The Case for Honest Timber

At Bryden Wood, we’re continually exploring the use of sustainable materials and are committed to the delivery of ‘honest’ buildings where all technical challenges are well-known, and thoroughly analyzed and addressed to avoid unwanted architectural or environmental impacts in design and operation. 

Our design approach supports construction systems where each component is designed for purpose, without the need for additional redundant materials that generate unnecessary waste or additional embodied carbon. Engineered timber fits this approach as it can perform as a structural element as well as an architectural finish. Engineered timber is a sustainable product due to its low carbon footprint. It’s also lightweight, strong, stable, easy to handle, suitable for prefabrication, has thermal properties, as well as great aesthetic appeal.

Extract of data published by Price and Myers for the embodied carbon of superstructures in projects with 2 to 10 stories and excluding basements.

Today, engineered timber is a suitable solution to reduce embodied carbon in buildings but this may change in the future once the construction industry moves towards new materials as an alternative to standard concrete and steel.

Constructing with timber presents its own technical challenges, but using a science-based approach to find the best solution means we can deliver not just an honest building but a resilient, high-quality asset. 

End-of-Life Global Warming Impact

Although timber captures carbon, if left to rot naturally in an open-air landfill, it releases the stored carbon as well as methane, which has a greater global warming impact than CO2. If incinerated, it generates energy and does not release methane but still releases the stored carbon along with other pollutants. However, if reused or upcycled, carbon remains stored offering long-lasting environmental benefits.

Some of these end-of-life difficulties with engineered timber relate to its size. The use of screeds, which are bound to the surface of timber and structural fixings also make it difficult to dismantle and reuse. 

Long-span engineered timber elements also undergo non-reversible long-term deformations that can limit its feasibility to be reused as a structural element. However, upcycling these elements is still a viable solution.

Our response is to: 

• Engage early with contractors to plan deconstruction scenarios and reduce waste sent to landfill. 
• Adopt circular economy strategies for deconstruction and reuse, including:
• Upcycling materials to put back into the local supply chain.
• Adopting the principles of buildings as material banks, urban mining, and use of material passports.
• Using bolted connections and smaller structural grids, which facilitate disassembly, cutting (if needed) and deconstruction. 

• Use membranes to decouple the slab from the screed and explore alternatives, such as dry screeds, sand and gravel screeds, floor dense boards, particle boards or cardboard and sand layers. 

Procurement of Timber and Distance

Currently, most timber used in the UK for construction is manufactured and imported from mainland Europe. Depending on the distance, this can have a significant impact on carbon emissions.

Our response is to: 

• Conduct detailed whole-life carbon analysis of buildings, including harvesting, processing, manufacturing, and transportation to end-of-life disposal.
• Prioritize locally produced engineered timber and strategically select timber that can be shipped instead of transported by road.

At Bryden Wood, we have explored local sourcing of engineered timber. The results show that distance is very important and the carbon emissions from northern France, Belgium and western Germany are relatively small. In the case of Spain or Sweden, even though these materials can be shipped by boat (lower carbon emissions per km), the distances are so large that they amount to more than the manufacturing carbon emissions.

Accounting for Biogenic Carbon Capture when Carrying Out a Whole Life Carbon Assessment 

Biogenic carbon capture is the process whereby trees absorb and store carbon dioxide during growth. Once cut down, part of this carbon (from leaves, roots, and small branches) is released and the remaining is stored in the trunk, which is used for timber. 

New trees are planted in its place to ensure a constant process of removing carbon dioxide from the atmosphere. As trees age, they absorb less carbon dioxide, so it is beneficial to replace them. This is known as a sustainably managed forest.

Our response is to:

• Recommend that biogenic carbon capture is only accounted for if the timber has a responsible sourcing certificate, a clear circular strategy, and a clear commitment to facilitate reuse.
• Conduct a comprehensive circularity plan that includes solutions for deconstruction and a plan for reuse, upcycle and recycle.

The Impact of the Timber Industry on the Natural Environment 

To meet the increased demand for engineered timber, we need to carefully consider land availability and the impact on ecosystems.

While sustainable forest management (FSC or PEFC) is the best tool available to ensure a reasonable exploitation of timber, these certifications aren’t perfect. Natural forests are complex biodiverse ecosystems that capture carbon, not just in trees but also within soil. A ‘tree plantation’ unlike a forest, may not enhance biodiversity and may have a reduced capacity to store carbon in the soil. 

One major risk related to carbon accounting and forest management is that Environmental Product Declarations (EPDs) may not adequately account for the carbon released from decaying root net and from the soil when cutting trees. This can be largely underestimated for most timber products. 

We must also acknowledge that sustainable foresting cannot produce enough timber to respond to global construction needs. Engineered timber cannot substitute or offset the use of concrete and steel but its use should be prioritized in the right type of buildings.

Our response is to:

• Ensure timber specified is of a sustainable nature, by using PEFC or FSC certification schemes. 
• Have a critical view about sustainable forest management and explore alternatives.
• Interrogate timber manufacturers on their root and soil carbon accounting.

 

Acoustic Performance

When designing with engineered timber special attention is required to reduce vibrations, noise transmission and reverberation time.

The layered nature of engineered timber can improve acoustic compartmentation and if properly designed, does not need the same mass as concrete to achieve the same airborne sound resistance. However, detailing is the main challenge and if not properly resolved it can generate sound flanking at the joints which is difficult to resolve without the use of wet trades. Creating reliable diaphragm action through slabs when trying to achieve acoustic separation and control movement is also fraught with issues.

Other acoustic considerations include the noise from building services and potential increased reverberation time due to smooth surfaces.

Our response is to:

• Engage in early discussions with specialist timber contractors and acoustic engineers to address all potential acoustic difficulties.
• Use additional mass (increase the thickness of the slab) and acoustic ceiling and wall panels.
• Decouple floor finishes from the slab with additional insulation.
• Use alternatives to wet screeds, slab breaks above partitions, and resilient strips between CLT panels. 
• Coordinate with engineers to reduce noise transmission caused by HVAC systems.
• Use in smaller size, lower buildings to reduce acoustic transmission and complexities.

Durability, Rotting, Installation in Wet Conditions

Significant volumetric changes can occur to timber exposed to changes in moisture, including swelling perpendicular to grain direction, warping, and bending of straight elements. With engineered timber overall deformations are less likely due to the controlled material use and grain directions, but it is still sensitive to swelling if exposed to moisture and water. This makes it necessary to adequately waterproof the envelope of the building and protect the material during transportation and construction.

Due to high demand, timber sold in the UK is typically not dried out properly. As it dries it changes geometry losing original accuracy. This is a problem as connections cannot be relied on geometrically and present a challenge, particularly for DfMA solutions and other situations where small tolerances are required.

Our response is to:

• Transport and store timber in waterproofed flat stacks.
• Engage early with contractors to produce an on-site maintenance plan with special attention to timber elements.
• Enable sufficient drying periods within the project program.
• Use preservative natural treatments to prevent woodboring and avoid the use of toxic chemicals such as chromium, chlorophenols, or arsenic. 
• Use preservative natural treatments on all sides to prevent water ingress. Oil-based primers penetrate wood better than latex or water-based primers, providing greater resistance and are less likely to be scraped off during construction. 
• Consider design details to prevent water ingress during construction and use.

 

Fire and Insurance

Since the Grenfell tragedy, public policy has shifted towards a zero-combustibility approach in certain higher-risk buildings, making the use of engineered timber more complicated. This has increased insurance costs for mass timber construction. It is common practice to solve the problem by enclosing timber for increased fire protection. Unfortunately, this generates additional carbon emissions and waste. 

Another issue with glulam and CLT is the debonding of layers during a fire.  The glue can start evaporating at a relatively low temperature causing engineered timber to fall apart even before it burns. If the timber only chars, it might remain stable but may not be suitable for compartmentation as smoke can break through at joints.

Our response is to:

• Use engineered timber in lower buildings with a decreased risk of vertical propagation. 
• Involve specialist contractors and fire specialists from early stages to ensure the material’s structural integrity in the event of fire and robust coordination between disciplines.
• Engage early with statutory bodies, supply chain, and timber contractors.
• Consider the use of sacrificial layers of timber to protect the rest of the structure and potentially result in self-extinguishment.
• Use timber fire treatment such as intumescent paint to prevent the spread of fire and reduce the amount of smoke produced, taking account of the impact on recyclability. 
• As a last resort, consider full fire bonding if there is a risk of debonding of layers. This scenario is appropriate in multi-story residential buildings.

Other general concerns when using CLT include:

• Volatile Organic Compounds (VOC’s)
• Shortage of construction expertise
• MEP co-ordination
• Cost

 

Conclusions for the Industry

Timber is a renewable material that has the potential to help decarbonize new buildings. However, should the industry move towards using more timber construction, local supply may not be able to meet demand leading to a less sustainable product. 

The technical difficulties don’t eliminate the use of engineered timber but it’s clear there are many challenges that require more complex integration and technical knowledge than conventional construction systems.  

The application of timber is wide-reaching from engineered timber solutions to simple uses of lumber in isolated instances. The right type and application of timber needs to be considered alongside the building typology. However, our prevailing view is that timber is the best structural option, particularly in designs driven by carbon. Timber should be used in low-rise buildings such as residential, schools and retail, and could be introduced as upper stories on a variety of taller builds mixed with other construction systems. It is not the solution for all buildings, but it has its place within the industry and should be maximized within its constraints. 

Technical areas that require further investigation and that may shift the public perspective of engineered timber include the adequate carbon accounting of root decaying and soil, and the impact of sustainable forest management on ecosystems. 

 

Article by Director of Sustainability Pablo Gugel, Head of Sustainability and Building Physics Helen Hough and Mya Patel.

 

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