U values for windows and walls, etc

[d000hg]

No idea if there is a specific forum area for this...

I've been looking into double glazing and the energy savings are one aspect of that. However our house also has solid brick walls (double brick, 12" or so) and the EPC report gave a very damning score due to this as well. Considering a window is quite a small fraction of the whole wall I'm wondering realistically how much difference that would make.

On this site (https://www.designingbuildings.co.uk/wiki/U-values#Typical_values) it gives typical U values (Watts per square metre per degree of temperature difference inside/outside):

A range of U-values are indicated below for the purposes of comparison only:

• Solid brick wall: 2 W/m²K.
• Cavity wall with no insulation: 1.5 W/m²K.
• Insulated wall: 0.18 W/m²K.
• Single glazing: 4.8 to 5.8 W/m²K.
• Double glazing: 1.2 to 3.7 W/m²K depending on type.
• Triple glazing below: 1 W/m²K.
• Solid timber door: 3 W/m²K.

Do those seem reasonable? And do they take into account draughts or for instance is that U=5 for single glazing on the assumption it's a well-fitted, sealed unit?

But anyway in our living room (5mx5mx3m) we have two external walls, one with a large original bay window totalling 2.5x2m. That means our window is 5m2 but each external wall is 15m2, excluding the window from the wall we have 25m2 of brick and 5m2 of window.

Ergo spending loads of money on a new window is going to have far less effect than spending a few hundred putting a stud wall in?

 

[JohnD]

spending money on a new window will have such a poor return that it will take about a hundred years to repay its cost in energy saving.

If it's a plastic window, it won't last that long.

 

[d000hg]

Some quick sums...

5 square metres of window currently U=5. Let's be generous and say I could get them to U=0. Further let's say there's a 20 degree temperature differential. That means I'm saving 5x5x20=500W. So 0.5kwh per hour of heading. If I have heating 12 hours a day, 8 months of the year that's .5x12x250 = 1500kwh.

Current prices are 12p for electric or 3p for gas per kWh. So my window is costing me £45 per year.

The quote I had so far to double glaze it is about 4 grand (replacement wooden sashes) so your hundred years figure is about right!

 

[JohnD]