U values and the economics of double/triple glazing

[StewartR]

Anybody out there understand U values? If so, I'd be grateful if you could sense check my understanding and my calculations.

The background is that we're building an extension to our house this year. It will be a "highly glazed" extension and we're going to run into issues of energy efficiency. We'll need to go beyond the minimum standards required by building regs, and one thing we want to consider is whether triple glazing might be cost effective.

The options we're looking at are double glazing with a U value of about 1.3, and triple glazing with a U value of about 0.8. We'll have about 30 square metres of glazing.

The way I understand it, the U value is the heat loss measured in watts per square metre per degree of temperature difference.

So the difference between the two types of window is 0.5 watts per sq m per degree. With 30 sq m, summed over all the glazing that's a total of 15 watts per degree.

Now I reckon we probably run the heating for about 7 months of the year, and over that time period I reckon the average temperature difference between indoors and outdoors is about 13 degrees. (I can refine both these estimates; for now it's the principle that's important.)

So with a 13 degree temperature difference the rate of heat loss is 15 x 13 = 195 watts. Let's say 200 watts to make the sums easier. Seven months is about 5,000 hours, so the total heat loss over that period is about 5,000 x 200 = 1,000,000 watt hours = 1,000 kilowatt hours. Our central heating runs on gas which costs about 3p per kilowatt hour. So the total cost savings by having triple glazing work out to about 1,000 x 3p = £30 per year.

Does that make sense? It doesn't seem like very much to me. The calculations look OK to me, but I can't help the vague feeling that I might be overlooking something important...

Any help or advice gratefully received. Thanks.

 

[Alastair]

There's a wee gizmo at the bottom of this page www.koemmerling.co.uk/ecomaXL/index.php?site=KOE_GB_GB_energy_savings_calculator .. select the closest option and correct the u-values and the energy price, then it gives you a savings curve.

Putting your numbers in gives £118 savings in the first year.

 

[shreds]

Yeah but have you checked how much triple glazing costs versus common or garden double glazing. On average folk are reckoned to move house every 7-10 years, so multiply your £118 by max of 10 and see if the difference tallies up? (Assuming energy costs are stable, which they never are).

 

[andrewk]

I suspect that your U values are more than a bit optimistic. Whilst it is many years since I practiced in this field, I remember typical U values for double glazed windows being around 2.5-3 W/sqM.degC. A quick wander around the Pilkingtons website suggests that this is still typical for current windows. Triple glazed windows are of the order of 1.5 W/sqM.degC. I think you need to check your source data. If the change in U value between double and triple glazing is say 1 W/sqM.degC, the change heat loss is twice the figure you used - around 390W.

Although a heating season for a poorly insulated property is typically 32 weeks (i.e. about 5400 hours), it will be much shorter for a well insulated house, as incidental heat gains (solar, people, cooking, lighting, TV, electronics) will provide the whole of the heating requirement for a much longer period at the ends of the heating season. I'd guess that a heating season for a well insulated property could be perhaps 4000 hours or maybe even shorter.

I have no way of knowing what the average efficiency of your boiler will be over a heating season, but if we assume that it is 80%, then your gas purchase price needs inflating to account for boiler and control efficiency. I don't know where you buy gas, but 3p/Kwh sounds low. Are you sure?

Your method of calculation is quite sound - but you have indeed overlooked something important. Although double and triple glazing have several advantages, they have never been economically justifiable. Even when I was professionally involved in the field (last in the early 1990s), payback periods for conversion from single to double glazing were typically 25 years or more. Conversion from double to triple glazing will probably not recover its initial cost in the lifetime of the glazing.

As an aside, you need to take savings calculations produced by a manufacturer who is trying to sell you his product with an enormous pinch of salt

Hope this helps.
Andrew

 

[bass_junkie83]

The U values you quoted are about right for modern glazing, but I suspect that they are centre pane U values.

Most heat loss is through the framing elements, so if you have smaller windows and therefore more frames your system U value would be higher than if you had larger windows with less framing.

So it's not quite black an white, the efficiency will be the result of the full design, of which the glass type is just one factor.

Saying that, if you can afford tripple I would. Every little helps and it is the way things are going to.

 

[StewartR]

There's a wee gizmo at the bottom of this page www.koemmerling.co.uk/ecomaXL/index.php?site=KOE_GB_GB_energy_savings_calculator .. select the closest option and correct the u-values and the energy price, then it gives you a savings curve.

Putting your numbers in gives £118 savings in the first year.

Thanks. Not sure I trust it, though.

I managed to reproduce the £118 you got by plugging in my figures. That's the difference between U=1.3 and U=0.8. But it also says that the difference between U=3 (probably what we have now) and U=1.3 is £209 per year, and the difference between U=3 and U=0.8 is £179 per year. That implies the difference between U=1.3 and U=0.8 is only £30 per year, not £118. It's not even remotely consistent.

And in fact it claims that going from U=3 to U=3 saves £78 per year. I'll have some of that, please!

 

[sirch]

It's not just about cost saving of heat loss through those windows though, how are you going to heat the extension? If it is off existing central heating then is the boiler up to it, it could be a balance between the cost of a new boiler and triple glazing. Also consider resale value and future energy efficiency rating and most importantly comfort. No point in building it if it is too hard to heat that you don't use the space - we have what we laughingly describe as a conservatory and there is about 1 hour a year when it is comfortable to sit in there, even if we heat it in cold weather then it needs so much heat input that it causes drafts and is still not very comfortable.

 

[Alastair]

Thanks. Not sure I trust it, though.

I managed to reproduce the £118 you got by plugging in my figures. That's the difference between U=1.3 and U=0.8. But it also says that the difference between U=3 (probably what we have now) and U=1.3 is £209 per year, and the difference between U=3 and U=0.8 is £179 per year. That implies the difference between U=1.3 and U=0.8 is only £30 per year, not £118. It's not even remotely consistent.

And in fact it claims that going from U=3 to U=3 saves £78 per year. I'll have some of that, please!

It's the app - you need to start again to clear all the previously held data, a fresh start calculating the u=3 to u=0.8 gives £219. If you've done one calculation and then start editing individual values the calculation borks. It needs a "clear data" button.

The calculations are about the values I've seen with work for a client in the energy saving business - maybe slightly generous but not by much. There's an official GDA calculation and I think BREAM has one as well, but I don't have direct access to those. As Andrew has pointed out the strict financial case for a lot of energy saving measures doesn't add up. At retail prices the quickest payback is loft insulation if you have little existing already installed. Everything else is debatable from the financials but don't underestimate the comfort value of a home with fewer draughts and that heats up quicker.

A good Green Deal Assessment is useful but I don't know if one applies to an extension before it's built.

Avoid Green Deal Loans, they're very poor value.

 

[StewartR]

I suspect that your U values are more than a bit optimistic. Whilst it is many years since I practiced in this field, I remember typical U values for double glazed windows being around 2.5-3 W/sqM.degC.

I think you might be surprised. Energy efficiency requirements have changed massively in recent years. The minimum standard for replacement windows (except for conservatories etc.) is a "C" rating on the energy efficiency label, which corresponds to U=1.6. An "A" rated double glazed window can achieve U=1.1, and triple glazed windows can achieve U=0.8.

Your method of calculation is quite sound

I'd guess that a heating season for a well insulated property could be perhaps 4000 hours or maybe even shorter. ... your gas purchase price needs inflating to account for boiler and control efficiency. I don't know where you buy gas, but 3p/Kwh sounds low. Are you sure?

Thanks. This can help me refine the calculations but it seems clear that it's not going to transform the conclusion. My latest quote for renewing my energy contract is £0.0302/Kwh.

Although double and triple glazing have several advantages, they have never been economically justifiable. Even when I was professionally involved in the field (last in the early 1990s), payback periods for conversion from single to double glazing were typically 25 years or more. Conversion from double to triple glazing will probably not recover its initial cost in the lifetime of the glazing.

I strongly suspect you're right.

Thanks for your help.

 

[StewartR]

The U values you quoted are about right for modern glazing, but I suspect that they are centre pane U values.

Most heat loss is through the framing elements, so if you have smaller windows and therefore more frames your system U value would be higher than if you had larger windows with less framing.

So it's not quite black an white, the efficiency will be the result of the full design, of which the glass type is just one factor.

Good point. Thanks. I think I'm OK though. I've been trying to ensure I compare like with like. I have one quote for a whole bunch of windows and doors where the U-values range between 1.29 and 1.37, with the higher U-values for the smaller windows. Since they're all using the same materials, they must logically be values for the whole units. So I just used 1.3 in my calculation as a simplification. The same manufacturer quotes U-values of 0.82 to 0.89 for triple glazed units, again depending on the size, so i simplified that to 0.8. I could have used 1.35 and 0.85, which would probably be a more accurate reflection of the average performance I'd get over a range of units, but that wouldn't make any difference.

 

[StewartR]

It's not just about cost saving of heat loss through those windows though, how are you going to heat the extension? If it is off existing central heating then is the boiler up to it, it could be a balance between the cost of a new boiler and triple glazing. Also consider resale value and future energy efficiency rating and most importantly comfort. No point in building it if it is too hard to heat that you don't use the space - we have what we laughingly describe as a conservatory and there is about 1 hour a year when it is comfortable to sit in there, even if we heat it in cold weather then it needs so much heat input that it causes drafts and is still not very comfortable.

Good points, thanks. We're effectively rebuilding the whole ground floor of the house so we'll be having a new efficient boiler to replace the existing 20-year-old inefficient ones, new efficient radiators to replace the existing 45-year-old inefficient ones, and underfloor heating in the big open plan area which will include the extension. So I'm expecting our total heating costs to come down if anything.

 

[StewartR]

On average folk are reckoned to move house every 7-10 years, so multiply your £118 by max of 10 and see if the difference tallies up? (Assuming energy costs are stable, which they never are).

Well, we're planning on staying here for ever, so we're willing to accept a longer payback period. But even at £118 per year for 20 years, the extra cost isn't justifiable. And I'm still coming up with figures of <£50 per year, which would make it even less justifiable.

 

[StewartR]

It's the app - you need to start again to clear all the previously held data, a fresh start calculating the u=3 to u=0.8 gives £219. If you've done one calculation and then start editing individual values the calculation borks. It needs a "clear data" button.

Oh, OK. However I tried again and I got U=3 to U=1.3 saving £190/yr, and U=3 to U=0.9 saving £219/yr, so the difference there is £29/yr. Still nowhere near £118/yr. And it still says that switching from U=3 to U=3 saves money!

Bottom line, I still don't trust the calculations. (Especially as the app is produced by a company who want to sell me stuff.) Sorry. But I appreciate the help.

As Andrew has pointed out the strict financial case for a lot of energy saving measures doesn't add up. At retail prices the quickest payback is loft insulation if you have little existing already installed. Everything else is debatable from the financials but don't underestimate the comfort value of a home with fewer draughts and that heats up quicker.

I think that's the bottom line, isn't it. There isn't a purely financial justification, so we'd have to decide whether we think it's worth spending the money to have a more comfortable home.

Thanks.

 

[bass_junkie83]

I do a fair bit of work with this in my day job, but I am much more focussed on commercial buildings with a lot more glazing than a domestic property.
I have just spoken with a colleague who is more experienced with domestic properties, and basically ignore what I said before about going for triple glazing.

In his opinion, you are unlikely to make back your investment in triple glazing over double due to the low glass to frame ratio. The frames will leak like a sieve thermally compared to the glass and if you wanted to spend a bit more you would be better off putting it towards a better framing system.

 

[andrewk]

The frames will leak like a sieve thermally compared to the glass and if you wanted to spend a bit more you would be better off putting it towards a better framing system.

It's only the very best framing systems that get near the U values used in the original calculations. I suspect that prices for those will be horrendous.

 

[bass_junkie83]

Indeed, it is one of those things governed by the laws of diminishing returns.

 

[srichards]

Modern condensing boilers only have a design life of 10 to 12 years. If the 20 year old one is a decent make then I'd just keep it unless it breaks catastrophically and will cost more than a few hundred to repair. The payback on the new one might be reached just when new one is ready to be scrapped. The older ones are much simpler so will remain repairable with a bit of common sense a lot longer than a new one.

 

[andrewk]

Indeed, yes - old cast iron heat exchanger boilers have few parts and will last pretty much forever. For example, I still have a Glow Worm Space Saver 40B, which has been outstandingly reliable and will probably outlast me. The big downside is that, despite the full load efficiency being around 78%, the average seasonal efficiency is probably little more than 60% if the boiler is lightly loaded most of the time (It will be now, as I have put in double glazing, roof and cavity wall insulation since I first installed it (circa 1986!!).

If the StewardR's existing boiler is a golden oldie like mine, he needs to redo all his calcs using a seasonal efficiency of maybe 60%.

 

[onomatopoeia]

Indeed, yes - old cast iron heat exchanger boilers have few parts and will last pretty much forever. For example, I still have a Glow Worm Space Saver 40B, which has been outstandingly reliable and will probably outlast me. The big downside is that, despite the full load efficiency being around 78%, the average seasonal efficiency is probably little more than 60% if the boiler is lightly loaded most of the time (It will be now, as I have put in double glazing, roof and cavity wall insulation since I first installed it (circa 1986!!).

My parents have a Glow Worm of a similar age, I helped my dad install it when I was in the sixth form at school. I did the electrics as I'm not colour blind and he soldered up the gas. It hasn't exploded or gassed anyone yet, although it's just about on its last legs now - the most recent casing seal has lasted less than three years and it's already leaking CO again, thermocouples fail more and more often (whether that's rubbish thermocouples or a symptom of some other problem is unclear, the RGI they had round to look at it seemed to have no idea), so they are looking at getting a condensing boiler installed now as they want to sell the house, and have a boiler that doesn't randomly stop working in the depths of winter.

 

[kendo1]

I still fail to understand why you would run your heating for 7 months of the year.
I run mine between November and end of Feb, but it's not on day after day. In fact, it's not been on today as it's been so warm for the time of year.

 

[onomatopoeia]

I still fail to understand why you would run your heating for 7 months of the year.
I run mine between November and end of Feb, but it's not on day after day. In fact, it's not been on today as it's been so warm for the time of year.

One of my colleagues barely runs her heating at all, but she lives in a modern purpose built flat so has only one external wall and no heat loss above or below.

I live in a bungalow, and despite having insulation to the level required by modern building regs in a 60+ year old property, plus double glazing etc, if I don't run the heating then within a day at this time of year the internal temperature will be about 13C, within two it will be 11C. Some people say they are comfortable sittng in that, but I need it to be about 19C which is where I have the thermostat when I'm at home. So, with potentially a > 20C temperature differential to outside the house, the heating gets run every morning and evening as the thermostat decides that it's colder than I like.

Maybe today was warm for the time of year as you say, my car said 1 Centigrade outside temperature. Is that unseasonably warm? Yesterday it was -4C, so it was warmer than the day before.

 

[kendo1]

I'm in Scotland...