How to evaluate the quality of European windows? Differences between US and EU specifications

how to evaluate the quality of the european window, window specification europe vs usa

The classifications and technical parameters should make it easier for the customer to understand the specs of a particular window, but when someone is following both the European and U.S. markets, they may feel confused by the plethora of technical terms: U-Value or U-Factor? Ug or Uf? Our dictionary will help you grasp the most commonly used parameters.

After reading this article you will know:

window specs used in europe iconography
window specs used in the usa iconography

U-Value/U-Factor (both terms are used interchangeably) – the lower the value, the better the window insulates heat. The size of the glazing plays a big role in lower U-Factor results, so profiles with a larger glazing area achieve better performances.

What is the U-Factor exactly? U-Factor determines how much heat is able to pass through 1 ft² of the flat surface of a window when the difference in air temperature inside and outside the room is 1 Fahrenheit. U-Factor is expressed in BTU per square meter per hour per 1 Fahrenheit (Btu/(h⋅ft² ⋅°F).

Its equivalent in the metric system is Uw. It is expressed in Watts per square meter per Kelvin (W/m²K).

Uw consists of the Ug (thermal transmittance of the glass) and Uf (thermal transmittance of the frame), which indicate how much heat escapes through the glazing unit and the window frame. Their area share is multiplied by the total area of the window to get the Uw which is most commonly used in the window specifications.

However, the Uw value is also affected by additional factors, such as the use of a warm edge spacer in the glazing packages, or additional insulators in the frames.

What if only European Uw values are given in the window specification?

It’s simple! You can get the U-Factor by the American scale by dividing Uw, e.g. 1.2, by 5.678, which in this case equals U-Factor 0.21.

U-Factor windows and the US climate

According to the recommendations of the international ENERGY STAR® program, the following U-Factor values are recommended in the States depending on the assigned zones:

usa climate zones according to energy star window specification specs iconography

R-Value heat resistance coefficient – with a higher R-Value, the window’s resistance to heat flow increases, and thus the structure is a better thermal insulator. The recommended R-Value for a window is usually higher than 5, but you don’t have to worry since the R-Value of triple-glazed windows available on the market is usually around 3-5.

What is R-Value exactly?
R-Value is a measure of an insulating material’s thermal resistance, therefore, it is estimated by the ratio of a given material’s layer thickness to its thermal conductivity λ.

So what is the difference between U-Factor and R-Value?

Both values are used to determine a window’s thermal performance and are the mathematical reciprocals of each other, but U-Factor uses scientific calculations to measure the level of heat transfer through a window, and R-Value measures the insulation capacity of a particular material and its ability to reduce heat transfer

how to calculate r-value based on u-value window specification

Specifications related to light and solar energy

Solar Heat Gain Coefficient (SHGC) – here, the choice of the window depends on the location of your investment – a window with a high SHGC index is more effective in collecting solar heat in the winter, and a profile with a low SHGC is more effective in reducing the heating of the room by the sun in the summer.

What is SHGC exactly? Solar Heat Gain Coefficient specifies the transmittance of the sun’s energy through the entire window (note that the European g-factor takes into account only the glazing and therefore SHGC is not its equivalent). This coefficient represents the fraction of solar radiation that can be transmitted through or absorbed by the window. It gives an idea of the window’s performance in terms of the amount of solar heat and sunlight that will enter your room.

u-factor and shgc recommended by energy star for windows

Another light-related parameter found in the U.S. market is Visible Light Transmission (VLT), which is a measure of the amount of visible light coming through a window. This measurement is not expressed in percentage, but is a rating between 0 and 1, with 0 blocking visible light and 1 allowing all visible light into the room.

an example of vlt of a window

Europe, however, uses G-Value, sometimes also called G-Factor or Solar Factor, this coefficient determines how much solar energy will enter the room through the glass and heat the area. The G-Value of Polish windows is approx. 50-55% in accordance with WT 2021.

What is G-Value exactly? The G-Value is the ratio of total energy transmittance to incident solar energy on a given partition.

Another parameter used in Europe is Light Transmittance (Lt) the higher its value, the more light enters the room. Current windows should have a Lt of 70-75%, but it is also possible to choose a profile with specially tinted glass, which will let less light through and reduce interior overheating.

What is Lt exactly? This coefficient indicates how much natural light is let through the glass into the interior. Lt largely depends on the thickness of the glass and its type. It is also worth noting that, for example, for a window on the north and south sides of a house, one usually has different expectations regarding light transmission.

Sound-related specifications

Weighted Sound Reduction Index (Rw) – the higher the Rw, the better the noise insulation of the window is. Rw is expressed in decibels and used in both Europe and the USA.

The glazing unit plays the greatest role in soundproofing, with layered glass being the best insulator, but the material, type of frame and careful installation are also important. To increase noise reduction, you can use soundproofing tapes or foam insulation.

recommended rw of a window window specification iconography

Another specification used in the US is Sound Transmission Class, which is an integer that indicates how well a window attenuates airborne sounds.

sound transmission class window specification iconography

Specifications related to air permeability and waterproofness

Air Leakage (AL) – this parameter is measured on a scale from 0.1 (best leakage) to 0.3. 

What is air permeability exactly? It determines the amount of air that gets through the gaps in the window. Air Leakage is calculated by measuring the amount of air (in cubic feet) that can pass through a window per minute, and then dividing that amount by the total area of the window.

classification of air leakage of a window iconography window specification

In Europe, the UNE-EN-1026 standard divides air permeability into classes 1-4, where 4 is the most airtight construction, it provides more heat in the room, but at the same time makes ventilation harder. European manufacturers usually offer windows categorized in classes 3-4.

air permeability classes iconography

In the US, Water Penetration Resistance (also called Watertightness) is expressed in psf (per square foot), it defines the highest allowable pressure caused by water that a window can withstand.

More details about this clasification can be found on the official website of the Fenestration and Glazing Industry Alliance.

air leakage and watertightness classes according to NAFS iconography

The European standard EN 12208 specifies 10 classes of watertightness, from the least watertight class 1A to class 9A along with the special class EXXXX (the pressure value is entered in place of X). Windows from class 1A-A3 are practically non-waterproof, for this reason, Polish windows are usually categorized as 4A-9A or EXXXX.

What is the watertightness of windows exactly? The watertightness of a window determines at what liquid load water will begin to penetrate the structure.  

The durability of the window is tested in a laboratory, the strength and speed of the wind to which the window is exposed during the test is also an important factor. In European norms, this resistance is expressed in Pa (Pa conveys the pressure used for the particular test), for illustration, a class 9A window is subjected to a test pressure of 600 Pa and a wind speed of 115 km/h for 5 minutes.

classes of watertightness in europe iconography

Have you ever caught yourself removing an NFRC sticker from a construction product and wondered what is the point in all of those numbers?

Now you don’t have to worry about it anymore since you know the basics! Above all, this knowledge will save you and your customer from potential problems resulting from the purchase of windows with specs that are not suitable for a given building, state or climate zone.

NFRC window sticker example window specification

about author

Bartek Michalak

Digital marketing manager at, a company from Poland that specializes in the export of windows and doors in the B2B segment to countries like Germany, France, UK, but also the USA or Seychelles.

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faq – who are we and what do we do?

1.What will I gain from working with you?

First of all, you’ll save your TIME. We will do the whole work with ordering doors, windows, gates, and blinds for you. Why should you outsource it to us? 
  • You will get products in a quality which was checked by us.
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We aren’t just a traditional producer. We order the production of doors, windows, gates, and blinds inside of a network of selected and verified suppliers in terms of production punctuality, their financial condition, and the quality of products. We call ourselves the broker on a polish joinery market. We’ll do all of the arduous work with ordering the joinery from Poland for you. We’ll choose relevant products at an affordable price, organise safe transport and we’ll deliver the product to any address in the world. 

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We do not have any branches in other countries. Our office and a magazine are in Poland in Oleśnica (30 km from Wrocław).We overcome thousands of kilometers separating us from our clients owing to modern technologies. Thanks to that you’ll save your money and time because we don’t have to add any extra costs to the services or products to maintain expensive offices all over the world.

Delivery time, payment, installation, warranty

1. What is the delivery time on

The time of order processing is highly dependent on its specification. For example, white PCV windows, standard colors (anthracite on one side) can be ready for pick up or a shipment from 7 working days. The waiting time for nonstandard PCV colors is about 4 weeks depending on the color. The production time of PCV doors is also about 4 weeks. Aluminium doors and windows - from 3 weeks, wooden windows from 6 weeks. Furthermore, the blinds about 2 weeks, garage doors - about 3 weeks, metallic doors from 4 weeks. You’ll find estimated production time next to every product on its subpage.

2. How does the payment look like? Do I have to pay the whole amount immediately?

We suggest two forms of payment: 1. 50% of advance for production of the product and 50% shall be paid before picking up/ sending the goods. 2. 100% payment in advance. This form is much more favorable because we give you an additional 5% discount depending on the group of products.

3. Will I buy the joinery with an installation on

No, on we sell only windows, doors, blinds, and gates, which we deliver to every place in the world. The installation of the windows depends on the client. We do not offer installation service because our offer is not directed to the building companies, installers, developers, and realtors that can install the joinery by themselves.

4. If the windows would be installed by a different company, do the products keep the warranty?

Yes, the only condition is installing the product in accordance with the art of assembly and applicable standards in the country of the installation in terms of the warranty.

5. Do the products that I buy have a foreign warranty?

Yes, the only condition is installing the product in accordance with the art of assembly and applicable standards in the country of the installation in terms of the warranty.

How to evaluate the quality of European windows? Differences between US and EU specifications

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