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One of them is a set of radiator fins designed to take heat away from
electrical devices. The other is part of a window frame, designed to... keep heat in? I've certainly always thought the job of windows is to keep it
warm inside when it's cold outside, and I suppose to keep it cool
inside when it's hot outside. Could you tell which picture was which?
Pretty tricky, eh!
It has been said that aluminium window frames destroyed Japanese
architecture. Apparently when aluminium was discovered, people made
jewellery out of it. It must have seemed like a great option to wood,
when it came to be used in windows. It must have been very cheap. But
when it comes to heat conduction, aluminium conducts about 1500 times
more than wood. Obviously, you can get away with using less aluminium,
but the surface area is likely to be similar, and heat conduction
depends a lot on the surface area.
Japan currently has ratings for window performance, based on their
heat conductivity. There is a star system, going from one star to four
stars. Four stars is the highest rating! This sounds really good,
until you look at what the stars are and are not measuring.
Read more about it here, and hear the annoying jingle! but no actually
meaningful numbers.
First of all, one star means more than 4 W/m2K. That means that at least 4
Watts of heat will flow through each square metre, for every degree of
temperature difference. One star just means that there is glass. A
single pane.
From 4 to 2.7 W/m2K gets 2 stars. This generally means double glazing.
2.7 to 2.33 W/m2K gets 3 stars. To get two stars, you need low-e
glass. This means low emissivity. I don't completely understand this,
but I think it's the opposite of reflectivity, so they could call it
"highly reflective" glass, and people would be able to understand
easily, but in their wisdom they don't. They add a thin film of metal
onto one of the sheets, so more heat is reflected back into the house,
or indeed reflected out of the house, if it's summer.
Less than 2.33 W/m2K gets 4 stars, the highest possible level.
Two-star glass is double glazed, with a 6 mm gap between. In the
example I've got, 4-star glass has a vacuum between the pains.
This is called Eco glass, where once again we have that feel-good
green feeling, but we're not sure whether it's the green trees
swishing in the breeze, or some green notes floating into our wallets,
or not floating out. In fact I think the motivation maybe neither and
the concern to keep the economy of the domestic window industry going.
In the grand scheme of glass thermal efficiency, 2.33 is far from the
highest possible level. I'm not sure how they arrived at the numbers
4, 2.7 and 2.33, but I imagine a few aging window makers got together
over a few beers and discussed the best windows they could remember
making. Anyway, let's say the next logical place for a mythical
5-star window would be around 2. To get to this performance of glass,
you need to start making a bigger gap between the panes. If the gap is
too big, the air in the gap starts circulating, so the convection
starts transferring more heat. The optimum thickness is something like
16 mm, but 12 mm with one low e pain will get under 2 W/m2K. Of
course once you start getting such a huge gap as that, the frame might
start to get bigger than the standard sizes of window frames.
Oh yes. What about window frames? Won't it make a huge difference what
the frame is made of? If you have an aluminium frame, the aluminium
will be conducting heat to its heart's content with something like 60
W/m2K, and condensation will be flowing like the Niagara falls. Since
April 2011, the window rating includes the frame, so it will make a
difference whether it's made of a thin sheet of aluminium, or wood
with a sandwich layer of insulation. The numbers have changed so the
whole window and frame assembly gets two stars if it's less than 4.65
W/m2K, three stars for 3.49 W/m2K and four stars for less than 2.33.
Until you get to the magical figure of 2.33, it seems they are
expecting the frame to conduct more than the glass. This contrasts
intriguingly with European manufacturers who consider that the frame
will conduct less than the glass until you get to triple panes.
It's great that they're taking account of the frames, even if they are
giving them more leeway than they give to the transparent bits that
are pretty much limited to glass. But they still haven't taken account
of how airtight the whole thing is. The window can have as many stars
as you like, but it won't help as much if cold air is blowing in. And
that's not even mentioning thermal bridges. I think we may have to
talk about those another time.
| Stars | Eco Glass(old) | Eco Glass Windows (new!) | Notes |
| **** | <2.33 | <2.33 | Double glazed, 6mm gap between pains, low e |
| *** | <2.7 | <3.49 | Double glazed |
| ** | <4 | <4.65</td> | Double glazed. Wait a minute, why are they allowing the frame to let more heat through than the glass?(1) |
| * | >4 | >4.65 | Credit where it's due, one star for not being a hole in the wall! |
Getting back to the glass, if you start using noble gases instead of
air, there are two advantages. First, the noble gase, usual Argon, Krypton
or Xenon, has single-atom molecules, while air is mostly made up of
double-atom nitrogen and oxygen, so the noble gases hold a lot less
heat. Because they hold less heat, they transfer less heat. Also they
are more viscous, so they don't start moving around until the gap gets
close to 20mm, and a thicker gap means a warmer house. The noble gases
are also inert, colourless and odourless, so they don't go off. Argon
is the cheapest and conducts one third less heat than air. Krypton is
more expensive but conducts half of Argon, so it is only used for high
performance, or extra-thin multi-paned windows. These gases will get
the glass down to 1.5 W/m2K, which should surely be worth 6 stars, and
we haven't even looked at triple glazing yet.
Let's give seven stars for glass under 1 W/m2K, even though we got an
extra star for an improvement of a little over 10% from 2.7 to 2.33.
If we were following the same progression, there would be so many
stars on the glass by now, you wouldn't be able to see through the
window! It's a good thing that 2.33 is the highest level! To get our
hypothetical seven stars, you need three layers of glass, two of them
should be low-e so they are reflecting back the reflected heat too,
and you need argon filling them.
Southwall Technologies are talking about U values under 0.5 W/m2K. They seem to be from the US, where the units are imperial--British thermal units per square foot degree farehneit--and different to metric by a factor
of six. It can be confusing as the units are usually missing when U
values are quoted.
The answer, by the way: Exhibit 1 is a set of radiator fins for cooling electrical devices. Exhibit 2 is an aluminium window frame.
