Triple or Double glazing?

on Monday, 04 December 2017. Posted in pages

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.

Bristol Road 2Bristol Road 1

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 C -18C in cold weather. This has FOUR very important benefits.

  1. Thermal comfort. If my home is heated to 20C, 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).  Firstly they look fabulous.

victoriansash-02-crop-u24309

Eksalta provided the windows and Ainars Locs gave an excellent service measuring and making sure the windows would fit in the bays properly as well as keeping an eye on the installation. Recomended by Ainars, Oriel windows undertook the fitting and Shaun Rainbow managed to get the windows to fit in a not so square opening as well as patiently deal with my many questions.