Shading, solar gains and overheating
The shading sheet is probably the trickiest within PHPP to get right, and it’s worth reading the PHPP manual carefully so you understand the instructions, especially about averaging reveal figures. It’s not sensible to be overly conservative and assume more shading than there is, because this can hide overheating problems. Be clear and reasonable in your assumptions and a Certifier is unlikely to disagree with you. If you are at all uncertain, get a Certifier appointed and agree the shading strategy early on.
For very complex shading it is possible to use separate software to calculate the shading such as IES, TAS or Ecotect. See page 94 of the PHPP manual for more information.
Whilst it is possible to get carried away worrying about shading accuracy, a good design should not be too sensitive to exact shading values as this suggests there is probably too much glazing. It is better to be more conservative in your assumptions.
The overheating check in PHPP is basic, and as such shouldn’t be used for non-residential or complex residential buildings. PHPP considers the building as one zone, distributing the gains evenly and as a result won’t identify high solar gains concentrated in one room. Be cautious about using high ventilation rates to solve an overheating issue, as this may not be realistic due to security or noise issues. A useful stress test is:
• Use the IHG sheet to accurately represent the actual internal gains – and consider modelling various scenarios to demonstrate differences in user behaviour. Note this will require several additional sheets to be filled in such as the hot water and electricity sheets. To represent small buildings with high occupancy, a value of 7W/m2 is reasonable if no other data is available.
• Input minimum user-operated summer shading. If you are building the house for yourself then you may be willing to open and shut blinds 10 times a day but the next residents may have a different view!
• In our experience the effectiveness of MVHR ‘summer bypass’ mode is overstated in PHPP so set at half normal vent rate (0.2ACH, for example).
• Assume no natural ventilation during the day.
• Assume half the achievable night time ventilation – again in most cases this is user determined, and be careful about assuming internal doors are open.
The upper limit for certification is to keep the building below 25°C for <10% of the year, and best practice is <5% of the year. It is likely the latter will become a certification criteria in future.
Whilst it is possible to build almost anything to the Passivhaus standard, be cautious about projects with large expanses of glass. These will have a strong reliance on high performance glazing but the structure that surrounds glass is more complex and therefore probably won’t perform as well. In addition, the building is highly reliant on the weather for both internal comfort and energy consumption – this does not make it easy to achieve the comfort and energy requirements!