If it’s all about the green, part 1…
A question was asked after publishing Is there such a thing as breaking even? “Does the carbon cost for windows repay sooner or later than the financial cost?”
I am going to find that a difficult question to answer, as we had some substantial building work done to put in a French door and take out a window and a French door on opposite ends of a wall and have a single big patio door put in.
So, let’s having a look at the energy saving compared to the old windows against what we can expect given the u-values for the new windows.
A quick reminder of what a u-value is: u-values are the rate at which heat is lost from a surface and is dependent on a few things such as materials, air gaps, and is measured in Watts per square meter a degree Kelvin or W/m2K. (Kelvin’s are used here as they are the scientific unit (also known as the SI unit) for temperature, unlike the Celsius unit and you then don’t need a pesky ° symbol when typing this out). The lower the u-value, the better.
From the previous article, we know that the temperate loss for the new windows will be, so we’re looking to compare that to the old windows. Some people who’ve studied this as part of their day job work this out to be: Annual kWh Savings = (u-value of old window – u-value of new window) * area of the window in question in m2 * “Degree Days” * 24 / 1000.
The first three terms of this equation are explained in full in the equation! The “24/1000” converts u-value into kWh. The “Degree Days” is a new term here and refers to the average temperature for the location in question. A good source is the Enman degree day data, which show how much a building needs warming up and cooling down based on the external temperatures. In the south of England, that’s approx. 15.5°C and can be sourced from the https://www.degreedays.net/#generate site or https://www.gov.uk/government/collections/digest-of-uk-energy-statistics-dukes. Of course, from the last post, you can see the UK is a properly seasonal place…
I hate to say this, but a spreadsheet is our friend here.
| Window | Area m2 | Old u-value W/m2K | New u-value W/m2K | Degree days °C | Annual saving kWh |
| Spare room north | 1.019 | 5.8 | 1 | 15.5 | 2.199 |
| Spare room south | 1.019 | 5.8 | 1.1 | 15.5 | 2.419 |
| Master bedroom | 3.500 | 2.8 | 1.4 | 15.5 | 5.104 |
| Ensuite (estimated area) | 1.000 | 5.8 | 1 | 15.5 | 2.158 |
| Bathroom | 1.019 | 4.8 | 1.1 | 15.5 | 2.002 |
| Sam’s office | 1.019 | 2.8 | 1.1 | 15.5 | 1.168 |
| Jon’s office | 1.019 | 2.8 | 1.1 | 15.5 | 1.168 |
| Landing 1 | 1.019 | 2.8 | 1 | 15.5 | 1.062 |
| Landing 2 | 1.019 | 2.8 | 1 | 15.5 | 1.062 |
| Kitchen doors | 3.471 | 2.8 | 1 | 15.5 | 3.615 |
| Kitchen by doors windows | 0.819 | 2.8 | 1 | 15.5 | 0.853 |
| Kitchen south not sink | 1.650 | 2.8 | 1 | 15.5 | 1.719 |
| Kitchen south by sink | 0.984 | 2.8 | 1.4 | 15.5 | 1.435 |
| Lounge French doors | 3.374 | 2.8 | 1 | 15.5 | 3.514 |
| Lounge windows by French doors | 1.649 | 2.8 | 1 | 15.5 | 1.718 |
| Lounge patio doors | 3.488 | 2.8 | 1.4 | 15.5 | 5.086 |
| Gym | 1.586 | 4.8 | 1 | 15.5 | 2.831 |
| Cloakroom | 0.643 | 4.8 | 1 | 15.5 | 1.148 |
| Grand total | 40.261 |
This is just plugging in the values in the table and getting the annual saving, then adding up the total.
The old u-values are estimated – a 2009 window is between 2.6 and 2.8 W/m2K, where the windows were failing, I have made them 4.8 or 5.8 depending on the level of failure. The ensuite windows are an estimate – these are shaped windows positioned in an opened area of roof. We know they were leaking.
Basically, the savings per annum in terms of power are going to be 40.261 kWh. Which is £10.47 per annum at £0.26 a unit. I could have used a different number for the old u-value, but mostly the windows were just about still working.
So, does it cost in quicker in terms of financial cost depends on how much the windows have cost and the cost of electricity or gas. The lower the u-value, generally, the more expensive the window. As you can see here, we had many windows being done! And that doesn’t include the building work.
What we are seeing is much better heat retention in the rooms where things were really bad and in every room, the energy needed to lift the temperature in the room is much less – in our bedroom, we’ve turned down the thermostat to make it more comfortable. In the spare room, where the windows were completely blown, the room holds its temperature beautifully. For us, with our EPC B rated house, this is more impactful than for someone in a house with badly insulated walls, floors, ceilings, and rooves. It should mean we retain our thermal envelop.
It’s also much quicker to vent the rooms, due to changing how the windows open, and significantly more light comes into the rooms.
But basically, it is going to take us a while, based on these values. And it is only worth it if your walls have a good u-value too, otherwise the heat leakage will come through the lower insulated path…
I’m tidying this up as I am sitting with the outside temperature hitting -2°C with the heat pump barely ticking over, so I think it’s been worthwhile.
Posted: January 3rd, 2026 under Driving off the grid.