Thermal conductivity of different types of windows

[Ian Stirling]

Only on the outside. Hopefully, on the inside, the gale doesn't get in.

[Ian Stirling]

On a sort-of-related topic, I'm insulating my house at the moment. As part of this, while I've got the plasterboard off, it's fairly easy to add a sheet of kingspan inside the wall, that slides out on concealed runners over the windowsill, forming a tightly fitting shutter.

In use, it looks like a 3cm or so deep alcove, with trim round it.

[The Natural Philosopher]

true, but that doubles the U value right away.

[The Natural Philosopher]

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..

so

Which takes us up to 0.84

, and that gets to .36

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

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.

[The Luggage]

The Natural Philosopher wrote: [snip]

The analysis is ONLY valid if your curtains trap the cold air efficiently. Unfortunately, the current fashion is for curtains to hang on a rail 3-4 inches outside the window recess, so the air trapping is very poor. It's even worse if there is a radiator under the window and long curtains that hang over it directing the heat towards the window, not the room.

Great original post, BTW.

TL

[Ian Stirling]

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.

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.

[The Natural Philosopher]

That depends a lot on how the curtains themselves are designed.

Ours hang against the window boards and there is almost no real space to allow a good air circulation, despite the cat that they ARE indeed 3" from he wall..but the 'ruffle' is deeper than that..and some curtains go down practically to floor level.

Oh and I don't have ANY radiators except in the bathrooms Its UFH or hot air convectors.

The difference oon a cold night betweenm a window with a drawin curtain and one without is astonishing.

It's even worse if there is a radiator under the window and

That is of course a complete waste of time ;-)

[Ian Stirling]

No, it doesn't - the outside R value is taken to be much smaller than the inside one - IIRC around 0.02, or something, compared to compared to

0.18.

The ~5W/m/k figure is pretty much the accepted one for a single pane of glass. Of course, double glazing has 3 'windless' surfaces, so that really helps it.

[Tony Bryer]

For a standard double glazed window the U-values in SAP2005 are:

6mm gap: 3.1; 12mm gap: 2.8; 16mm or more 2.7

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.

[Ian Stirling]

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.

[The Natural Philosopher]

I don't think so.

I have been watching the digital stat this winter..a sunny clear day as opposed to a dull one nets only about 1-2C rise in the room. And thats from the solar systems best 'radiator'

I guess the reason why we put on anoraks is to stop our bodies losing heat by radiation eh?

multi layer things like coats and jackets that trap air in spaces are known to be better than one thick layer..curtains when lined are simply an example of the same.

When hanging naturally our curtains are about 1cm thick. I can't begin to consider that is worse than a 12mm gap between two pieces of glass.

Incidentally, my figure of 14C average annual temperature is optimistic..here's the reality.

it only makes insulation even more important.

[The Natural Philosopher]

and triple layer curtains and a SG window have 7..

[Mark T.B. Carroll]

I was wondering, is there any real value to these powdered paint additives that are sold as imbuing the paint with insulating properties?

FWIW with windows I'm wondering how regular heavy curtains compare to these honeycomb shades and blinds with the air cells in them that sit actually in the frame.

-- Mark

[Tony Bryer]

Yes, but curtains, mine away, are not trapping air behind them to any extent. You've got gaps at the cill and at rail level a continuous gap (with most curtain rails anyway).

[Mark T.B. Carroll]

Tony Bryer writes: (snip)

Mmmm, true. Sometimes when I see insulating/thermal curtain and blind installation described, it turns out that some come with a magnetic strip you install to have the edge snugly meet the wall or frame. How available this is, I don't know: I don't think I've seen it in real life.

-- Mark

[Ian Stirling]

Certainly - if you put on 10cm thick of it.

[CWatters]

I can confirm that. We are putting in some metal framed windows that only take 4-6-4mm panes (eg 6mm spacer). To meet 2.0 we had to use coated glass.

I believe Krypton is slightly better than Argon. Not too much more expensive (if you can find someone who already has a cylinder of the stuff).

[Andy Wade]

The t/k term! For 100 mm of Celotex you have t = 0.1 m and k = 0.019 W/mK, if memory serves. So t/k = 5.26 m^2K/W and adding the boundary layer resistances make a total resistance of 5.44 m^2K/W or a U-value of about 0.18 W/m^2K.

The point is that with the single-glazed window the glass contributes about 2% of the total thermal resistance whereas the Celotex is providing nearly 97% in your example. Taking up Nat Phil's point, with the Celotex wall it doesn't make much difference if the outer boundary layer is blown away in a gale; with the s-g window it does.

It helps to think in terms of R-values rather than U-values. To use an electrical analogy, we're talking about resistances in series here, so to think in terms of their conductances would be a bit silly.

[Grunff]

Ok, I got it now - thanks very much for the explanation.

[Alex Selby]

Great post. It's really good to cut through the hype with some calculations, but I don't totally agree with all of your figures.

In the above case, you've assumed an average temperature difference of

5C, but 10C is nearer the mark. (This improves the benefit of insulation but doesn't alter the calculation of the benefit of turning your heating down by a degree.)

You've assumed a cost of DG of £600/m^2, but I've just been quoted for a job at £270/m^2.

You've assumed that you are replacing a fairly good existing SG unit. This may be the case, but if you want to understand why DG is insisted on then what about the case of replacing a drafty SG unit at the end of its life, or the case of a new building. In these cases you need to subtract from £270 the cost per m^2 of SG to make a proper comparison.

On the other hand, you've assumed 10p per unit. This should be a lot less if you used gas, which reduces the ROI.

So the minimum ROI is

10*.3*2.5/1000*365*24/270 = 2.4% (assumes you're replacing a perfectly good SG unit and heating with gas at 3p per unit).

To work out the maximum ROI we need to know the cost of SG, which I don't. Taking it to be £135/m^2 we get

10*.1*2.5/1000*365*24/135 = 16% (assumes you're building from new or the existing SG unit needs replacing; assumes heating with electricity).

There are also benefits in terms of quality of heat. If you just replace heat lost from the windows with central heating then you get temperature differentials - colder nearer the floor than at head level etc - which is not as snug as uniform heat (in my opinion).

And there are benefits of DG in terms of extra sound insulation too.

Alex