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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthortony
    • CommentTimeJan 15th 2015
    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
    The trouble with extending the payback period is that you are asking the person who pays for the works to give part of the benefit (and pay for it) for future occupiers. So the term needs to be limited to the time the person paying for the work expects to live there.
    • CommentAuthorsnyggapa
    • CommentTimeJan 15th 2015
    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

    • CommentTimeJan 15th 2015
    payback is my problem, I only expect to be in my new build for 10 to 15 years and as others have said when property is sold not much attention is paid to things like a very good EPC. I want comfort in my older years but at what cost?
    • CommentAuthoradwindrum
    • CommentTimeJan 15th 2015
    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.
    • CommentAuthorGarethC
    • CommentTimeJan 15th 2015
    Payback's so important, and as people suggest, it's important for different reasons to different people. The only time it's not important is when you don't have a budget limit (who care's about payback when you're already a multi-millionaire?).

    But put it this way, if it were possible to reduce total domestic emissions/energy use in existing buildings, and therefore bills, by 90% for an investment of £6k on average (given average utility bills of £1.3k), then, since the payback's just five years everyone would do it pretty damn quickly; they would expect a net benefit from it after five years and, compared to other ways to invest £6k, the returns are spectacular. Then the UK would be a long way to hitting it's target CO2 reduction targets.

    Since it's not, and the payback's generally MUCH longer, it just won't happen quickly.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 15th 2015
    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.
    • CommentAuthorGarethC
    • CommentTimeJan 15th 2015
    I'm sure it will have an impact, but I reckon it will be pretty marginal. Might make the payback go from 60 years to 'only' 40 or something.

    On the comfort issue, this page: http://www.homebuilding.co.uk/advice/key-choices/green/triple-glazing

    Has the following table. Assumes internal temp is 21 degrees and outside is 'cold'. I dunno, but two degrees difference between 2G and 3Gfeels pretty marginal... I realise if you were right up against the window you would notice, but really, you wouldn't have to be far away before you didn't notice (and just in our house at least we don't sit all that near the windows!)

    •Next to a single glazed window, the internal surface temperature is around 1°C.
    •Next to a double glazed window (2000 vintage), the surface temperature is around 11°C.
    •Next to a modern, energy-efficient double glazed window, the surface temperature is 16°C.
    •Next to a triple glazed window, with a centre-pane U value of just 0.65, the temperature is 18°C.
    • CommentAuthorGreenfish
    • CommentTimeJan 15th 2015
    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.
    • CommentTimeJan 15th 2015
    There are already methods to work out this sort of thing, the usual caveat being that anything over 5 years starts to get uncertain, 20 years time it is a complete guess.

    Just run the figures using PV, NPV and FPV and you will soon see what a nonsense it all becomes quite quickly.

    If I had said this time last year that oil and gas would be cheaper in real terms, I would have been laughed at (I did and I was).

    What people are really talking about is not 'value' in the monetary sense, but value in the societal sense.

    Would be easily solved if we just put a price on emissions and their cash cost to society.
    • CommentAuthorEd Davies
    • CommentTimeJan 15th 2015
    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.
    • CommentAuthorjamesingram
    • CommentTimeJan 15th 2015 edited
    The upvc 3G I fit is about 10-20% more expensive than 2G , I don't really care what the financial payback is
    as I can supply and fit for what many charge for 2G upvc .

    There's more to life than financial payback
    Energy return on energy invest is the important one for 'green' building , costs need to weighed up against
    other potential options to get best solution ( energy efficient) for the budget.
    • CommentAuthorjamesingram
    • CommentTimeJan 15th 2015 edited
    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 :)
    • CommentAuthorjamesingram
    • CommentTimeJan 15th 2015 edited
    Ed re your 3G , 2G point, yes on paper Pv might save more kW than the 3g in your bedroom.
    But after several weeks of wiping the condensation off your 2G in the winter months you'll be thinking perhaps
    3G might have been a good idea after all :bigsmile:
    • CommentAuthorEd Davies
    • CommentTimeJan 15th 2015
    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.
    • CommentAuthorjamesingram
    • CommentTimeJan 15th 2015 edited
    I think the 2G will be a weak spot. heard others with more experience than me saying it's a problem in super insulated home , 2G isn't up to the job in this situation.

    Re blinds (claimed thermally superior), I've fitted some in my place makes the condensation worst behind them as the internal pane of the windows is slightly colder.
    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
    Ah well, if it isn't condensation on the inside of the 2g it will be condensation on the outside of the 3g. Either way you might have to wipe windows to see out!
    • CommentAuthorSeret
    • CommentTimeJan 16th 2015
    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.
    • CommentAuthorRob_14
    • CommentTimeJan 16th 2015
    We replaced the original windows in our house last year – hardwood frames and 2g – they lasted 50 years.
    • CommentTimeJan 16th 2015
    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.
    Posted By: fostertomas 3G is 'standard' in Europe,
    3G unavailable other than as a costly special order mid-Italy (my local guy who just makes ali windows and doors has never made a 3g unit), maybe you mean north western Europe (excl UK) - so less than 20% of europe really.
    3G is rare and much more expensive choice in France.

    Agree that when people say Europe they are talking about Holland, Germany, Scandinavia etc
    • CommentTimeJan 16th 2015
    Yeah, and consequently it's the 'warm' southern states (plus UK) that boast by far the biggest energy bills spent on space heating.
    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.

    • CommentAuthorGotanewlife
    • CommentTimeJan 16th 2015 edited
    Posted By: fostertomconsequently it's the 'warm' southern states (plus UK) that boast by far the biggest energy bills spent on space heating.
    For sure they do but not because people choose 2g over 3g!!! Here they don't care about simple financial payback over even 3 or 4 years, let alone the esoteric 2g vs 3g debate. I was unusual getting Argon fill and even more unusual going for warm perimeter. I don't live in a rural backwater, big University city nearby (Perugia).
    • CommentTimeJan 16th 2015 edited
    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'.
    Yesterday I was out and about in Toulouse in T-shirt and sunglasses :)
    • CommentTimeJan 16th 2015 edited
    French sailor's T-shirt - Toulouse and Toulon
    French sailor's motto - A l'eau! c'est l'heure!
    I think what needs to be taken in consideration in Med climates is the slightly higher sun angle, even in winter.
    At the moment the mornings can be very very frosty, ice all over the place. But if the sky is clear during the day, that direct sunlight can really warm things up in a way that is not the same in more northern latitudes.

    What comes out in the PH research paper that takes in account several example locations across the Mediterranean , is that external shading becomes much more important, when combined with adequate insulation, ventilation and DG fenestration.
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