HPH186 : Choosing windows for a Passivhaus home – function versus aesthetics
Ben and Kay Adam-Smith have a difference in opinion over which windows to specify for their new build Passivhaus home. While Ben is keen to minimise mullions, transoms and window frames, Kay believes this takes away from the aesthetics of the building.
A visualisation of Ben and Kay's house
Today's episode is really a short story about aesthetics. Life would be boring if we all liked the same thing. However, what happens when you're working with your other half on a self build and you strongly disagree about something? How do you move on? Well we haven't answered that one yet, but we can share the disagreement and let you know what happens over the course of time!
I (Ben) also feel like I might have said what my wife is saying a few years ago. I believe I've grown in my understanding since then and so now my opinion has changed.
Mullions, transoms and window frames
Firstly what are mullions and transoms? Well, they are the structural elements that divide up sections of a window. Mullions are vertical while transoms are horizontal.
There's no denying that the use of mullions and transoms can make some beautiful windows (as in the stained glass window above, where it's taken to an extreme).
In the past it was just not possible to manufacture large single pieces of glass and so to achieve a big window, a number of small ones needed to be ‘stitched' together.
Large glazing is now possible
Probably a window's most important function is to provide that visibility from one side to the other.
Windows without mullions and transoms therefore do a better job of connecting the inside to the outside (and obviously bring in more light).
As we spend 90% of our time indoors, you could argue that the view out is more important than the aesthetic appearance of the window from the outside.
Large single pane windows are still not the norm
In a country where our period properties make up a large proportion of the housing we live in today, it is hardly surprising that we gravitate towards their features.
That's perhaps why Kay would like two of three ‘lights' (subdivisions) in each window.
She also thinks that a lot of houses that use large panes of glass just don't look right; they look like they need to be sub-divided.
Reducing the amount of glazing in a window affects its performance
The frame is generally the weak point of the window.
Chris Parsons of Parsons + Whittley says: “We want more glass and less frame in the way, because the more glass we have the better the performance of the window. When you start dividing them up with mullions and transoms, you now can't insulate that part of the frame by wrapping anything around it.”
How much glass is actually in a window?
PHPP models the amount of glass and the amount of frame
Even if a window has a good central pane U-value (a measure of how effective it is as an insulator), it is only referring to the glass. Therefore the Passive House Planning Package (PHPP) requires information on both the amount of glass and the amount of frame in every window. This is the only way to produce an accurate model of the thermal performance.
The ease of opening and closing windows depends of their size
While performance is important when creating a Passivhaus, it must also be balanced with practicality.
When windows get too big, they take a bit more manoeuvring so this could become a valid reason for introducing different lights in certain windows.
Aesthetics will always be a key driver in window choice
There is no doubt that windows with mullions and transoms produce a pretty pattern on the window.
It is also possible to get Passivhaus sash windows, for example. However, for new build, does it make sense to specify sash windows (given all the arguments above)?
This is a choice you'll have to make . . . and a choice we have to make on our build!
Keep an open mind
This can be a great attitude to anything in life and it's just the same with architecture.
I made this video in 2013, when I realised how my past shaped what buildings i liked.
If you enjoy this video, you'll probably get a lot of benefit from taking our 10-day Eco Design 101 e-course.