Triple Glazing
I’ve just had an illuminating conversation with the technical Director at our glass supplier. He has software which calculates the efficiency of double glazed and triple glazed units, dependent upon the type of glass, the thickness of the glass, the space between the glass, the type of gas and the type of spacer used.
It would appear the benefits of triple glazing are being marketed heavily by the glass manufacturers to increase their glass volume output by 50%. However, the true benefits of triple glazing are only evident if the combination of certain factors are at optimum.
Assuming the gas is argon, which is the only economic gas to use, and that the spacer bar is warm edge, the best available, then the following examples are interesting:
Example 1: 4:16:4:16:4 – 44mm overall
Provides the optimum performance for a unit combining two sheets of soft coat low-e glass and one standard sheet of glass on the outside, having a 16mm gaps between the panes (44mm overall) and argon gas fill, provides a centre pane u value of 0.8 and a typical window overall u value of 1.1. compared with a double glazed window with 4:16:4 configuration which will have a centre pane u value of 1.16 and overall u value of 1.46, a net improvement of 24.6%
Example 2: 4:20:4:20:4 – 52mm overall
Centre pane u value of 0.9, overall u value of 1.2, when compared with a double glazed window with 4:16:4 configuration which will have a centre pane u value of 1.16 and overall u value of 1.46, a net improvement of only 17.8%
The problem comes when we consider that most domestic windows are typically 70mm thick and can only just manage a 32mm maximum width sealed unit. In timber we would require a more expensive 90mm frame to achieve the same. But let’s look at another example to see the reduced benefit. A timber window designed to take a 44mm overall unit would have to be at least 94mm thick.
Example 3: 4:10:4:10:4 – 32mm overall
Centre pane u value reduces to 1.1, overall u value of 1.4, when compared with a double glazed window with 4:16:4 configuration which will have a centre pane u value of 1.16 and overall u value of 1.46, a net improvement of only 4.1%
We can now identify the hype put out by the glass manufacturers.
Some other considerations.
Unit cost: Triple glazing is typically 75% more than double glazing. There is 50% more glass, double the spacer tube and double the labour.
Frames cost: Will typically have to use 28% more material.Hardware cost: Because of the increased weight, heavier duty hinges are required, costing 15-20% more.
Labour cost: Handling the heavier units will typically increase the labour cost of handling the 50% increased weight, especially when working through awkward areas of a house, upstairs or in conservatory roofs. Under health and safety a man should only carry a maximum of 28kgs. A typical 2.4 sq.m double glazed unit will weigh 57.6kg, just manageable by two men, but a similar sized triple glazed roof pane will weigh 86.4kgs instead and require 3 men to handle or the hire of lifting equipment, which takes doubles the handling time anyway. So typically this will at least add 50% to the installation cost.
So we can see how the increased on-cost of triple glazing can significantly outweigh the spurious benefits.
Conservatories: will usually have a more efficient overall u value where the panes of glass, which are more efficient, are larger when compared to the frames of a normal window, which are less efficient and are typically 33% of a window area.
Whilst I will not deny both the acoustic and insulation benefits of triple glazing when the optimum glass and gap is applied, I’m pleased to have carried out the above exercise as I now have a much clearer picture of the current benefit to cost relationship of triple glazing. I’ve tried to analyse as factually and as objectively as possible from the information acquired and hope you have found it useful. I welcome any enlightened feedback. Certainly, whilst we have triple glazing as an option we shan’t be encouraging it’s use as an economic aternative. Perhaps the payback can be compared to the true return on wind turbines and solar panels.
