HEM Deepdive: Obstacles

With the Home Energy Model edging towards its official release, I’ve been playing with some of the new inputs to understand how impactful they are on a dwelling’s energy performance.

The first section that will be unfamiliar territory to SAP assessors is Obstacles. 🏣🏠🏢

In SAP, we have to enter the ‘Sheltered Sides’, which is ranked from 0 to 4.

0 means the dwelling is positioned in the middle of an open field, and

4 means the dwelling is surrounded on all sides.

This rating considers how exposed the building is to the elements.

Previous versions of SAP gave better results to a well sheltered house, but in SAP10.3 it feels like you'd get the same outcome using a random number generator!

HEM takes a more sophisticated approach, but the downside is it’s a time consuming extra step for assessors.

For every building or obstacle that surrounds the dwelling, HEM wants to know its height, how far away it is, and the orientation.

It combines this data with information from the weather file to reduce solar gains (due to shading) and also reduces the strength of wind hitting each façade (which impacts the infiltration rate).

If any CSH assessors are still out there, the form-filling process is very similar to the old HEA1 daylight assessment.

 🌄 But does all this extra assessing effort make a difference?

The energy performance of the same dwelling changes when the surroundings are considered.

I’ve built a HEM model and run it twice through the HEM:FHS Python wrapper*. Once with zero obstacles, and once as if it’s totally surrounded by skyscrapers.

The heavily-shaded house has an increased heat demand by 23%; mostly due to the reduction in solar gains.

Total energy demand is 28% higher and, as you'd expect with a heavily shaded building, the performance of the PV drops through the floor.

So, considering the two extremities, a dwelling that's free from obstructions will give better energy performance overall, but actual results will vary for each individual plot.

This new section certainly meets the brief of making HEM results more accurate than SAP, but we need the final version of HEM before we can unpick how this may impact compliance with Part L and alter EPC results.

*I've used FHS per 1.0.7a which is not the final version and can't be used to measure compliance with Building Regs.