Thermal transmittance of windows: the definitive calculation

Thermal transmittance (technically known as the U-value) is the precise measure of how much heat or cold passes through the materials in an enclosure over a given period. When insulating your home, knowing the thermal transmittance coefficient of your windows is essential: the lower this numerical value, the better the insulation provided by the glass and frame. 

This means that a structure with a low U-value effectively prevents heat from escaping, keeping your home warm in winter and cool in summer.

The direct impact of insulation on your energy bill and everyday well-being

Keeping energy loss under strict control not only improves thermal comfort in your rooms, but also transforms your household finances. When you have inefficient windows and doors, the cost of heating and cooling your home skyrockets month after month.

• Financial savings: reducing heat loss drastically lowers your daily reliance on heating and air conditioning.
• Acoustic comfort: materials designed to prevent the transfer of cold and heat often block outside noise too, ensuring a good night's sleep.
• Sustainability: using less electricity or gas directly reduces your carbon footprint, actively helping protect the environment.

Improve your home's energy efficiency with Brikum materials and tools

When you decide to seal gaps, insulate walls, or repair old frames to improve overall thermal transmittance, having the right tools and equipment makes all the difference between a DIY job and a truly professional finish.

At Brikum, we provide everything you need to carry out these improvement projects with confidence. From the best thermal insulation materials, such as polyurethane foams and high-density sealants to prevent the dreaded thermal bridges during installation, to the most precise power tools on the market. Proper insulation is an investment that pays for itself in a very short time, and we can help you achieve it by working exclusively with the best brands in the industry.

The basics: understanding the U-value of your windows

The U-value of your windows measures the exact amount of energy lost through one square meter of window for every degree of temperature difference between the inside of your home and the outdoors. The lower this number, which you will always see expressed in W/m²K, the better the structure’s insulation, blocking winter cold and summer heat much more effectively.

Thermal transmittance explained simply

To understand it without complex technical terms, imagine that this coefficient is simply the rate at which heat escapes from your living room to the outside of your home. If the value is very high, the heat from the heater disappears quickly; if, on the other hand, it is low, the comfortable temperature stays right where you need it most.

The key difference between the U-value of the glass and that of the frame

The overall calculation never depends on just one component, since any window assembly is divided into two main elements that behave very differently in response to the climate:

• Glass (Ug): determines the insulation level of the glazed surface alone, which is usually the largest and most exposed area.
• Frame (Uf): measures the resistance of frames to heat transfer, whether they are classic wood, aluminum or polyvinyl chloride (PVC).
• The spacer’s effect on overall performance.

There is a third, almost invisible but entirely decisive factor known as the spacer (Ψg), the strip that separates the panes when you install double glazing:

• Metal edges: traditional aluminum spacers transfer the outdoor temperature very quickly and worsen the final result.
• Warm edges: modern plastic spacers interrupt metal conductivity and significantly improve the thermal comfort of the entire unit.

The mathematical formula: how to calculate the thermal transmittance of windows

To calculate the thermal transmittance of windows technically and accurately (the result of which is the Uw value), you must multiply the glass area by its U-value, add the frame area multiplied by its U-value, add the spacer length multiplied by its coefficient, and finally divide the entire result by the total opening area. 

Although it sounds like pure quantum physics, this is the mandatory European standard that ensures the product you buy complies with current energy regulations.

The final calculation using a weighted sum

The overall value (Uw) is the only figure you really need to care about when requesting a quote for a new enclosure, since it represents the combined performance of all the materials installed in the wall. If a manufacturer gives you only the glass's impressive coefficient but leaves out the frame's, they're hiding half the vital information from you.

Practical examples to understand the formula's result

Let's see how dramatically the figures change depending on the materials you choose for a standard-sized window:

• Poor insulation: an old aluminum sliding window without a thermal break and with single glazing usually has a disastrous value of five.
• Optimal insulation: a state-of-the-art PVC frame with triple glazing and argon gas can easily bring the value down to zero point eight, retaining heat wonderfully.

Spanish regulations and the Technical Building Code

Current law (CTE) requires newly built or fully renovated homes to become increasingly energy-efficient to reduce pollution and costs:

• Cold regions: in inland or mountain cities, regulations require installing windows with extremely low transmittance coefficients to prevent heat loss.
• Warm regions: in the Mediterranean coastal areas or the south of the Iberian Peninsula, the limits are somewhat more lenient, although exceeding them is always highly recommended to spend less on air conditioning during August.

Key materials for reducing thermal transmittance as much as possible

High-density PVC and aluminum with a thermal break are the star materials on the market for dramatically reducing thermal transmittance, especially when paired with low-emissivity glass containing argon gas in the cavity. Choosing the right combination of these three components is the most important and smartest step in protecting your home against severe weather.

PVC frames versus aluminum frames with a thermal break

The structural frame of your enclosure largely determines its energy performance in response to daily temperature swings:

• Plastic insulation: PVC is an excellent natural insulator that does not transmit cold, prevents water condensation, and requires very little maintenance over the years.
• Metal solution: aluminum is a great conductor, so it must incorporate internal plastic components (the famous thermal break) to stop the transfer of cold from outside to inside.

The vital role of modern glazing

Glass usually covers almost eighty percent of the total surface area, so choosing the best available technology is vital to prevent costly heat loss:

• Insulating cavities: installing double or triple glazing with sealed cavities filled with argon gas greatly slows the passage of cold.
• Invisible treatments: special solar-control and low-emissivity coatings reflect heat from the heater back into your home in winter and block the sun’s powerful rays in summer.

Get the best finishes and sealants at Brikum

There’s no point in buying the most efficient, most expensive window in the world if you leave gaps between the frame and the brick wall during installation:

• Expanding foams: in our online hardware store, you’ll find high-quality polyurethane cans for filling structural gaps and preventing treacherous drafts.
• Professional-grade silicone sealants: finish the exterior and interior joints with our long-lasting waterproof sealants to ensure a truly perfect seal.

Frequently asked questions about your window insulation

The most common questions about window thermal transmittance tend to focus on finding out the exact, ideal figure for each type of climate, whether replacing only the old glass is worthwhile for saving money, and how dreaded roller shutter boxes contribute to energy loss in a room. 

Knowing how to address these issues will save you a lot of money and quite a few headaches when comparing quotes from your installers.

What is the ideal value for a very cold climate?

If you live in areas where winters are harsh and nighttime frosts are frequent, you should always look for windows with a Uw value of 1.2 or lower. Getting below that technical threshold guarantees a warm home and huge savings on your gas or electricity bill from the very first month of use.

How can I find out what level of insulation I have now?

It’s not easy to tell the exact figure at a glance, but if you touch the glass or metal frame in the middle of January and they’re literally freezing, the value is extremely high and very poor. To get the precise figure for newly installed modern models, always check the energy efficiency label or the technical specifications provided by the manufacturer.

Is there any point in replacing just the glass without changing the frame?

It’s a highly inadvisable stopgap and often a waste of money. If you install spectacular double glazing in an old aluminum frame without a thermal break, the cold will keep coming straight through the metal, and condensation and black mold may even appear around the edges.

Does the roller shutter affect the overall structural calculation?

Yes, decisively. The roller shutter box is often the most overlooked part of renovations and the biggest source of drafts in Spanish homes. A poor-quality, hollow box without sound or thermal insulation can completely undermine the performance of a top-of-the-line window, so make sure it comes factory-reinforced.

Conclusion: invest in efficiency for a more sustainable and cost-effective home

Minimizing thermal transmittance is not just a matter of aesthetics to make your home look more beautiful, but one of the most profitable, logical, and sensible investments you can make to increase your property’s value and protect it financially against constant rises in energy prices. Understanding how these figures work gives you the power to choose the right materials for your renovation without anyone pulling the wool over your eyes.

The perfect balance between thermal comfort and long-term savings

Replacing exterior windows and doors is a significant upfront expense for any family, but the numbers are always in your favor:

• Real savings: the substantial monthly reduction in heating and cooling bills means the renovation practically pays for itself in just a few years.
• Quality of life: completely eliminating icy drafts and street traffic noise transforms the peace and comfort of your living spaces.

Make your full renovation project a reality with Brikum

Preparing your home to withstand even the most extreme temperatures is an exciting project when you have the right materials and support:

• Specialized catalog: visit Brikum to discover all the tools and equipment, polyurethane foams, and fillers you need to carry out your installations and minor repairs with complete confidence.
• Guaranteed success: always equip yourself with top-brand products and ensure a completely airtight installation that keeps the cold at bay, winter after winter.