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I have been considering installing secondary double glazing units in my home in an attempt to turn my double glazed windows into triple glazed windows.

I understand the law of diminishing returns, and the fact that sealed units are inherently more efficient, but is this a practical solution to reducing energy bills?

Amongst my concerns are:

  • How I avoid condensation between the secondary glazing and the existing double glazing.
  • Whether I can find secondary glazing units which won't prevent the existing windows being opened and or cleaned easily.
  • Whether I should try to fit the secondary glazing unit as close as possible to the current double glazing unit, or whether a larger air gap would be preferable.
  • If close placement is better, whether I can fit a secondary pane to the existing frame (i.e. fitting it so that it opens with the existing glass).
  • If frame placement is possible, whether inside or out would be preferable or even allowed.

If it makes any difference, my current double glazed windows have Trickle Vents, so the windows are not as sealed as one might hope.

† I could see that bolting an extra pane of glass onto the outside of an existing double glazing unit may pose a risk of the pane falling out and thus might this be banned under planning regulations.

‡ These appear to be required in my block of flats, though I'm not sure whether this is due to local building codes or something specified in the lease for the block, and I'm not sure why we seem to like them so much in the UK.

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Short answer: it's unlikely to be cost-effective, if you consider only the energy savings.

Longer answer:

Yes, it's possible. Let's look at the issues one by one.

Condensation

If you only open the secondary glazing when the inside air is cool and dry that will reduce the chances of condensation. Put some dessicant in the gap, and dry it out occasionally, if you like. The specific risk of condensation will depend on the temperature and humidity when you open the secondary window, and the temperature to which the air gap subsequently drops.

Positioning

The building of the secondary frame, and clearance for opening, will determine the air gap.

Don't fit it to the existing frame: have it in its own frame. The hinges on your double glazing aren't designed to handle the extra weight of another sheet of glass, and you won't get a good seal around the new pane.

Place the secondary glazing inside: your existing double glazing is already weather-proof. Your secondary glazing needn't be.

Energy savings

You've already got most of the energy savings that you're going to get, from the double-glazing. And they probably came as much from the improved air-tightness, as from the decreased U-value. With secondary glazing, you'll only have savings from additional U-value decreases. Here's the calculation:

Combined U-value = 1 / [ (1 / secondary-glazing U-value) + (1 / double-glazing U-Value) ]

So if your secondary glazing would have a U-value of 2.0, and your double-glazing has a U-Value of 1.0, then the combined U-value = 1 / (1/2.0 + 1/1.0) = 1 / 1.5 = 0.67. So you'd reduce the window losses (not total losses, just losses through the window) by a third.

That's unlikely to be economic, on energy savings alone, unless your unit cost of heat energy is immense, and/or it gets to -20°C outside for a few weeks a year.

But then there's comfort

if you currently find that sitting right under the window is uncomfortable because of cool down-draughts, then the effective increase in usable room space you get from the secondary triple-glazing might make it worth it.

And taking it to the extreme

If the place is already super-insulated all over except the window, and you get lots of sun in in winter, and you've already installed an MVHR, then adding secondary triple-glazing might be the thing that effectively turns your home into a Passivhaus. And at that point, you probably wouldn't need a central heating system. The money saved in not having to renew your central heating system, will go a long way to paying back the cost of the secondary glazing.

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  • "You've already got most of the energy savings that you're going to get, from the double-glazing". 2G varies wildly in performance, from Uw=3.1 to Uw=1.7 (both values approximate) which means if you go from low performance 2G to high performance you can half the energy loss. So I'm not sure I agree with that. Of course, best to replace with 3G if you're going to do it. – Dan Gravell Aug 9 '17 at 10:22
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EnergyNumbers pretty much covered it all. I can only add two things. If you have vinyl windows already, adding a very thin sheet of rigid plastic (lexan, polyethylene, etc.) larger than the original glazing cut to fit the interior of the sashes using an adhesive (epoxy works best, but it's permanent) can reduce conductive and convective loss. It's easy to ruin the aesthetics so practice on windows less visible first.

At the risk of going off-topic, an alternative option is replacement sashes. Contact the manufacturer and you can usually purchase replacement sashes with more efficient glass packs for about the cost of good storm windows. The energy benefit is usually better.

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In terms of whether this makes sense, it depends on the current glazing. 2G varies quite a lot.

Condensation

This is an issue; it depends whether the tertiary glazing is fixed or openable.

Getting a proper seal is difficult, you have to consider air infiltration from the outside. To that end I would suggest openable units so you can manage the condensation.

Positioning

A bit more on positioning. Generally the optimum gap is about 18mm for energy saving. Below that you get more conductivity through the air, above it circulation can occur leading to convection aided energy transfer.

If you are optimising for sound reduction, you would want a larger gap.

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