In uk.d-i-y The Natural Philosopher wrote:
Ian Stirling wrote:
In uk.d-i-y The Natural Philosopher wrote:
Jonathan Tong wrote:

The Natural Philosopher wrote:


This is U values not K. Watts per sq meter per degree K.

DG windows range from 2.2 to 4.2..depending on gap, filling and frame.
Hmmm, I hope not... I understood that current building regs requires
windows with U values of less than 2.0 to be installed. Our DG is
apparently approx U=1.2 using Pilkington Optitherm (soft-coated glass),
argon filled and a super-spacer-bar-thingy. The units are called
Pilkington Insulights.
snip
If you trap 6" of air behind a set of thick curtains, that's not far off
the same insulation as 6" of rockwool. Far better than any window itself
can achieve.

This is rubbish.

15cm of rockwool has a U value of .266.

Internal surfaces have a R value of .12, and curtains have two,

six. Plus about 3 mm of insulating material. I am talking about REAL
curtains..outer fabric, interlining, lining. Not a bit of chintz slung
up hopefully.

so that's 0.72..



Alas, it's not.
The thermal resistance is not inherent to the surface, it's the still
layer of air next to it.
Two surfaces close together do not have four times the insulating
capacity as simply multiplying the per-surface value for a large cavity
by 4.


so
that's .24 thermal resistance. Assuming the curtains have the same
resistance of 5mm of rockwool, that's another .12

Which takes us up to 0.84

, and that gets to .36
a 50mm cavity has a thermal resistance of .18, taking it to a total of
.54.

My cavity is 150mm, so that's another 0.54 taking it up to 1.38..

You can't simply multiply the thermal resistance of a 50mm cavity by
three to get the value for 150mm, otherwise the middle of a room would
be warmest.

Annoyingly I lost the nice graph I had from somewhere that gave thermal
resistance of a cavity as it scales with size.


Or a U value of 1.85, or around 2cm of rockwool.

so a U value of 0.72, way better than any DG window.

And that's without adding in the window itself.

add in the SG window at say 5 U value, and it's down to 0.63

Ok its not as good as 150mm of rockwool, but its BETTER than 50mm of
rockwool.

Thick curtains are in every way better than DG, except for condensation.
The windows WILL get ICY.