HPH038 : What Type of Windows Do You Need in a Passivhaus? – with Bronwyn Barry from One Sky Homes
Bronwyn Barry from One Sky Homes explains the evolution of windows and why modern windows are really pieces of equipment. She also shares what's involved when it comes to choosing high performance windows for a project.
Interview with Bronwyn Barry
Bronwyn Barry is originally from South Africa but she studied architecture in America and never left!
Her passion is for integrating good design with comfort and performance. When Bronwyn discovered Passivhaus, she felt it had the whole package.
The Smartwin window system
Windows Have Become More Sophisticated Over Time
A window traditionally has two functions. Firstly it lets in light or air, and secondly it allows the dweller to see outside (or vice versa).
In a high performance building, the windows are likely to be the weakest link in the chain when the whole building envelope is considered. So how the windows are built and how they are designed into the envelope is critical to making the full package work.
Historically Windows Have Even Told Stories!
Early windows were just tiny openings in buildings with no glass. As glass became more available, windows even became a status symbol for the wealthy.
Bronwyn talks about the glass tax in Ireland where people were taxed on the number of windows they had!
Gothic cathedrals really pushed the boundaries of what windows were, using them to tell biblical stories.
Building Science is Nothing New
Bronwyn reflects on a trip to Frankfurt in Germany where she visited the Goethe House. On the top floor are the original leaded glass single pane windows from the 1600s which have a channel at the base and weep holes to allow the interior condensation that would form to drain to the outside.
Bronwyn found this fascinating that building science was being used to come up with solutions centuries ago.
Domestic Markets for High Performance Windows are Growing
When Ben asks whether a Passivhaus needs different windows to any other building, Bronwyn says not necessarily. However, the windows must be very carefully designed and selected, taking in account the climate, orientation of the building and the materials.
The good news is that there's more and more choice on the market. While imports (from countries such as Austria and Germany) have dominated Passivhaus projects in recent years, local producers have cottoned onto the growing market and have started producing appropriate windows and even customised products for their specific markets.
Windows Need to Function Differently According to the Project
There are variables to take into account such as the solar access of the site. The energy balance will change according to the size of the building. Taller buildings, multi-family units etc. will have large internal gains compared to relatively small gains on single family, detached residences.
Bronwyn explains with an example of one of her recent projects which had no solar access: “I’ve recently done a retrofit where our windows weren’t net energy producers. What I mean by that is the overall energy gain from the solar gain of the windows was actually less than the amount of energy that gets lost out of the windows, just because they are a weak insulation component in your full envelope. That was in California, where we’ve got really great solar access and a relatively mild climate but this was a project that had a whole line of redwoods across the southern border of the property completely shading the house. So we really had to choose our windows that would give us lots of natural light, which people still always want no matter where you are on the planet, and wouldn’t lose too much heat and maintain a relatively comfortable interior temperature.”
Aesthetics Tend to Come First When Choosing a Window
In Bronwyn's experience choosing windows for a project generally follows this sequence:
- Select the ones you like the look of
- Look at the performance data to see they are adequate
- Consider the price
Another factor to bear in mind is what materials the windows are made from. Some people do not want any uPVC on their projects!
Glazing Selection has a Big Impact on Heat Gains
The glass that is chosen to go in the frame has a huge influence on how much heat gain there will be from that window. Again, this is due to a combination of factors.
For example, quite often there are double pane or triple pane. There are a variety of films and coatings that can be applied to the glass too. Different gas fills can also go in between the panes, adding an extra layer of insulation.
Do Gas Fills Leak Out Over Time?
Ben raises the concern that these gas fills in the windows may not last and Bronwyn responds that in a recent study of a 10-year old window it was found that only a very small amount of the gas had escaped.
There is currently debate that at high altitudes this could be more of a problem but even then there are workarounds.
Bronwyn says: “If your glazing panel already has very poor performing spaces, so if they’re aluminium spaces holding the two panes apart that’s already probably a worse thermal bridge kind of heat loss opportunity than no gas in the window. And if your frame is a really poorly insulated frame that’s another bigger opportunity for worse heat loss. Then if the installation, how the window’s installed within the wall opening, if that has all sorts of metal pans that go from the inside to the exterior and big air leaks there again is another bigger offender.”
Rough opening framing with air-sealing tape
Everybody in the Chain of Custody has to do a Good Job
It is not a case of just buying a really high performance window and the job is done, because if it is put in a sub-optimum position in the wall or is not sealed properly to the building fabric, the overall performance will be diminished. So it is very much a team effort. Good communication and a little education can go a long way.
As Passivhaus requires a blower door test it quickly reveals any poor workmanship, both in the production of the window and the installation.
Solar Gains Need to be Carefully Controlled
With the opportunity for windows to be great solar gain mechanisms they essentially become the heating system for these buildings. However, if this is not controlled carefully it will become an over-heater for a building which is also not desirable.
This was Bronwyn's childhood experience in South Africa, where overheating and being uncomfortable from excessive heat was more the issue rather than lack of heat.
So being able to shade the windows carefully and control the solar gain is just as critical as the window design and where they are located in the building envelope.
Exterior operable shades in Berkeley, CA
Your Shading Mechanism Should be a Long Term Proposition
There are various shading mechanisms that can be used on a project such as an eaves overhang and brise soleil to exterior louvered blinds or a neighbouring building. Even trees can be used as part of your strategy. The important point is that it is a lasting solution.
On high rise buildings it can actually be a coating on the glass itself that modulates the solar gain.
Balance Project, New Mexico (Image courtesy Jonah Stanford)
The Importance of the Window Frame is Often Over-Looked
While architects and energy modellers can get engrossed in the glass specification the actual frame design itself can get skimmed over. The frame can make or break a high performance window.
The glass can be replaced and most manufacturers provide a good selection of glass inserts but when it comes to the frame there are normally only a few options.
That's why Bronwyn's top tip is to pay attention to how well a window is put together and also how it drains at the sill.
Download a transcript of the interview with Bronwyn Barry.
Our Question for the Comments Section
How have you chosen windows?