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