Triple Glazing, Right?

An Eccentric Anomaly: Ed Davies's Blog

Normally for a well-insulated house you'd think putting in triple glazing would be a bit of a no-brainer as the price penalty for 3G is getting smaller, even in the UK. However, for the roof windows required for an A-frame-type house things are a bit more marginal and it seems worth doing the arithmetic.

Considerations

  • 3G is considerably more expensive for roof windows than 2G, particularly for the top-hung windows needed for fire escape.
  • 3G loses less heat (of course) though the difference isn't that huge, especially if you put some sort of insulated covering behind the window at night (which is a large part of the day at the times of year when the insulation effect matters).
  • 3G has less solar gain.
  • 3G is said to reduce drafts and radiative heat loss so contribute to better comfort at lower temperatures.

From what I hear the UK window industry likes to push that third point. I don't think it makes too much sense - yes, you'll get more heat gain with 2G but that's on the days when it matters least as you've got external solar collectors pumping energy into the house anyway (haven't you?, … ). We need insulation to survive runs of days of poor insolation.

Here's a similar calculation I did earlier.

Cost Penalty

At the manufacturer's list price the difference in cost between top-hung double and triple glazing is eye-watering. At more-sensible dealer prices for the 780 × 1398 mm-frame-size pine windows I have in mind they're £345.75 for 2G and £464.10 for 3G giving an extra cost of £118.35.

For the centre-pivot windows the extra cost of 3G is only £67.28.

(All prices here are VAT exclusive as VAT can be reclaimed on new build. Overall, it cancels out anyway.)

Extra Heat Loss

The overall window U value for 2G is given as 1.4 W/m²·K and for 3G as 1.1 W/m²·K. (For the centre-pivot windows 3G gives a UW of 1.0 W/m²·K - presumably the extra weight of the 3G and the resulting extra springs required means an increased loss through the frame somewhat offsetting the reduction through the glass for the top-hung ones.)

Here are the heating requirements, in degree days, for EGPC (Wick Airport) to a 20 °C base temperature for some recent winter months from www.degreedays.net.

Winter 2010/11 2011/12 2012/13 2013/14
Nov - 332 425 426
Dec 591 507 504 -
Jan 508 500 480 -
Feb 426 419 464 -
Mar 453 387 530 -

The two worst months were 2010-December with 591 degree days and 2013-March with 530. Both were 31 days long so the average temperature differences were 19.07 and 17.09 °C respectively. Let's say a “typically bad” month has an average temperature difference of 18 °C.

The frame area of the windows I have in mind is 1.09 m² so the heat loss for 3G is 1.09 m² × 1.1 W/m²·K × 18 K = 21.6 W and for the 2G is 1.09 m² × 1.4 W/m²·K × 18 K = 27.5 W giving an extra heat loss of 5.9 W or 141.6 Wh/day.

That's ignoring the effect of blinds, curtains or shutters put behind the window at night which would reduce the heat losses for both windows types and the difference between them even more.

Still, working on an average of the equivalent of 45 minutes of solar generation per day in midwinter (noting in passing that one of the worst months was a March which would typically have considerably more) we would need 188.8 W of PV to compensate for that heat loss.

Reasonable PV panels can be had from a known supplier at £0.52/W (ex-VAT). Add in some mounting hardware, cables, immersion heaters and some electronics to give, say, £0.80/W.

PV to compensate for the extra losses from 2G would therefore cost 188.8 W × £0.80/W = £151.04.

So double glazing and the extra PV would cost £151.04 - £118.35 = £32.69 more but would give an extra 180 W or so of electricity the rest of the year.

I don't know for sure but the PV is likely to have a longer lifespan than the window glazing unit. On the other hand, when the window glazing goes it's not difficult to replace and it's only the glazing unit itself which will be needed (one hopes). Maybe we can look forward to better technology for both the window and the PV on replacement, as well.

Conclusion

There's really not a lot in it either way. Just get the 3G and enjoy the increased comfort which is particularly significant in a house which has been designed to have small window areas and therefore is likely to be used sitting close to windows.

Update 2013-12-11

A chap who goes by the name of DJH on various forums I frequent points out in private e-mail that the U-value of roof windows increases depending on the angle at which they're mounted. The first reference I found was BR 443 which says, for windows at my intended angle of 60°, to allow an extra 0.3 W/m²·K for double glazing and 0.2 W/m²·K for triple.

That increases the difference in U-values between double and triple glazing from 0.3 to 0.4 W/m²·K so will increase the cost of PV that would be needed to compensate for double glazing by a third to about £200 so the overall cost penalty of that combination becomes £80 rather than £32 but for an extra 250 W of PV in summer, rather than 188 W.

In other words, this tweak is worth bearing in mind but doesn't radically alter the basic conclusion that 3G is probably a bit better than 2G + compensating PV but there's not enough difference to get wound up about the matter one way or the other.