The Home-Grown Homes Project Shows How to Deliver Zero-Carbon Homes

Buried in this big report is a lot of really useful information about how to build.

Net Zero Whole Life Carbon

Woodknowledge Wales

The Home-Grown Homes Project is described as "a 33-month research study to design and test out interventions that could substantially improve the timber construction supply chain in Wales." The study, completed in December of 2020, was put together by Woodknowledge Wales (WKW), a non-profit with the mission to "champion the development of wood-based industries for increased prosperity and well-being in Wales." Much of the project is about promoting the use of wood. The introduction explains:

"The purpose of the project has been to identify and test out interventions that, if applied, could have a transformative impact on the Welsh timber construction supply chain in particular and on the delivery of low carbon social housing in general."

However, a key component of the project is the development of a framework for what they call Net Zero Whole Life Carbon Homes, with a comprehensive look at every aspect of a building, summarized in five easy steps:

1. Minimize Embodied Carbon

Embodied Carbon

Woodknowledge Wales

Embodied carbon is composed of upfront carbon, the emissions that come from the raw materials supply, transport and manufacturing of building products, the transportation to site, construction, and installation. Other sources of embodied carbon come from maintenance, repair and refurbishment (which is why one wants durable products), and end-of-life emissions. They use a numerical target set by the RIBA 2030 Challenge. Woodknowledge Wales has also produced one of the most comprehensive guides to embodied carbon that I have seen yet.

2. Minimise Energy Demand

Building form
Comparison of fabric characteristics associated with two form factor typologies to deliver the same space heating demand.

Woodknowledge Wales

Here again, they pick up the targets from the RIBA 2030 Challenge which are essentially the same as Passive House. They provide a terrific guide to zero-carbon homes written by Dr. Rob Thomas of Hiraeth Architecture Ltd and James Moxey of Woodknowledge Wales, which on its own is a keeper if you want to understand these issues. They remind the reader that form factor matters, that it is easier to hit the numbers with simple buildings and multifamily buildings, noting "the direct relationship between form factor and space heating demand – the higher the form factor, the greater the requirement for a high-performance fabric. This is a simple concept – detached houses have a much greater heat loss area than a mid-terrace property sharing two party walls with neighbours," which is one reason we were recently discussing bans on single-family houses.

The document also deals with building fabric or envelope, foundations, and a lot more information about how to reduce embodied carbon.

3. Only Use Renewable Energy

This would be pretty straightforward in most of the world, but this is the UK where burning wood is considered renewable. So they write:

"We recommend Welsh Government review the support given to wood burning (as the least carbon-efficient use of timber) in favor of wind, solar and tidal alternatives. Equally, the biomass subsidy as currently applied is diverting timber away from the manufacturing and construction sectors and doing little to bring under-utilized woodland into management."

4. Minimize the Performance Gap

Here they address the question of build quality, which in much of the UK is pretty awful. Again, they have produced a thorough and important document, a Building Performance Evaluation Guide that's valuable wherever you are building. Fionn Stevenson writes in the introduction that "the performance gap between expected carbon emissions from new housing and what actually happens is often shockingly underestimated" and asks:

"How did this happen? How could there be so much ignorance about the reasons for this vast performance gap? And how are we going to reduce the gap in time? One reason it’s there is that no-one bothered to find out what the gap was, and so housing development in the UK continued for decades in ignorance of the design and construction failures mounting up."

It's long and detailed and technical, all about teaching, testing, and meeting standards. This should be part of every building program, but in many places, they don't even require a basic blower door test. Fionn Stevenson gets it right in her conclusion to the forward, about how important this all is:

"My hope is that this guide finds its way into every housing organisation’s computer and onto every housing building site, to help clients, design teams and contractors produce the best possible housing that they can by understanding what is really going on."

5. Offset to Below Zero

Here they are putting their Woodknowledge Wales hat on, noting that "a factor of safety should be applied to take us below zero to account for uncertainties in calculation methods." They would do this through afforestation, noting in yet another thorough document that "new woodland creation is one of the most cost-effective ways of reducing carbon emissions and offsetting our nation’s carbon footprint."



Woodknowledge Wales

The document concludes with the statement that "The Home-Grown Homes project is the work of many passionate, patient and persistent individuals across a number of organisations in Wales and beyond." This is an understatement; these people have created a document of significant value.

The documents here lay out a framework for Wales, but could be applied just about anywhere; if you want to learn about how to design, build, and ensure you actually have a zero-carbon home, go to Woodknowledge Wales but prepare to be stuck there for a while, there is so much to learn here.