What u value

[Russell griffiths]

So confused as usual. 

I read a thing recently regarding, what is the point in having super dooper walls if you have 75% glazing in that wall. 

 

Well I think I may be in that position, I’m trying to decide on a buildmethod and the approximate u value achieved with that build up, but if you look at the pic below this is a design we have been following and the rear wall will have a very similar look to the pic. 

Would you consider the amount of glass area to have such an effect on the overall heat loss that making really low u value walls will be a waste with all that glass, even with good quality windows and doors I don’t believe they come close to matching the walls. 

Any thoughts. 

Edited December 18, 2017 by Russell griffiths

[NSS]

If anything, I'd say it makes it all the more important that the other elevations (and roof/floor) are well insulated. Works both ways too as unless your rear elevation faces North then you'll benefit from solar gain in the colder months.

 

Stunning looking property BTW. 

[Jeremy Harris]

It's always a trade off between performance and cost, but windows and doors are areas where the benefits of improved performance are generally greater than elsewhere, not just because the losses are lower, but mainly because they improve the comfort level more than the bare figures tend to suggest.  The impact of having two layers of low e glass, reflecting long wavelength IR back into the house, are very noticeable indeed, and add a fair bit to the way the house feels in cold weather.

 

The argument that it's not worth improving insulation elsewhere just because doors and windows won't be as good doesn't really stack up at all, as the areas of all the other surfaces are so much greater than that of the glazing.  There are also other factors to bear in mind, like ensuring that the decrement delay factor of the wall and roof construction and insulation is adequately long, as that makes a significant difference to the comfort level.

[Ferdinand]

On average the performance of a wall is several times better than a window, and they need to balance out to give you whatever performance level you want.

 

IF you are not very well insulated at say BR basic spec or a little better then you need to balance windows ... even 3G windows are well behind BR requirements walls .. against walls for overall performance.

 

If you are basically well insulated say much better than BR Levels but not reaching what some people on here achieve, then you still need to balance, but in money terms you could potentially spend the money on wall insulation rather than upading from high end 2G to 3G.

 

Then there are questions about low inclination sunlight overheating your house in the spring / autumn etc.

 

And hence we all have sore heads from the calculations, but the tradeoffs need to be explored to the point where each self builder is satisfied.

 

Ferdinand

[LadyBuilder]

I have this in my living room. I did it about 5 years ago and it was low U-value for glass (can't remember precise value). Building control was not happy, but after a very thorough SAP report, it was passed as SAP thought that it actually adds heat to the building.

In actual fact, despite it being quite air tight, temps are always about 2 degrees lower in that part of the living room in the winter and in the evening I have a blanket. In the summer it is impossible to sit directly underneath.

I love the look of it though, so I am happy to use that blanket in the winter…It is down to a personal choice more than building regulations IMO

[Cpd]

I really really like the look of the photo @Russell griffiths But how on earth do those massive H beams not cause a world of trouble.....

i know its the design you like and am sure you would not make such a thermal bridge ! But its still impressive.  

[Russell griffiths]

6 hours ago, Cpd said:

I really really like the look of the photo @Russell griffiths But how on earth do those massive H beams not cause a world of trouble.....

i know its the design you like and am sure you would not make such a thermal bridge ! But its still impressive.  

We are thinking of changing the beams for gluelams, this pic is from the states where anything seems to be ok. 

[Russell griffiths]

Thanks as always everyone for some very informative answers, 

 

so would there be a comfortable u value, where trying to achieve more would be a waste. 

For arguments sake if I could achieve 0.15 for the budget I have in mind would I be better off spending another £5000 on more insulation to pull the walls down to say 0.13 or spend the extra £5000 on a higher spec glass. 

[Jeremy Harris]

18 minutes ago, Russell griffiths said:

Thanks as always everyone for some very informative answers, 

 

so would there be a comfortable u value, where trying to achieve more would be a waste. 

For arguments sake if I could achieve 0.15 for the budget I have in mind would I be better off spending another £5000 on more insulation to pull the walls down to say 0.13 or spend the extra £5000 on a higher spec glass. 

 

Generally it's a hard question to answer, as it's not just the U value that matters, but the overall heat loss.  Much of this will be ventilation loss, and there is very definitely a trade off between the volume of a house and the insulation level needed - as buildings get larger insulation values become less important and ventilation heat loss becomes more important.

 

It's partly why I wrote the simple heat loss spreadsheet, as that allows you to fairly quickly see the effects of changing things.  You can pretty quickly do "what if" checks to see what effect making the walls, roof or floor better insulated have, or what effect improving the windows has (although bear in mind that there is a subjective comfort factor with having 3G glazing with two low e panes that is not shown by the U value difference).

[Stones]

+1 on spending the time doing some 'what if' heat loss calculations.  I built on Jeremy's spreadsheet to include household gains, solar gain and the effect of wind speed.  It was a very useful exercise and let me compare various permutations, which resulted in swapping out one build up for another that saved several hundred £ vs a 50 year + payback.

[LadyBuilder]

I would say on the glass. The difference between 0.15 and 0.13 is negligable on the bills or how warm/cold it feels. A lot of glass will have a noticable difference to the temp inside, even if that is somehow offset in the bills

[LadyBuilder]

https://www.homebuilding.co.uk/does-triple-glazing-make-sense/

 

thought of you

[Jeremy Harris]

8 hours ago, LadyBuilder said:

https://www.homebuilding.co.uk/does-triple-glazing-make-sense/

 

thought of you

 

 

It's a great shame that the major benefit of triple isn't emphasised in that article, and is just ignored.  Triple glazing isn't primarily about the U value, in fact I think the U value is probably one of the least important factors in most of the UK.  Double glazing can achieve good U values, but cannot get close to the comfort improvement that triple glazing gives, because it only allows a single pane of low emissivity coated glass to be used (because the coating is fragile and has to be on the inner face of the outer pane).  Triple glazing allows two low emissivity coated panes to be used, the inner face of the outer pane and the inner face of the centre pane.  That just about doubles the long wavelength IR reflected back into the room, and as far as comfort goes, our bodies are very good at detecting high radiated heat loss, so standing in front of glazing that reflects most of our radiated body heat back feels, subjectively, a great deal warmer.

 

We still have our old house, with decent double glazing and a single low emissivity coated pane.  Although the U value of that glazing isn't bad, it feels, subjectively, pretty cold when you stand or sit in front of a window, and it encourages you to pull the curtains early, even when the room temperature is OK.  In the new house you cannot notice any difference between standing in front of a window on a very cold day and standing in front of a wall, as the windows reflect back the majority of your body heat, more than the walls do I think.

 

I think more needs to be made of this point, as it often seems to be missed in our obsession with U values.

[Russell griffiths]

On 19/12/2017 at 19:50, Stones said:

+1 on spending the time doing some 'what if' heat loss calculations.  I built on Jeremy's spreadsheet to include household gains, solar gain and the effect of wind speed.  It was a very useful exercise and let me compare various permutations, which resulted in swapping out one build up for another that saved several hundred £ vs a 50 year + payback.

Could you elaborate on what your change of build up was,

its reading your blog @Stones that has prompted a lot of my questions. 

I might have to ask a few more. 

Thanks russ. 

[Stones]

We had initially specified 200mm full fill rigid insulation between rafters and 25mm across rafters for all our vaulted roof areas.  I swapped it out to 180mm earthwool batts and 50mm across the rafters.  The U value wasn't as good, but given the running cost difference was only £18 a year, it was going to take well over 40 years before there was any payback.  I also concluded that the mix of insulation types gave better decrement delay, and would avoid the requirement to have to foam in each and every board between rafters.

[Redoctober]

5 hours ago, JSHarris said:

We still have our old house, with decent double glazing and a single low emissivity coated pane.  Although the U value of that glazing isn't bad, it feels, subjectively, pretty cold when you stand or sit in front of a window, and it encourages you to pull the curtains early, even when the room temperature is OK.  In the new house you cannot notice any difference between standing in front of a window on a very cold day and standing in front of a wall, as the windows reflect back the majority of your body heat, more than the walls do I think.

 

@JSHarris I agree and we are currently debating whether to go double glazed or triple glazed and I suspect the price may well have a factor in that decision. Anyway, although we have currently priced up for triple glazed windows, the French doors are having to be double glazed due to the weight involved we are told. If that is the case,  then other such doors like Bi Folds are also double glazed. Or are they? If so, how do people overcome the "coldness feel" you rightly mention above, when faced with a larger expanse of glass?

 

PW.

[PeterW]

4 minutes ago, Redoctober said:

 

@JSHarris I agree and we are currently debating whether to go double glazed or triple glazed and I suspect the price may well have a factor in that decision. Anyway, although we have currently priced up for triple glazed windows, the French doors are having to be double glazed due to the weight involved we are told. If that is the case,  then other such doors like Bi Folds are also double glazed. Or are they? If so, how do people overcome the "coldness feel" you rightly mention above, when faced with a larger expanse of glass?

 

PW.

 

I have triple glazed French Doors and they are fine - glazing units were 38kg each but they have worked well and pleased with them. 

 

With no heating other than a 2kw heater it’s not cold in here and we have upped the insulation to 400mm of fibre anywhere flat with 25mm of PIR over the top of 200mm on the sloping bits which gives a variety of uValues from 0.09 to 0.18 depending on which element you look at. 

[Jeremy Harris]

We also have triple glazed French windows, but they are slightly poorer thermally than the main glazing, as they have to have a 6mm toughened inner and outer pane, so they are 6 -16 - 4 - 16 - 6, rather than the 4 - 20 - 4 - 20 - 4 that we have for the windows.  The U value is slightly poorer, but subjectively they feel as warm when standing next to them on a cold day as the other windows.