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Choose well

Window joinery is a product with a very long life cycle. It is usually bought with the intention to be used for 30 – 40 years, and its replacement involves serious renovation works. There is no room for mistakes and corrections with such an investment. You have to choose once and wisely, whereas selecting the optimal system of window profiles is a complicated undertaking – difficult even for professionals. There is a lot of information about the advantages and disadvantages of different profiles which an investor comes across at the market – such information is impossible to verify or compare and often contradictory. A large part of this information is purely of a marketing nature and sometimes purposefully misleading. What’s more, it is practically impossible to check the properties of profiles in a finished window which have been used for its production. How to deal with such a situation then?

Thermal insulation

Energy safety is one of the favourite subjects of discussion between investors and window sellers.

We will try to explain what it is and how the knowledge of a slightly mysterious thermal insulation rate can be useful for investors and what it results from in in each window.

Let’s explain first what the thermal insulation rate is.

It’ best to describe it as a quantity specifying the amount of heat lost through a square metre of a window in specified conditions, when the difference between outside and inside surroundings is 1 * K (1°C). The thermal insulation rate unit is W/m2 *K.

The lower the thermal insulation rate expressed in W/m2 * K and determined through tests for a specific window structure, the better the window protects us against undesirable heat losses.

In accordance with the applicable regulations, the maximum value of the Uw thermal insulation rate for windows in residential buildings should be as follows:
- for buildings for which the construction permits were issued before 31 December 2008 
Uw <  -2,6 W/m2 * K
- for buildings for which the construction permits were issued after 1 January 2009
Uw <  -1,7 W/m2 * K

We already know what the thermal insulation rate is, we know its maximum values permissible for windows. Now we should move on to the most important point. Is it worthwhile paying for each 0.1 of this rate, struggling with the knowledge about glass panes, chambers and other similar window wonders?
The practical application of the thermal insulation rate allows investors to answer the question whether a window for which we know the thermal insulation rate, with a specific surface, in a specific time period and at a specific difference between outside and inside temperatures can protect us against losses of energy used for heating rooms.

When we know the cost of kWh of energy, it is very easy to calculate how much of the investor’s money will not just fly through the window you purchase without any purpose.

For fun and also for investors to understand better what it is about, let’s calculate how 0.1 of the Uw rate can translate into investors’ wallets, i.e. how much you will save fighting for each 0.1 of the Uw rate and paying considerable amounts of money for it.
Let’s assume that the thermal insulation rate of your window is Uw = 0.1 W/m2*K, its surface is 1 m2, the average difference between inside and outside temperatures will be 20 °C during each heating season, the season lasts 4800 hours and the price of  1kWh of energy is PLN 0.45.

The amount of energy W = 0.1*1*20*4800 = 9600 = 9.6 kWh = 9.6*0,45 =  PLN 4.32

This calculation shows the value the investor gets from fighting for... 0.1 W/m2*K for each 1 m2 of a window at the assumed conditions. One may say now that PLN 4.32 is ridiculously next to nothing!
Not so fast about such hasty conclusions.
We have only calculated by 0.1 what if we decrease the rate by another 0.5?
What about the surface of windows? 
The surface of windows in an average M-4 flat is 13m2 and about 25m2 in a single-family house!
Decreasing the thermal insulation rate by 0.1 for 13 m2 of windows gives the savings of PLN 56.16 per year.
Decreasing the thermal insulation rate by 0.1 for 25 m2 of windows gives PLN 108 per year.

So, dear investor with a small house, if you decide that you will install windows with the rate of U=0.8 instead of windows with the rate of U=1.7, you will surely pay more for the windows, but in return, you will save about PLN 1000 each year at the heating bills.

We wish energy-saving investors pleasant calculations and encourage everyone to think this through and decide whether it is worthwhile fighting  for a lower Uw of your windows.