This is a Passivhaus Basics blog post that gives an overview of a specific aspect of the Passivhaus Standard.

Windows, doors, rooflights, curtain walling and any other glazed elements often lose (or gain) significantly more heat than the surrounding walls or roof of the thermal envelope. For this reason, the international Passivhaus Standard pays particularly close attention to the design and specification of glazed elements.

This blog post looks at Passivhaus Windows as these are usually the main glazed element of a Passivhaus building. Other glazed elements can be considered along similar lines.

Reducing heat loss conserves energy, but it’s not just about energy efficiency. Reducing heat loss is also about providing optimum comfort for the people using the building. This is, after all, what the international Passivhaus Standard is all about: providing exceptional comfort whilst being radically energy efficient.

The international Passivhaus Standard also provides healthy living environments. Passivhaus buildings have plentiful clean fresh air and are free from mould. And the rigorous quality assurance of the standard results in highly durable buildings.

Passivhaus Windows have an important role in all of these aspects: Energy Efficiency, Comfort, Health and Durability.

Note: This blog post is specifically about window components so does not cover the glazing design of a whole building, orientation, shading etc.. I have written about solar orientation here though.

How is a Passivhaus Window Energy Efficient?

Windows are intended to do many things, including Passivhaus Windows, and energy efficiency is just one of those things. Ideally, windows would provide wonderful views and abundant daylight, whilst at the same time losing very little heat and providing just the right amount of solar heat gain.

Views and daylight depend on the site and the design rather than the specific windows. While heat losses and gains do depend on the site and the design, the window specification has more impact.

Gains

Solar gains are primarily dependent on the solar heat gain coefficient of the glass, the g-value, and how much glass there is. The building design and where and how the window is located, orientated, shaded etc. are also key but not the subject of this post.

A Passivhaus window is certified on the basis of the frame with a set glazing specification that allows like-for-like comparison between Passivhaus windows. As the glazing specification will be project-specific, the g-value isn’t part of the certification. When the glass is specified for a Passivhaus building it is always a balance between the U-value (minimising heat loss) and the g-value (optimising solar heat gain).

Passivhaus Windows need to have g-value to suit the building location and climate. In a cool-temperate climate such as the UK, this would typically be 0.5 or higher. Glass with a g-value of 0.5 means that 50% of the solar heat is transmitted through the glass. In a hot climate, a lower g-value might be more suitable. However, lower g-values do start to impact on the view through the glass.

Losses

Heat loss through a window is primarily dependent on the heat transfer coefficient of the whole window, the U-value. As above, other aspects relating to the design of the building are key also but not the subject of this post. Note that heat loss is through the whole window and not just the glass. Typically the glass is the area of the least heat loss of a window and this is why the international Passivhaus Standard goes into much more detail than just the glass. Windows lose heat through four different parts:

• the glass
• the frame
• the glass spacers
• the installation of the window in the wall

The Glass

For certification purposes, a glass U-value of 0.7 W/(m².K) is used for cool-temperate climates such as the UK. However, in practice a U-value of 0.6 W/(m².K) or lower is often used in the UK. The U-value is achieved through the use of triple panes of glass. An inert gas, such as Argon or Krypton, that transmits less heat than air is sealed between the panes. A low emissivity (low-e) coating is also applied to some of the glass surfaces.

In warmer climates, such as northern parts of NZ, high-performance double glazing can be used. It is the actual U-value (and g-value) of the specified glass that is used in the Passive House Planning Package and for certifying a Passivhaus Building.

The Frame

For certification purposes, both the frame and the glass spacer are considered together as the opaque elements of a window. Combined they give a Psi-value (linear heat transfer coefficient) which must be at least 0.2 W/(m.K). A certified Passivhaus window has an “efficiency class” (from phC “Certifiable component” up to phA+ “Very advanced component”) which is based on the performance of the opaque elements since the glass specification is project-specific.

The frame is the structure of a window, it is used to install the window in a wall and to support opening sections of the window. (High performance fixed glazing can help structurally.) For these reasons, the material is usually a poor insulator and loses more heat than the glass. However, frame technology has developed considerably and now frames can be thermally broken or insulated so that much less heat is lost through them. Frames have also got thinner so there is more glass and less frame in the same size of window.

The Glass Spacer

When more than a single pane of glass is used in a window, something needs to hold the panes apart: this is the spacer. These are often aluminium, which, unfortunately, is a very good conductor of heat and therefore, a thermal bridge. Passivhaus Windows require ‘warm edge’ spacers made from plastic or composite materials which conduct close to half as much heat as aluminium spacers.

The installation of the Window

While the installation of a window is project-specific, for certification purposes three scenarios with the window installed in Passivhaus suitable walls must be considered. This gives the opportunity for like-for-like comparisons between windows.

The installation detail matters because there are additional surface areas at the junction between the wall and the window and often there is more structure needed in the wall. This means there are additional heat loss areas in the window reveals and there is less wall insulation due to the additional structure.

The recommended installation detail includes wall insulation covering some of the window frames to further mitigate or eliminate heat transmission through the frames and the window reveals. This isn’t the only way to detail the installation of a Passivhaus window though. And it should be noted that insulating over the frame doesn’t have any effect on the performance of aluminium clad windows.

Window cills are not so easy to insulate and usually the heat loss through the cill detail is worse than through the reveals and head detail. The different details each need to be accounted for in the Passive House Planning Package. It is that detailed.

The Installed Window U-value

A Passivhaus certified window U-value is not the actually U-value that a project will actually have. It is a value that allows for like-for-like comparison. And it provides critical information to use when inputting the project-specific details into the Passive House Planning Package.

The installed window U-value takes into account the dimensions of the window, the glass, the frame, the installation details, the U-values of the glass (centre pane) and the frame, and the Psi values of the glass spacer and the installation detail.

For a Passivhaus certified building the required U-value is climate dependent. In the UK and most of Europe, it must be less than 0.85 W/(m².K).

A Passivhaus certified building does not require Passivhaus certified windows. However, if the window isn’t certified it can be more difficult to track down all the necessary and correct information about it.

What about Air leakage?

A Passivhaus certified window doesn’t have a specific requirement for airtightness. Some other glazed components such as sliding doors do. However, if a window wasn’t airtight, it wouldn’t meet the exacting comfort and hygiene requirements as discussed below. It must also be suitable to form part of the airtight construction of the building and pass the blower door test.

Heat loss and gain through a window is significant and therefore needs detailed consideration for Passivhaus.

What does a Passivhaus Window have to do with Your Comfort?

Typical windows create discomfort in a number of ways. As they transmit more heat than the walls, they are cooler than the inside temperature. This results in three areas of discomfort:

1. Cold draughts. As the warm air in the room contacts the window and cools down, it sinks to the bottom of the room creating cool draughts. Radiators are often located under windows to counter this effect.
2. Temperature asymmetry. As the window feels colder than the rest of the room it is uncomfortable to have one side of your body feeling cooler than the other side.
3. Heat Loss. Our body radiates our heat out to the colder surfaces of the window and we feel colder.

The high-performance and climate specific requirements of Passivhaus windows are in part determined by addressing these discomforts. A certified Passivhaus window can only be 4.2 degrees cooler than the average surface temperature of the surrounding space. This ensures that there are:

1. No draughts caused by air contacting cold window surfaces.
2. No temperature asymmetry as the window feels as warm as the rest of the room.
3. No heat loss from our body radiating heat to a colder window.

This degree of comfort also means we can sit in a window seat or near a window, even when it is freezing outside. And we won’t need to put on another layer of clothing just to be near the window.

This high-performance is only achieved by taking into account all the detailed aspects of energy efficiency noted above. This is core to the international Passivhaus Standard, radical energy efficiency provides the means; exceptional comfort is the outcome.

As well as providing great views and daylight, Passivhaus windows enhance our comfort.

How is a Passivhaus Window Good for Your Health?

Aside from all the discomfort noted above, typical windows can also be bad for your health. The colder surface temperature means that the relative humidity on the window surfaces can be high enough for condensation to occur. I see this in my non-Passivhaus house very regularly and I am sure you will have seen it also. Condensation brings with it the risk of mould and the associated dangers to our health. This is part of the reason why asthma rates are so high in countries like the UK and NZ. (I’ve written about this before here and here particularly.)

Passivhaus Windows eliminate this risk. The integrated design approach of the international Passivhaus Standard encompassing the thermal envelope, thermal bridge free construction, airtight construction, good ventilation and high-performance windows ensures a condensation-free indoor environment. This is a healthy and mould-free environment that all people would benefit from.

For certification of Passivhaus windows, the coldest point on the window frame (typically in a corner where there is both frame and spacer) must not have a “water activity” of greater than 0.80. What this means, is that the relative humidity at the surface of the material, or just below it, must not exceed 80%. Mould growth can start once the surface RH exceeds 80%. The certification criteria include climate-specific values for the internal surface temperature factor (f_Rsi) which ensures the relative humidity is acceptable.

The relative humidity increases as the surface temperature goes down, so it is directly related to the window staying warm. The internal surface temperatures are directly related to the installed window U-value. So once again, energy efficiency is the means, but in this case, hygiene and a healthy indoor environment is the outcome.

Passivhaus windows provide a healthy mould-free indoor environment.

Passivhaus Windows: Energy Efficient + Comfortable + Healthy = Durable.

Passivhaus windows take into account the thermal performance of the glass, the frame, the glass spacers and the way the window is installed. This level of detailed consideration ensures Passivhaus windows performs as intended to: at a very high standard. Passivhaus windows are part of the reason why the international Passivhaus Standard eliminates the performance gap between predicted and actual building performance. The international Passivhaus Standard has integrity.

Passivhaus windows also play a major role in the international Passivhaus Standard delivering exceptional indoor comfort. Passivhaus windows aren’t cold to be near and they don’t cause draughts.

There is no condensation on the inside of Passivhaus Windows. Eliminating condensation is good for hygiene as it also eliminates mould, a serious health concern.

The combination of energy efficiency, comfort and health has another benefit also: durability. While lower energy bills, warm surface temperatures and an absence of mould are all good for people, they also provide good conditions for building materials to last. Eliminating damp, and mould, and the associated material degradation ensures Passivhaus windows continue to perform over their long lifespan.

Passivhaus windows: high performance in energy, comfort, health and durability.

Some additional resources:

• This blog post is now available in French: Qu’est-ce qu’une fenêtre Passivhaus? after being translated by Damien Chaveron on the Maison passive Ozalée blog.
• Slideshare presentation: High Performance (Product) Matters – How and why we need to change our current window design for Comfort, Health & Profit by Bronwyn Barry
• Slideshare presentation: NFRC vs. PHI vs. PHIUS Window Certification for the US by Bronwyn Barry
• Youtube video: Passive House Windows for Mediterranean Climates with Dr. Benjamin Krick from the International Passive House Institute. There is a lot presented that is still relevant for other climates.

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