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