What windows for a passive house - basic concepts.
Every house builder asks himself a question at some point - which windows for a passive or energy-saving house will be appropriate?
The question is important, because windows are one of the weakest elements of a low-energy house. To choose, we first need to know the basic concepts:
1. window heat transfer coefficient, i.e. Uw. The lower the better (for passive houses recommended less than 0,8), but remember, that:
a) U applies to the entire window, and thus also to the glass and frames (profiles). Often sellers provide the value of thermal transmittance for the glazing unit, which will almost always be lower than the thermal transmittance of the profile. Because each window has different proportions of glass and frame, The Uw for each window will be different. It is also good to know the Uw glass and Uw profile separately.
b) Unfortunately, we cannot check the accuracy of the data provided by the window manufacturers. It is not uncommon, when a window made of the same profile and glazing unit in two different companies has different Uw, everything is a matter of research - one Polish company (I'll keep silent about which) She "tested" her windows at an institute in the Czech Republic, where it obtained amazingly good results considering the ingredients used in the production of windows. Please notice, that there are over a dozen important producers of window profiles, and window manufacturers in Poland - several hundred if not more. Knowing the base profile, we can check its parameters with the manufacturer - the risk of distortion is then lower.
2. energy transmittance coefficient (coefficient g) shows how much of the solar energy falling on the surface penetrates through the glass into the interior of the house. Here, the opposite - the higher - the better. In traditional technologies, as the Uw is lowered, the g coefficient is also reduced, which results in lower solar gains. That is why special windows are produced for passive houses, which at low Uw offer high values of the g-factor. Unfortunately, they are still quite expensive. Of course, installing such windows only makes sense on the south side or a slight angle to the south.
One note - sellers like to talk about triple or even quadruple packets, outdo each other in the number of chambers in the profile (in fact, often the difference between the six profile and e.g.. there is practically no eight-chamber one). This is only sales marketing, we do not care about it and, above all, we look at the above parameters.
Caution! However, buying windows with the most demanding parameters is often unprofitable.
We have two variants of windows - one with an average Uw = 0,9 and others about Uw = 0,7. The second option costs, for example 4 thousands of zlotys more and will allow us to save the order 100 PLN per year - the cost of returning such an investment is theoretically 40 years . So it is always worth calculating all this for your home and your heating source. There are many such calculators on the Internet (e.g.. on the manufacturers' website), and if we don't have the head to count it ourselves - let's ask the seller. It will be an approximate estimate, but he will always give us some idea of the matter.
If we want accurate calculations (taking into account our specific house design and its location) - let's use the OZC programs (but this is for real enthusiasts, or let's order such a service - it will also be useful for making other construction decisions (e.g.. insulation thickness)
Finally, a piece of advice: don't spend weeks looking for the best windows - spend half of that time looking for a crew, which will install them correctly for you. Improper or inaccurate assembly will cause, that all the financial effort put into buying good windows will be wasted.
I will also write about assembly in other articles.