Is there really much difference between A and A+ rated double glazing?

gm0

Triple glazing can often be quite cheap to add as there is a lot of fixed cost in custom sized windows and the installation which doesn't shift with the 2G to 3G change.  They do work a bit better.  We now get occasional (to weather) external condensation on the outside on cold glass some morinings.  Never used to happen with the old (not great) double glazed units which were (relatively) nice and warm on the outside and so not cold enough to show it.  So less heat is escaping.  A marginal gain.

Glass thickness and spacing are limited by the vendor and profile as to choices.  Upping this spec will generally add cost and sometimes weight.  Some of these profiles are better at acoustic damping as different thicknesses and spacing affects resonance.

Trickle vents are problematic at window replacement spec stage if you are in a property which is that era of building control for ventilation (with TV and extractor fans to kitchen, bathroome etc.). i.e. no designed in ducting and MVHR. 

Some modern and attractive window systems don't really suit them as designed for the newer scenario and for non-UK BC applications.  Passive haus. Outward openers, thin edge profiles as examples.

When shopping around I was consistently appalled by the dodgy quality and design of many "after thought" trickle vents which are added to profiles. The windows I replaced softwood timber with TV would "thrum" in the wind and did not shut properly.   Poor from day 1. Worse once the rubber perished.  And that style is still in use sold new of various qualities of manufacture..

I fitted some new windows which have the trickles on the side of the window and a rigid plastic shield over the interior aperture so not the tilting rubber/spring clip design. This seems much better.  I mostly leave most of mine shut as we are not underventilated so they are there mostly for code not comfort. 

Anecdote aside - if speccing these I do recommend you see one pre-sales - at a sales/demo centre/show/customer reference.
If you are saddled by planning with glazing bars/astragals (crosses) then this too is worth looking at closely as on some systems it was part of the design brief and again - on others it is an afterthought if supported at all

canaldumidi

Is the extra £900 the difference in cost of your selected supplier? What was the differece in price of the other quotes you got?

You have got at least 3 quotes to compare yes?

I also recommend visiting several supplier's sales rooms and looking at samples of the windows you are considering.

I had 4 quotes and visited 2 sales rooms before deciding.

Went for double glazed/no vents but high rating (I forget exactly what). They had a facility to open the window a fraction and lock it, so effectively a vent though as freebear says, based on hinges and handles!

https://trustedtraders.which.co.uk/articles/what-is-which-trusted-traders

https://www.buywithconfidence.gov.uk/

Apodemus

In terms of difference between grades of DG units, is there not also a difference in the gas used to fill the units and the degree of heat transfer allowed by the spacers?  The latter will also affect the likelihood of condensation on the lower edge, which is an issue for longevity, particularly for wood-framed DG windows. 

BUFF

afaik the grades are governed by the calculated results (they include allowing for solar gain but depending upon aspect your actual window may not achieve that gain) & there are different ways of achieving them - e.g. different spacings, argon or krypton fill (or vacuum if you are feeling flush), different no. of panes, different coatings/no. of coatings on the panes etc. etc.

donnablossomm

Having trickle vents in all windows just means we'll have to invest more in things to draw and redirect water away from the windows, like the little beads you can get that absorb the moisture from the vents

princeofpounds

I went and looked in a lot of detail into manufacturers' specifications for thermal performance of windows a couple of years ago.

What was particularly interesting to me was that having specialist thermal glass or thermal film coatings were actually typically more effective than moving from double glazing to triple glazing. But double glazing was the clear best first step - the returns from there were obviously diminishing, but still noticeable up to triple glazing (apologies I can't quantify them from memory).

I remember one manufacturer (Pilkington?) produced data for up to 9 layers of glazing! 

The frame is also remarkably important.

Anyway - it looks like the A rating means 0-9kWh/m2/yr, A+ means 10-19 (windows this efficient are considered a plus due to solar gain).

Let's assume a difference of 10. Then maybe 30m2 of window area. Then maybe 5p/kWh for gas (maybe more right now, and of course maybe 3-4x more for electric heating). So you would maybe save £15/yr on your energy bill, or a 1.7% return on your investment. Obviously change the figures as you see fit.

So financially it's probably not worth it, unless you have loads of money in the bank doing absolutely nothing. Of course if energy prices inflate the return will do a bit better over time, but remember that windows don't have an infinite lifespan so may not last long enough to repay the whole investment over 30-60yrs.

However, there will also be some minor amenity gains in terms of 'feel' in terms of keeping temperature constant across the room, particularly if you are going for large French windows or Bifolds. Again, probably not too much to worry about.

Trickle vents... that's a whole other topic. Yes, they blow a hole (literally) into the thermal performance of a window. But then the ventilation problem has to be solved somehow, particularly in a cool, damp country like ours and that is a separate requirement to thermal performance - if you don't have the vents, you need something somewhere that lets air move. A lot of newbuilds when the government fad for mandating insulation began have condensation damp problems even though they are warm, because the moisture just can't escape easily enough.

British houses typically don't have proper HVAC systems with active ventilation and heat recovery unfortunately. That's worth spending some money on in my opinion.

greenface2

FreeBear said:

greenface2 said: Soon trickle vents will be in every new window regardless of like for like or new build . They are becoming compulsory . Why change your draughty windows for draughty new ones . I can see big customer complaints due soon . Throws all trickle vented windows into C rated windows . They have a big hole ripped through them.   

Whilst current regulations require trickle vents if the old windows had them, I really hope it doesn't become the default on all replacement windows. I don't see the point in fitting a window, sealing up the edges to stop draughts, then punch a ruddy great hole through the frame.... There are better ways of ventilating a property - That is why most windows have hinges & handles.

Absolutely . The industry has improved its product to such a degree that windows and glass used to be the outlet ( also inlet) for ventilation even when well fitted .  Nothing better than seeing a sock stuffed into a trickle vent when you go to inspect a damp issue but they cant seem to find the handle on the window to crack it open a bit .