Confused by Triple Glazing Justification

[Oz07]

2 hours ago, tonyshouse said:

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

Theyre economical and do a job. If my nan has had 30 years usage I'd say that's good enough for a lot of people. 

 

They recycle the old plastic frames now don't they?

 

Bit like mvhr here, people spend big then will convince themselves it's justified. I know the good Windows are better, but not everything always needs to be the best of the best. 

[SteamyTea]

1 hour ago, Oz07 said:

but not everything always needs to be the best of the best. 

Perfect is the enemy of good

[Simon R]

On 14/04/2021 at 12:14, MortarThePoint said:

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

Thought I would just throw in my pennies worth. I'll start by admitting I'm no expert, but the importance of keeping inputs to a minimum is key.

When we looked at the building cost and energy consumption, I did very similar sums to your own. It depends on site orientation etc, but our build thermal input worked out at a theoretical 68w/per degree for the structure. So a 10 degree difference is 680 requires 680w input. We have quite a bit of glazing and doing it all triple glazed meant we did not require the cost of complexity of an ASHP significantly simplifying the build.  A couple of small towel rails are all we need to heat the house. Looking at our gas use over the past two weeks the heating has only come on twice and our total gas use has been  just 70kWh mainly for hot water.

[MikeSharp01]

On 14/04/2021 at 12:14, MortarThePoint said:

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?

Great piece of work but the overall benefit may hinge on more than the pay back time for you personally. The house will be around for perhaps 100 years at which point it will have paid back more that 10 times over on a planetary scale rather than just for your pocket - assuming the windows don't get replaced (our daughter has just replaced several rotten sashes that where installed in 1887). If you have children, they will see the benefit in an improved environment and future generations more so. Maybe there is additional gain here to help justify.

[MortarThePoint]

16 hours ago, Simon R said:

Thought I would just throw in my pennies worth. I'll start by admitting I'm no expert, but the importance of keeping inputs to a minimum is key.

When we looked at the building cost and energy consumption, I did very similar sums to your own. It depends on site orientation etc, but our build thermal input worked out at a theoretical 68w/per degree for the structure. So a 10 degree difference is 680 requires 680w input. We have quite a bit of glazing and doing it all triple glazed meant we did not require the cost of complexity of an ASHP significantly simplifying the build.  A couple of small towel rails are all we need to heat the house. Looking at our gas use over the past two weeks the heating has only come on twice and our total gas use has been  just 70kWh mainly for hot water.

 

I see this as were triple glazing comes into its own. This approach is about eliminating weaknesses and that would invariably be the windows if not more than double glazed. In a house that is designed to be less thermally performant, something like an ASHP is needed in which case the windows can be evaluated on their own merit rather than trying to minimise there impact on a broader passive house type objective.

[MortarThePoint]

15 hours ago, MikeSharp01 said:

Great piece of work but the overall benefit may hinge on more than the pay back time for you personally. The house will be around for perhaps 100 years at which point it will have paid back more that 10 times over on a planetary scale rather than just for your pocket - assuming the windows don't get replaced (our daughter has just replaced several rotten sashes that where installed in 1887). If you have children, they will see the benefit in an improved environment and future generations more so. Maybe there is additional gain here to help justify.

 

Windows will hopefully last a long time, but glazing units don't do they? Does anyone have data on the life expectancy of glazing units. Naively, one could assume a triple glazed unit has half the mean time to failure of a double glazed unit if there were two independent points of failure. They won't be independent though, but I'd be interested to know how they compare.

 

My calculation only considered the carbon cost of the glazing unit.

[MikeSharp01]

2 minutes ago, MortarThePoint said:

Does anyone have data on the life expectancy of glazing units. Naively, one could assume a triple glazed unit has half the mean time to failure of a double glazed unit if there were two independent points of failure.

Good question, I just wandered round here and looked at our windows, double glazed and fitted in 1993 (close to 30 years), there are over 40 individual units and none of them are blown. I would have thought that modern manufacturing would get to 30 years MTBF for triple glazed without a problem although I cannot see any guarantees that go out that far. 

[craig]

Glazing units will generally come with a 5 or 10 year warranty, older units as mentioned by Mike will with a high degree of probability be supplied without any coating and air filled. So not very efficient. However, glazing units will generally last a considerable period of time.

 

I have not seen any evidence anywhere that a double glazed unit is less likely to fail over a triple glazed unit. It's not generally the glass that fails, it's the bonding (hot melt) around the glass unit that fails.

Edited April 19, 2021 by craig

[jayroc2k]

Adding a different perspective, whatever the glass is surrounded by is actually more important than the number of panes.

 

metal frame with poor thermal breaks are the worst. I have 3 examples installed at home.

all double glaze examples.

1. new 'sealed' sash Windows - this is the worst, its draughty around the edges, the seal is very difficult to get right as the panes still needs to slide

2. metal double glaze conventional window, this is the second worst, you feel the air movement from the cold metal 'convection'

3. wood frame large sliding door with great seal due to the 'lift and slide' mechanism. the best of the bunch here by a country mile. 

 

pay more attention to the thermal breaks, seals. then pick 2G or 3G. 

 

 

[ProDave]

I will just add my thoughts to this.

 

Most of out house has triple glazed Rationel windows.  I faced this 2G / 3G question when looking for roof windows for our en-suite.  To be frank I found the market offerings disappointing. But anyway, at a time when we were short on funds I compared the heat loss of a small 2G roof window with a 3G version, and concluded the heat saving of 3G in this small window was tiny, so i bought 2G.

 

That was a bad decision.  This is the ONLY window in the house that gets any condensation.  Only a small amount around the edges, but I wish I had paid a little more for the 3G version which would hopefully have prevented that.

 

There is more to choosing than pure heat loss vs "payback" time.

[DragsterDriver]

2 hours ago, jayroc2k said:

Adding a different perspective, whatever the glass is surrounded by is actually more important than the number of panes.

 

metal frame with poor thermal breaks are the worst. I have 3 examples installed at home.

all double glaze examples.

1. new 'sealed' sash Windows - this is the worst, its draughty around the edges, the seal is very difficult to get right as the panes still needs to slide

2. metal double glaze conventional window, this is the second worst, you feel the air movement from the cold metal 'convection'

3. wood frame large sliding door with great seal due to the 'lift and slide' mechanism. the best of the bunch here by a country mile. 

 

pay more attention to the thermal breaks, seals. then pick 2G or 3G. 

 

 

 

which lift and slide doors do you have?

 

I’ll go triple glaze due to the proximity of a dual

carrogeway.

[ProDave]

1 hour ago, DragsterDriver said:

 

which lift and slide doors do you have?

 

I’ll go triple glaze due to the proximity of a dual

carrogeway.

Happy with this Rationel triple glazed lift and slide door I just fitted, timber frame aluminium clad.

 

[Scoobyrex]

On 14/04/2021 at 13:42, 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.

 

You only have to comply at the time the house was built, you do not need to update the house to the current regs when you sell the house.

 

not sure what additional value triple vs double would add to the value of the house,  I would look at it has potentially saving x amount on heating costs, planning to live there Y years.

[Thorfun]

1 hour ago, Scoobyrex said:

 

You only have to comply at the time the house was built, you do not need to update the house to the current regs when you sell the house.

 

not sure what additional value triple vs double would add to the value of the house,  I would look at it has potentially saving x amount on heating costs, planning to live there Y years.

it wasn't about complying with building regs later down the line it was more about having to compete with new build houses with triple glazing if we only had double. I was more than happy to pay the 10%-15% uplift for triple glazing to 'future proof' the house which also has the added benefit of reducing our heating costs etc now.

[gc100]

On 14/04/2021 at 12:37, MortarThePoint said:

 

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.

Arn’t wood burners now the number one polluter above cars etc now in the Uk 

[PeterW]

52 minutes ago, gc100 said:

Arn’t wood burners now the number one polluter above cars etc now in the Uk 

Depends which AQ measure you are using ..? PM2.5 yes, NOx no, but “pollution” has many factors. 

[Jilly]

On 19/04/2021 at 09:33, MortarThePoint said:

 

Windows will hopefully last a long time, but glazing units don't do they? Does anyone have data on the life expectancy of glazing units. Naively, one could assume a triple glazed unit has half the mean time to failure of a double glazed unit if there were two independent points of failure. They won't be independent though, but I'd be interested to know how they compare.

 

My calculation only considered the carbon cost of the glazing unit.

Two of my new triple glazing units have blown already ?

[gc100]

23 hours ago, PeterW said:

Depends which AQ measure you are using ..? PM2.5 yes, NOx no, but “pollution” has many factors. 

yes PM2.5. Very unhealthy. 
 

I have my own wood as well but didn’t install a stove. They don’t belong in this country anymore given the population IMO.

 

I know everyone is very defensive of them and I’ve had them in the past. But I used the money to build a solar array instead.

 

[SteamyTea]

1 minute ago, gc100 said:

I used the money to build a solar array instead

And, for the same area, it will give a greater energy yield per year than growing timber, by a factor of at least 40.

[gc100]

2 minutes ago, SteamyTea said:

And, for the same area, it will give a greater energy yield per year than growing timber, by a factor of at least 40.

but soon as you burn it surely it then adds to all the pollution so not valid vs a clean energy?

 

 

[SteamyTea]

1 minute ago, gc100 said:

but soon as you burn it surely it then adds to all the pollution so not valid vs a clean energy

Oh yes, timber burning is a dreadful pollutant, not just CO₂/kWh either.  The overall efficiency is so low that it just seems a bonkers way to heat something when we have viable alternatives.

I usually say to people that are advocates of wood burners to count the rings in the logs, and that is the number of years it took to grow.  Then compare that to the hours that the log lasts in the burner.  Rough guess, an hour a year.  That is 0.01% 'time efficiency'.

[PeterW]

16 minutes ago, gc100 said:

yes PM2.5. Very unhealthy. 

Depends on perspective but NOx is probably worse in an urban environment. There is also a whole load of issues with road vehicles and PM5/2.5 coming from brake disk and tyre wear which is not being reduced by the move from IC engines. 
 

22 hours ago, Jilly said:

Two of my new triple glazing units have blown already ?

 

That’s not good ..!! Fixed or opening windows ..? They should still be under warranty. 

[Jilly]

1 minute ago, PeterW said:

 

That’s not good ..!! Fixed or opening windows ..? They should still be under warranty. 

The company have said they will replace them. One was an opening window, the other a leaf of a French door. 

 

On the WBS discussion, has anyone researched masonry stoves? They seem to have a pretty clean burn.

[SteamyTea]

1 minute ago, PeterW said:

There is also a whole load of issues with road vehicles and PM5/2.5 coming from brake disk and tyre wear which is not being reduced by the move from IC engines. 

Except 'one peddle driving' reduces the use of brakes, and tyre wear may be reduced because of the different power delivery to the road (this depends more on driving style).

But yes, the motor type does not solve all the problems, just reduces them.

[SteamyTea]

Just now, Jilly said:

On the WBS discussion, has anyone researched masonry stoves? They seem to have a pretty clean burn.

What do you think 'clean' means?  Non-visible?