Condensation on room facing side of secondary glazing film

[Jo Wills]

This is something happening in my own house, however the reason for the post is because we recommend secondary glazing film (i.e. the Stormguard product from Bunnings etc) to the communities we work in, based on personal experience. Up until recently, I have not experienced condensation on the ‘room facing’ side of the film (I’ve had to wipe the film down with a towel), I’ve always been able to say ‘it stops condensation’…now I can’t. I’d really appreciate some discussion about what could be causing this. I understand BRANZ has done some work on all forms of secondary glazing, so there might be some insight to be found there.

These are the conditions:
– the film is over all wooden framed windows in our bedroom
– the room is north/west facing and receives all day sun
– the curtains are triple lined, closed at the top, and puddle on the floor
– 90% of windows in the house are curtained and curtains in all rooms are pulled each night
– blast ventilation is practiced everyday
– windows are draught stopped with v seal and probably not 100% tight but the wind doesn’t blow through (anymore)
– we practice top notch extraction and containment behaviours in the bathroom and the bathroom is about 4 – 5 meters away from the bedroom
– we have and use an extraction fan in the kitchen
– heat source is a 8kw wood burner in the lounge (opposite end of the house to the bedroom but we close all other bedroom doors and the heat does make it down there)
– temperature in the lounge sits around 20 – 24 in winter with the fire, in our bedroom 17 – 18 (by the time we go to bed, after the fires been on)
– no drying clothes inside
– 2 people and 1 cat in the home
– 100sqm brick house, 1960’s, new corrugated iron roof, good ceiling and floor insulation, GVB, no wall insulation.

This has happened twice, the first time about 3-4 weeks ago, both times the morning has been very cold (but no frost).

This is the second time I’ve applied secondary glazing film to these windows, the first lot was up for a couple of years and I never experienced room facing condensation during that time.

The windows do get a light misting (on the glass) which I understand is due to the wooden framing releasing moisture, this has always just taken care of itself.

Has anyone else experienced this? I’m a starting to feel reluctant to recommend this product now, which is a real shame.

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[Jo Wills]

Responding to my own post! After having a conversation with Vicki, I want to validate my comment about ‘stopping condensation’…I only ever recommend this product after focusing on all of the sources of moisture first. I am completely/100% aware that double/secondary glazing doesn’t manage moisture. I realise that above comment may have indicated otherwise, but there is no confusion at all (its more the source of my frustration around what is actually happening). I understand what double/secondary glazing does. But these are my questions:
1) how is the warm air from the bedroom escaping down the back of the curtains and cooling because the curtains are triple lined, full, long and closed plus they cover a fair chunk of the walls on either side.
2) how is the surface of the glazing still so cold?? (the gap between the glass and glazing is significant)
3) WHERE IS ALL THE MOISTURE COMING FROM???
and a 4) why is this happening now when never before – more rain?
I guess I am at a loss as to what is happening from when I look at everything together. Our humidity readings are usually sitting in high 60s, so yes, a bit high but that is with a digital monitor, I have just started using a hygrometer, and the humidity is reading lower, in the low 60’s. So I can’t accurately say what it is, but having lived in this house now for ten years, I have turned it from being a cold damp house, to a warm not-damp one…I know what a damp house feels like.
So – is the question of the secondary glazing completely irrelevant and its just a moisture problem? Then…from where? My only guess would be in the walls…which has always been a suspicion.
Any input would be greatly appreciated, learning from this will be so important when sharing information in our communities.

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[Ian McChesney]

Hi Jo
I’ve had plastic film secondary ‘glazing’ in my house in Christchurch for quite a few years and experience the same thing occasionally. The condensation is always at the bottom of the glazing, which is the coldest part of the window – is that your experience?

I’ve put it down to this:
– sharp drops in outside temperature following warmish, humid weather where the house may have been quite warm and the air in the house may be carrying a lot of moisture (the warmer the air the more moisture it is capable of holding – remember RH readings only tell you the amount of moisture in the air relative to the maximum amount it can hold at that temperature). In these conditions condensation on windows may occur at high indoor temperatures than you might normally experience.
– your v good curtaining will be exacerbating the likelihood of condensation. They do a great job of keeping the room heat away from the window (which is their purpose), but the down side is that the window surfaces (including the secondary glazing layer) can get colder because they don’t have exposure to the warmer air in the bedroom, and hence be more prone to some condensation, sometimes
– note also the bedroom will have extra moisture load during the night because of sleeping bodies.

So, what you describe is similar to my own experience. I just put it down to these circumstances that will occasionally cause some condensation, and continue to have confidence in the product(s). I continue to use secondary glazing and recommend it. I presume the secondary glazing in other parts of the house is still performing OK?

If the problem persists, say on one window more than another, or if the fogging between the glazing becomes more consistent, it could mean the secondary glazing airtightness is compromised and needs attention.

Whether there are other moisture issues in your house exacerbating the problem? Difficult to tell at this stage, but if there were I would think there would be other signs of dampness in the house e.g. mould in wardrobes etc.

Hope this helps

all the best
Ian

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[Jo Wills]

Thanks Ian, appreciate your response. On the two occasions it’s happened, there has been significant drops in temperature overnight (and rain) and into the morning, however this morning there was no condensation anywhere in the house and similar conditions (cold, wet, no changes to indoor behaviour, humidity reading about 60 – 64% at night, when I woke up and checked it was 70% but dropped into the low 60’s again)…every window dry. But after going over and over everything in my mind, the two instances have been connected to very drastic temperature drops.

It’s the relationship I have been trying to understand with the secondary glazing and everything else going on inside and out of the house. These two instances of condensation have been the first times in about 3 years, and Nov last year we renovated our bathroom = no more damp shower curtain and a super powerful extraction fan. So I have been expected reduced humidity readings in the house…

With regards to where the condensation sits, its from the bottom up about 1/2 of the way. Which could then be my triple layer curtains making themselves known.

Its helpful to know you have experienced it as well, and the circumstances surrounding it.

Thanks again for responding!

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[Vicki Cowan]

what a master class – thanks both

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[alexking]

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.

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