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Passivhaus and small houses – is small beautiful?

May, 2019|

My last article was about houses with poor Form Factors & the associated impacts on meeting Passivhaus. Our project at Dart House had a Form Factor of 4.0 (the ratio of external thermal envelope to usable floor area) in contrast to our lowest project Form Factor to date of 1.8 (a row of terraces). The conclusion was that all things are possible, but there will be a price tag attached – in this case an extra £300/sqm of GIA (gross internal floor area) for the supply of our PH15 construction system.

Since then we have been working on another PH15 project, Studio Cottage, that trumps even Dart House, at a whooping Form Factor of 4.6! How did it get so poor, you may well ask, & how can it be worse than that house you showed us last time? The answer is that it is small (<100 sqm GIA), whereas Dart House is large (298 sqm GIA). With Passivhaus you can simply get away with more, by being large.

If you want to dig deeper into the maths (you might), then have a look at the excellent article on the subject, written by Nick Grant and Alan Clarke, available on the UK Passivhaus Trust website under the ‘Guidance’ tab. The title is not enticing, ‘Internal Heat Gain Assumptions in PHPP’, but not everything good comes with great packaging! I will try to summarise briefly here.

In any house you are balancing out energy losses with energy gains, that is if you want to maintain your interior temperature. In energy-efficient buildings, like Passivhaus, solar gains (heat from the sun arriving through your glazing) and incidental gains (you, your children, lights & appliances) become much more significant and help reduce the amount of heating energy you need to add-in (hopefully using a minimal heating solution like towel radiators etc.). The article deals with the question of how PHPP quantifies these incidental gains. The PHPP method is to link incidental gains to occupancy, and then to base occupancy on a presumption of 1 person for every 35 sqm of floor area. Applying this to Dart House we would assume nine occupants, but in fact the house only has two! This means that PHPP bestows a gift to Dart House, and the required heating input will be offset by the assumed internal gains (nine people not two!), a much larger amount than justified. On the other hand, poor old Studio Cottage gets no such ‘leg up’.

On top of this blow, maths hands out a further hurdle for the small house – it will struggle with a higher surface area for a given floor area (honestly, its true, work it out if you don’t believe me). With Studio Cottage the owners also wanted a double height living space, a further loss to that important usable floor area, & up the Form Factor climbs again. Now add in a final blow, a difficult plot with limited ability to maximise solar gain from orientation, in this case a very small urban plot with lots of shading. Oh dear, that modest budget and modest ambition is not getting you any help from Passivhaus is it! Small is not beautiful, in our Passivhaus world.

In a situation like this, we end up looking at all sorts of ways we might shave off those final few kWh of heating energy in order to pass the magic 15 kWh/m2 per year, that Passivhaus demands. But the reality is, with hurdles like this, the extra work starts to get a bit silly, incurring significant costs for only minor gains. That is when you need to ask yourself the hard question of what makes practical & financial sense. You may have set your heart on Passivhaus Classic, but how critical is it?

In this scenario, we might advise that you accept the miss & apply for the Passivhaus Low Energy Building Standard. It is still an excellent achievement and just the ticket for those tricky small houses on challenging sites. The standard will still take you through the certification process and ensure that quality approach. We calculated the miss for Studio Cottage as within a range of 2 kWh/m2 a year. (The maths is then 2 kWh x 92 m2 = 184 kWh each year extra @ 4 pence/unit (assume gas) x 184 units = £7.36). It really is not worth fretting over in my view, and certainly not worth the disproportionate interventions in the build that could well cost £1000’s. It is a shame that the Passivhaus Institute has not (yet) been open to adjusting PHPP to tip the balance more fairly towards the smaller scale house, it is after all what we should be encouraging, building for sufficiency. Maybe the referenced paper should be resurrected and pushed at the PHI one more time – or at least we should be aware that sometimes the Passivhaus Low Energy Building Standard might just be the right answer. 

Jae Cotterell 

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Buildoffsite Property Assurance Scheme (BOPAS) was developed to address these concerns and perceived risks associated with innovative construction. BOPAS is recognised by the principal mortgage lenders as providing the necessary assurance underpinned by a warranty provision, that the property will be readily mortgageable for at least 60 years.