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.

http://tinyurl.com/2rkpv7

Putting all this in a steel/upvc frame might raise the U value to around
1.4-1.5?

I can only go by what the building regulations data says. Mine is 2000 data.

they give 2.2 for a double glazed argon filled 12mm gapped window in a
wood or UPVC frame..

I am sure the GLAZING is better, but the frame area represents a
significant cold bridge.

However the point of that post was to demonstrate that really it makes
sod all difference to the overall house heatloss.

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.

I didn't go into air changes either. Now its hard to translate the
regulations into actual airflow of ventilation but e.g. a fan of 15l/s
minimum is what is required for otherwise unventilated bathrooms etc.

If we take that as a minimum ventilation requiremennt, that is .015 cu
meters or .019 kg of air per second, with a specific heat of around 1000
joules per kg per deg ..which is 15 joules per second, per degree C or
75Watts for a 5C internal to external temperature difference.


In the case of our room, insulated down to a U value of less than .75
overall, 25 square meters of external surfaces.. the lossse are 93watts.

Adding in ventilation of 15l/s NEARLY DOUBLES THE HEATLOSS.

Or to put it another way, in a perfectly insulated room of that size and
shape that loses no heat except by ventilation, the ventilation alone
will increase the U value to 0.6 all by itself.

So whether you go for a calculated U value of 0.7, or the recommended
0.3 or less becomes completely irrelevant as long as you have the actual
ventilation that the regulations insist on.

The conclusion is that much beyond a U value of less than 1, unless you
also either break the ventilation requirements or arrange in addition
some heat exchanging on the ventilation, you are wasting your time largely.

I can foresee in the future that some form of twin coaxial tubes will be
used to allow hot air to rise out of rooms, heating up incoming air as
it does so...









J