In uk.d-i-y Tony Bryer wrote:
On 17 Jan 2007 15:54:34 GMT Ian Stirling wrote :
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.

For a standard double glazed window the U-values in SAP2005 a
6mm gap: 3.1; 12mm gap: 2.8; 16mm or more 2.7

Taking this apart - neglecting the glass.

6mm 12mm 16mm
R value .32 .357 .370

Delta-R
.037 .013

Total cavity inside thermal resistance. (-.18)


.14 .177 .19


Interestingly, the cavity is a worse insulator in many cases than the
interior face.


My understanding has always been that beyond this you get next to no
improvement since the extra space allows for convection currents up
the warmer face and down the colder one.

My guess is that the biggest effect of curtains is to kill the heat
loss from radiation.