A building physicist has come back on a project saying that the payback time for 3g over dg is so long as to make it not worthwhile, over 20 yrs does not stack up.

I asked why 20 yrs, 60 would be more realistic. then they do add up!

Same for walls again calculation for 20 tears, what would happen to the walls in 100? demolition possibly but nothing else.


Is justifying U-values, heat losses, etc even vaguely sensible using payback, no mention of upgrading/replacement costs which blow all calcs out of the warer

Payback is partly in the eye of the beholder, but also is relative.

It is possible that the money would be better spent on something else - so it makes no sense to spend say an extra £5,000 on triple glazing which may save a small amount a year, if that money could be better spent on PV or more insulation or other technology to achieve a greater saving - that may be what he is getting at

-Steve

Pay back over 60 years is highly theoretical. A few triple glazed window manufacturers may make windows that last that long, a few owners may look after them properly to give them the chance to last that long and a few owners will like those original windows enough for 60 years to not want to change them, not to mention the obvious reason to change windows in the next 60 years as its a no brainer to follow the latest tech developments.
60 years isn't realistic for me.
20 is.

When I've come across figures for payback on 2G versus 3G, it usually assumes that the temperature and setting of thermostats) inside a house will be the same for either construction. My anecdotal assertion is that the sensation of "coolth" next to a 2G window will be greater, and so the tendency will be to push up the room stats. Likewise due to the perceived drafts due to moving air currents at the inside face of 2G. What would those user generated increased heat inputs do to the 2G versus 3G arguement? I would also suggest that with the ever greater demand for large expanses of glazing, people will want to feel more 'temperature' comfort, standing close to the glass, taking in the views.

Posted By: adwindrumA few triple glazed window manufacturers may make windows that last that long, a few owners may look after them properly to give them the chance to last that long and a few owners will like those original windows enough for 60 years to not want to change them, not to mention the obvious reason to change windows in the next 60 years as its a no brainer to follow the latest tech developments

Shame that we can't make windows that will last 60 years. Traditional windows were (still are) in place for longer than that. Greater shame that we are so fashion fickle that we change windows for apperance sake. The myth of progress!!

If we are saying windows get replaced every 20 years, for whatever reasons, do we not need to look at how windows are installed to make replacement easier? Mine are well air sealed into the opening but as a consequence I don't see how they could be replaced (in an air tight way) without distroying and reconstructing the reveals. Quite a motivation to keep the window frames I fitted. Hope they last.

For my house, which will have quite a small amount of window area for its size (though still approaching double the Scottish building regs minimum) I plan to have 3G for the four main rooms (study, living room, kitchen and bathroom) and 2G for the bedrooms at each end.

This is despite the calculation that the increase in energy loss for the 3G windows if I used 2G instead could be replaced by PV, even in mid-winter, for slightly less money. The main reason is comfort - I do expect to sit close to the windows to get extra light at least sometimes in the winter. That's part of the trade-off in having small window area.

The bedrooms will have 2G because at the times when the rooms are in use the blinds will be closed giving a bit more insulation anyway and also because they need to be escape windows which are significantly more expensive in 3G because of weight (they're all roof windows).

In other words, by my arithmetic the 2G/3G decision is not a simple calculation and you have to think carefully about how the windows are likely to be used.

why are many 3G windows considerably more expensive than 2G , because people are greedy
and want to make as much money as possible for the least effort. This can easily be achieved by
placing the words Premium, Eco or Green in front of your product name :)

I'll be a bit disappointed if I get much condensation on even the 2G windows. A reasonably warm house, MHRV and blinds/internal insulation which seals reasonably well really ought to prevent it.

Problem with condensation is if you increase the temperature of the glass then the vapour will just condense in a different place. (assuming there's a colder surface than the glass). If there isn't then the air can/will take up more vapour until it reaches saturation point (at a higher temperature) and then condense on the surface which has reached dew point anyway. This assuming the vapour isn't vented out or mechanically controlled

Posted By: tony
I asked why 20 yrs

That sounds a pretty reasonable period tbh. What's the average length of occupancy? How many years are they guaranteed for? Certainly I'd say 60 years is too long. How much 60 year-old glazing do you see still fitted? My place is about 60 years old, and doesn't have any of its original windows.

FWIW I tend to agree with the assessment that 3G isn't preferable over 2G in straight fuel cost payback terms. You go for 3G if you're spending extra money to get fabric values down low enough to make passive heat a dominant factor, you don't do it for the sake of the fuel you're burning to heat a building.

Once you get away from UK made windows or any plastic windows - i.e. to basic European made timber 1.1Uw, then 3G costs no more than 2G, maybe even less, as 3G is 'standard' in Europe, 2G is 'special order'. But I think this discussion is comparing 'PH standard 3G' with UK made 2G - there is the supreme-value European 1.1Uw alternative in between.

3G is rare and much more expensive choice in France.

Agree that when people say Europe they are talking about Holland, Germany, Scandinavia etc

From what I have researched, including research papers prepared by the PH institute, for Mediterranean climates don't need 3G and it can risk over heating.

http://passiv.de/en/02_informations/05_ph-mediterranean/05_ph-mediterranean.htm

Yet Vitrocsa (Portugal) make the only seriously insulating all-aluminium windows/glazing in Europe (world?)

Posted By: GotanewlifeHere they don't care about simple financial payback over even 3 or 4 years

Because they think they live in a 'warm' climate, even though winters at any kind of altitude (and there's a lot of mountains) can be bitter like we don't know in UK. That being the case, even PHInst can't make blanket statements about 'Mediterranean doesn't need 3G'.

French sailor's T-shirt - Toulouse and Toulon
French sailor's motto - A l'eau! c'est l'heure!