Windows 101 – R U confused?


With so many types of windows (glazing or fenestration) available on the market, how does your average punter compare apples with apples? Should you trust the claims of different manufacturers? Much of the info I read, and hear, suggests you should be cautious and do your homework.

R-values and U-values? Single, double or tripled glazed? Aluminium, thermally-broken aluminium, timber, PVC, or composites of aluminium with timber or PVC, for the frames? Clear, tinted or lowE for the glass? What does thermally-broken or lowE even mean? Let alone VT, CR, or AI!?

I’m going to break this down into a series of posts over the next few weeks, because, while I love the science and a bit of data, I know it isn’t everyone’s cup of tea. Meanwhile, if you want to get ahead of the class, I recommend the independent resources and searchable databases developed by the Australian Window Association. They have a terrific Window Energy Rating Scheme and their FAQ page is a very good starting point. 

The AWA even have a tool that calculates the predicted energy savings from switching windows in particular climate zones. I just used it to predict the annual savings for a small house in Canberra, of switching from 1. aluminium framed 3mm single glazing (SG) to 2. basic aluminium double glazing (DG), 3. basic PVC DG or 4. PVC DG with a larger air gap and lowE coating.

  1. Basic alum SG (3mm clear glass) = starting situation
  2. Basic alum DG (3mm clear glass/6mm air gap/3mm clear glass) = $90 savings/year
  3. Basic PVC DG (3mm clear glass/6mm air gap/3mm clear glass) = $181 savings/year
  4. Fancier PVC DG (3mm clear glass/12mm air gap/4mm lowE glass) = $245 savings/year

How is it so? I’m not going to bombard you with all the details in one go but we’ll start with the first big point of confusion.

R-value, U-value and Conductivity
Lots of people are familiar with use of R-value when talking about the insulation we put in our ceilings and walls. R-value is a measure of how well something resists the flow of conductive heat; the thicker the insulation batt, the higher the R value and the slower the transfer of heat. When it comes to R-values, bigger is better. 

U-value is the inverse of R-value or 1 divided by R. So an R5 batt, that we typically put on Canberra ceilings these days, has a U value of just 0.2. When talking U-value, tiny is terrific.

A typical brick and plasterboard wall insulated with batts with an R-value of 2 has a total (or whole or system) R-value of 2.24 and, inversely, a U-value of 0.45. Let’s compare it to the total window (frame + glass) performance of some common windows (the most basic versions).

Aluminium SG                                                                                        U 6.7       R 0.15

Timber SG                                                                                                U 5.4       R 0.19

Aluminium DG                                                                                        U 4.8      R 0.21

Thermally broken aluminium or composite DG                      U 3.6      R 0.28

Timber or PVC DG                                                                                 U 3.0      R 0.33

Brick and plasterboard wall insulated with R2 batts             U 0.4      R 2.24

This means that windows, even double glazed windows, when the sun isn't shining through them, lose heat between 6 and 15 times faster than a typical insulated brick and plasterboard wall. That’s why, when I was learning to do thermal performance simulations of house designs, my instructor told me “All windows should be thought of as a HOLE in the wall!” Some of them are just a bit less leaky than others (hence the importance of glazing to floor area ratios and window dressings but those personal passions are for future posts!).

It also means that a basic aluminium double glazed window transfers heat 1.6 times faster than a basic PVC or timber double glazed window - losing it in winter and gaining it in summer.

Why does the aluminium frame make such a difference? Aluminium is an extremely conductive material. Aluminium conducts heat 4 x faster than steel, 200 x faster than glass and 1100 times faster than PVC or timber. Aluminium will transfer heat out of your house in winter, and into your house in summer, much faster than timber or PVC. As the weather gets cold, Aluminium also gets very cold making it very prone to condensation… but that’s a whole other story.

Enough fenestration flapping for this Friday! Windows 102 to follow soon.