Passivhaus Consultant and Designer, Bristol.  Very Low Energy Buildings.  Domestic and non-domestic.  New build and refurbishment. 

07502 425158

Bristol, UK



Passivhaus and the Law of Increasing Returns

I was recently discussing Passivhaus with an architect with strong green credentials, who argued that the Law of Diminishing Returns meant that Passivhaus was not the mainstream answer to our need to reduce building energy demand.  This got me thinking...

I concluded that Passivhaus turns this law on its head - hence the title of this blog post.

The law of diminishing returns can be illustrated by considering solid wall insulation:

  • uninsulated solid wall of U value 2.1W/m2
  • add 100mm of generic insulation of k=0.03 W/mK and achieve a U value of around 0.3W/m2
  • add another 100mm of insulation and achieve a U value of 0.15W/m2K

So the first 100mm insulation achieves a reduction in U value of 1.8W/ m2K and the second achieves a reduction of 0.15W/m2K.  The first part of the insulation therefore achieves 10 times more energy benefit than the second.  [Of course the second 100mm gives us other benefits around comfort and condensation risk which are a subject in their own right].

But Passivhaus isn't only about increasing the thickness of the insulation.  I prefer to think of the uninsulated house as a leaky bucket.  Mend it badly and it is still full of holes.  Mend it well and it works properly.  The benefits or returns are maximised and increased only when all the holes are completely filled.  This isn't a perfect analogy, but I still believe it is a useful one.

Passivhaus is about attention to detail during design and construction, particularly continuity of insulation and airtightness at junctions.  With the right team these things come relatively cheap.  If we take a similar example but for a new build:

  • 200mm insulation on the wall meets Building Regulations with a bit to spare;
  • thermal bridging at junctions is worth an additional 0.15W/m2K on the U value if the junctions are not well designed (according to SAP - and in my opinion, a useful approximation for this example)
  • air leakage at junctions could be worth the difference between 0.6 air changes per hour (ACH) and, for example 6 ACH
  • based on a house of wall surface area 150m2 and volume 350m3 in Bristol with 68kKhrs annual heating degree hours with the air leakage through the wall junctions accounting for a quarter of the total, the heat loss through the poorly detailed wall is 3885kWh/yr, whilst the heat loss through the well detailed wall is 1612kWhr/yr.  

So paying attention to those junctions, which requires little more in the way of materials, lots more in the way of design consideration and more skilled workmanship results in a 60% reduction in annual energy use.   This is the Law of Increasing Returns in the context of Passivhaus.  





Indoor air quality in the news and blogosphere - what's the answer?

The Guardian and Sustainable Homes blog have both recently published pieces about indoor air quality and energy efficiency.  Both articles highlight that increased energy efficiency is linked with poorer indoor air quality which is associated with health problems, particularly asthma.  The issue arises as more energy efficient buildings are designed and constructed with higher levels of air tightness.  This means that air cannot leak through cracks and gaps in the building fabric to refresh the internal air with air from outside.

However, the air we breathe and the fresh air which flushes out contaminants from your house are not supposed to be replaced by flow through gaps in the fabric, the fresh air is supposed to be drawin in through background ventilators, usually trickle vents in windows, by extract fans in kitchens and bathrooms.  The problem is that the trickle vents are often not sufficiently large and the extract fans are set on low speeds to avoid noise.  With increasing airtightness the gaps in the building are not there to provide a safety net.  There is also a problem when the fans are off, when wet rooms are not in use.

One answer is to use continuous fans which run quietly and continuously at a background speed and then increase the speed to purge ventilate when bathrooms and kitchens are in use.  This can provide sufficient ventilation, but it also purges heat from the builidng.

The best solution - well designed, high quality, silent, heat recovery ventilation (HRV) - such as this unit at Toronto Road, Bristol.  Come and see it on Saturday 26 September when it is open for Bristol Green Doors.

Ventive is an innovative alternative which can be installed in chimneys as part of a refurbishment, with less disruption than full HRV. 


Reedley Road - performance gap eliminated

Predicted specific heat demand 36 kWh/m2/yr.  Actual specific heat demand 24.5 kWh/m2/yr.

This Passivhaus inspired refurbishment project exceeds the design expectations with over 80% of the heating provided by a wood burner in the kitchen/living area.   The specific heat demand is just below the threshold for the Passivhaus refurbishment standard EnerPHit.

Come and see it at Bristol Green Doors 26 September 2015. 

Reedley Road Design Stage PHPP Modelling



Our houses on show at Bristol Green Doors 26 September 2015

EWI back of Toronto RoadCome and see the completed Passivhaus inspired refurbishment at Reedely Road and the almost complete refurbishment at Toronto Road at Bristol Green Doors.


Mind the Gap - Talk at Constructing Excellence Gloucestershire Club

Come and see Piers's talk tonight at Constructing Excellence Gloucestershire Club where he will present the findings of his Innovate UK Building Performance Evaluation work undertaken with Quattro Design Architects on 'The Main Place', Coleford Community Centre.