Re-glazing windows

Question: We had double glazed (about 14 mm) hardwood windows installed at home over 25 years ago and some units are now misty (inefficient) so need replacing. We should like argon filled units but the total rebate is only about 30mm deep, so a joiner has suggested only 20mm units reducing the depth of the external beading. We are reluctant to replace the windows, as the hardwood is wearing well.
Please would you kindly give us suggestions as to how we could make the best of what we have? Reduce energy loss, but not entail too much expense/carbon emissions? Perhaps put a double glazing unit on the inside as well? The frame is 5cm deep. Also any advice on getting a good insulating seal around the window frames, as the mortar around the frames is becoming loose, and wonder if we should use something other than mastic to seal and insulate?

Bill Butcher, Director, Green Building Store

Bill Butcher, Director, Green Building Store

Answer: You probably expected me to say this, but there is no simple answer to your question! A lot will depend on how much you want or are able to spend, and also what else, if anything, you are planning to do on the house. From an energy efficiency point of view it is all about getting the most ‘bang for your bucks’. You can look at the windows in isolation but really you need to consider your budget and then consider how you can get the best carbon reductions for your money.

Heat losses from your windows will occur in three ways.

  • Through the window which is directly proportional to their insulative properties. This is measured by the U value, in W/m2K
  • Through air leakage, both through the window itself and around the window. Most modern windows have good draughtseals and are fairly airtight. Air leakage through the installation can be eliminated by good installation practise.
  • Through the installation detailing which can act as a ‘thermal bridge’ (which is very much what it sounds to be, allowing a bridge between the inside and the outside). This is more significant when installing low U value windows into well insulated buildings.

The U value of your current windows will be around 4-4.5 W/m2K. The best double glazed timber windows are now around 1.4 W/m2K and the best triple glazed windows can be as low as 0.65 W/m2K, so you can see there is a long way to go from what you have to what you could have.

The glazing units you have at the moment will be made up of 2 sheets of plain glass separated by an air gap of 6 mm, with highly conductive (yes, the exact opposite of what is needed) aluminium spacer bar.

Modern high performance glazing units now have the following features:

  • Selective ‘low emissivity’ coating which allows the radiant heat from the sun into your home (even in Pennine Yorkshire where we live!), but allows out almost no radiant heat from your rooms.
  • A cavity from 16 to 20 mm filled with argon gas, which hugely cuts down the heat loss through conduction and convection.
  • A spacer made of a material which is insulating – normally a plastic, so cuts down the heat loss at the edge of the glass.

The 20 mm units you are suggesting would, if using the best low emissivity coating and argon fill have a U value of around 1.2 W/m2K. You can expect your overall U value of the window to be about 1.7 W/m2K with hardwood frames. There are a couple of things you need to consider if you do this.

It will be very tight to get a bead thin enough to allow for some sealing of the glazing unit. You have to be sure that the new bead will fully cover the edge seal of the glazing unit. If you use a smaller glazing unit, say 16 mm, the U value of the glass will rise (get worse) to around 1.7 W/m2K. This might still be worth considering as it would bring your windows up to something like 2.0 W/m2K ie the backstop value in the current Building Regulations.

The main cause of glazing unit failure is trapped water between the unit and the frame. There are two really good approaches to dealing with this. The first is to use a ‘drained and ventilated rebate’ . This means that the bottom rebate is sloping outwards and has a bead (normally aluminium) which allows ventilation and drainage. The other way is to internally bead, and to bed the unit in silicone to a standard developed by the Institute for Window Technology in Germany, which is what we use on our PERFORMANCE range. There are other ways of glazing, including full silicone bedding, but you need to be sure they are robust enough to stop water getting around the unit. Much UK practise is not good enough, and you find after a few years that the units fail as a result.

Featured Image: PERFORMANCE casement outward opening triple glazed timber windowIf I were you, I would first cost up the replacement of just the glazing units against the cost for replacing the windows with high quality double (or better, triple) glazed pine windows like our PERFORMANCE range before you decide. Good new windows should last longer, be more airtight, and of course can be carefully installed to avoid air infiltration and thermal bridging. If you decide to replace the glazing, then I would check very carefully what standard your joiner is fitting the glazing to (Hodgson Sealants have some good silicone bedding systems). Absolutely avoid the use of any form of putty! With regard to the sealing of the existing frames, I would carefully rake out the mastic, carefully fill the void with expanding foam and then re-point probably with a silicone sealant.

As I said, a complicated subject, which we have, with some trepidation, tried to summarise. I hope this is helpful when you have digested it!

All information is provided in good faith. However, any use of the information is entirely at your own risk, no guarantee is given as to its suitability and Green Building Store does not accept any liability whatsoever for building performance or safety, regulatory compliance, or any other outcome which may result from its use or implementation.

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