Bio-based materials improve the comfort and carbon footprint of buildings

Redu­cing the car­bon foot­print of the con­struc­tion sec­tor relies on a num­ber of dif­fer­ent factors. Cir­cu­lar eco­nomy, eco-design, energy renov­a­tion… and new mater­i­als. The man­u­fac­ture of mater­i­als for build­ing con­struc­tion accoun­ted for 10% of glob­al CO₂ emis­sions in 2020, equi­val­ent to 3.2 Gt of CO₂¹. And demand is grow­ing: glob­al cement con­sump­tion rose from 900,000 to almost 2 mil­lion tonnes between 2000 and 2015 (it dropped slightly in 2020 due to the pub­lic health crisis). Cement and steel are the main con­trib­ut­ors, fol­lowed by alu­mini­um, glass and insu­lat­ing mater­i­als. Biobased mater­i­als are a good altern­at­ive for the sector.

The struc­ture, insu­la­tion and clad­ding of a build­ing can be made from biobased mater­i­als. Derived from bio­mass of anim­al or (more often) plant ori­gin, biobased mater­i­als on the con­struc­tion mar­ket are var­ied: tim­ber, cel­lu­lose wad­ding, shives or flax shives, wheat straw, etc. How­ever, they are cur­rently used very little: only 12% of the mater­i­als used in con­struc­tion are biobased². Moreover, these mater­i­als are sorely lack­ing in a sound reg­u­lat­ory frame­work for the envir­on­ment: the European stand­ard (NF-EN 16575) con­siders as a bio­sourced mater­i­al any product derived entirely or par­tially from bioresources… 

Each kilo­gram of wheat straw con­trib­utes to stor­ing about 1.4 kg of CO₂.

A private label goes fur­ther by dis­tin­guish­ing mater­i­als incor­por­at­ing a min­im­um threshold of biobased mater­i­al, set at 70% for insu­la­tion or 25% for plant-based con­crete³. In France, the state label “Bâti­ment bio­sourcé” (bio­sourced build­ing, in Eng­lish) con­cerns new build­ings. To obtain it, a single-fam­ily house must incor­por­ate between 42 and 84 kilo­grams of biobased mater­i­al per m² of floor area, depend­ing on the grade required. These fig­ures fall to 18 to 36 kg/m² for a col­lect­ive dwelling. 

“The man­u­fac­ture of con­ven­tion­al mater­i­als con­trib­utes to the remov­al of car­bon stored for mil­lions of years in the soil and its injec­tion into the atmo­sphere,” explains Thibaut Lecompte. “The sec­tor’s pri­or­ity must be to reuse mater­i­als, and then to use bio-based mater­i­als”. The main advant­ages of biobased mater­i­als? Unlike con­ven­tion­al mater­i­als, they store car­bon. “Plants cap­ture the CO₂ con­tained in the lower atmo­sphere dur­ing their growth,” he explains. It then remains stored with­in them for the entire life of the build­ing, and even longer if the mater­i­al is reused!” For example, each kilo­gram of wheat straw con­trib­utes to stor­ing about 1.4 kg of CO₂⁴. The cap­tured car­bon is even­tu­ally released into the atmo­sphere at the end of the mater­i­al’s life, for example if the wood is burnt for heat­ing. Biobased mater­i­als there­fore have a double advant­age: they sequester atmo­spher­ic CO₂ for dec­ades, but also avoid deplet­ing soil car­bon by repla­cing con­ven­tion­al materials. 

The oppor­tun­it­ies offered by the dif­fer­ent altern­at­ives vary. Let’s look at wood, the most com­monly used bio-based struc­tur­al mater­i­al. “Tim­ber fram­ing lim­its thermal bridges and allows large open­ings to be made while facil­it­at­ing imple­ment­a­tion, unlike steel or con­crete,” points out Thibaut Lecompte. Forests are the second largest car­bon sink in the world, help­ing to store 10 bil­lion tonnes of CO₂ equi­val­ent⁵. How­ever, vir­tu­ous forestry must be man­aged sus­tain­ably, by lim­it­ing clear-cut­ting, mono­cul­tures and the use of phytosan­it­ary products ⁶.

In Europe, we remove less wood than is pro­duced each year (73%): this equi­lib­ri­um needs to be pre­served if the car­bon bal­ance is to be maintained.

“The bio­mass extrac­ted must also be com­pensated through pro­duc­tion,” adds Thibaut Lecompte. “In Europe, we remove less wood than is pro­duced each year (73%): this equi­lib­ri­um needs to be pre­served if the car­bon bal­ance is to be main­tained for example by redu­cing our con­sump­tion of fire­wood in favour of tim­ber.” A European research team estim­ates that the area of forestry needed to sup­ply tim­ber or insu­la­tion can fully meet the demand: it rep­res­ents only 23% of the land already used for con­struc­tion, and would there­fore not gen­er­ate any land pres­sure or inter­sect­or­al com­pet­i­tion⁷.

Anoth­er approach is bio­sourced walls, incor­por­at­ing bio­sourced insu­la­tion and plant-based con­crete. “Bio­sourced mater­i­als are par­tic­u­larly effect­ive when it comes to improv­ing thermal com­fort,” says Vin­cent Pic­an­det. Thanks to their abil­ity to absorb and release water in the form of vapour, humid­ity inside the build­ing is bet­ter reg­u­lated. Even though con­ven­tion­al insu­la­tion mater­i­als often have lower thermal con­duct­iv­ity, they are much less effect­ive, if not totally inef­fect­ive, at reg­u­lat­ing humid­ity,” he con­tin­ues. Unfor­tu­nately, the great­er com­fort offered by bio-based mater­i­als is not reflec­ted in cur­rent stand­ards, which only meas­ure thermal con­duct­iv­ity. To date, only 11% of roof­ing renov­a­tions are car­ried out with biobased insu­la­tion (this fig­ure falls to 7% and 4% respect­ively for walls and floors)⁸. 

How­ever, they do have one draw­back when com­pared to con­ven­tion­al mater­i­als. “Prop­er install­a­tion must be observed to ensure that the entire wall breathes prop­erly,” says Vin­cent Pic­an­det. This makes their use more com­plic­ated in renov­a­tion. “But if the right install­a­tion con­di­tions are respec­ted, these mater­i­als have the same dur­ab­il­ity as con­ven­tion­al mater­i­als over a peri­od of half a cen­tury,” Vin­cent Pic­an­det explains. We don’t have the neces­sary hind­sight for much longer than that. For the research­er, the “Hemp con­crete is less dur­able than tra­di­tion­al con­crete: but the stand­ards require us to build build­ings that are more res­ist­ant than neces­sary!” Of course, con­ven­tion­al con­crete will always be needed for civil engin­eer­ing works or struc­tur­al func­tions. But con­crete made from plant aggreg­ates can be used as a sub­sti­tute for con­ven­tion­al insu­lat­ing mater­i­als or for the com­pos­ite walls that are com­monly used.

From a cli­mate point of view, the life cycle ana­lys­is (LCA) of the annu­al plants used for insu­la­tion or bio­sourced con­crete (hemp, wheat, flax, sun­flower, etc.) is very pos­it­ive. In a few months of cul­tiv­a­tion, they sequester car­bon, which is then stored in the mater­i­al through­out the life of the build­ing. The car­bon impact of the imple­ment­a­tion of an entirely bio­sourced wall (com­posed of a wood frame and straw insu­la­tion, with a lifespan of 50 years) is eval­u­ated at nearly 4 kg CO₂e, com­pared to 49 kg for a con­ven­tion­al solu­tion⁹. 

The con­struc­tion sec­tor would only need 12% of the land already avail­able for con­struc­tion if wheat straw were used as insulation.

The poten­tial is there: the con­struc­tion sec­tor would only need 12% of the land already avail­able for con­struc­tion if wheat straw were used as insu­la­tion, as a large part of it is not cur­rently used. This is not the case for hemp and cork, which are not widely cul­tiv­ated today. “To main­tain an attract­ive car­bon foot­print, it is neces­sary to main­tain a bal­ance in land use: do not replace a forest with a field, for example,” warns Thibaut Lecompte. For example, hemp and flax can be intro­duced as an inter­me­di­ate crop. In an art­icle in The Con­ver­sa­tion¹⁰, research­er Hélène Len­or­mand gives an over­view of all the plants avail­able for insu­la­tion: “If there is com­pet­i­tion between dif­fer­ent applic­a­tions, the annu­al sup­ply is so huge (15 mil­lion tonnes in France) that all the dif­fer­ent uses can be sup­plied without fear of any com­pet­i­tion. The dif­fer­ent sources are com­ple­ment­ary both geo­graph­ic­ally and tem­por­ally, ensur­ing the avail­ab­il­ity of the resource. This is a major advant­age com­pared to oth­er con­ven­tion­al impor­ted materials.

Des­pite this, the bio-based mater­i­als sec­tor is still under­developed. Few fig­ures exist. Biobased insu­la­tion has grown by almost 40% in 3 years, to reach 10% of the mar­ket accord­ing to industry pro­fes­sion­als¹¹. The arrival of the new French thermal reg­u­la­tions could change things: the RE2020 – applic­able to new build­ings – integ­rates the car­bon impact of the mater­i­als used through their LCA, as do Den­mark and the Neth­er­lands. And around the world, more and more coun­tries are intro­du­cing reg­u­la­tions – includ­ing the car­bon foot­print of mater­i­als – aimed at achiev­ing zero car­bon by 2050¹².

Anaïs Marechal