Friday, December 4, 2015

Secondary glazing

My house has lots of windows. Windows are very poor insulators, and my windows let a lot of heat out of the house in winter and a lot of heat into the house in summer. Improving windows' thermal performance is very worthwhile if these things are already done:
  • drafts reduced/eliminated
  • Summer sun is excluded from the house (no direct sun on the windows in summer)
  • roof insulation is improved
I've done work on all these. The drafts project is not complete, but I've gone around 2/3 of the house with gapfiller, and have closed all the internal air vents. We've put external blinds on all the East-, West- and North-facing windows (though most of them were there when we moved in) and I've put a lot of insulation in the roof (not a fun job, but a very worthwhile one).

Secondary glazing is a cheap retrofit to get some of the benefits of double glazing. It doesn't insulate as well as double glazing, but it's a lot better than single glazing. The basic principle is that a second pane of glass is installed close to the first. It needs to be about 10 - 20 mm away. Too close, and heat transfer is too great (the air barrier isn't wide enough), too wide and an air current is established within the window and heat is transferred by convection.

For my house, I ordered acrylic sheeting which was cut to size. It was about $1100 for about 23 windows. I had an idea to attach it using double sided sticky tape so that it would look neater (than having screws in the corners). This seemed to work well, and I put some desiccant into the window to absorb moisture and prevent condensation. They looked great and I was very happy with them. Very schmick, so I kept going and did more windows.

Problem 1

What I found was that all the desiccant absorbed water (perhaps from the dobule-sided tape itself, though I'm not sure) and the desiccant turned to drops of sticky crap within the window. To cut a long story short, I don't recommend that people attach secondary glazing with double-sided tape. It may be that if I had done it on a cold winter's day and foregone the desiccant that condensation would not have been a problem. However, it will be very difficult to remove the glazing if there are problems down the track.

Hence, I've put all the rest on with small screws, one in each corner. This still looks good, and performs well.

Problem 2

We have sliding sashes in our windows, and the way they work means that the secondary glazing on the upper window needs to go on the outside so that it doesn't interfere with the opening of the window. This raises another problem. The acrylic is now exposed to the hot outside air, which causes it to expand more than the indoor sheeting (acrylic expands much more than glass as it heats -- up to 5 mm per meter of length). Because the corners are fixed in place, this is causing the sheeting to bow out in the middle (which means there are air gaps down the sides of the sheeting. To improve this, I plan to drill larger holes in the acrylic, and use rubber washers underneath the screws so that the acrylic can move relative to the screws, and hopefully avoid bowing. I will post about the success (or otherwise) of this plan.

Potential problem 3

The other potential issue with having the secondary glazing outside is that it is exposed to the elements. It is fairly UV stable, but will lose clarity in time if in direct sun (I'm being careful to keep it out of the sun). Also, I'm a little concerned about rain ingress into the glazing. I'm unsure how best to prevent this, but plan to put a bead of silicon along the top of the acrylic on the outside and see if that is enough (all the glass is under eaves so shouldn't get much direct rain on it).

What's left?

We have two enormous windows (1500 W x 1800 H) which it is possible to secondary glaze. I'd need to use 10 mm acrylic (vs 6 mm for the smaller windows) which would be extremely heavy and unwieldy and expensive (the two windows would cost nearly as much as the rest of the house). For this reason, I'm getting a quote to replace the existing glazing with a double-glazed plane. This is possible because the existing frame is nearly 100 mm wide and there is plenty of room. This will perform better than the secondary glazing, but it is not feasible to replace the glazing in all our sashes without replacing the sashes themselves (prohibitively expensive).


Our house definitely has better thermal performance than last summer. We've had a few days up around 40 already this summer, and the house has stayed about 26. This weekend, we'll have two consecutive days at about 40, it will be interesting to see how it goes. It's about 23 inside right now.
Note also, that our secondary glazing does not have the benefit of low-e glass, which is reflective to infra-red frequencies and helps prevent heat transfer. If we buy double glazing for the large windows, we will definitely get this (our's are South facing. For windows benefiting from winter sun, you may want to avoid low-e glass).

Cheaper options

Instead of installing secondary glazing, it would be much cheaper to make and install kume blinds, which provide possibly better insulation (at the cost of light and the view). We decided to improve the glazing.


  1. Hi Angus,

    Mate, that acrylic is really cheap for so many windows. Being a flamezone BAL-FZ rated bushfire resistant house means that I have few windows at about 34m2 in all and they cost about $1,000/m2 which I almost had a freak out about, but they do provide good heat insulation...

    Out of interest was the dessicant, the little silica dessicant balls because I've used them in secondary glazing applications in the past and wondered how they went over the long term. Did it take long for them to break down? Incidentally I used them in the double glazed acrylic sheeting in the experimental bee box but haven't noticed them breaking down yet.

    Hey, also if you have closed up the ventilation gaps, I recommend having a check during late winter in the structural timber to make sure that there is no build up of moisture - if only because moisture is a problem from a fungal and termite point of view. Dry rot is a myth, timber rots because it is damp.

    Wow, you are doing a really interesting experiment with those windows so it will be interesting to read how it performs on a really hot (and really cold day).

    Man, how are you coping with the heatwave? It was 41 in your part of the world and I read that you had the second hottest night on record... Not good.

    That large window is a very standard size - which you can probably pick up double glazed on the cheap. Ebay advertises plenty of windows that size. They are fairly easy to install, but if you need some guidance, no stress, give me a yell by email and I can send you some pictures. It may be a two person lift though.

    Actually, that is a pretty good result and about what I get here when there are a couple of days past 40. Top work. No actually, that is really good performance. I open up the house in the evening to let in the cool night air after those days too and that helps a lot - but you have to have insect screens...



  2. Hi Chris,

    I forgot to mention -- I may not have used the right desiccant. It was hard to find, and the one I bought was more like white flakes. I assumed it would be functionally equivalent, but perhaps not...

    Your point about damp/mould is a good one. My house is double-brick, so there are also vents on the outer wall that I've kept open (I've only closed the internal vents). I would definitely not stop ventilation under the floor either!

    We just had 4 consecutive days of about 40. One night had a minimum of 31. It was hot! The inside of the house reached about 30 C (though we have a split system in the lounge that I ran a few times at a thermostat of 25/26 during the day when our solar was producing). We installed ceiling fans at the end of last summer and I found sleeping at 29 C was warmer than ideal (by about 5 degrees), but ok with the ceiling fan on low.

    Hmm -- good point about the second hand option -- I will definitely check it out.

    Yeah, I'm pretty happy with the performance overall. The next big win would be to paint the roof white ;-) It's just very hard when you get consecutive very-hot days...

    Cheers, Angus


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