisture is condensing on that surface. This happens if there is too little ventilation between the panes to outside. There will always be some leakage of moist air from the room into the air gap. What you need is a way for that moisture to escape. So if your inner seal is a bit leaky, you want the external seal to be a bit more leaky. If your original window is very air tight and your secondary glazing system is not, and can't be fixed, then you have a problem. One solution is to drill a small hole in your outer window frame to make it more leaky. It can be quite small - no more than 5mm. It is best to fill this with something loose like cotton wool so that insects don't get in.

I suspect you may not always get a good seal with the clip-on glazing system illustrated above - it depends on the condition of your window frame.

Secondary glazing causing condensation elsewhere

I have also come across cases where putting secondary glazing on some windows has exacerbated condensation on other windows that are still single glazed. This is because fitting the secondary glazing has fixed the draughts in those windows, reducing ventilation generally. Hence the air in the home is now more moist than it was and you get worse condensation in other places. The solution is to improve ventilation and/or upgrade the remaining windows.

It all comes down to ventilation - having enough in the right places. Sometimes loft or wall insulation can lead to moisture problems and ventilation is usually at least part of the problem then too - but that is another topic for another post.