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Powell Dobson PassivHaus Papers

Week 4 Triple Glazing

Feb 2021

Let’s talk glazing. There are a few key things we need to consider when looking at this aspect of a PassivHaus. As already discussed in Paper 3, the external fabric of a PassivHaus is very thermally efficient, to ensure minimal heat losses. The windows are of course punched through this external fabric, so it makes sense that these would need to be providing a good performance too, so as not to undo all your hard work on the walls and roof.  

Why triple glazed? For PassivHaus, we need to be targeting an overall window U Value (Uw) of 0.8 W/m²K. To put that into context, a standard building regulations home would probably be specified with double glazed units, with a performance in the region of 1.6 W/m²K. Triple glazed windows are more efficient at stopping heat loss, thanks to the additional air gap produced by the extra pane of glass. This air gap is typically filled with an inert gas (Usually Argon or Krypton) which are poor conductors of heat, ensuring less energy transfer through the glass build-up. 

Frame to Glazing Ratio. Typically for PassivHaus, we have thermally broken/insulated frames, which of course further limit heat losses, but solid timber units can also be used, with the unit still performing really well. One of the challenges to overcome with PassivHaus, is making sure that the windows are kept nice and simple where possible. Typically, the frame of the window will perform far worse thermally when compared to the glass, so we try to keep the fenestration simple, and avoid any ‘surplus’ frame where we can. The example adjacent is something that crops up quite often.

Something else that we take notice of when looking at glazing on a PassivHaus, is the thermal bridge at the glazing edge. To ensure the airgap is the correct width between each individual pane, we have something called spacers. In a standard window, it’s not unusual to simply have an aluminium profile in this location, however for PassivHaus it’s a good idea to have something that doesn’t let energy move as freely across it. The image on the left shows a ‘warm spacer’, these consist of a specialist rigid foam, with a foil backing to ensure the gas between the glass stays within the cavity.

This kind of uplifted specification (something like a SWISS SPACER) is the norm when looking at certified PassivHaus components, but also used in highly efficient units that may not be certified. The heat loss at this glass edge is calculated by the manufacturers, and available for anyone who enquires, we call this heat loss a PSI value.

As well as simply allowing for the window dimensions, and the glazing performance, we also plug the frame thermal performance, the PSI value for the glass edge (as explained above), as well as the installation PSI value (also available from the window suppliers) into our PHPP for a really accurate heat loss to be determined for the each overall installed unit. 

There is a snip taken from one of our projects (shown on the PDF attached), showing the level of information needed to be plugged in for the PHPP for one window type, as explained above. PassivHaus tries to capture everything!

MYTH BUSTER. ‘You cannot open windows in a PassivHaus?’ Yes, of course you can open your windows. When developing the PassivHaus design, and working through the PHPP, we need to keep a watchful eye on overheating. We can calculate the frequency of overheating likely to take place in the finished building, and use the opening of windows as part of this exercise to offset any perceived comfort issues. Quite often, an allowance of opening a first floor landing window for example overnight is used to keep things balanced (in ‘special’ circumstances), and internal conditions comfortable for the building users in the warmer summers.

With the above being said, you shouldn’t typically ‘need’ to open your windows day to day. The MVHR allows for very regular fresh air changes, and filters out pollen and dust, so in some locations, such as busy cities for example, it may actually be better to keep them shut, and allowing for a more comfortable internal climate.

If you are an ‘open windows’ kind of person, that’s fine, but it does mean the efficiency of the building drops a little. It’s not forbidden! I use the example of driving down the motorway in your car at 70mph in 3rd gear, yes it gets the job done, but with higher revs and the car working harder, it’s not allowing the engine to run efficiently, and the MPG drops as a result. This is what happens in a PassivHaus, we lose energy through the open windows, and have to work harder as a result, getting heat back in the building, and rebalancing the heat losses and natural gains.

That’s glazing finished for now, hope you found it interesting. For extra reading, and to look at some pretty impressive units, swing by the ‘Internorm’ website.

Look out for the next instalment, coming soon. 

Next week: We'll be taking a little break next Friday for half term so check back on 26th Feb for our blog on MHVR. 

Oliver Henshall, Technical Associate and Certified PassivHaus Designer
E: oliver.henshall@powelldobson.com T: 033 33 201 001

 @HenshallOli 

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