Internal Heat Gain Assumptions in PHPP
Passivhaus experts Nick Grant and Alan Clarke have written a paper in which they examine the effect of internal heat gain assumptions across a range of building sizes and usage.
One of the ways in which PHPP has closed the performance gap between modelling and reality is to assume low internal heat gains. The simplicity of a fixed W/m2 internal gains figure for each building type is attractive, however in practice we have seen this lead to anomalies at extremes of building size and usage. One example of this is the “small house problem” where a design for a two person retirement dwelling of 40m2 had no sensible design solution to meet the Passivhaus standard, yet Passivhaus components would clearly deliver a very low energy design with Passivhaus comfort levels.
Another case is the difference between schools in the UK and in Germany: occupancy in the UK is usually higher in terms of pupils per m2, and the higher levels of internal heat gain mean the optimum building design is different.
In order to design comfortable and cost effective low energy buildings we need a useful estimate of internal heat gains. This estimate can never be accurate because the occupancy and technology associated with a building will vary over its lifetime. However there is potential to improve considerably on the current assumptions in PHPP in terms of accuracy, and in terms of delivering the most effective building design. As we will see, heat loss and internal gains are very non linear for small buildings.