Site shading matters, a lot. Not modelling it accurately is likely to cost money and risks straining client relationships. PHPP can only handle the very simplest of shading scenarios. Passive House designers need to get familiar with DesignPH and use this software for modelling site shading. It’s not difficult to use but it does presuppose basic familiarity with SketchUp.
What’s the problem?
If shading is under-estimated, because important elements are left out or not modelled accurately, the model will predict more solar gain than the building will receive. This risks the house being colder than expected in winter and it may need more heating input to run than the clients (and the model) expected.
Over-estimating shading is arguably worse because the problem may not be noticed until summer arrives and your client starts to roast. The model calculates less free heating energy from solar gain than the building will actually receive. That might mean you decide to increase insulation or specify higher-performing components, if the model is close to heating demand limits. That all costs money and it might be wasted. Worse, it could lead to overheating problems in summer, when the building gets more sun and heats up more than expected. Air-conditioning may need to operate more often and at higher intensity than predicted. In worst cases, expensive remediation is required to solve the problem, such as retrofitting shutters or external blinds. That comes with monetary and other costs, like loss of daylight and potentially very valued views. This is not a conversation you want to have with your clients.
The solution
PHPP is acceptable if your project’s site shading is very simple. You can specify only height and distance for any given object that shades the building. You cannot model things that slope, wing walls, semi-permeable objects and it’s very hard to accurately estimate things at a distance.
Site shading doesn’t get much simpler to calculate than on this site, a flat paddock with a uniformly sized hedge. Yes, you could model this in PHPP.
Anything more complicated and our team’s recommendation is to do the site shading modelling in DesignPH. It’s not hard to learn but it can be confusing at first to know what shading objects to include and what you don’t need to bother with. So, here are some tips to help you be efficient and accurate in Design PH.
- If you can see it from the site, you need to model it.
For instance, in a built-up urban area, if surrounding topography is not visible from the site, it doesn’t need to be included. All visible buildings and vegetation need to be included in the model.
Distant terrain to the north can be excluded, even if visible, because the sun will never dip below it. To make sure, use the Sun Seeker app (iOS or Android) or in NIWA’s online Solarview tool.
Make sure to include all visible shading objects to the south, given the path of the rising and setting sun in mid-summer—this is especially the case in New Zealand’s lower latitudes. That late summer sun is strong and is an overheating risk.
2. Modelling nearby topography
For terrain (shading objects) close to the building, it’s useful to sketch nearby topography using SketchUp. Geo-locate your building then use the location terrain feature. The following steps are carried out in SketchUp, without the need to run DesignPH.
A. Turn on the terrain
B.Draw a polyline on the surface of the terrain, along the whole extent of any ridges:
C. Turn off terrain; the line will be visible. Draw lines on the vertical (blue) axis down to the origin height, where your building should be vertically located.
D. Do this for all intersection points on the ridge. Now join the bottoms of the lines together to make a surface:
E. Final check: turn terrain back on to verify it’s been accurately captured in the model.
Now turn on DesignPH. Assign those new shading surfaces to a non-thermal group. That way DesignPH doesn’t recognise them as part of the building thermal envelope and treats them as shading.
3. Modelling far away topography
SketchUp’s terrain tool is only useful for a limited area around the site, depending on what you pay for. An alternative for modelling further away terrain that shades the site, import a topo map image into SketchUp. Scale it to actual size (1:1) and rotate it so the top of the map faces north. Repeat the process described above for nearby terrain, using ridge and valley heights to create surfaces. Here’s an example of what you might end up with. Below is a Wanaka site with Roy’s Peak visible in the distance to the west of the building:
Pro Tip: Don’t run DesignPH with the terrain tag turned on in SketchUp, because it’s likely to crash the programme, or take forever—DesignPH will try recognise thousands of flat surfaces in the terrain.
Unless the building is at sea level, don’t forget to adjust the height of the simplified topography: minus the number of metres ASL of the build site from the ridge height on the topo map.
Then assign all your topo surfaces to a non-thermal group in DesignPH.
Arranging the simplified topography into groups is useful eg near and far, modifiable or non-modifiable or shade with and without impact for the site. You can turn a cluster of shading objects on and off in the model and observe their impact.
If a shading object has no impact, don’t include it in the model you send through for pre-construction certification. Ideally include a short report explaining your shading assumptions and noting what was investigated and found not to be relevant. At least include a note on the certification platform when you upload your DesignPH files for review.
Need more help?
The course for new Passive House designers launched by Sustainable Engineering last year will be back later in 2023. The Practical Passive House Design Process Course is now called Passive House in Practice. We’re designing it so people can sign up at any time and complete the coursework at a pace that suits them. They’ll still have access to live expert advice from our team. Keep an eye on Sustainable Engineering’s social media channels (Facebook, LinkedIn) for more info.
21/08/2023 Addendum
In designPH, it is possible to retain extremely large terrain models and objects such as trees with realistic foliage in the designPH model and still have very short designPH analysis time. This is done by manually selecting only the thermal envelope, TFA, and project footprint, then clicking analyse. DesignPH then infers that everything not selected is a shading surface.
Many thanks to Marcus Strang of Hip v Hype of Melbourne, for this pro tip