Glazing choices - Uw, Wer (category) and EPC ?

[gcm]

Selecting glazing - still rather a mystery even after some forum reading and googling
I have the following back from Rationel on Alu clad FORMA Plus w.r.t glazing options

Sample window 1.12 m2 area, side hung, twin outward opening sashes  Data: Uw Gg Ug LTg  WER   

1 Glass (fillings) 4-24-4 Clear/Energy WE w. argon (1, 2)  1.4 0.64 1.17 0.82  -27.8  as a base cost from an earlier quote and discount offered thereon
2 Glass (fillings) 4-22-6,8 Toughened Clear/Laminated Energy WE w. argon (1, 2)  1.4 0.64 1.16 0.8  -29.5  at 5% uplift
3 Glass (fillings) 6-18-8,8 Tougenend Clear/Sound Laminated Energy WE w. argon (1, 2)  1.38 0.61 1.13 0.79  -29.4  at 12% uplift

*4 Glass (fillings) 4-18-4-18-4 Energy/Clear float/Energy WE w.Argon (1, 2) 1.03 0.53 0.53 0.74  -13.8  at 2.50% uplift

5 Glass (fillings) 4-18-4-16-6,8 Toughened Energy/Clear float/Laminated Energy WE w.Argon (1, 2)  1.07 0.53 0.6 0.73  -16.5  at 9.30% uplift
6 Glass (fillings) 6-14-6-14-8,8 Toughened Energy/Clear float/Laminated Energy WE w.Argon (1, 2)  1.08 0.51 0.63 0.72  -19.8  at 15% uplift

*Upon first look option 4 (triple glazed) - 4-18-4-18-4 at 1.03 Uw appears to be the "value" pick (based on their "offer") - this feels like a very small uplift on an already "high" base quote as this seems a rather trivial difference between 2G and 3G.

My questions: 

1 Verify my understanding of WER and categories and feedthrough (or not) to EPC.

Option 4 at -13.8 WER is still category "C" (-10 to -20) if I have understood WER correctly and not been misled by the web  - and options 1,2 would be "D" or worse on this calc but still "pass" for an install on u-value criteria.  So the option 4 window has good (but not great Uw) (1.03) but it doesn't look so great on WER presumably due to its lower solar gain

So is WER and the category actually significant in either real or "regulatory" world ?   I am keen not to discover that I bought the wrong windows at a future EPC assessment (house doesn't have one).  I am also keen to grab easy EPC "points" where I can - help ensure access to RHI/FIT and similar.  If I get and keep the manufacturer glazing test documentation and the Uw numbers for each window presumably these can go into the EPC SAP estimator? (Scenario is NOT new build so will be working with the reduced "rules" on when "real" vs category estimate data can be applied and the allowed documents.

2.  We have specified as safety requires - low level glazing etc.  What's the rationale to upgrade from 4 to 5,6 above more generally. 

I am guessing more "secure" as in harder to break but that is not an especially critical requirement in our location. 
Also one of the options *may* be better acoustically (but how much vs a 3G base is a question) it seems it is mildly less good on energy (narrower units presumably) and it also seems a lot more expensive.  Am I missing something as to why these latter choices and the 2G equivalent are a good idea ? 

[AliG]

I must admit that I don't know about WER and SAP, but a couple of comments.

 

The windows with laminated glass will be more secure as they will be harder to break.

 

3G is better for sound transmission, but sound transmission is also reduced by having different thicknesses of glass that reverberate at different frequencies, so options 5 and 6 should be quite than option 4 and 3 will be quieter than 2 which is quieter than 1. 3 might even be quieter than 4.

 

I personally would go for option 5 as over 4 due to the better sound transmission and higher security of the laminated glass. 6 would offer very little over 5. Depends how much you value security and sound transmission.

 

I would note that 3G is quite a bit heavy and the thicker glass will be heavier again which might affect the cost to install the windows if the quotes are supply only.

 

TBH 9.3% uplift from 2g to 3G laminated seems like a very good deal.

 

 

 

[A_L]

1 hour ago, gcm said:

My questions: 

1 Verify my understanding of WER and categories and feedthrough (or not) to EPC.

Option 4 at -13.8 WER is still category "C" (-10 to -20) if I have understood WER correctly and not been misled by the web  - and options 1,2 would be "D" or worse on this calc but still "pass" for an install on u-value criteria.  So the option 4 window has good (but not great Uw) (1.03) but it doesn't look so great on WER presumably due to its lower solar gain

So is WER and the category actually significant in either real or "regulatory" world ?   I am keen not to discover that I bought the wrong windows at a future EPC assessment (house doesn't have one).  I am also keen to grab easy EPC "points" where I can - help ensure access to RHI/FIT and similar.  If I get and keep the manufacturer glazing test documentation and the Uw numbers for each window presumably these can go into the EPC SAP estimator? (Scenario is NOT new build so will be working with the reduced "rules" on when "real" vs category estimate data can be applied and the allowed documents.

 

All your windows "pass" on U-values. IIRC WER band "C" or U_w<=1.6W/m².°C to pass. Windows are the very last place to look for "easy" SAP points, the difference between your worst and best will be marginal.

[gcm]

Thank you - perhaps I should have been clearer on "easy" - what I meant was that I don't through lack of understanding want to miss some (arbitrary) threshold and lose what are for this building - precious points (such as to be eligible for incentives, to rent, even to sell (one day) - for want of an avoidable slight spec issue. We don't have an EPC at present (occupation pre-dates) but have read the media on renting, RHI eligibility etc so the concern is raised and I find it difficult to back trace EPC - SAP - WER - Uw to this or that specification of window as offered to manage that risk.  Other than the <1.6 rule which is clear and binary. 

Still finding it odd that a 1.03 Uw triple glazed window like this one is simultaneously expensive (in Alu clad) has an apparently decent Uw (vs the BC minimum) and yet is still "four categories down" A++ A+ A B C" on WER) - that does however explain why they don't advertise it in that way.  Either I am looking at the wrong window ? Or WER is a bit weird ? Or both ?

1.4 vs 1.03 x 45m2 x 16 deg = ~266W difference 2-3 people sat about - or an evil older plasma television 

1kw vs 740W losses - worth having if cheap - draught proofing a better bet.

Just don't want to be told - you should new better windows on the EPC when it is done - I need to replace due to blown units and expiring timber so want to invest in the right stuff. 

Reading 2012 SAP guidance - 3G didn't seem to buy you anything special. In some circumstances you could use the Uw actuals in the modelling (I was left unclear on the actual rules and practices as to when you can or cannot and which product documents or whole window estimates are proof) and indeed whether this helps to any material extent anyway.  Actuals and assumptions used on walls and roof presumably matter a lot more.

Marginal - I ran the one of the web calculators and it was £50 / £80 vs the existing 90's stuff  (if performing to spec and well sealed which it now isn't hence replacement).  
My cost compare might in reality be a bit more favourable due to kerosene + wood burner heat inputs and if I put in my actual glazing some of which are on the large side as web calculators assumptions about walls + glazing may be off.

Thanks again

[gravelld]

You also need to consider things like comfort. A good 3G window won't produce a downdraught, 2G likely will. The additional air movement creates a slight draught which compromises comfort.

 

Plus there's the radiative element - standing next to them. A better quality window means you can use more of your house.

 

I would read the reasons Ph choose the parameters they do, and I'd always trust Ph over SAP, which is a methodology designed to support a wide range of stakeholders... not necessarily people building quality houses.

 

Also bear in mind that some programmes incentivise worse scores, e.g. RHI.