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Window systems is a product for many years. It is usually bought with the intention to be used for 30 – 40 years and the replacement is connected with serious construction work. There is no room for error or corrections in such an investment. It is important to choose once but wise. But choosing an optimal window profiles system is a complicated venture – difficult even for the professionals. An investor can find different kinds of unverifiable or even contradictory information about the cons and pros of each system. A large amount of the information has marketing as its goal and sometimes such information are intentionally misleading. Moreover, checking the profiles in a ready made window is almost impossible. So, how to deal with such a situation?
Energy saving is one of the favorite discussion topics between the investors and the window sellers.
We will try to explain what is the mysterious thermal insulation rate, where does it come from and how this knowledge can be used.
First, let us explain what is the thermal insulation rate.
The easiest way to describe it is to say it is a value defining the amount of heat lost through one square meter of the window in a set conditions when the temperature difference between the outside and the inside is 1 * K (1°C). A unit of the thermal insulation rate is W/ m2 * K.
The lower the set Uw thermal insulation rate described as W/m2 * K, the better the window protects us from unwanted heat loss.
According to current law the maximum value of the thermal insulation rate Uw for windows in residential buildings should be:
- For buildings for which a construction permit was issued prior to 31.12.2008
Uw <- 2,6 W/m2 * K
- For buildings for which a construction permit was issued after 01.01.2009 r
Uw <- 1,7 W/m2 * K
Now we know what is the thermal insulation rate and we know the maximum allowed values for windows. It is now time to move on to another aspect. Is it worth it to pay for each 0.1 of the rate, to deal with the technical knowledge about windows, chambers and other window related awkwardness?
The practical use of the thermal insulation rate allows the investors to answer the question if a window having a certain value of the insulation rate and a given size, will prevent the loss of energy used to heat the building over a certain time.
Knowing the cost of 1 kWh of energy it is easy to calculate how much of the investors money will be lost because of the selected windows.
Just for fun and to make the investors realize the point, we shall calculate how much does actually this 0.1 of the Uw rate cost, or how much the investors can save by fighting for each 0.1 Uw and baying significant amount of money for this fight.
Let us assume that your window’s insulation rate is Uw = 0.1 W/m2*K, the window is 1 m2 large, and the temperature difference between the inside and the outside will be 20°C on average during the entire heating season, while the season lasts 4800 hours and 1kWh of energy costs 0.45 PLN.
The amount of energy W = 0,1*1*20*4800 = 9600 = 9,6 kWh = 9,6*0,45 = 4,32 PLN
This calculation show the value gained from fighting over 0.1 W/m2*K for each 1 m2 of a windows in the previously assumed conditions. Someone may say that 4.32 PLN is nothing!
Let us not take early conclusions.
W calculated only for a 0.1 difference, and what will happen if we reduce the rate by another 0.5?
And what about the window surface?
In an average M-4 size apartment the windows have a total surface of 13 m2 and in an average detached house it is around 25 m2!
Reduction of the thermal insulation rate by 0.1 for 13 m2 is a gain of 56.15 PLN annually.
Reduction of the thermal insulation rate by 0.1 for 25 m2 is a gain of 108 PLN annually.
Dear investor having a small house, if you are making a decision of putting windows which have U=0.8 instead of U=1.7 then you certainly will pay more for the windows but each year you will reduce your bills for heating by 1000 PLN.
We wish a pleasant calculation time to all the energy saving investors, and we encourage the rest to think it over and make a decision whether the fight for a lower Uw rate is worthwhile.