Thermal Gain


I live in the UK. We have a new Part O building regulation to comply with. This got me thinking about the orientation of windows and which has more solar gain or not or does it not make any difference.

I would like to know if there is a difference between a landscape window and a portrait window regarding solar gain. (windows are of the same area)

I think the landscape window has a greater chance of capturing more sunlight than a portrait window. Hence portrait windows are better for reducing solar gain and landscape windows are better at increasing solar gain and illuminating a room. Please let me know your thoughts on this.

It depends on the wall thicknes. If this is zero (a single face) then there would be zero difference. But walls do have thickness in reality. Then for same orientation it all depends on how much of the wall is blocking sunlight from entering the opening at certain times of the day.

Orientation north\east\south\west do matter!

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I would guess that a landscape oriented window should block solar gain faster as the sun rises since its height should be less than that of a portrait oriented window having the same area.

No doubt you are aware but to be clear – Part O only applies to new residential buildings.

But, for sure, good to think about overheating for additions to existing dwellings.

yeah, I would apply the same logic as sun blockers.

in the south, the sun tend to be higher, so we use horizontal protection. Therefore, an horizontal window would get less sunlight through it, through the year. maybe not a lot, in winter it wouldn’t make a difference, and it also depends on the thickness of the wall (a 150mm wall won’t make a diff, a 600mm one will)

East west, we use vertical protection. on instinct, I would say that a vertical window would get a bit less light then.

In both cases, the thickness of the wall (assuming the window is mounted on the inside) acts like a sun protection, both horizontal and vertical.

a pair of quick and dirty tests :

late august, midday, south, you see the shadow takes almost the whole horizontal window. but less than 50% of the vertical.

I turned east, switched to winter and the vertical now looses more.

So, without any dimension, context, thicknesses or location, it’s all very theoretical, but I would say that in the south vertical get more light, and east west it’s horizontal. Inbetween (SW and SE) you’ll get pretty much the same over the year.

Wall thickness, exact orientation, time of the year will give more contrasted results, but still.

overall, on instinct, to reduce sun exposition in summer (but not in winter), I would pick horizontal windows and thick walls. Sure, in summer, it means more sunlight, but here in marseille, we would use blinds and curtains regardless. And in winter, a sunny day would be a heating-free day.


I think the landscape window has a greater chance of capturing more sunlight than a portrait window.

This sounds correct to me. The landscape shape/orientation ought to allow for longer insolation of the interior. However, I’m not an architect, so I would not be surprised to be proved wrong!

This whole discussion can go on endless when trowing in all sorts of parameters/conditions etc.
One thing is for sure: wall thickness zero means there would be zero difference between portrait and landscape.
OP needs to come up with the real situation, location on earth, orientation and what the purpose would be: blocking sunlight to keep out the heat or getting in as much sunlight as possible to get the work done without additional light.

So @hilltop_watt, can you come up with additional information?

One cannot just say a portrait window lets in more sunlight, nor can one just say a landscape window lets in more sunlight.


Thanks for the reply that’s interesting and you could be right. As the wall thickness is the same in both cases perhaps there is no difference. Thanks for your reply

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The question is theoretical only. I was wondering, when designing new housing if one was better than the other. But as you say, over a year with the sun’s movement and wall orientation, it perhaps averages out to be the same.

Obviously, the square window is optimal or neutral, I guess.

Having said that, I am now thinking, with a sun-facing wall, perhaps a landscape window would create more shadow in summer and gain more thermal energy in winter when the sun is lower, which here in the UK would be beneficial.

In my mind, I am picturing a window 2 ft by 6 ft with a 1 ft thick wall. The landscape window would benefit from solar gain early and later in the day. Where the portrait window would have more solar gain midday. This is with the assumption of a south-facing (sun-facing) wall (northern hemisphere)
North-facing (non-sun-facing) walls with window I guess does not matter so much.

Thank you for all your replies.

Thank you for all your replies.
I think the consensus of opinion has changed my thinking on this.

Ah Marseilles !!! Australians have a fond attachment to Marseilles :slight_smile:

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Have you tried PreDesign?

It can provide some insights to communicate with your clients real quick and easy based on location and the (8) facades.
PreDesign is included in a subscription plan of SketchUp.

If you want to dive deeper and explore several options and the impact of the energy balance, there is also Sefaira.
This would need an additional plan, though.

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You could make a light study, there are some plug-ins or even pre design as it was suggested to make that kind of analysis. If the windows are the same size and only the orientation is different it depends on the size, with small windows there’s not much difference it with bigger ones depends on where they’re going to be to see what shapes lets the sunlight pass for the longest time.

You can test different options with Sefaira. It has a plug-in for SketchUp.
With the same model you can test different orientation and you can also make a daylight analysis