Thermal conductivity of different types of windows

Hi all,
Anyone know where I can get k values for different types of windows? Specifically, I'd like to compare:
- single pane leaded windows (real leaded, with the lead all the way through, not stuck on) in a steel frame
and
- 18mm double glased sealed units in an oak frame
TIA
--
Grunff

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The second will win.
:)
As to the leading - it's largely irrelevant. Glass has a K of 1, so a 6mm pane has a U value of 1*.006= .006. The air sitting next to the inside of this utterly swamps this in insulation.
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Ian Stirling wrote:

You don't say...

I dunno - lead has a K of ~35. Steel has a K of ~46. If we take one opening pane, which is 49cm wide by 90 cm high:
Total area: 4410 cm^2 Glass: 3563 cm^2 Steel: 540 cm^2 Lead: 307 cm^2
That means the glass only makes up 81% of the window area, with the rest of the area being made up of materials that are over an order of magnitude more conductive.
--
Grunff

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You can't quite get there from here - as a very significant factor at this level of conductance is the air next to the window/frame. Also - the much wider section of the metal, and the fact that it's not (usually) solid means that it's less conductive.
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Ian Stirling wrote:

Yes, for single glazing it's the air boundary layers that dominate.
1/U = Rsi + Rso + sigma(t/k)
Where Rsi is the resistance of the inside air layer Rso is ditto for outside sigma(t/k) is the sum of the thickness/conductivity ratios of the intervening layers
For horizontal heat flow Rsi = 0.12 m^2K/W, Rso = 0.06 m^2K/W. The sum of these is 0.18 m^2K/W.
Taking a 4mm (t = 0.004 m) pane of glass with k = 1 W/mK (given earlier in the thread) t/k is 0.004 m^2K/W so contributes negligibly to the sum. The U value is 1/0.184, i.e. about 5.4 W/m^2K
Replacing the glass with a sheet of 16 swg copper (t = 0.0016 m, k ~ 200 W/mK) only increases U to about 5.5 W/m^2K.
--
Andy

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Andy Wade wrote:

Thanks for the input Andy. While I understand your logic, something still feels instinctively wrong - can't the same calculation be done to show that the difference between copper walls and 100mm celotex walls is negligible? What am I missing?
--
Grunff

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Grunff wrote:

The fact that celotex traps a far far larger 'boundary' layer - about 100mm in fact ;-)
And what GRUNFF is missing, is that in a 30mph gale, the boundary layer gets stripped away on copper, and glass, both.
It also gets pretty bad with rockwool in the loft.
Its not often mentioned, but I did get significantly better insulation when I boarded over mine..noticeable in strong winds..
Likewise my suspended (and insulated) concrete floor is significantly colder when the wind blows..the underfloor space is a good insulator when its calm.
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Only on the outside. Hopefully, on the inside, the gale doesn't get in.
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Ian Stirling wrote:

true, but that doubles the U value right away.
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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.
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Ian Stirling wrote:

and triple layer curtains and a SG window have 7..
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Grunff wrote:

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.
--
Andy

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Andy Wade wrote:

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

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Grunff wrote:

I have this somewhere..I have SG leaded..but I haven;t time to look it up.
Its in the building regulations tho..handy guide..
IIRC the SG U value is around 5-7 and the DG is around 1-3 depending
A good insulated wall is less than 0.3...
If you are considering installing new leaded lights, like wot I did you WILL need to get the whole building energy calculated to convince the BCO that it is within spec, and this WILL mean extra insulation elsewhere.
If its an upgrade to an existing building, you may also need to convince him/her that enough extra insulation is applied elsewhere at the same time to bring the modifications to at least no worse than what was there before, and preferably a lot better.
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The Natural Philosopher wrote:

Ok, thanks.

I'm not, I'm just curious about the relative heat loss in different rooms of our house, and the choices the architect made. It seems to me that the leaded windows lose several hundred Watts more than the double glased ones, and I wanted to see if this is really the case.
--
Grunff

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Grunff wrote:

Oh yes, definitely.
But sick a lined and interlined curtain over them, and they are actually much better than a triple glazed unit without curtains.
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Ok, found the book
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.
SG windows are in the range 4.7-5.8 depending on frame.
Note a good wood frame SG is almost as good as a crap DG metal frame...
solid doors are about 3.
9" solid brick wall about 3.5
4.5" single brick wall 7.
10mm expanded polystyrene or rock wool or 6mm Celotex 3.5
..yes folks, thats how little will HALVE the heat loss through a 9" solid brick wall. and if its a single 4.5" brick wall..just 5mm of polystyrene will halve it, too..
15mm plasterboard has a U value of about 10.
15mm of wood paneling is about a U value of 10 too.
Its very instructive to see how a brick wall without cavity, a window and a door are all very similar..this is typical Victorian style construction, and how little 15mm of wood flooring will help with an underfloor vented cavity, or 15mm of plasterboard ceiling will help with a vented roof cavity..Brr!.
Its also very instructive to see how little insulation is needed to make a substantial difference to this sort of property.
Now building regs are trying to get U values down below 0.3 overall..about TEN TIMES better than a Victorian 'as built' standard.
You can instantly see that uninsulated plasterboard ceilings to a vented roof are by far and away the worst losers of heat. Which vindicates the emphasis on loft insulation.
Its also easy to see why Britain, with loads of suspended wooden floors has a penchant for fitted carpets with thick underlay..
Its very hard to see why double glazing is so insisted on.
Its very easy to see just how bad solid brick walls are as well..and remember a cavity wall with exterior air bricks is not far off a 4.5" single brick wall, in a moderate breeze..you don't need a lot of insulation to radically improve these sorts of wall..dry lining with just 1/4" of Celotex will halve the heat loss through a solid brick wall. Add in 15mm of plasterboard and U value is down to 1.5 from 3.5. For a total loss in room dimensions of less than an inch all round the exterior walls. No brainer innit?
Now lets consider a room - say its 12x8 ft. and 8ft high, with two external walls. So external area is 96+646 sq ft..and lets consider it has a suspended floor and be generous with carpeting and put that as U value of 3.5 same as the walls.. so another 96 sq ft takes us to 270 sq ft.
Thats a total area of 25 sq meters. Lets assume an average annual internal temp of 19C and 14C average annual external..so 5C drop..so the watts required is 5x3.5x25= 437.5 watts. And a peak requirement at -6C external of 5 times that..2187 watts.
Consider how we might improve all this.
Let's say we have two windows totalling 2.5 sq meters. At a U value of 5..so thats just 8% of the total heatloss from the room. A GOOD DG unit should more than halve that..netting us a 4% gain, or ariound 17.5 watts average, and annualized around 155KWh..say at 10p per unit..15 a year gain.For probably about 1500 outlay. So a 1% ROI.
Now let's dry line the room with 2" celotex on the external walls..thats 50mm. Thats a U value of 0.4, so with 15mm plasterboard at 10, and our wall at 3.5, neglecting cold bridging by studs we can achieve an overall wall U value of 0.35. 186sq ft (17.2799654 sq meters) less 2.5 sq meters of windows nets us 15.2..and the saving in heat will be an average of 240 W average. Or 2097KWh over the whole year.
Thats for 5 sheets of celotex and 5 sheets of plasterboard..and some studwork..say 400 quid in all? and a hundred quids worth of skim and paint..well anyway its WELL under 1000, and at 10p a KWh, it will save 200 per annum. An ROI of around at LEAST 20%.
Similar gains may be expected from doing the same to the floor.
In short even if my figures for energy costs are high, based on electricity, the gains to be had from drylining are about 20 times as cost effective as double glazing.
If we add in an insulated floor as well..then our gains are about 234 watts out of the 437..such that all that is left is the window really..about 63W average...and our walls are now losing just 40 watts average.
In short we have come from 437.5 watts down to 103W..75% of the heating bill has gone. Adding SG might net us a further 40W or so, but so would a decent set of nice lined curtains.
My points are these.
1/. Loft wall and floor insulation represents ROI of up to 30% or more..
2/. Loft wall and floor insulation on an uninsulated property represents up to 70% energy reduction. More if you do it to full building control specs. With a typical figure of less than 10% of wall area and only a factor of two improvement, double glazing represents at best a 5% energy saving on an otherwise uninsulated or just loft insulated house, and probably less than 1% ROI. It is in fact a total waste of money and will never pay for itself..unless you had to replace the windows anyway.
3/. Fitting a new boiler is easy enough..going from a 50% efficient boiler to an 80% efficient one is a net energy improvement of 37.5% in bills..the ROI will be easy to calculate from your annualised fuel bills.
4/. Let's say our 437W room has two 100W lamps, used an average of 4 hours a day..291.2 KWh per year..and we replace then with two 17W CFLs..costing a fiver each. So we come down to just 49Kwh per annum. AND we have to make good the heat no longer added to the room..well anyway the net saving IS about 24 on electricity...not bad returns for a tenner..but mitigated by the fact that we have to add the heat back with the boiler..in terms of saving the planet we don't really save that much after all..as our boiler is not a great deal better than the electricity generating plant. Still, it's something.
5/. Here's another interesting calculation. Let's say our house is a 4 bed detached one comprising 8 rooms on two storeys as calculated. so it's total heating is 8x437 watts. Annualised that is 30MWh. About 3000 to heat then with electricity (and as anyone who has used storage heaters, in a house like that, thats not far off true). Now you get about 10KWh per liter of heating oil (and similar for a cu meter of gas actually) so at say a 50% boiler efficiency, that's around 5Kwh per liter..which equates to 6000 liters of oil to heat that house. Again those of us who have heated houses like that know thats not unrealistic.
That's 1320 gallons..enough to take a nice tidy 45mpg diesel car 60,000 miles...let's say you insulate your house and knock that down by 70%..you can afford to run a car for 42,000 miles a year and still be using less oil.
Makes you wonder sometimes why car fuel is 90p a liter and heating oil is 30p a liter.
That actually puts a new 80% efficient boiler into perspective. Say it costs a grand. But puts out 8.5Kw/liter. You save 1500 liters a year. or around 450. On an uninsulated house.
On your 1800 liters a year insulated house, you will save just 450 lites, or 150. Not that great a saving..15% ROI.
6/. Wearing a 50 pullover that you replace every year, and knocking your stat down by one degree, to 18C..saves you 20% of your annual fuel bill. If its at 1800 a year (30p/l and 6000 liters) and you are a family of 4, that's 360 a year off your fuel bill for a cost of 200 of woollies. :-)
Of course, once you insulate the house and are running at a mere 540 a year heating bill, the savings of 108 are not worth the cost of buying (and washing) the pullovers..;-)
7/. One annual trip of 2000 miles by plane (at about 70mpg per passenger)is peanuts compared with the 12,000 miles you do to commute to your job at 45mpgh, or less in congestion..
8/. Lets say you do 60 miles a day, 200 days a year ..a nice 12,000 mile commute. And you elect to stay at home and work 3 days a week from home. That takes you to 3000 miles a year commute. The direct savings on fuel at 45mpg are 200 liters. About 180 a year..but with motoring costs in total running at around .20 a mile your real savings are nearer 1800..and since you pay out of taxed income, that's about 3600 off your gross salary..and 4000 of what you cost your employer..before the cost of office space., heating and lighting, and kit is taken into account. Probably another 1800 or so. So he could afford to pay you another 2200 a year to work from home, and you would be directly 1800 better off..so the equivalent to a 4k pay rise to you, and a gain of about 6 hours a week....240 hours a year on a 200 day working year..or about 6 weeks extra holiday in gain of leisure hours, to you.
Why ARE we commuting then? No real answer.
9/. What does a hot bath cost? well mine is 1.3 long x .5 wide x .3deep 195 liters. But I take up a lot of that so lets say 100l for a really good soak. I like my bath to be as hot as I can stand..lets say 45C and we will assume the average incoming water temp is around 14C ..so 39 c rise and 100liters is 3900 calories or 16.4 Mjoules. That's getting on for a liter of fuel with a 50% efficient boiler. Gosh. Almost 30p.
Could cost as much as 100 quid a year to have a real soak every day.
10/. Do showers save money and the planet? Well that depends on how good they are. we know that a mingy electric shower soaks up 10KW..so on a 6 minute shower thats 1KWh..3.6MJ. Most decent showers will do at least twice that..a typical combi today might do 30KW..so a 6 minute shower would be 10.8MJ. In short unless you simply use showers for a quick brush up and are in and out quickly, they don't save you any money or water really at all over a medium bath.
11/. Does an electric kettle half full save the planet? Let's say your kettle is a liter. 2 pints or thereabouts. And the water in it is at room temp..say 20C because you left it there from the last cup of coffee. And you make 10 cups of coffee or tea a day. That's 800 kilocalories of heat a day. 3.36MJ. At a 50% fuel to electric conversion ratio that's almost a 1/6th of a liter. 5p!! almost 15 a year on coffee boiling!!! so lets say we save half of that directly..30 liters of fuel a year..In fact we don't, because a lot of the time we are heating our houses and the kettle is part of that..the net gain is probably less. say 15 liters of fuel a year. about a fiver. Or to put it another way thats about 3.3 gallons of fuel a year, or 150 miles of road fuel usage.
Taking two days off work saves nearly that. or going to the supermarket at a 5 mile round trip one time less a week saves more.
Why did I taker the time to write all this?
Well..in cam.misc someone complained their gas bill was too high, and in UK.D-i-y, someone wanted to know how much better double glazing was than single glazing..and I really thought.."we get bombarded with green crap, told to buy CFL's take showers not baths, half fill kettles, buy new boilers, fit double glazing and not fly"
And yet the reality is that the massive dominant and overriding two things we do that chew up oil and cost us a bloody fortune, are heat uninsulated houses, and drive to work every day. And the supermarket every other day and the kids to school half the year..
The rest is completely irrelevant as long as we don't insulate the walls ceilings and floors, and continue to use the car on a daily basis to do an average of around 50 miles a day.
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The Natural Philosopher wrote:

<snip great post>
Thanks v much for the detailed info, and the work-through. What book is the data from? Sounds like a very handy book to have on the shelf.
--
Grunff

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Grunff wrote:

"Building Regulations Explained" 2000 revision South Edition.
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<big snip>
Kudos for that !
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