Windows are a crucial element of our homes, significantly impacting our comfort and energy efficiency. In this article, we’ll delve into the importance of windows, their influence on our indoor environment, and how to select the best ones for your project.
2 Reasons Why We Need High-Performance Windows
- Energy Efficiency – High-performance windows can save up to 30% on heating and cooling energy. Poorly insulated windows with inadequate airtightness can feel like you’re throwing money out the window. Investing in high-performance windows not only enhances comfort but also significantly reduces energy costs, making them a smart and cost-effective choice for any home.
- Comfort – You will literally FEEL the difference between high-performance window and conventional one. I`m sure you know the feeling of sitting near a window and suddenly feeling cold air blowing in, making one’s hands freeze. Passive House windows eliminate this issue, preventing cold drafts, uneven room temperatures, and outside noise. Instead, you will enjoy a consistent, comfortable indoor climate.
The 2 Factors Defining the Window`s Performance.
- Airtightness – In conventional buildings, radiators or other heating units are typically placed below or around the windows to compensate for the cold air entering from the outside. They serve two purposes: actively heating up the incoming cold air and acting as a heat barrier to maintain an even room temperature. However, in a Passive House, those heating units aren’t necessary because the high-performing windows are so airtight that cold air doesn’t penetrate through them. This superior airtightness eliminates drafts and contributes significantly to the overall energy efficiency of the building.
- Insulation – Comfort in a home is greatly influenced by the differences in surface temperatures. The Passive House standard addresses this by measuring the Radiant Temperature difference in Kelvins (K). A comfortable value is defined as below 4.2K, meaning the difference between the surface temperature of the window and the indoor air shouldn’t be more than 4.2K. In a conventional house located in a cool-temperate climate with standard windows (U value of 1.6 W/m²K), if the internal temperature is 21°C and it’s -14°C outside, the Radiant Temperature difference is 5.5K. This significant temperature difference can create drafts and an uncomfortable living environment. In contrast, a Passive House with triple-glazed, high-performance windows (0.8 W/m²K) achieves a Radiant Temperature difference below 3K. This substantial improvement not only reduces heat losses but also enhances comfort, providing a more stable and pleasant indoor environment.
Anatomy of a Window
It`s not just a window!
When we talk about windows, we need to understand each component of the unit to fully grasp its function and its impact on energy performance.
Key components of a High-Performance windows:
- Glazing: The glazing unit itself, often multi-layered, where triple glazing is common to enhance insulation.
- Frame: The material and design of the frame significantly impact thermal performance, where narrow and deep frames are preferable. When it comes to the materials… with any of the common window materials, e.g. uPVC, wood, and thermally broken aluminum frames, can be achieved good Uf values.
- Spacer: The spacer separates the layers of glazing. Warm-edge spacers, often made from materials like plastic, help minimize thermal bridging and prevent condensation from forming around the glazing edge.
- Gasket: High-quality gasket are essential to prevent air leakage and ensure the window is airtight, which is a critical factor in Passive House design.
- Thermal Breaks: Found in frames, thermal breaks are barriers made from low-conductivity materials that separate the interior and exterior parts of the frame, further reducing the heat losses.
- Low-E Coating: A microscopically thin, transparent coating applied to the glazing that reflects heat back into the room, improving thermal efficiency. In warm and hot climates can be used coatings that minimize the solar loads.
- Gas Fill: The space between glazing layers is often filled with inert gases like argon or krypton, which are more effective than air at reducing heat transfer.
Below is an image of Alumil that is a certified Passive House Component where you can see all of the above components illustrated.
Passive House Windows: Which Values To Look For
- U-Value: Thermal transmittance, also known as U-value, is the rate of transfer of heat through a component. This component can be both a single material and multi-layered composite. In other words – how much heat can be transferred trough. The lower the value, the less heat is transferred so the better the insulation is!
- g-Value: Also called SHGC (solar heat gain coefficient), the g-value is the sum of the energy transmitted from direct solar radiation and from secondary heat emissions from the outside.
So, one stands for the heat losses and the g-value for the solar gains through the window. Therefore, we could say that the perfect window offers low U value and high g-value. Then we will have almost no heat loss and significant heat gains through the windows.
How to Calculate The U-value Of Windows?
For this calculation we use the following formula:
To easily calculate that you need to first understand what each latter stands for in the formula and where to get the numbers from:
- Uw, installed – the thermal transmittance of a window after installation.
- Ug – the thermal transmittance of the glass of a window. We get this value from the window supplier.
- Uf – the thermal transmittance of the frame of a window. We get this value from the window supplier.
- A – Area.
- L – The length of the thermal bridge in the spacer or the length, i.g. perimeter, of the window.
- Ψ (Psi) – thermal bridge value. It is a linear thermal transmittance measure in W/(mK). It is not a material-specific parameter but rather depends on the type of window installation. The Ψ of the spacer is provided by the manufacturer whereas the Ψinstallation value we get from software (like Flixo) or we can work with references.
Once you have all those numbers collected, it`s just a matter of calculating it.
Why Is It So Important?
Gathering all calculations (installed windows, thermal bridges, seasonal data, etc.) is crucial for understanding the energy performance of your building. This step is mandatory for every Passive House project, and we use the PHPP software to accomplish it. Every Passive House Designer/Consultant has an in-depth knowledge of the building parameters influencing energy efficiency and how to calculate these to determine energy use.
For example, if you are designing a house and find your heating demand too high you will want to optimize it to have lower energy bills, while not overpaying for high-quality components. With PHPP, you can easily adjust the numbers to simplify your decision-making and of course see if the building meets the Passive House criteria. You might decide to switch stainless-steel spacers for PVC ones, and the software will also allow you to calculate the cost effectiveness and your ROI (return on investment).
A common mistake is that solar control glazing is used in cool-temperate and cold climates. Yes, this type of glazing minimizes the solar gains and is helpful in lowering the cooling demand in summer, however, it also minimizes the solar gains in wintertime and the energy consumption for heating is higher. As the energy bill for heating is much higher in cooler climates, we should prioritize for the winter case and therefore this type of glazing should be avoided.
What Are Optimal U-values For Passive House?
Optimal U-values vary depending on the climate, the size and the orientation of your project, the insulation thickness and other factors. Therefore to achieve optimal energy efficiency levels without overpaying for components, it is important to have a complete PHPP model of your building.
In warmer climates, the acceptable U-values can be slightly higher due to less severe temperature differences between inside and outside. However, in cool-temperate climates, stricter U-values are essential to maintain energy efficiency and indoor comfort. To give you an idea, here is optimal values for cool and tempered climate:
Uf ≤ 0.8 W/m2K
Ug ≤ 0.8 W/m2K
Uw ≤ 0.8 W/m2K
Uw, installed ≤ 0.85 W/m2K
g-value = 0.5-0.6
In conclusion, windows are not just a feature of our homes but a critical component that influences our comfort and energy efficiency. By understanding their impact and choosing the right ones, you can significantly enhance your living environment. Choose wisely to ensure your project benefits from the best in both performance and comfort.
If you want to learn more about energy efficiency and windows, take a look at our Certified Passive House Designer course, where we explore this topic in full detail.
