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

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Bristol, UK

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Bristol Local Plan Review - Passivhaus as an attractive alternative route to building compliance

Below is my response to the Bristol Local Plan review which is out for consulation. 


General Comments:


I note that the original plan pre-dates the declaration of a climate emergency in November 2018 and therefore does not mention the climate emergency.  For the climate emergency declaration to be relevant it must shape the local plan and realistically must be over-arching and weave through the local plan.  The local plan needs to be updated to reflect this.  

I would also propose a general position where if climate change policies are at odds with other policies then climate change should take precedence.  An example of this in my area of expertise would be for double glazing to be allowed in listed buildings as climate change is more important than conserving the visual appearance of the buildings.  This is a sensitive area and considerable thought needs to be put into the hierarchy of potentially conflicting policies.  This appears to be absent from the local plan and the council’s decision processes in general.

The local plan needs an energy section which sets out a vision for the city’s energy generation, storage and distribution and how the needs of the growing city population will be met.

With the exception of the chapter 13 (see below) my impression is that, with regard to the climate emergency, the policies support incremental changes in the way things are done in the city.  This incremental approach is not commensurate with the climate emergency and I would like to see much more radical proposals for transport, green infrastructure and health and wellbeing.  The transport section in particular looks like business as usual, when presumably it could include further radical proposals for large scale low energy rapid transit systems or perhaps something like an aspiration to make the city centre car free by 2030 and use the space of the roads to create green cycle corridors.

It would also be helpful in the document to make the important links between climate justice and social justice, setting a clear pathway to protect poor and marginalised communities from the impacts of climate change as well as providing better quality social housing and work places.

Chapter 13: Climate Change and Sustainability:

The tile of this chapter is a little misleading and as mentioned above I think climate change (and also sustainability) need to weave through every aspect of the local plan to reflect the climate emergency.  I suggest this chapter is renamed with ‘built environment’ in the name.

I strongly support the overall aims and approaches in this chapter, although there are a few items I disagree with or where clarification is required.  My comments are set out below:

Page 107, Policy text first bullet point: reference to ‘passive ventilation…and passive solar design’ does not take design towards zero carbon and these phrases are actually harmful in the quest for very low energy and low carbon buildings.  The reference to 'passive cooling' in the middle of these is important and worth maintaining.  Passive ventilation is normal in homes but is much improved upon by well designed heat recovery ventilation such as that used in Passivhaus buildings, which provides lower energy consumption, lower climate change impacts, better comfort and better indoor air quality.  In offices passive ventilation is a significant improvement on active ventilation, but again well designed heat recovery ventilation is a much more effective way of conserving energy and improving indoor air quality. 

However heat recovery ventilation often suffers from poor design and there would be merit in Bristol City Council developing its own simple heat recovery ventilation design guide.  

Passive solar design is a vague and outdated idea which involves maximising solar gain and often using thermal mass to store and re-release the solar energy gained.  It sounds positive but is often an excuse to over-size windows, risking over-heating and increasing heating demand in cold and cloudy weather.  

In view of the above I suggest replacing the phrase 'optimal levels of thermal insulation, passive ventilation and cooling and passive solar design’ with ‘high levels of gap free thermal insulation, thermal bridge free and airtight design and construction, heat recovery ventilation, triple glazed windows and insulated doors; combined with careful design of glazing to avoid over-heating and passive and night cooling where needed.’

13.8 I suggest replacing 'AECB CarbonLite’ with the 'AECB Building Standard’, however it is not clear how these standards can be used to help achieve compliance when only Passivhaus is explicitly identified in CCS2.

Paragraph 13.10 - a clear definition of 'zero carbon' is needed in the policy or supporting documentation.  Reference is made halfway down the policy text to 'regulated and unregulated energy’, but there is still considerable ambiguity about the meaning of the term, for example, how is the unregulated energy demand calculated?.   As well as the regulated and unregulated energy being matched by the renewable energy generated or offsetting, there are two other factors which need to be considered when defining zero carbon buildings (Passivhaus: the route to Zero Carbon?, Passivhaus Trust, 2019):

- performance gap (the difference between the energy demand as estimated using SAP/SBEM, and the actual annual energy demand) is estimated by the Passivhaus Trust (2019) to be about 40%; 

- seasonality effect - this accounts for the storage losses required to store energy between demand and generation.  The storage losses account for a further 40% energy (Passivhaus Trust, 2019).

This means that true zero carbon buildings require about 80% more energy to be generated than estimated according to most methodologies.  If these two aspects are not accounted for then the annual demand will exceed the supply.  For the 33,500 new homes proposed by 2036 this would amount to a shortfall of about 3,300 kWh/yr per house or 115 giga watt hours per year.

There are two further issues that need to be considered when deciding on the best low or zero carbon housing policy according to the Passivhaus Trust (2019) :

- peak demand - the higher the energy demand of houses the higher the peak energy demand and therefore the higher the capital costs of implementing the required energy infrastructure;

- space for renewable energy generation - the greater the offset energy amount, the more pressure on land to generate renewable energy and the less land available for building, growing food and green spaces.

The arguments above and more thoroughly set out in Passivhaus Trust, 2019, are why the option of compliance via Passivhaus Certification should be the preferred route in the policy.  

The policy should acknowledge that the approach advocated will not remove the need for energy production for these new homes.  

The requirement for 100% offset of regulated energy needs to be strictly enforced with Certified Passivhaus being the backstop position if this cannot be met for economic or other reasons.

Local afforestation should be considered as a method of carbon-offsetting as this is one of the more reliable means of achieving negative emissions which will be required for Bristol to meet its zero carbon aspirations.

CCS2 - Heating and Cooling Systems - this appears to have been written as if heat networks were the only and certainly the best method of providing low carbon heat.  Two aspects which need to be emphasised here are that the heat comes from a low carbon source and that heat losses are minimised.  In my opinion site QA is the only way to demonstrate that the losses are low and provision of photographic evidence should form part of the requirement to demonstrate low losses.

Options available for low carbon heat are currently large scale inter-sesonal storage of solar thermal heat , heat pumps and hydrogen produced by excess renewable electricity generation.  This is a developing market and the policy should ensure that heat networks do not become the lazy option for developers.

Where demand is very low, eg Passivhaus, this approach is very likely to be counter-productive (De Selincourt, 2018). 

It would be appropriate in this section to recognise the part to be played by air source heating in the heating of homes (Committee on Climate Change, 2019).

Cooling systems should be designed out as far as possible by reducing glazed areas, solar shading and night cooling.  Active cooling of buildings with glazed facades should not be allowed: the building should be redesigned to reduce solar gain.

Paragraph 13.28 - this should include heat pumps as well as cooling units.

CCS3 Adaptation to a changing environment

The reference to over-heating does not require any particular standard to be met and therefore will probably allow the fashion for glazed facades and over-sized windows, which cause over-heating, to continue. 

I suggest that over-heating assessments should be mandatory if glazing percentages exceed the following:

South facade 25% 

or East/west 20% 

(window to internal wall area)


CCS4 Resource efficient and low impact construction

The single most important factor is reduction of cement and concrete usage.  This should be measured and perhaps a method developed for how to determine reasonable quantity.  This could be an aspiration initially with best practice developed over time.  


References (2)

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