When we model a building envelope to measure it against the Passivhaus Standard, we model the external surface of the thermal envelope along with any shading (which can be seen in grey). Within the home we model volumes and Treated Floor Area as per Passivhaus methods. On the outside we draw the thermal bridges as lines in space so that we can calculate a total length.

Above is an image showing that the volume is much greater than it would be if the ceiling were flat. In a Passivhaus it is important to ensure that the volume is calculated so that the capacity of the Energy Recovery Ventilation unit can be measured amongst other things.

Cadwork is the software we use to model in 3D. Like other 3D modelling programs it is pretty much a database, though in particular, Cadwork is raw and well suited to self-definition of elements. Each element is assigned an attribute which can then be recalled in the PHPP.

After inputting climate data, areas, thermal bridges, windows and volumes into the PHPP we input U-Values and fix any issues which need to be addressed to ensure the PHPP is valid. We then proceed to iterate options in an additional worksheet and test combinations of variables. We make sure that it is easy to move back to any of the early iterations so that we can review.

With this project it was realised that the windows were the weakest point. We even got to a point where we found that eliminating one glazed element would make a huge different to the performance of the overall building envelope. That is simply the power of the PHPP.

The PHPP can be used to provide energy balance advice even if the project does not meet Passivhaus standard, here’s a blog post about it. Hopefully this house will meet the Passivhaus Standard in the future.