Natural Materials in Construction: Demystifying Performance, Cost, and Application

As a RIBA Chartered Practice, we are committed to designing buildings that are not only beautiful and functional but also fundamentally sustainable. Increasingly, our clients are asking about the benefits of natural materials — timber, stone, straw, lime, lime, clay, and innovative plant-based insulations like hemp. There is growing interest, but also persistent questions that are often asked: Are they more expensive? Do they perform as well as modern synthetic products? Do they suit a retro fit project, whether it’s an old stone cottage, a Victorian terrace, or a larger Estate House?

This article sets out to address those questions, highlight some of the most exciting natural materials we are detailing today, and show why we believe they should no longer be seen as niche or experimental and open for a wide use of applications.

What are Natural Building Materials

Natural building materials are those derived with minimal processing, from renewable or naturally occurring sources, unlike synthetic products that rely on petrochemicals or intensive industrial manufacturing. Common examples used historically include timber, earth, straw bales, hempcrete, clay, lime, stone, and wood-fibre insulation. Their use reconnects construction to ecological cycles while reducing reliance on high-energy synthetic alternatives such as plastics, foams, and concrete composites.

What are the Benefits using Natural Materials in my Building

Using natural building materials in construction projects offers five key advantages:

1.      Healthier Indoor Air – low Volatile Organic Compounds (VOC) emissions and natural humidity regulation.

2.      Thermal comfort – strong insulation and breathability.

3.      Sustainability – renewable, low-carbon, and biodegradable.

4.      Durability – resistance to mould, damp, and structural decay.

5.      Aesthetic and Biophilic Qualities – tactile, calming finishes that foster connection to nature.

Are Natural Materials more Expensive?

There is no denying that some natural products carry a higher upfront material cost, particularly in a market still dominated by conventional building systems. However, it’s important to look at the full picture:

• Lifecycle value: Buildings constructed or retrofitted with natural materials often have lower running costs thanks to improved comfort and moisture management.

• Reduced maintenance: A wide variety of natural based materials facilitate breathable construction which means walls stay dry, reducing the likelihood of future damp-related defects and costly repairs. This is especially important where the original building was built to be breathable.

• Market trends: As demand grows and supply chains strengthen, the price gap is closing. Companies like Ecological Building Systems and H.G. Matthews are a couple of the suppliers we have worked with recently.

Labour can sometimes be more specialised, but in retrofit and conservation contexts, traditional trades (lime plasterers, stonemasons, etc.) already work with these materials. Increasingly, self-builders and forward-thinking contractors are embracing them too.

Do natural materials perform as well as modern synthetic products?

A common misconception is that natural products can’t match modern petrochemical-based materials in terms of thermal performance. The reality is more nuanced.

• Thermal performance: Many natural insulations — such as wood fibre, hemp, or cellulose — achieve U-values that compare favourably with mineral wool or rigid PIR boards. For example, Gutex wood fibre boards (available in the UK via Ecological Building Systems) achieve thermal conductivities in the region of 0.038–0.043 W/mK, similar to mineral wool.

• Vapour permeability: Where natural materials excel is in their hygrothermal performance. Unlike closed-cell foams or plastic membranes, natural insulations are vapour-permeable. This means they allow moisture to migrate through a construction build-up, reducing the risk of interstitial condensation and mould growth. In practice, this often results in better performance over time than what the standard calculation suggests.

• Air-tightness: Although vapour “breathable,” natural build ups are detailed to be airtight. An excellent airtightness is required to retain heat in buildings to reduce heating bills and achieve an optimum, moderated internal environment.

• Thermal mass and comfort: Wood fibre boards, for instance, add significant thermal mass. This helps buffer internal temperatures, reducing overheating risk — a critical issue given the UK’s increasingly hot summers.

• Internal Air quality: Natural insulations like wood-fibre and straw-bale walls generally emit lower VOCs than synthetics, buffer indoor humidity through their hygroscopic, breathable nature, and can even adsorb or neutralise pollutants—improving indoor air quality compared to synthetic materials. For further reading, see Richter et al. (2021), “Natural Building Materials for Interior Fitting” and Foster et al. (2025) “Indoor Environmental Quality in an Occupied Whole Life Straw Bale House in England, UK”.

In short: yes, natural materials perform well, and in many cases outperform their synthetic counterparts when whole-building comfort, durability, and health are considered.

Where Can Natural Materials Be Used?

One of the most exciting aspects of natural materials is their versatility across different building types. Can I use them in my own home, whether it’s an old stone cottage, a Victorian terrace, or a new-build project?

• Retrofit: With the UK’s vast stock of Victorian and Edwardian housing, retrofit is arguably the biggest challenge we face. Vapour-permeable systems like wood fibre internal insulation or lime render externally are particularly well-suited here. They improve thermal performance without trapping moisture — a critical failure of many past retrofit attempts.

• Historic Buildings: The use of appropriate Lime mortars and lime render are essential when working with stone or brick heritage buildings. These materials accommodate movement, manage moisture, and ensure historic fabric can continue to shift and breathe. The Society for the Protection of Ancient Buildings (SPAB) has long advocated for their use.

• New Build: Natural materials are not limited to conservation work. Strawbale walls, timber frames with wood fibre insulation, or unfired brick walls are increasingly popular in new low-energy homes. Their performance is robust, and their aesthetics — earthy textures, depth, tactility — bring something contemporary materials often lack. The material’s carbon footprint is also significantly less due to reduced processing required and potential local availability.

• To address National Planning Policy – The use of low carbon natural materials helps us to address National Planning Policy, promoting truly sustainable design, unique place making and Climate resilience in the Planning balance. Application of the design philosophy behind natural materials provides a great angle in favour of sustainable development, especially when we consider eventual ‘end of building life.’

Spotlight on Materials

Wood Fibre Insulation - Gutex

Wood fibre insulation, like Gutex, offers a combination of thermal efficiency, vapour permeability, and acoustic performance. It is particularly well-suited to internal wall insulation in retrofit projects and as an external wall insulation board in timber or masonry new builds.

Strawbale – Dense Construction Ready Bales

Strawbale construction is no longer a fringe experiment. With tested fire resistance, exceptional insulation (often 0.13 W/m²K or better in finished walls), and widespread UK precedents, it’s a serious option for one-off self-build house projects, mass housing, commercial and Public buildings. The material is detailed for a life-span of 200 years+, can be locally sourced and allows for beautiful internal spaces. Ecococoon is a supplier that offers a high-density, engineered, off-site solution for strawbale building, reducing time on site and increasing precision, with the ability to reach Passivhaus standards with correct detailing - ecococon.eu/gb/

We’ll be dedicating a whole separate article — Getting Serious About Straw — to this remarkable material.

Lime - Lime Mortar, Lime Render, Lime Wash

Lime has historically been the cornerstone of breathable construction. Unlike cement, lime mortar allows moisture to move in and out of masonry, reducing trapped damp. It also provides a joint that is able to shift and adapt with the building, preventing structural cracking in the building fabric. Lime render provides flexible, vapour-permeable protection to walls. For historic buildings, it is indispensable, but we are also specifying lime renders in contemporary projects for their performance and beautiful finish. The use of Lime wash internally provides a natural based finish, which is able to enhance indoor air quality through low VOC emissions and inherent resistance to mould and bacteria. The breathability and moisture-regulating qualities even a surface finish of Lime wash provides, helps prevent dampness, by moderating excess humidity, creating a healthier, more resilient indoor environment.

Unfired Bricks – (Strocks by H.G. Matthews)

Unfired clay bricks have an exceptionally low embodied carbon, since they bypass the energy-intensive firing process. H.G. Matthews produces beautiful unfired bricks in Buckinghamshire, supporting local supply chains. Clay-rich earth is combined with chopped straw creating a brick to regulate internal humidity, store heat, and provide a low carbon alternative to traditional blockwork — ideal for both heritage-sensitive retrofits and contemporary sustainable internal uses. The block can be used as the inner skin of an external wall and for internal load-bearing walls, typically up to 3 storeys.

www.hgmatthews.com/natural-building-blocks/strocks

Why Work with Architects Experienced in Natural Materials?

Designing with natural materials requires a different mindset. It’s not just about swapping one insulation for another — it’s about creating a whole building fabric that breathes, balances moisture, and performs in harmony with its environment.

As Architects, our role is to:

• Select the right material for the right context.

• Detail junctions and interfaces carefully to avoid cold bridges or unintended moisture traps.

• Work with contractors and suppliers who have the expertise to deliver high-quality finishes, or provide information and supplier guidance for Contractors to work confidently with new applications.

• Create buildings of their place, using locally available materials to significantly reduce the carbon footprint of projects and create distinctive and meaningful designs.

Ashman Architects is passionate about sustainable design. We believe that natural materials aren’t an “alternative” anymore — they’re a core part of the toolkit we need to meet the RIBA 2030 Climate Challenge and create buildings that are healthy, low-carbon, and future-proof.

Summary

Natural materials are no longer just the preserve of heritage specialists or experimental eco-builders. They are viable, high-performing, and increasingly mainstream. Yes, they sometimes cost more upfront, but the benefits — healthier homes, longer-lasting construction, lower carbon impact — are undeniable.

Whether you are considering a deep retrofit of your Victorian terrace, conserving a listed stone building, or embarking on a new-build home, natural materials offer solutions that are elegant, practical, and sustainable.

We are always delighted to discuss these options with clients who share our enthusiasm for low-carbon, breathable, and resilient Architecture.

Contact us at Studio@ashmanarchitects.com | Tel – 01733 913576

Visit www.AshmanArchitects.com

References & Further Reading

• Ecological Building Systems – Gutex Wood Fibre - www.ecologicalbuildingsystems.com/

• H.G. Matthews – Unfired Bricks

• SPAB Guidance on Lime

• RIBA Sustainable Outcomes Guide

• LETI Embodied Carbon Primer