Designing a structural timber project to low-carbon or even Passivhaus standards is not the cheapest or the fastest way to build - but it doesn’t aim to be. What it does deliver on is exceptional levels of longer-term affordability, interior comfort, health, wellbeing and durability.

Having a client who embraces these benefits, rather than considering only the up-front cost, is the first step to success. And herein lies the challenge, especially in this time of extremely tight budgets.

Architects and contractors can help clients be more ambitious by showing what can be achieved with commitment, and encouraging them to take a longer-term view of potential cost and carbon savings.

A tight-knit design team is also essential. Clear collaboration and buy-in is needed across the whole build and design team, including subcontractors and stakeholders.

Our design for the first Passivhaus primary school project in Scotland to use a cross-laminated timber structure, led by project director, Clara Garriga, demonstrates what can be achieved with an ambitious client like The City of Edinburgh Council on board.

The two-storey extension to the Grade B-listed Sciennes Primary School – which offers four additional classrooms and flexible teaching areas – opened its doors in September, having achieved an impressive air tightness rating of 0.28 ach/hr@50Pa. To compensate for the lack of winter solar gain, the design centred on a fabric-first approach, addressing air tightness and thermal performance – including triple glazing.

The CLT structure creates a warm and inviting atmosphere, which also contributes to carbon storage and environmental preservation, with every cubic metre of CLT locking in one tonne of CO2.

Showcasing the timber within the classrooms was positive for pupils and staff. It provoked questions about how the building was constructed and where the timber had come from, which we made into a factsheet for the kids.

Timber really is the perfect choice for Passivhaus, as it generally achieves a better air tightness than a steel structure. As it can be replenished and recycled, it’s also good for embodied carbon. However, there were some initial nerves from the council over getting a largely timber building to meet fire regulations.

We overcame this through a combination of treating the CLT with a fire retardant and covering some of it up with fire-rated gypsum board. We did have to compromise on the amount of CLT we could leave exposed internally - around 25%, when we had hoped for more. The process was more complicated than expected, but as more industry guidance emerges on timber buildings and fire, this will get easier.

Also, just complete – and currently awaiting Passivhaus certification - is Buntingford First School, Hertfordshire’s first fully net zero school which has been worked on by our architect James Bowie. This trailblazing sustainability project for the county again uses a CLT frame construction.

Part of our brief was to reduce the embodied carbon of the construction materials, so innovative trackers were used to enable the site team to produce as-built carbon data to compare with the initial project targets.

The project was design-intensive up front, as everything had to be manufactured prior to construction work beginning, which required great effort and co-ordination across the design team. However, as one of the biggest advantages of structural timber is obviously the speed of assembly, this time was clawed back once on site, with the structure flying up within 5-6 weeks.

As more projects like Buntingford and Sciennes reach completion, they will serve as important exemplars across the rest of the UK on just what is possible.

We’re already seeing more clients switch on to the benefits of timber Passivhaus design. In June, construction work began on Blairgowrie Recreation Centre, Scotland’s first ultra-low energy swimming pool and leisure centre, for Perth & Kinross Council.

With its exposed wooden beams, we aim to create a soft and welcoming space which is markedly different from traditional designs for civic leisure facilities.

Although construction is at an early stage, Holmes Miller architect Yida Zhou and the team have already learned from the development of the design. For example, having studied CLT's hygroscopic behaviour to identify a suitable external wall/roof membrane approach when CLT is used as both a structural and finish material in the Pool Hall.

Applying a breather membrane to CLT is a conventional approach widely recommended by industry including the Structural Timber Association, however, it was demonstrated that this approach would result an accumulation of interstitial condensation and significantly compromise the performance of the thermal envelope.

Responding to this, we proposed using a vapour barrier which initially caused some apprehension within the design team. Despite this it was agreed upon, following scientific-based analysis demonstrating how it limits the amount of moisture moving from inside the building to outside, preventing interstitial condensation and reducing risk of excessive moisture accumulation within CLT.

Because this is only the second Passivhaus leisure centre in the UK, we’ve had to go through a steep learning curve to demonstrate everything to the certifiers, but once this process is complete, it will become easier for others to follow suit. We hope more councils will be inspired to consider Passivhaus standard for leisure projects, especially in the face of high energy bills when it’s estimated to save up to 70% compared to a conventional leisure facility.

The work by Holmes Miller architect Guillem Arraez on the Michelin Scotland Innovation Parc (MSIP) in Dundee, a joint venture between Michelin, Dundee City Council and Scottish Enterprise, has also taught us a lot about successful use of structural timber.

Our Innovation Hub project at MSIP will open its doors in November. When initially discussing the project with the client, we suggested a structural timber approach would be most appropriate for a building that will be used to promote a green agenda.

While the Hub was not briefed to us as a Passivhaus project, we followed a Passivhaus approach. Due to the building’s compact shape, it achieved similar air-tightness values (1.87 ach/hr@50Pa, when Passivhaus would target a score of under 1).

With this project – unlike a school, which has stricter acoustic requirements – we were able to leave the glulam structure, CLT panels and even the junctions exposed internally. The high-quality finish of the timber means it still looks attractive, and visitors can see how the building was put together - rather like a giant Lego set.

The entire structure of the building went up in just three weeks. Because the client wanted an industrial look for the exterior, the building was then wrapped in sinusoidal cladding which took only a week to achieve.

As well as its green credentials and speed of assembly, structural timber is increasingly popular due to its positive impact on user wellbeing. When you are in a space created mainly with natural materials, you simply feel better.

Ultimately, if you put the technical expertise, intricacy, and hard work aside, it’s clear what it really takes to make low carbon timber buildings a reality - commitment.

As architects, it’s our role to demonstrate that commitment is not only possible, but essential if we are to deliver net zero across the built environment. Hopefully that job will get easier as more successful low-carbon timber projects open their doors.

Use of structural timber in low carbon design