Windows
For most of us our windows are the least insulated part of our house boundary. They are also expensive to replace, have environmental consequences to replacement and to what benefit? Some of this depends on how old the existing windows are and in what state of repair.If the windows are wooden with gaps then they may well be repairable by a joiner, if they are aluminium, like the one we are looking at, then if they are damaged they are damaged. If they are still wind and water tight then how do we make a decision about if it is worthwhile changing them?
There are several places you can start. I started by calculating how much energy (kWh) we are loosing through this window wall. This is done by getting hold of or estimating the U-value for the window. A U-value tells you how much energy (in Watts) is lost through one metre square area of the window for every degree Kelvin (equivalent for these purposes to a degree centigrade). The lower the value the less energy is being lost. For a window this can be a centre pain U-value, which will be better than a whole window U-value as the whole window includes things like openings and the metal spacers between the panes. All these allow energy (heat) through thus the condensation round the window on cold mornings.
So I want the whole window U-value, the size of the area (2.48 x 2.3m) and the average temperature difference between the outside and the inside. Inside is easy our house sits at an average of 18 degrees centigrade. Our outside value I googled the average temperature at our local weather monitoring station. It gave me 10degrees centigrade.
There are several places you can start. I started by calculating how much energy (kWh) we are loosing through this window wall. This is done by getting hold of or estimating the U-value for the window. A U-value tells you how much energy (in Watts) is lost through one metre square area of the window for every degree Kelvin (equivalent for these purposes to a degree centigrade). The lower the value the less energy is being lost. For a window this can be a centre pain U-value, which will be better than a whole window U-value as the whole window includes things like openings and the metal spacers between the panes. All these allow energy (heat) through thus the condensation round the window on cold mornings.
So I want the whole window U-value, the size of the area (2.48 x 2.3m) and the average temperature difference between the outside and the inside. Inside is easy our house sits at an average of 18 degrees centigrade. Our outside value I googled the average temperature at our local weather monitoring station. It gave me 10degrees centigrade.
How to do the calculation
The amount of heat lost is:
The whole window U value x the area of the window x the average temperature difference. This doesn't take inot account ait gaps.
Written this is: Uw x A x (T inside - T outside)
So our window U value is 5 Wm2/K
The Area is 4.5m2
T inside 18C
T outside 10
So Uw xA x (T inside - T outside) = 5 x 4.5 x (18 - 10) = 180 W/hr
So a year has 8760 hrs in it roughly
So if we loose 180 W/hr for 8760 hours gives a loss of 1576800 W/yr.
Our fuel bills usually come in kW or 1000W. so that is 1576.8 kW/yr.
This window is an extreme window, it is old, damaged and aluminium. Having said that this represents nearly 10% of our heating bill!
If we had double glazed window at todays specification it would have a Uw value of 1.5. Plugging this into the formula gives: 473 kW/yr, a saving of over 1000kW a year!
If we had a passivhaus standard window with a Uw value of 0.7 this would reduce our losses to 220 kW/yr.
However, window replacement is not a cheap option. The cost to replace this window in autumn 2012 to passivhaus standard would be about £2000. Interestingly the difference in cost between wood and uPVC is minimal.
The whole window U value x the area of the window x the average temperature difference. This doesn't take inot account ait gaps.
Written this is: Uw x A x (T inside - T outside)
So our window U value is 5 Wm2/K
The Area is 4.5m2
T inside 18C
T outside 10
So Uw xA x (T inside - T outside) = 5 x 4.5 x (18 - 10) = 180 W/hr
So a year has 8760 hrs in it roughly
So if we loose 180 W/hr for 8760 hours gives a loss of 1576800 W/yr.
Our fuel bills usually come in kW or 1000W. so that is 1576.8 kW/yr.
This window is an extreme window, it is old, damaged and aluminium. Having said that this represents nearly 10% of our heating bill!
If we had double glazed window at todays specification it would have a Uw value of 1.5. Plugging this into the formula gives: 473 kW/yr, a saving of over 1000kW a year!
If we had a passivhaus standard window with a Uw value of 0.7 this would reduce our losses to 220 kW/yr.
However, window replacement is not a cheap option. The cost to replace this window in autumn 2012 to passivhaus standard would be about £2000. Interestingly the difference in cost between wood and uPVC is minimal.
uPVC or wood frames
Superficially this looks like a daft question. Clearly wood is the more ecological material.Trying to find facts and research rather than rhetoric is realy difficult. However, the University of York have brought together a relatively balanced arguement at www.sustainability-ed.org.uk.
They look at sustainability from three angles. It's environmental impact, economic sustainability and how a product fulfills people's social needs. The outcome?
Enviromental impact - very similar other than water eutrophication (a measure of one aspect of water pollution) wood is about 30 time more polluting! uPVC produces about 75% more waste. Economically ( it doesn't state when these figures are from) it quotes "The uPVC industry emplys 530,000 people and contributes to sustainable development in the EU". By sustainable development they mean "no accumulation of man-made materials in nature, no accumulation of substances from the earth's crust in nature, biodiversity not reduced and human needs met fairly". Greenpeace disagrees with several of these points, which are set out with the uPVC industries responses.
So what did we do? Our 1965 house has uPVC windows throughout. It would not be enviromentally or financially sustainable to replace them all in wood, so to maintain continuity we went for uPVC.
They look at sustainability from three angles. It's environmental impact, economic sustainability and how a product fulfills people's social needs. The outcome?
Enviromental impact - very similar other than water eutrophication (a measure of one aspect of water pollution) wood is about 30 time more polluting! uPVC produces about 75% more waste. Economically ( it doesn't state when these figures are from) it quotes "The uPVC industry emplys 530,000 people and contributes to sustainable development in the EU". By sustainable development they mean "no accumulation of man-made materials in nature, no accumulation of substances from the earth's crust in nature, biodiversity not reduced and human needs met fairly". Greenpeace disagrees with several of these points, which are set out with the uPVC industries responses.
So what did we do? Our 1965 house has uPVC windows throughout. It would not be enviromentally or financially sustainable to replace them all in wood, so to maintain continuity we went for uPVC.
Trickle vents
Trickle vents on windows are required on building regulations on new build houses. Replacement windows it would appear that if they were present before they should be fitted but no clear regulation. So what do they do? They allow air in from outside ventilating the house. Do we need them? Well we need some form of ventillation to stop our houses getting stuffy and pollutants building up. So if there is no other source of ventillation then yes.
However, they do let outside un heated air into the house and are a source of draughts and depending on situation may be noisy. So if you don't need them then don't have them.
We have a heat recovery system in our house, and despite all out insulating modifications the house often felt draughty and cold. I blamed the heat recovery system and thought we had to live with it, until I blocked up two of the vents and transformed the feeling of the livingroom. I went on and did the same with all the other windows bar one - in the un ventillated porch - and the feel of warmth in the house improved significantly.
However, they do let outside un heated air into the house and are a source of draughts and depending on situation may be noisy. So if you don't need them then don't have them.
We have a heat recovery system in our house, and despite all out insulating modifications the house often felt draughty and cold. I blamed the heat recovery system and thought we had to live with it, until I blocked up two of the vents and transformed the feeling of the livingroom. I went on and did the same with all the other windows bar one - in the un ventillated porch - and the feel of warmth in the house improved significantly.
What else can we do?
How we cover our windows makes a huge difference to how much heat they loose. In the Spring to Autumn many days
