What is the heat transfer coefficient windows (Uw)?

The heat transmission coefficient, denoted by the symbol Uwis one of the most important technical parameters to be taken into account when selecting the windows. It defines, how much heat escapes throughout window per square metre of surface area with a temperature difference of 1 degree Kelvin (K). Im lower Uw coefficient, including improved thermal insulation windowsand therefore - less heat loss and lower heating bills.

It is worth knowing that Uw is the total value, which includes both the frame and the insulating glass unit. It consists of two other parameters:

• Uf - the thermal transmittance of the window frame. frame),

• Ug - the thermal transmittance of the glazing. glass).

To calculate Uw accurately, the following is taken into account percentage of individual elements windows, their surface and their thermal conductivity. This is why even the best glazing will not guarantee a low Uw if a poor quality frame profile is used.

Examples of Uw values and what they mean in practice:

By comparison: contemporary requirements for new buildings in Poland (as at 2025) specify that Uw must not be higher than 0.9 W/m²K. For passive houses, the recommended value is 0.6-0.8 W/m²K.

In practice, this means that by choosing windows with a low Uw coefficient, we not only invest in thermal comfortbut above all in long-term savings. Heat loss through partitions, including through leaky windowsmay be responsible for even 25-30% of the total heating energy demand.

Why is Uw factor crucial in 2025?

In 2025, the topic of energy efficiency windows is becoming even more important than ever. Rising energy prices, climate change, new building regulations and greater environmental awareness are making low heat transfer coefficient Uw is becoming not so much an option as a necessity for investors and homeowners.

Reduced heating bills

The simplest and most noticeable effect of low Uw is real financial savings. Windows with a high U-value can "drain" hundreds of kWh of energy from a building every year. The lower the Uw, the lower the heat loss and thus the lower heating costs, by up to 20-30% per year. At a time when gas and electricity prices are unpredictable, this makes a huge difference to the household budget.

Year-round thermal comfort

Good windows not only lower bills, but also higher quality of life. Windows with low Uw effectively retain heat in winter, but also protect against overheating in summerespecially if they are fitted with solar control coatings. This ensures that the temperature in the home remains stable and that householders do not have to agonise over the chill at the window in winter or the stifling heat in summer.

Energy-efficient and passive construction - a standard, not a luxury

There is no longer any place for outdated solutions in modern construction. Energy-efficient and passive houses is the future of the real estate market. In order to obtain the relevant certificates or subsidies (e.g. from the Clean Air Programme), investors have to choose windows with very low Uw - usually below 0.9 W/m²Kand in passive houses even less than 0.8 W/m²K.

Thanks to modern production technologies - such as those used by the GM PLAST - Achieving such performance is entirely possible, without compromising on aesthetics or durability.

New regulations in 2025 - what has changed?

In accordance with the current Technical Conditions in force from 2021 and tightened in 2025, all newly constructed buildings must be nearly zero-energy buildings (nZEB). This means not only modern heating systems, but also windows with very good thermal performance.

From 2025, in practice:

• Maximum Uw coefficient for façade windows must not exceed 0.9 W/m²K,

• For roof windows, the limit is 1.1 W/m²K,

• More and more banks and institutions are making the granting of a green loan conditional on the use of energy-efficient windows.

In a nutshell. if you want to build legally and in line with trends, you need to bet on windows with low Uw.

Which Uw values are considered good and which are considered excellent?

Choosing new windows, it is worth paying particular attention to the Uw heat transfer coefficient, as this largely determines the energy efficiency of a building. But what values are considered goodwhich can be described as excellent?

Below is a practical table of the most commonly used Uw thresholds:

Uw recommendations for different building types:

🔹 Residential block (thermal modernisation):
If you mention windows in a large-panel block of flats, the recommended Uw value is approximately 1.0 W/m²K. In such buildings there is often no need for the most expensive windows passive, but it is worthwhile to ensure that the insulation is clearly improved over the old wooden windows.

🔹 Single-family house (modern construction):
In the case of house construction, it is worth choosing windows o Uw less than 0.9 W/m²K. This will ensure thermal comfort, lower heating costs and compliance with current standards. In houses with recuperation and heat pumps, windows will work best with Uw 0.7-0.8 W/m²K.

🔹 Office or commercial premises:
Commercial buildings are increasingly using energy-efficient windows. For a good energy balance and savings on heating, we recommend the values of less than 1.0 W/m²K, especially if the facility has air conditioning and zoned heating systems.

🔹 Passive / zero-energy house:
There are no compromises here. In order to meet the stringent passive house standards, it is necessary to windows with Uw ≤ 0.8 W/m²Kand preferably 0.6-0.7 W/m²K. Only such parameters allow the building's thermal energy requirements to be zero.

What factors influence the heat transfer coefficient?

The Uw value is influenced by many elements - not only by the glassbut also the profile of the frame, the type of materials used and even the method of installation. It is worthwhile to understand each of these factors, as together they form a coherent insulation systemwhich determines the comfort and energy efficiency of the entire windows.

1. window profile type - PVC, aluminium, wood

• PVC (plastic) - is currently best price/thermal performance ratio. PVC profiles with multi-chamber construction and thermal inserts achieve very low Uf (up to 0.92 W/m²K). Our systems are an example Kömmerling and economical Decco series.

• Wood - natural material with good insulating properties, but requires more frequent maintenance. Uw can be very good, but at the expense of thicker frames.

• Aluminium - aesthetically pleasing, durable, great for large glazing, but without thermal breaks have a high Uf. Modern systems with thermal insulation (e.g. with a polyamide spacer) significantly improve this parameter.

2. type glazing - 2-glazed vs 3-glazed, low-e coatings

• Package 2-pane - most common in older windows. Ug approx. 1.1-1.3 W/m²K - does not meet current standards.

• Package 3-pane (triple-glazed) - used in modern windows, often with noble gas (argon, krypton). Ug even 0.5-0.6 W/m²K.

• Low-E coatings - special layers that reflect heat into the building. Significantly improve glazing insulation without changing the thickness of the glass.

3. Warm frame spacer

It's glazing separator in an insulating glass unit, often underestimated but with a real impact on the final Uw.

• Traditional aluminium frames conduct the cold, leading to the formation of thermal bridges.

• Warm frames (e.g. stainless steel, plastic or composite) reduce heat loss along the edge of the glass, improving Uw by up to 0.1-0.2 W/m²Kand also eliminate the risk of windshield dew.

4 Method installation - traditional vs. warm fitting

Even the best windows lose their properties if they are badly fitted.

• Traditional assembly (for mounting foam and anchors) is the minimum today, which in many cases does not ensure full tightness.

• Warm installation with vapour-proof and vapour-permeable strips (so-called layering) eliminates thermal bridges between the frame and the wall, significantly reduces heat and moisture loss.
  At GM PLAST we use both standard and warm installation - always advising the customer on the best solution for the building.

  How to lower the Uw coefficient when buying new windows?

  When buying new windowsMany investors focus on the appearance, price or completion date. Meanwhile a key technical parameter which has a real impact on thermal comfort and heating bills is precisely the heat transfer coefficient Uw. So how do you approach the purchase wisely to choose windows with the lowest possible Uw?

  1. Choose the right window system

  Not all windows are created equal. Which profile system and glazing package you choose, has the greatest impact on the final Uw value.

  • For customers who value energy efficiency and modern design, we recommend systems Kömmerling - are premium windows that, in triple-glazed variants with warm frames, reach up to Uw of 0.7 W/m²K.

  • For those looking for a reasonable compromise between price and qualityprofiles are the ideal choice Decco - robust, aesthetically pleasing and attractively priced, with Uw within 0.9-1.0 W/m²K.

  2. trust in professional advice and installation

  Parameters windows is one thing - but only professional selection and installation guarantees that the low Uw will also be maintained under actual conditions.
  At GM PLAST we always offer technical support in selecting the glazing package, frame and installation method.
  We advise on whether, for a given building, a better passive windowWhether they are energy-efficient, which installation method works best and whether it is worthwhile to use, for example, an energy-saving system. warm frame or sunscreen coating.

  3. Don't skimp on quality - it doesn't pay off

  Purchase of the cheapest windows with poor performance is an apparent saving.

  • Such windows have high Uw (1.3 W/m²K and above)which translates into greater heat loss and higher bills for years to come.

  • In addition, they wear out faster, have inferior seals and are less weather-resistant.

  Instead, it is better to invest once - in tested profiles, triple-glazed packages, warm frames and professional installation. The difference in price can pay for itself after just a few heating seasons. In the long term, you save energy, money and... nerves.

  Summary - what to keep in mind when choosing windows with good Uw

  Selection windows with a low Uw heat transfer coefficient is the key to comfort and economy in any building. Here is Top 5 things to keep in mind when buying windows:

  • Choose windows with certified low Uf profiles - PVC, wood or aluminium with thermal breaks.

  • Opt for modern glazing packages - preferably triple-glazed with low emissivity coatings and warm spacer.

  • Pay attention to professional installation - preferably so-called warm installation eliminating thermal bridges and ensuring airtightness.

  • Don't skimp on quality - good windows are an investment that pays off over the years thanks to lower heating bills.

  • Benefit from expert advice and service - choose a proven manufacturer and contractor who can help you choose the optimal solution.

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  If you're looking for windows that combine high quality, state-of-the-art technology and value for money - then you're in for a treat. Contact GM PLAST!
  Our experienced team will be happy to advise you on the best solutions and help you order windows tailored to your needs. Together, we will ensure the warmth and comfort of your home or office.