Sunday 26 February 2023

Scotland to Adopt Passivhaus Standard for new Houses


Linn Way, Garelochhead - a Passivhaus Development of 10 homes by Argyll Community Housing Association (ACHA) 

On 15th December 2022, Patrick Harvie, Minister for Zero Carbon Buildings, Active Travel and Tenant's Rights (there's a job title for you), wrote to Alex Rowley, a member of the Scottish Parliament to inform him that his campaign for new homes in Scotland to be built to Passivhaus standard would become law.

Back in November, Rowley had  introduced a Member's bill to the Scottish Parliament, the "Proposed Domestic Building Environmental Standards (Scotland) Bill" – which would introduce new minimum environmental design standards for all new-build housing to meet a Scottish equivalent to the Passivhaus standard.

The bill followed on from a public consultation which Rowley had organised earlier in the year.  

The consultation received 629 responses which included submissions from  Cala Group Ltd,  Barratt Developments PLC, the Scottish Federation of Housing Associations and The Royal Incorporation of Architects in Scotland - people were taking this seriously!

Having garnered the support of more than 18 MSPs from at least two political parties, the Bill was due to be debated in the Scottish Parliament.  Instead the Scottish Government choose to avoid the debate by announcing that it intended to give the Bill effect through subordinate legislation within two years.

"I hereby state that the Scottish Government will make subordinate legislation within two years, to introduce new minimum environmental design standards for all new-build housing to meet a Scottish equivalent to the Passivhaus standard"  Patrick Harvie MSP, 15 December 2022

So where does this leave the future direction of Scottish Building Regulations?

The Passivhaus standard is remarkably simple to summarise

1. Maximum space heating demand cannot exceed 15kWh per square metre of net living space per year, or 10W/m2 of peak demand

2. The Primary Energy Demand (total energy used for all domestic applications (heating, hot water and domestic electricity) must be less than 60kWh/m2 per year.

3. Airtightness cannot exceed 0.6 air changes per hour at 50 Pascals pressure.

A key difference between the Passivhaus standard and the Scottish (and rest of UK) building regulations is that Passivhaus is absolute while Building Regulations are relative.  

UK building regulations work by taking a house shape you want to build, calculating the annual energy use if that house was built following an approved specification (the notional house).  The specification for the house you build cannot be worse than the notional house.  

You can read more about how UK building regulations for energy work in this earlier post.

Buildings that minimise the ratio of their surface area to their inside volume are intrinsically more energy efficient than buildings with a high surface area to volume ratio.  Simple rectangles outperform complicated (but architecturally appealing) building forms with cross wings, porches, bay windows, dormer windows and the like.  Large buildings out perform smaller buildings on this measure too.  Linked buildings (terraced, semi-detached) outperform individual buildings.

Credit: BRE Passivhaus Designers Guide

Passivhaus rewards efficient building form because it is concerned with the absolute energy use, UK building regulations ignores building form because it compares the performance of that house design with a notional house of the same shape.

In terms of the resulting specification that might achieve the standard, the Passivhaus Institute estimates that for most cool-temperate climates

Walls, floors, roofs: A heat transfer coefficient (U-value) of 0.15 W/(m²K)

Windows:  U-value of 0.80 W/(m²K) 

The notional house specification for Scottish Building Regulations 2022 uses a U value of 0.15 for walls, 0.12 for floor, 0.09 for roofs and 1.2 for windows.   So with the exception of windows, Scotland will be building homes that already largely meet or exceed the specification, so long as building shape and orientation does not require higher levels.

Air permeability is 5m3/h.m2 in Scottish Building regulations notional house.  Taking a storey height of 3m, this translates into 1.67 air changes/hour.  To get to 0.6 air changes/hour will require a big improvement to an area that has proven to be challenging in the past, especially when building in volume with traditional methods rather than off-site manufacture.

The final big change is  that Scottish (and UK) Building Regulations only consider energy used for space heating, domestic hot water, and electricity for lights and services (pumps and controls).  It ignores the main part of electricity used in the house to run appliances and plug-in devices - a weakness that becomes increasingly noticeable as homes become better insulated.

By contrast the Primary Energy Demand requirement in the Passivhaus standard considers all the energy use in the property with allowances for electricity consumption by residents using appliances and electronic devices.

It is worth noting that energy generated by Photovoltaic (PV) systems may not be counted against the Primary Energy target in the Passivhaus calculator (called PHPP). This is a deliberate implemented to prevent poor standards of energy efficiency being offset by the use of renewable energy.  As emerging technology maximises the use of solar generated electricity (for example solar diverters and battery storage), this is looking increasingly anachronistic.

The Passivhaus Institut is moving towards including PV  generation in Passivhaus certification. This will be in the form of new classes "Passivhaus Plus" and "Passivhaus Premium". These standards require the same fabric standard as any other Passivhaus but higher reductions in the primary energy demand compared with the existing Passivhaus standard, normally achieved using on-site generation.

This decision by the Scottish Government has really has the potential to overturn an orthodoxy that for years has based energy efficiency standards for homes on the government's own calculation - the Standard Assessment Protocol (SAP), which has been the overarching mechanism to demonstrate compliance with building regulations on energy for decades.  

How revolutionary this change turns out to be, well, time will tell - after all the Scottish government only committed to a 'Scottish version of Passivhaus' which could be as limited as a new version of SAP with exceptionally high U values and airtightness in the notional house.  Alternatively if the founding principles of Passivhaus are followed, it could turn out to be a radical shift in the way the building regulations are delivered in the UK.