What is a Passive House window and how do I pick the right one for my project?
Passive House window and door requirements vary across climate zones.
With rising energy costs and an increased awareness of the environmental footprint of buildings, the Passive House movement is gaining more traction in the United States. Window manufacturers and distributors are quick to meet the rising demand for windows and doors on Passive House projects with new or rebranded Passive House product lines. While most of these products do offer excellent thermal efficiency, it doesn’t mean that they are the right fit for your Passive House project. Why? Because the United States stretches across multiple climate zones (10 different zones, in fact, if you follow the Passive House US Institute’s climate zone map for windows and glazing). What works for a Passive House project in Minnesota isn’t necessarily a good fit for a Passive House project in North Carolina. In this article, we’ll walk you through all the important aspects and performance criteria for windows, explain the available Passive House product certifications, and help you find the right products for your specific project.
But first, here’s some context on passive houses in general that will help to explain the performance criteria for Passive House products. The Passive House concept strives to create net zero or net positive buildings by employing rigorous energy efficiency standards. The term ‘Passivhaus’ was coined by the German and Swedish scientists Feist and Adamson in the 1980s. The first ‘Passivhaus’ was built in Darmstadt, Germany and Feist went on to found the Passivhaus Institute in the same city in 1996. In 2007 the Passive House Institute US (PHIUS) was founded to adapt the Passivehaus standards to reflect the diverse climate-specific requirements throughout the US.
Modeling your Passive House
Passive House models consider a large number of variables to determine the building performance and compliance with the Passive House standard. Source
The Passive House methodology relies on building energy models to quantify the effectiveness of the employed design principles and building products and to verify the thermal efficiency before construction is even started. Therefore, building according to the Passive House methodology does not only require high efficiency products but also sufficient data to run the building energy modeling. The data that are required not only pertain to the products, but also to the building location and layout.
Understanding how location and layout affect a building’s energy efficiency is key to picking the right window and door products for your project. We have already mentioned one of the key drivers in the intro: your project’s climate zone.
Clearly, there is a difference between trying to achieve net zero in Alaska compared to achieving net zero in Florida. What’s less obvious is why it matters. Most people initially come to the conclusion that it shouldn’t matter. After all, a window with a low thermal transmittance (also called U-value) should always be the way to go, whether you want to minimize heating energy consumption in Alaska or cooling energy consumption in Florida, right?
That isn’t wrong, but it ignores two other factors, the solar heat gain and budget constraints. The solar heat gain (represented by the Solar Heat Gain Coefficient or SHGC) of a window represents how much heat is generated on the interior by the sunlight. This effect isn’t captured by the thermal transmittance or U-value. Windows can have a very low thermal transmittance but still a fairly high solar heat gain. That’s great in Alaska where it’ll reduce your heating cost, but not so great in Florida, where your AC will have to work much harder if there’s a lot of solar heat gain (check out our article on window glazing if you want to learn about this effect).
Of course, the solar heat gain is not only determined by your climate zone. The orientation, size and shading of your windows and doors are equally important. If you have perfect shading, the SHGC of your windows might not matter at all, even if you’re in Florida. Again, it all depends on your project.
Beyond solar heat gain, your climate zone also matters in terms of the window and door prices that you’ll need to budget for. While it’s true that windows and doors with a low thermal transmittance are always better than windows and doors with a higher thermal transmittance, it doesn’t always mean that it’s where you should allocate your budget. If you live somewhere in the mild climate of California’s bay area, you might be able to get closer to your net zero or net positive target by buying windows with a higher thermal transmittance and investing the freed up budget into additional solar panels instead.
Using product data and certification to pick the right Passive House windows and doors
Example of a Passive House windows performance label. Source: PHIUS
As you have gathered by now, making the right decision on Passive House windows and doors depends largely on having (and using) as much information as possible in your decision making. And that’s where product marketing and certification come in. Generally speaking, the Passive House methodology is more popular in cold climates. This is due to where it originated and where people can save the most money with net zero building. Therefore, most companies that market Passive House windows and doors design them for cold climates. That means they will have a very low thermal transmittance (=low U-value) and high solar heat gain (=high SHGC). Accordingly, these are also the products submitted to the Passive House Institutes around the world for certification.
However, once you take a closer look at those certifications, you will see that they provide more than just a confirmation that these windows will perform well in Passive Houses in cold climates. The certification actually breaks down the window performance into a set of data points that give you all the information you need to include it into your specific Passive House model. The most important ones are the thermal transmittance (U-value) of the frame (Uf), the center of glass U-value (Ug or Ucog) and solar heat gain (SHGC) of the glazing that was used in the certification process, and the overall U-value (Uw) of the tested sample.
With these criteria, you have all the data that you need to determine if a window is the right fit for your Passive House project (-- there’s actually more relevant data on the product certificate, but we’re focusing on the most important data points for clarity. Contact us in case you want to learn more about things like PSI-spacer and PSI-opaque). By looking at the U-value of the frame and the overall window, you can determine the thermal transmittance of the product. Again, lower is better if you’re just looking at the window, but keep your overall budget in mind. Look at the U-value and SHGC of the glazing next to determine if the window was tested with the right glazing for your needs. If it doesn’t meet your solar heat gain requirements (too high or too low), it’s not the end of the world. Manufacturers who are serious about energy efficiency can deliver their products with different glazing options and provide you with the corresponding performance data, even if they didn’t get all the options certified. In terms of getting all the performance data for a Passive House, you actually don’t need the certification at all as long as the manufacturer can provide the information from a reliable and verifiable source. The product certification just makes the verification process a bit easier.
What are the Passive House performance requirements that my window and door products should meet?
We have not given any specific numbers for U-values and solar heat gain in this blog post. As you can probably tell by now, that’s because performance requirements for Passive House windows and doors depend on your specific project. Technically, you’ll only know exactly what product performance levels are needed once you (or your architect / engineer) have created the Passive House model. At the same time, you cannot model your home without picking the products to model. To resolve this chicken and egg problem, the Passive House Institutes issue climate zone-specific product performance recommendations for all important components. If you follow these recommendations and employ all Passive House design principles, you can be confident that your model will confirm that you’re meeting the Passive House standard.Here’s the link to the performance criteria for windows by climate zone from the Passive House Institute US that contains the table below and other helpful information on window performance.
Window and glazing performance recommendation per climate zone. Source: PHIUS
If you’re having issues navigating the requirements or getting all the different steps of building a Passive House lined up, feel free to contact us. We’re happy to help with recommendations for architects, builders and component suppliers, whether you’re considering our windows and doors or others. If you’re browsing our products, use our climate zone-specific Passive House product categories to find products that meet the Passive House Institute US standards for your climate zone. Gathering the right product information for your Passive House project up-front will help ensure a smooth process as you pursue overall project certification.
About Bauwerk Building Solutions
Bauwerk is a multinational business that connects US clients to an extensive network of European manufacturers of windows, doors, curtain wall systems, and more. We specialize in innovative and refined building products of the highest quality. Our offices are located in Charlotte, NC and Berlin, Germany.