Triple or Double glazing?

Like everyone I have a limited budget and I needed to replace some first generation UPVC windows in my Victorian terraced house. Apart from being ugly, cold air is coming in around both the window frames and the seals. There is also water ingress in the south west facing corners which take the brunt of the weather. The internal walls are damp, and the two rooms are cold compared to the rest of the house. The UPVC window were fixed into the original box sash windows which is causing some of the problems and they are of particularly poor quality.

The questions were, should I replace the windows with

1. Sash or casement windows
2. Double or triple glazing
3. Wooden or UPVC

Sash or casement windows?

The first decision was relatively easy, I decided that I wanted sash windows to be in keeping with a Victorian house, but not necessarily sliding sash windows as you can get replacement windows that look like a sash but are not.

Double or triple glazed?

This was slightly more difficult and needed some thinking about, for me the two issues were not only the thermal performance of the windows but comfort as well

Wood or UPVC

I live near to the sea, so I get a lot of weather, which means that standard guarantees on paint may be reduced, in one case from 10 years to 2 years. However, wood was still my preference for the following reasons

• It just looks and feels so much nicer
• I can repair it if something goes wrong
• It can be recycled at end of life

Typical U values and costs

Several quotes gave me an idea of costs and performance, some window suppliers could not provide a U value, so that went no further. These prices are only a rough guide based on my measurements (which proved to be too conservative). Cost where known include VAT

Window type	Glazing	Material	U value	Cost with installation
Current window	Double	UPVC	2.8 W/m2K
Sliding sash	Double	UPVC	1.5 W/m2K	£4,500- £5,500
Sliding sash	Triple	Softwood	1.0 W/m2K	£10,500
Mock sash	Triple	Softwood	0.8-0.91 W/m2K	£6,400- £7,500

‘Mock’ sliding sash windows

There are several triple glazed Victorian sash ‘look alike’ replacement windows. The Green Building store do a mock sliding sash which in reality is a tilt and turn casement window  https://www.greenbuildingstore.co.uk/products/mock-sliding-sash/

Eksalta windows do the VictorianSASH which is a triple glazed timber window with a fixed upper window and tilt and turn lower window  http://victorian-sash.co.uk/

The wooden triple glazed sliding sash was just too expensive, so the choices came down to two options

• UPVC double glazed sliding sash
• Wooden triple glazed mock sash

Thermal performance and comfort

The difference in U values between new double and triple glazing is about 0.6 W/m²K. My windows are about 9.25m² so using annual degree day calculations, the heat losses are as follows

Window	Glazing	U value	Area	Degree hours South East UK (kka)	Heat loss kWh/year
Current windows	Double	2.8W/m2K	9.25m2	61	1580 kWh
Green building Store	Triple	0.8 W/m2K	9.25m2	61	451 kWh
Eksalta windows	Triple	0.91 W/m2K	9.25m2	61	513 kWh
Double glazed window	Double	1.5W/m2K	9.25 m2	61	846 kWh

Installing triple glazing will save me between 333 kWh and 400 kWh per year in heat loss. At 6p a kWh for gas, that’s £20 and £40 pounds a year – so based on cost savings it’s not necessarily the right thing to do.

Its not all about cost, windows can be the coldest spot in a house in winter and the warmest in summer. The reason why triple glazing is specified in very low energy homes like Passivhaus, is because this will maintain a surface temperature on the internal pane of glass of about 16 ⁰C -18⁰C in cold weather. This has FOUR very important benefits.

1. Thermal comfort. If my home is heated to 20⁰C, then as a human I probably won’t notice the temperature difference between the internal surface of the window and the rest of the room. This means I will feel comfortable
2. Draughts. In houses without triple glazing, warm internal air will hit the cold surface of the window, cool and fall, this creates air movement that we would feel as a draught. That’s why radiators were usually placed under window, to counteract this. If I have a triple glazed window this should not happen, and I can place the heat source anywhere in the room.
3. Space. Now the bay window area is cold and damp, with no draughts and better thermal comfort I can use the bay ares for sitting. This means more of the limited floor area is useful.
4. Climate Change. If we really want to stop climate change, we need deep retrofit of our existing homes. Windows are an expensive item, I wanted to put in the best quality and performing product now, as I don't want to replace them later on.

Triple or double glazing? – triple, its just the right thing to do.

So after (some may say excessive) deliberation I chose the Eksalta triple glazed Victorian(SASH).