Confused by Triple Glazing Justification

[MortarThePoint]

I'm confused by the motives of going for triple glazing as I like to do the maths behind these sorts of things. I see two primary reasons economic, reduced energy bills, and environmental, reduced 'carbon footprint'. I am pro green tech so start from a bias of wanting to include them.

 

Improved U-value: The U-value of a 1m2 window might typically improve from 1.2 W/m2K for double glazing to 0.8 W/m2K and so an improvement of 0.4 W/m2K. Based on heating the house for the coldest 6 months of the year (October to April inclusive):

Difference in U-values, dU = 0.4 W/m2K

average temperature difference across window, dT = 13 K (around my area for these 6 months, 19C inside, 6C outside)

Duration, t = 183*24 = 4392 hours

Window Area, A = 1 m2

Average heat flow, Q = A.dU.dT = 1*0.4*13 = 5.2W

Annual thermal energy, E = Q.t = 4392*5.2 = 22.8kWh per year

[Ignored: A double glazed window lets in more light (8% nominally) so there will be more solar gain in those months. That might work out as (guess) 50W/m2 and so an 4W difference between the two windows, but only for a fraction of the time so may amount to 1W average].

 

Ecomonic

If heated by ASHP the 'efficiency' will vary, but I pick a value of 3.5 to be representative. With an electricity price of 14p/kWh that equates to a cost of thermal energy of 4p/kWh. That makes for a cost saving of 4p/kWh * 22.8kWh = 91p per year. Electricity prices could go up, but the cost uplift of triple glazing is >100 times that annual figure. The cost is all upfront as well.

 

Environmental

A triple glazed unit contains many extras, but most obviously includes an extra sheet of glass.

Energy required to make glass, 21.9MJ/kg [1]

Density of glass, 2.5kg/(m2.mm) (2.5g/cm3)

Area of glass, 1m2 [overestimate as neglects frame width]

Thickness of glass, 4mm

Mass of glass 2.5*1*4 = 10kg

Entrained energy in glass, 21.9MJ/kg * 10kg = 219MJ = 60.8kWh

Energy debt payback time based on oil heating, (60.8kwh / 22.8kwh) = 2.7 years

Energy debt payback time based on ASHP(SCOP=3.5) heating, (60.8kwh / (22.8kwh/3.5)) = 9 years

There will be other uplifts in manufacturing 'carbon cost', but 9 years is a significant payback time and all that 'carbon' is upfront in manufacture and doesn't consider renewable sources of power generation that will come online over the next 9 years. Also, I would guess that glass manufacture is less likely to be using renewable sources of energy than the domestic electricity suppliers. I could believe that payback time heading beyond the lift of the unit.

 

What am I missing here? Are the perceived benefits different?

 

[1] https://www.sciencedirect.com/science/article/abs/pii/0166309781900614

Edited April 14, 2021 by MortarThePoint

[Declan52]

My house is beside a very busy road and the triple glazing really helps to kill the sound of lorries going over the speed bumps that my lovely council decided to place right outside my bedroom window.

 

[PeterW]

Sound insulation ..? 
 

Comfort value ..? They feel “less cold” when you stand near them

 

 

[PeterW]

10 minutes ago, MortarThePoint said:

I am pro green tech so start from a bias of wanting to include them.

Haven’t you installed a wood burning stove that negates all that “green tech” anyway so is this clearing your conscious of that by installing something to offset that ..?? 

[MortarThePoint]

9 minutes ago, PeterW said:

Haven’t you installed a wood burning stove that negates all that “green tech” anyway so is this clearing your conscious of that by installing something to offset that ..?? 

 

We have our own bit of woodland and will likely mostly be burning fallen timber that would rot in a less environmentally friendly fashion otherwise.

[MikeGrahamT21]

Yeah there are many factors other than savings to take into account... comfort, sound reduction.

 

Also you say a reduction in 0.4U as if its nothing, i would imagine if you had the chance to reduce a wall's uvalue from 0.4 to zero you'd be there in a shot ?

[Tom]

I think looking at "heat" or the temperature of the air is oversimplifying how we find our environment comfortable. I'm reminded of that recent GD episode where the chap tried to build a completely passive house, insulating the earth around it to act as a thermal store, he said that even though the temperature of the air was lower, the warmer walls radiated heat and the actual comfort of the space was equivalent. The same could be extrapolated to the effect of windows on the heated space - a warmer triple glazed one radiating heat (or at least more than a double glazed one). Likewise the draughts/convection currents that would happen against a colder surface. 

[Thorfun]

for us the choice to go triple glazing included future proofing as well as all the other reasons given above. as the cost wasn't a huge uplift between double and triple glazing (10% - 15% approx) and as cost differences come down in price it probably won't be long before building regulations specify U-values for windows that can only be achieved with triple glazing. Therefore in order to ensure we comply in later years if we ever came to sell the house it was an easy choice to pay the uplift now rather than having to replace all the windows with triple glazed windows in 15 years or so.

[MortarThePoint]

36 minutes ago, Tom said:

I think looking at "heat" or the temperature of the air is oversimplifying how we find our environment comfortable. I'm reminded of that recent GD episode where the chap tried to build a completely passive house, insulating the earth around it to act as a thermal store, he said that even though the temperature of the air was lower, the warmer walls radiated heat and the actual comfort of the space was equivalent. The same could be extrapolated to the effect of windows on the heated space - a warmer triple glazed one radiating heat (or at least more than a double glazed one). Likewise the draughts/convection currents that would happen against a colder surface. 

 

My wife mentioned that she read an article that said a triple glazed (18C) unit is about 2C warmer than a double glazed (16C) unit when a single glazed unit would be at 1C. It was based on a 21C room temperature, but didn't say what the outside temperature was. That means it's 3C colder than room temperature on the surface of the triple and 5C on the double. I can see that would affect convection and the feeling of warming if right up near the window.

[MortarThePoint]

36 minutes ago, Thorfun said:

for us the choice to go triple glazing included future proofing as well as all the other reasons given above. as the cost wasn't a huge uplift between double and triple glazing (10% - 15% approx) and as cost differences come down in price it probably won't be long before building regulations specify U-values for windows that can only be achieved with triple glazing. Therefore in order to ensure we comply in later years if we ever came to sell the house it was an easy choice to pay the uplift now rather than having to replace all the windows with triple glazed windows in 15 years or so.

 

I have wondered about the perceived value in the market if the house was up for sale.

[Thorfun]

2 minutes ago, MortarThePoint said:

 

I have wondered about the perceived value in the market if the house was up for sale.

yeah, it's a valid musing especially when you consider how clued up the general house buyer is on building regs/EPC/SAP etc. but as it will be a highly insulated, airtight house the thought of having to rip it apart years later to then install triple glazing to save some money now fills me with dread! we're following the 'get the fabric right at the start' mantra as it's a lot easier to rip out a kitchen or bathroom down the line. ? 

[MortarThePoint]

On 14/04/2021 at 13:42, Thorfun said:

Therefore in order to ensure we comply in later years if we ever came to sell the house it was an easy choice to pay the uplift now rather than having to replace all the windows with triple glazed windows in 15 years or so.

 

You wouldn't be required to meet the prevailing building regs at that point, but are you thinking it would make the house more sellable if you did?

[tonyshouse]

3g is the mainstream product in central and Northern Europe with good reason. 
 

extra sheet of glass in a more robust frame makes perfect sense 

[Thorfun]

13 minutes ago, MortarThePoint said:

thinking it would make the house more sellable if you did

yeah. this mostly. who knows what the future holds but it might be that people are more likely to buy with triple glazing than double. maybe not though!

[Mr Punter]

On 14/04/2021 at 14:19, MortarThePoint said:

I have wondered about the perceived value in the market if the house was up for sale.

 

I put in 3g because it sounds better than 2g.  Nobody asks about the actual u values or coatings or wall insulation or airtightness but 3g gives buyers the impression of extra thermal efficiency.

[Bozza]

3g has a 3 in it.  2g has a 2 in it.  3 is bigger than 2.  
 

but seriously it was a nominal increase £400 if I recall to go from 2g to 3g.   I have a couple of 2g Fakro roof windows in the house.  I do note a difference in particular in respects of sound and condensation.

 

 

[Iceverge]

As usual it depends. 

 

The less energy your house takes to heat the more of a difference it will make. Going to double glazing would increase our annual heat demand by 30%. 

 

On a poorly built house this would represent a far lower % difference. 

 

The numbers can only tell so much. A cheap double or triple glazed window will leak air, catch when opening, and fall off the hinges in a few years. 

 

Whimsically there needs to be a point at which you draw the line at where the inner accountant gets to rule all.  Otherwise you'd be mental to install windows turning on the light is far more economical!

[Dreadnaught]

Heat loss through frames is often greater than through the glass. Manufacturers who make triple-glazed windows probably care more about frames than those that only make double-glazed ones. Buy good frames.

[MortarThePoint]

On 16/04/2021 at 00:25, Iceverge said:

Otherwise you'd be mental to install windows turning on the light is far more economical!

 

? I don't fancy living in a house with all the charm of nuclear bunker

 

1 minute ago, Dreadnaught said:

Heat loss through frames is often greater than through the glass. Manufacturers who make triple-glazed windows probably care more about frames than those that only make double-glazed ones. Buy good frames.

 

Lots of manufacturers appear to use the same frame and hardware for double and triple glazing meaning that 3g may actually be more 'flimsy' as the frame and hardware are working harder.

 

On 15/04/2021 at 20:06, Bozza said:

3g has a 3 in it.  2g has a 2 in it.  3 is bigger than 2

 

Feels a bit like a Gillette advert and look where that's ended up. Are they on 5 blades now?

 

On 15/04/2021 at 17:38, tonyshouse said:

3g is the mainstream product in central and Northern Europe with good reason. 

 

In countries with <0C for prolonged periods of time it makes a lot of sense. If you double the dT in my calculations the COP of the ASHP suffers and so the 'carbon' payback time will come down a lot, perhaps to ~3 years. Also the apparent warmth will be more significant too.

[Russell griffiths]

10 minutes ago, MortarThePoint said:

 

 

Lots of manufacturers appear to use the same frame and hardware for double and triple glazing meaning that 3g may actually be more 'flimsy' as the frame and hardware are working harder.

 

 

 

 

 

I would say then that you are looking at the wrong manufacturers, my windows are have a 125mm wide frame, you won’t find any 2g windows with frames like that. 

 

We went 3g and at about 10%  uplift on 2g, they are silent and just feel completely different from any other 2g windows I’ve owned.  

[Iceverge]

An important part of glazing is thermal comfort. 

 

Someone in the passive house movement (cult!?) figured 4 deg was the maximum difference between adjoining surfaces in a house for maintaining a scientifically calculated level of comfort. 

 

The attached image I found in a Smartwin doc suggests that for almost all of the UK double glazing is sufficient to achieve this. 

 

However, I challenge you to source a suitably high quality double glazing supplier that the price uplift to triple glazing isn't marginal. 

[Oz07]

I'd be more interested in what the frame is built from and the longevity of this if it was a forever home. Do the high quality window suppliers just use the same size rebate with deeper beading for 2g as opposed to 3g?

 

My nans been in her place over 30 years now. Got the rosewood upvc they put in when renovated before moving in. Changed a few units but the frames still serviceable now. How long are the "good quality" 2g Windows expected to last above the standard upvc product? Got the standard upvc in mine and got a good air test result. 

 

I think when it comes to Windows be careful not to overspend and don't believe all the hype. 

[dpmiller]

upvc works fine for me too.

 

Isn't it tru tho that a poorly- designed 3g unit can be bettered by the best of the 2g options? Without going to "fancy" frame options (aluclad etc) there's only so much depth to work in and filling the 2g's airgap with another sheet of glass doesn't leave much space...

 

Other than the asthetic of the more expensive frame options, are we not into the realm of diminishing returns as with other insulation thicknesses?

[tonyshouse]

It is fake news that 3g can be bettered by any 2g and nonsense 

 

Our uk plastic windows are poor quality and short lived, proper windows should last many decades 

[SteamyTea]

TL;DR 

But two things struck me, apart from the bollocks about fallen timber bring better burnt than add to the carbon store in the ground (wrongly called rotting).

People tend to close curtains at night, so that can help energy losses.

The big one though us that we need to reduce continuous energy usage, not only to save money, but to make implementing renewables easier and cheaper.

The less we have to generate, the less we need to build.

So decent windows, decent fitting curtains and leave trees to do what they do best.