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).

Performance

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.