Listed Building - Secondary Glazing, Condensation/Misted Up Inside

[stevehayter]

My wife and I recently moved into a grade II listed cottage, which has timber casement windows, with secondary glazing attached to each glass pane internally. When we moved in, the majority of the windows had been neglected for 15-20 years - all mouldy and black. I have recently embarked on a project to refurbish each one - notably cutting out any mouldy wood, sanding and cleaning them, and repainting them with a water based satin paint.

I have completed 4 sets of windows so far - 2 of them are fine - however 2 have started misting up on the inside of the secondary glazing - as shown in this picture:

I can't really figure out why two are fine, and two suffer from this issue. Of the two that are misting up, one is on the front of the house and one is on the rear. So it doesn't seem to be location specific. All the windows are very similar, and all have the same secondary glazing. But it's obviously an issue as the condensation forms, and then forms drops of water inside the glazing which are starting to cause mould again.

I should point out I re-applied the secondary glazing after a dehumidifier had been on in the room for 24 hours, but I have read that these secondary glazing units are not 100% sealed anyway, and water will always find a way in. But how do I stop this? I've read about adding a small hole inside the glazing to the outside, but does this work? I've also read about putting silica gel inside, but surely this wouldn't stop all the condensation?

If anyone can shed any light on this it would be appreciated .

 

[Gazman16]

This will always happen no mater how well you seal them, Atmospheric moisture will always find a way in. It may just be that this window is nearer sources of moisture (kitchen, bathroom, sleeping people etc) so it has shown up here first.

You will probably want to go for a secondary glazing system that is quick and easy to remove and put back for cleaning. Clips, Magnates or a proper sliding window system

 

[JohnD]

The way to do it:

Seal the secondary glazing on the house side as well as you can. For example draught strip.

Allow the space to ventilate to the OUTSIDE. Usually old windows are pretty leaky, but if not, you can drill a small hole, sloping downward and outward, in the frame and stuff it tightly with mineral fibre to prevent dirt and insect penetration. It need only be a pinhole, because the expansion and contraction of the air in the cavity will give all the movement you need.

The outside air is colder and contains less water vapour than the inside air.

Your objective is to keep air from the house out of the cavity, and let outdoor air in. The air in the cavity is practically still so it still works as an insulator.

Make sure there is no rain ingress to the cavity or you may still get misting on rainy days.

 

[stevehayter]

This will always happen no mater how well you seal them, Atmospheric moisture will always find a way in. It may just be that this window is nearer sources of moisture (kitchen, bathroom, sleeping people etc) so it has shown up here first.

You will probably want to go for a secondary glazing system that is quick and easy to remove and put back for cleaning. Clips, Magnates or a proper sliding window system

Thanks Gazman. We have sliding glazing downstairs, but are not in a financial position to get it installed upstairs yet (plus I'm informed one of the windows is too high (in terms of length) for sliding glazing. But will check out other mechanisms if JohnD's advice below doesn't get me anywhere.

The way to do it:

Seal the secondary glazing on the house side as well as you can. For example draught strip.

Allow the space to ventilate to the OUTSIDE. Usually old windows are pretty leaky, but if not, you can drill a small hole, sloping downward and outward, in the frame and stuff it tightly with mineral fibre to prevent dirt and insect penetration. It need only be a pinhole, because the expansion and contraction of the air in the cavity will give all the movement you need.

The outside air is colder and contains less water vapour than the inside air.

Your objective is to keep air from the house out of the cavity, and let outdoor air in. The air in the cavity is practically still so it still works as an insulator.

Make sure there is no rain ingress to the cavity or you may still get misting on rainy days.

Thank you . The glazing is sealed very well on the inside - there aren't any obvious areas where it would leak. Did want to try drilling a hole, but I can't tell where the glass runs to, and am worried I'll drill a hole into the glass. But I'll look at it a bit more closely and try and figure it out.

 

[JohnD]

the hole can be in the frame or mullion. Need not be near the glass.

 

[stevehayter]

Thanks JohnD. The hole has to be inside the secondary glazing though? As there isn't a lot of room to play with. The glazing pretty much just covers the glass + a cm or so for the seal.

 

[JohnD]

yes, the hole has to ventilate the cavity between the inner and the outer glazing.

Looking again, I see your secondary glazing is attached to the casements, not the frames.

 

[bernardgreen]

One caution about drainage holes through the timber window frame. The tube drilled through the timber should be sleeved to prevent water being absorbed from the tube into the untreated timber wall of the tube. This can make the timber damp and thus accelerate decay of the timber frame.

 

[endecotp]

Remove the sec. glazing, dry everything, and then wait untill moisture level is very low before refitting. You'll then trap dry air in the gap.

Note that you want low *absolute* humidity, not low *relative* humidity. A cold, dry morning after a few dry days for example - but before you've used the shower!

 

[stevehayter]

Hey endecotp. Everything I've read online though suggests these units will always let moisture in eventually, regardless of airtight you try and make them. Is this not correct?

I'm coming to the conclusion I'll just have to take the glazing off occasionally and clean them

 

[endecotp]

How are they attached?
I have some magnetic panels and they've never suffered condensation - though they are in relatively dry rooms.

 

[stevehayter]

They're attached with clips screwed to the window. Though I am considering changing to magnetic strips for ease of removal.

They are near the bathroom which I suspect is the main source.

 

[JohnD]

having looked at the way they are fitted, I think they are worse than most secondary glazing, because they are fitted to the sashes, not the frames.

Secondary glazing is most often fitted so that the whole of the original window assembly is on the "cold" side of the secondary glazing. So the typical leakage and draughts around openable windows is able to ventilate the cavity sufficiently to keep the moisture content more or less at outdoor levels.

 

[JohnD]

drainage holes

It's a ventilation hole, not a drainage hole.

 

[stevehayter]

having looked at the way they are fitted, I think they are worse than most secondary glazing, because they are fitted to the sashes, not the frames.

Secondary glazing is most often fitted so that the whole of the original window assembly is on the "cold" side of the secondary glazing. So the typical leakage and draughts around openable windows is able to ventilate the cavity sufficiently to keep the moisture content more or less at outdoor levels.

So the glazing covers the entire window? How do you open the windows though with this sort of glazing?

I'm going to have to just save up for the sliding secondary glazing aren't I .