Effects of Internal Heat Gain assumptions in PHPP

Passivhaus experts Nick Grant and Alan Clarke have written a paper in which they highlight the effect of internal heat gain assumptions across a range of building sizes and usage.

The paper addresses that basing internal heat gain (IHG) calculations on floor area leads to misleading predictions of heating demands for small buildings. The Passivhaus standard favours large dwellings as well as those with more efficient shapes. A small building will have a higher external surface area for a given floor area and this has led to the use of extreme U values and additional areas of south glazing on what would otherwise be low cost modest dwellings. 

The proposal for the rate of internal gains to be dependent on floor area in the domestic PHPP is not meant to be a “cheat” to get small houses through a loophole, it is simply a corollary of looking more carefully at the model and reality. PHPP has demonstrated the worth of assuming a realistic internal heat gain figure for making good predictions of heating demand. However as we have seen, basing IHG calculations purely on floor area leads to serious errors for small buildings. We propose that as PHPP is used for a wider range of building sizes and uses, that the basis for internal heat gain calculations is reviewed. 

The paper concludes that PHPP can better reflect the non-linear relationship between Treated Floor Area and Internal Heat Gains. The refined PHPP model shows that in reality, even tiny Passivhaus buildings should be no more difficult to achieve than large ones.

Download the full paper here.

Download the presentation slides from the 2014 International Passivhaus Conference here.

29th April 2014

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