Glazing percentage: handy rule of thumb to prevent overheating

19 July 2023 by Jason Quinn

Prevent overheating by keeping the window area to 20% of the floor area. That’s a very old passive solar rule-of-thumb from New England (Eastern USA) where I grew up. 

So I was interested to discover that Mark Siddall has researched this same metric for homes in the UK and found it holds true there. He details this in a brief LinkedIn post you can read here.  

Mark Siddall is a building science researcher and Passive House expert, well known to most readers of this website. His work on thermal bypass and the issues it causes is epic.

Mark does say the 20% rule is a whole-of-house guide and suggests doing a Daylight Factor calculation to understand the room-by-room results. This can be done using PHRibbon if there’s a PHPP file for the project.

I’ll check this sometime against a dynamic model. At Sustainable Engineering, we usually just look at the glass-to-floor area for the room. If it is much higher than the rest of the house we tend to add external shading or specify low g-value glazing.

Comments 3

  1. Hi,

    Thank you for these regular newsletters, I look forward to receiving them, they are a great read and very educational.

    Acknowledging that with increases in the thermal efficiency of homes there is also an associated increase in risk of homes overheating through solar heat gain, especially in the hotter regions of New Zealand.

    However, decreasing the percentage of window area also carries the increased risk of reducing the amount of natural light into the home.

    Natural light is widely acknowledged as being an important component for the wellbeing of a home’s inhabitants and it is important this is addressed during the design process as well.

    Like a lot of things in life it is about balance.

    There have been some impressive advancements in solar control glass, as is touched upon above, and certainly the solar control performance of the window insulated glass units will become a focal point moving forward.

    It would be a step backwards to see houses that are being designed losing the opportunity to take advantage of the allowing the occupants to continue to experience the psychological benefits of having a connection to the outside world and losing the option to enjoy increased levels of light entering the home.

    Understanding that in the days before solar control glass limiting the percentage of window to floor area would be a rational solution, the opportunity now exists to increase the percentage of window to floor area.

    We have certainly invested in stocking these types of glass in New Zealand in readiness for the increased risk of overheating of homes being a concern and welcome the opportunity to contribute to the advancement of this type of technology into the local construction industry as and when opportunities arise.

    1. sometimes a fantastic view is also a highly desirable consideration and the spatially aware design to enhance the feeling of spaciousness.

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