The KNX shading advice for passive houses is written for the wrong hemisphere

The KNX Association has an article on automated shading in passive house design that I mostly agree with. It also quietly assumes you live in Europe, and if you're reading this in Johannesburg, that changes which blinds do the work.

The article is dated 8 March 2026. It describes a passive house as highly insulated and airtight, with large south-facing windows that bring in free winter heat. The risk, it says, is overheating, and its answer is automation: blinds open on sunny winter days so the floors and walls soak up warmth, closed at dusk to trap an insulating layer of air against the glass, and external blinds that come down before the room gets uncomfortable. It also has shading and the ventilation system talking to each other, so that if the blinds can't hold the temperature, the house pulls in cool night air.

That's sound design thinking. It also contains no numbers, no case study and no temperature thresholds, and it comes from a body whose members sell KNX products. So I went looking for something harder.

What the passive house people measure

The Passive House community's own reference site, Passipedia, gives the benchmark. A house should not spend more than 10 percent of the year above 25 °C, and a summer comfort factsheet hosted on Passipedia recommends staying below 5 percent. Passipedia's worked example is a mid-terrace house in Darmstadt, Germany. With no shading and no window ventilation it lands at 10.77 percent, just over the limit. Interior roller blinds bring that to 6.8 percent. Temporary external shading brings it below 0.5 percent.

I'd be careful with those figures. They come from one reference model in a German climate, and the page itself is undated and refers to a 2007 version of the Passive House planning tool. I wouldn't read them as predictions for a house in Fourways. What they do show, clearly, is the order of effectiveness: stopping the sun outside the glass beats stopping it inside the glass by a wide margin. Passipedia also says temporary shading works for all window orientations, while fixed overhangs help south-facing windows but do little for east and west.

Now flip it

Here is the part the European articles can't tell you. In the southern hemisphere the sun sits in the northern half of the sky at midday. The equator-facing side of a South African house is north, not south. The passive house logic of letting the winter sun in through the big glass and keeping the summer sun out applies, but to the north facade.

The KNX Association's seasonal efficiency article, from October 2025, describes control by facade: east blinds lower in the morning, south at midday, west in the afternoon. For us, read it as east in the morning, north at midday, west in the afternoon. The west facade deserves its own line in the programming. It's the one that takes the hot late sun in summer, and it's the facade where a fixed overhang does the least, according to the Passipedia material.

I haven't run a thermal model of a Johannesburg house for this post, so treat the facade switch as my reasoning from where the sun sits, not a measured result. What I can say from site work is that a west-facing living room with big sliding doors and interior curtains is the room clients complain about first. That's an observation, not a study.

What I'd put in the KNX design

Outdoor blinds or external shading on the west and north glazing first, and on the east glazing if it faces bedrooms. A sun and brightness sensor with a facade-based schedule rather than one global rule. Winter behaviour that leaves the north blinds up on sunny days. Wind protection, because external blinds in a Highveld storm without it are an insurance claim waiting to happen. The seasonal article mentions wind and rain sensors for the same reason.

One honest limit. A passive house is a certified, tightly sealed building, and most of my clients aren't building one. Their houses have large glass, modest insulation and no mechanical ventilation. The principle still carries over: shade it from outside, before the heat gets in, and let the bus handle it so nobody has to remember. I wouldn't promise a number to a client.

If you're planning a build and want to talk through facade by facade shading, just a conversation about what the project needs.

wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za

Sources:

KNX Association, The role of automated shading in passive house design (8 Mar 2026), knx.org

KNX Association, Seasonal energy efficiency: Automating your KNX smart home for winter and summer (29 Oct 2025), knx.org

Passipedia, Summer comfort basics, passipedia.org/basics/summer

Passipedia, Planning for summer comfort factsheet, passipedia.org