Unless the current windows were very poor, I would not put in 3G until a window needed replacing.

However if there was one window that I felt cold when sitting near it, I would consider it. 3G saves very little heat compared to most other upgrades you can do.

Also I expect the cost of 3G to reduce a lot over the next few years, so choose between upgrade 2 windows this year, or put money in a CASH ISA and maybe upgrade 4 windows in 2 years time....

Posted By: ringi3G saves very little heat compared to most other upgrades you can do.

Exactly. Plucked out of the air round numbers to get an idea: ratty old 2g, about 2.8 W/m²·K; decent but not absolutely top-notch 3g about 0.8 W/m²·K so 2 W/m²·K saving. For 4000 heating degree days that's 192 kWh/m²/year or less than £30/m²/year even if you heat with standard rate electricity. £10/m²/year if you heat with gas.

I would set your heating going on a cold day, get the temperature of each room to be the same, then starting with the rooms you use the most, find the windows that the feel the coldest when you are near them.

The real saving with better windows, is not that they let out less heat, it is that you can feel comfortable at a lower temperature. None of the energy rating systems for buildings have a good way of taking this into account.

To get most benefit you need to fit a heat recovery ventilation system, as cold drafts from window vents rather defeats the point.

Also unless you are going for frame that allow thicker glazing units than normal, 3G does not give much better U value then the best 2G. So I question if 3G is worth the cost over 2G unless you are going for high end frames.

I think what people are saying, and I agree with them, is that it is not worth going down the 3G route for you, period. Better to get high end 2G, and get them fitted really well (which will make a mess!). Ref now or later, it is the same as PC speed and the answer is invariably get them now not later. Spend the money you save on air tightness measures and HR (however achieved). 2 FreshR units is a lot of money! Unless you are going to reap the benefits of the lower running costs because of high end insulation and airtightness I would think again or at least have a conversation with yourself that starts: OK I have to fit HR how am I going to do it without the FreshR Unit. Ref the FreshR units, easy enough to shoot an E Mail to them asking if it can be mounted landscape (assuming it is not covered in the on-line documentation).

I remain sceptical as to the 'game changing' nature of FreshR units, so far I see them only being used in bespoke new build PH scenarios, where they are designed in or in retrofit scenarios where there really is no other way and significant efforts are being made with airtightness coupled with all those compromises ref big gaps under doors etc.

In short: I think you have asked the wrong question. Perhaps better would be something like: How should I prioritise the renovation of my house to improve comfort and energy saving....include in your question lots of details about your house, your plans and qualitative flavour of your budget. Otherwise it is a bit like your 20 year old son asking which Merc he should buy...would you be doing him a favour if you gave him a straight answer?

Posted By: GotanewlifeRef now or later, it is the same as PC speed and the answer is invariably get them now not later.

In isolation I agree, but, say, if the property was also requiring re-rendering soon, and that would be an opportunity for EWI, then best to hold off until the new windows can be fitted with the EWI, right? (Just a hyopthetical example).

Why do you think that something approaching 90% of Central European windows made are now 3g?

If you are going to a job do it well, 3g cut out more noise, last longer, are more robust and last longer.

How come they are almost standard in the countries Central Europe?

When deciding which to get triple I was advised that those with more frame to glass area should be triple as lots of heat escapes via frame...so small windows and those with long thin glazing.
I suspect Europe probably has triple as its cheaper to upgrade than us.

Circumstances are solid wall victorian house for considerable refurbishment, single glazed throughout except one pvc double in bedroom which will stay. Windows are large relative to the amount of external wall. Original sashes have already gone so no reason to keep single glazing. Walls to be insulated internally, possibly some externally at rear. Rear side-return extension with rooflights and patio doors to be added.

Actually, I'm guessing that rooflights would be top priority if i was to bother with any 3G, and then patio doors, especially if we were to go for trendy bifolding.
Thanks for the comments anyway. I can see 3G brings out some strong opinions!

3G rooflights now seems to be much more of a standard option, they are also good for reducing the sound of rain.

Will anyone be sitting next to the patio doors? If so comfort may be a bigger driver then saving heat.

Posted By: ringireducing the sound of rain.

Oh but I love the sound of rain on a (2G) rooflight!