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Volumetric CLT, Passivhaus design and the challenges of offsite construction

Beechwood village is an exemplar for use of volumetric CLT in custom build housing. We've asked the project team at Pollard Thomas Edwards architects about their experience, the challenges they encountered and the opportunities they see for this type of construction.

Interview partners from left to right: Tom Dollard, Associate: Head of Sustainable Design; Matthew McColl, Associate; Justin Laskin, Associate Partner. 

What drove the choice to design and build with wood for Beechwood Village?

  • Flexibility – Considering the number of custom-build options at Beechwood we wanted a structural solution that offered us maximum flexibility for fixings, connections, and changes in internal and external materials.
  • Dimensional stability – CLT is hard-wearing, long-spanning, and very accurate, so well suited to the exacting tolerances that are achievable in the factory.
  • Sustainability – We wanted to use a renewable material; CLT is made from a fast growing softwood and has good thermal performance, even before it is coupled with the rest of the thermal envelope.

What do you perceive as the main challenges when building with wood and what are the main opportunities and benefits in your perspective? 

Getting the timber to the factory has a high embodied energy since the panels are currently coming from Austria. It also takes a considerable effort to make amendments to the panels after they are cut. It is essential to thoroughly coordinate the models across disciplines well in advance of “putting saw to wood.” All of the disciplines on Beechwood are using BIM to make sure we have fully coordinated solutions to each of our million possible options, which in turn reduces factory assembly times and the amount of waste.

Looking at different timber offsite solutions, where do you see the particular strengths and differences between volumetric and flatpack?

Volumetric CLT at Beechwood gave us the ability to complete far more in the factory: improving quality and taking weather out of the equation. We are able to put in the windows, insulate and waterproof the modules, and complete all of the internal finishes regardless of the time of year or outside temperature. Time on site is considerably reduced.

As Passivhaus experts, what are your 5 top recommendations when designing and building a Passivhaus with timber? 

  1. Airtightness: Design and install a continuous, robust airtightness layer. 
  2. Service Zone: Create a dedicated internal services zone, so the service holes or internal finishes do not affect the airtight layer.
  3. Continuous Insulation: Minimise amount of timber or steel fixings bridging the insulation zone.
  4. Collaborate: Work with suppliers and manufacturers to understand advantages and disadvantages of their system before you start designing. 
  5. Prefabricate: Consider offsite manufacture to minimise site work and improve quality assurance.

Which (timber) build systems lend themselves best for Passivhaus design?

Passivhaus can be achieved with all types of construction, but the most economical can achieve both continuous airtightness and insulation with minimal effort. SIPS or closed panel timber frame systems with a twin stud or I-Joist are very thermally efficient, but have structural and airtightness constraints. CLT offers better structural performance and inherent airtightness, but requires considerable insulation to the external face to meet the thermal performance.  

If you had 3 wishes, what would you ask the timber industry to do?

  1. A clear and simple process for quality assurance, warranties, and mortgage provision to assist in raising the profile and perception of solid timber / volumetric housing.
  2. Increase forestry to ensure that the supply of home grown timber meets the demand for homes within the U.K.
  3. Work collaboratively to standardise systems and detailing.
  4. Develop a hybrid modular system that combines the high strength of CLT and the reduced weight of timber frame. e.g. A modular system with improved strength to weight ratio that reduces waste whilst achieving increased spans. Advancements in LVL (Laminated veneer lumber) technologies could potentially make this possible.

 

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