The Natural Philosopher wrote:
...
Its very hard to see why double glazing is so insisted on.

...

Let's say we have two windows totalling 2.5 sq meters. At a U value of
5..so thats just 8% of the total heatloss from the room. A GOOD DG unit
should more than halve that..netting us a 4% gain, or ariound 17.5 watts
average, and annualized around 155KWh..say at 10p per unit..£15 a year
gain.For probably about £1500 outlay. So a 1% ROI.


Great post. It's really good to cut through the hype with some
calculations, but I don't totally agree with all of your figures.

In the above case, you've assumed an average temperature difference of
5C, but 10C is nearer the mark. (This improves the benefit of insulation
but doesn't alter the calculation of the benefit of turning your heating
down by a degree.)

You've assumed a cost of DG of £600/m^2, but I've just been quoted for a
job at £270/m^2.

You've assumed that you are replacing a fairly good existing SG unit.
This may be the case, but if you want to understand why DG is insisted
on then what about the case of replacing a drafty SG unit at the end of
its life, or the case of a new building. In these cases you need to
subtract from £270 the cost per m^2 of SG to make a proper comparison.

On the other hand, you've assumed 10p per unit. This should be a lot
less if you used gas, which reduces the ROI.

So the minimum ROI is
10*.3*2.5/1000*365*24/270 = 2.4%
(assumes you're replacing a perfectly good SG unit and heating with gas
at 3p per unit).

To work out the maximum ROI we need to know the cost of SG, which I
don't. Taking it to be £135/m^2 we get
10*.1*2.5/1000*365*24/135 = 16%
(assumes you're building from new or the existing SG unit needs
replacing; assumes heating with electricity).

There are also benefits in terms of quality of heat. If you just replace
heat lost from the windows with central heating then you get
temperature differentials - colder nearer the floor than at head level
etc - which is not as snug as uniform heat (in my opinion).

And there are benefits of DG in terms of extra sound insulation too.


Alex