How can a Climate-Neutral Building Look Like?

Abstract

The climate crisis is urging us to act fast. Buildings are a key leverage point to reduce greenhouse gas (GHG) emissions, but the embodied emissions related with their construction remain often the hidden challenge of any ambitious policy. Considering that a complete material substitution is not possible, we explore in this paper a material GHG compensation where fast-growing bio-based insulation materials are used to compensate building elements that necessarily release GHG. Looking for analogies with other human activities, different material diets as well as different building typologies are modelled to assess the consequences in term of bio-based insulation requirement to reach climate-neutrality. The material diets are defined according to the gradual use of herbaceous materials, from the insulation up to the structural level: omnivorous, vegetarian and vegan. Our results show the relationship in terms of volume between the climate intensive materials and the climate-negative ones needed to neutralize the overall building GHG emissions. Moreover, they suggest how climate-neutral building can look like and that it is possible to have climate-neutral buildings with wall thickness within the range of current construction practices.

Full text

ICBBM 2021
Proceedings of the 4th International
Conference on Bio-based Building Materials
16-18 June 2021, Barcelona, Spain
Editors :
Sofiane Amziane & Mohammed Sonebi
Associate Editor :
Jonathan Page

4th International Conference on Bio-Based
Building Materials
ICBBM2021
16-18 June 2021
Barcelona, Spain
Edited by
Sofiane Amziane, Mohammed Sonebi
Associate Editor: Jonathan Page

4th International Conference on Bio-Based Building Materials
June 16th - 18th 2021
Barcelona, Spain
HOW CAN A CLIMATE-NEUTRAL BUILDING LOOK LIKE?
O.B. Carcassi1*, G. Habert2, L. Malighetti1, F. Pittau1,2
1 Department of Architecture, Built environment and Construction engineering (ABC), Politecnico
di Milano, 20133 Milan, Italy
2 Institute of Construction & Infrastructure Management, Chair of Sustainable Construction, ETH
Zurich, 8093 Zurich, Switzerland
*Corresponding author; e-mail: olgabeatrice.carcassi@polimi.it
Abstract
The climate crisis is urging us to act fast. Buildings are a key leverage point to reduce greenhouse
gas (GHG) emissions, but the embodied emissions related with their construction remain often the
hidden challenge of any ambitious policy. Considering that a complete material substitution is not
possible, we explore in this paper a material GHG compensation where fast-growing bio-based
insulation materials are used to compensate building elements that necessarily release GHG.
Looking for analogies with other human activities, different material diets as well as different
building typologies are modelled to assess the consequences in term of bio-based insulation
requirement to reach climate-neutrality. The material diets are defined according to the gradual
use of herbaceous materials, from the insulation up to the structural level: omnivorous, vegetarian
and vegan. Our results show the relationship in terms of volume between the climate intensive
materials and the climate-negative ones needed to neutralize the overall building GHG emissions.
Moreover, they suggest how climate-neutral building can look like and that it is possible to have
climate-neutral buildings with wall thickness within the range of current construction practices.
Keywords:
climate-neutrality, embodied GHG emissions, LCA of 3BM, bio-based insulation, fast-growing
biomass
1 INTRODUCTION
Considering the greenhouse gas budget left (Habert et al., 2020) that can be emitted before reaching the tipping
point, we need to reach the climate-neutrality by reducing to net-zero the GHG emissions in every sector of the
economy within 50 years. Buildings are a key leverage point to reduce greenhouse gas (GHG) emission, but the
embodied emissions, related to their material manufacture, transportation, construction and end-of life disposal,
remain often the hidden challenge of any ambitious policy. In fact, conventional building materials, such as concrete,
steel, or mineral insulations, represent a massive source of GHG emissions due to both their manufacturing energy
intensive processes (De Wolf et al., 2020) and releasing of chemical reactions (Davis et al., 2018). Here, we referred
to them as climate-positive materials and they were divided according to their GWPnet values in high and low climate
intensive materials. To mitigate the embodied emissions, recent studies demonstrated the efficiency of substituting
climate-positive materials with bio-based ones, e.g. wood and straw, due to their carbon storage potential and
reduced life-cycle emissions (Churkina et al., 2020; Pittau et al., 2018). However, Pomponi and coauthors (Pomponi
et al., 2020) demonstrated that the related increase of wood in the construction industry could intensify the
deforestation and illegal logging, whereas they suggest the use of fast-growing (or herbaceous) bio-based materials,
such as hemp and straw, that have greater yield. Moreover, inside the controversy either to consider or not the
biogenic carbon in the different bio-based product life-cycle stages (Hoxha et al., 2020), Guest et al (Guest et al.,
2013) showed that by adding the time factor with the regrow of plants and considering the carbon storing within the
building boundaries in the life cycle assessment (LCA) methodology, the herbaceous biomass are the most
promising to regenerate the climate. To calculate this potential, they defined an index, the biogenic global warming
potential (GWPbio), to consider the storage period of harvested biomass with different rotation periods in the
anthroposphere as a negative value to be considered at the beginning of a classical LCA. Hence herbaceous
biomass can be considered as climate-negative in virtue of the carbon uptake through photosynthesis and they
exhibit a great potentials as insulation material (Schiavoni et al., 2016). Unfortunately, not all the construction
materials can be substituted with the herbaceous ones. Consequently, we aimed to propose a new way of
approaching the design of climate-neutral buildings based on the use of the adequate amount of herbaceous
materials, or climate-negative, as insulation to neutralize the emissions resulting from the climate-positive ones. To
this end, different material “diets” were designed according to the gradual use of herbaceous materials, from the
insulation up to the structural level: omnivorous, vegetarian and vegan (Fig. 1). For all the diets, the insulation
materials are the herbaceous ones, in particular we used three different biomasses, namely cotton stalks, straw
and hemp fiber. By leveraging their negative GWPbio, this research quantified the herbaceous biomass needed to
bring to net-zero the total embodied emissions of buildings. In literature, there is no similar approach to design the
insulation finalized to reach the building climate-neutrality instead of the energetic performance.
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ICBBM2021
Fig. 1: From the left: omnivorous, vegetarian and vegan material diets. Materials’ classification according the Net-
GWP value, that divides them to Climate Positive or Climate Negative materials
2 METHOD
2.1 Building models
We tested the climate-neutrality of the three material diets on new residential buildings in the European context.
Therefore, the four typical European Building Typologies (BT), namely single-family house (SFH), terraced house
(TH), multi-family house (MFH) and apartment block (AB), were used to create the geometrical reference buildings
form the Tabula/Episcope database (Intelligent Energy Europe, 2016). The data extrapolated from this database
used to set the dimensions of the building models for the different BT are: Reference Energy Surface (RES) [m2],
Number of conditioned stories (NCF) [-], Floor Surface (SSCF) = Roof Surface (SR) = Basement Surface (SB)
[m2/m2RES], Exterior Walls Surface (SW ALL) [m2/m2RES], Window Surface (SWIND) [m2/m2RES]. Moreover, the single
area for every floor was kept the same for each storey. All these geometrical data collected from Tabula/Episcope
database were normalized according to the RES, except for the NCF. RES is the total surface of the conditioned
building, which in this case is the single conditioned storey surface multiplied by the number of conditioned storeys.
Usually, the materials used for the windows have high environmental impacts. Hence, first the emissions resulting
for finishing, waterproofing membrane and the structures for the three diets were calculated, and, later we assigned
the higher window surfaces to the most polluting geometric configurations for each building typology. In this study,
only the MEDIAN geometrical configuration for the 4 BT were reported as the statistically significant values of data
sets, for a total of four building models (Tab. 1).
Tab. 1: Geometrical parameters of the four building models
MEDIAN
SFH
MFH
AB
TH
OMN
VEGT
VEGA
OMN
VEGE
VEGA
OMN
VEGE
VEGA
OMN
VEGE
VEGA
RES [m²]
145
842
1702
137
NCF
2
4
5
2
SSCF = SR =
SB [m²/ m2RES]
0.50
0.25
0.20
0.50
SW ALL [m²/ m² RES]
1.13
0.68
0.65
0.72
SWIND [m²/ m² RES]
0.15
0.16
0.15
0.16
0.17
0.16
0.17
0.17
0.17
0.15
0.15
0.15
2.2 Structural volume incidence
To define the carbon footprint of the different structural systems of the three material diets, a parametric model was
set up in MATLAB. The omnivorous diet was designed as in-situ cast concrete columns and walls supporting a
reinforced concrete plate; the vegetarian one, as a platform timber frame system composed of walls with offsite
assembled load-bearing elements (massive solid wood and OSB panels) and beams in solid wood; the vegan one,
with the engineered cross-laminated bamboo (CLB) modelled as load-bearing walls and floor panels. The
foundation was constantly in reinforced concrete. The parametric model defined the minimal load-bearing areas of
columns, beams, walls and slabs, to support the structural loads under two combinations: service state limits and
ultimate state limits. The model was based on simplified modular geometries, with a mesh 10x10m and a floor
height fixed of 3,2m and variable number of storeys according the ones collected in in TABULA for the MEDIAN
geometrical configurations. All the values were finally normalized according to the gross floor area of the module to
obtain normalized values and were applied to the different BT. Since for the rest of the materials were normalized
according to the RES, we assumed that the structural normalization is equal to the normalization to the RES.
Therefore, the structural incidence was expressed in m3/m2RES. In addition, no specific design for fire safety was
performed, since all structural elements were protected with fireproof finishing.
2.3 GWPnet computation of construction materials
For the representative MEDIAN geometrical configurations of the four BT, the non-structural material volume was
computed. After that, the structural and non-structural GHG emissions (kg CO2e /m3) were determined. The latter
are depending on the potential carbon uptake of materials used, which have been here classified into three main
categories: i) high climate-intensive, ii) low climate- intensive and iii) climate-negative according to their resulting
GWPnet (Fig. 1).
CLIMATE POSITIVE
CLIMATE NEGATIVE
HIGH climate
intensive
GWPnet >
300 C02
eq/m3
CLIMATE
NEGATIVE
GWPnet <
0 kg C02 eq /m3
LOW climate
intensive
0 < GWPnet
<300 kg C02
eq/m3
Omnivorous material diet:
Structure
:
C
oncrete C25/30 and
C30/37 (f oundation)-
Internal and external nishing :
Mineral plaster
Insulation:
Bio-based
materials
Membrane:
Polyethylene -
Structure
: steel -
Internal pavement:
Ceramic tiles -
Window:
Insulated
triple glazing + PVC frame
Vegetarian material diet:
Structure
:
C
oncrete C25/30 and
C30/37 (f oundation),
Wood OSB -I
nternal nishing:
Gypsum plast erboard
Structure and external
nishing:
Solid wood
(hard) -
Internal pavement:
Solid wood (soft ) -
Insulation :
Bio-based
materials
Membrane:
Polyethylene -
Structure
: steel -
Window:
Insulated triple glazing,
Wood aluminum frame
Vegan material diet:
Structure
: CLB ,
C
oncrete C25/30 and
C30/37 (f oundation)
Internal nishing:
Clay plaster
Internal pavement:
Bamboo ooring,
Bio-based
insulation
materials
Membrane:
Polyethylene -
Structure
: Steel
-
Window:
Insulated triple glazing,
Wood frame -
External
nishing: bamboo cladding
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ICBBM2021
GWPnet Calculation
The GWPnet of construction materials measures the consequence on climate change of fossil GHG emissions and
biogenic CO2 emissions/removals during the lifecycle of a product. To calculate it, three steps were followed. First,
we collected the GWP at 100 years (GWP100y) index of each material, according to the IPCC 2013 assessment
method (Joos et al., 2013). Afterwards, we computed the CO2 removal of bio-based materials according to the
GWPbio method (Guest et al., 2013). And finally, the two obtained values were summed up and multiply for the
material density to obtain the net-value, here called GWPnet in kgCO2eq/m3.
Life-Cycle Assessment (LCA)
In this study, the cradle to gate stages (A1-3) were taken into account as well as the waste disposal (C1-4) to
perform the LCA. The GWP values for non-bio-based materials have been assumed from the
“Koordinationskonferenz der Bau- und Liegenschaftsorgane der öffentlichen Bauherren” (KBOB)
(Eidgenossenschaft, 2016), which is the “Coordination Conference of Building and Real Estate Bodies of Public
Builders” in Switzerland. Unfortunately, the KBOB does not contains neither a huge selection of bio-based insulation
materials nor the bamboo ones. Therefore, the research was also extended to Environmental Product declarations
(EPDs) (Cavac Biomatériaux, 2018) in the market, and to the scientific papers (Schiavoni et al., 2016; Vogtländer
and van der Lugt, 2015). As a conclusion, three bio-based insulations with different negative GWPnet values were
chosen to cover the material variability, namely the cotton stalks, which exhibits the highest GWPnet, hemp fibers,
which exhibited the lowest value, and straw, with a value in between the two others.
Carbon Sequestration
Guest and coauthors (Guest et al., 2013), with the GWPbio index, prosed a method that combines, by means of a
Dynamic LCA (DLCA) (Levasseur et al., 2010), the annual CO2 uptake in the land due to the biomass regrowth and
the delayed biogenic CO2 emissions through biomass incineration at end of life of a building. The storage period in
the anthroposphere was here assumed to be 60 years while the rotation depends on the different regeneration
periods for each material used, namely 90 years for the wood, 5 for the bamboo and 1 for the fast-growing or
herbaceous species. The herbaceous spices, e.g. hemp and straw, need a shorter time than slow-growing ones
(wood), resulting in a more advantageous effect in lowering the radiative force remaining in the atmosphere in a
limited period. Therefore, the GWPbio was extracted for every bio-based material by entering in the graph at 60
years, i.e. the chosen building lifespan and extracting the GWPbio index for the different biomass according their
rotation period. To calculate the carbon sequestration of bio-based materials, the following Equation (1) was
considered, which calculate the mass of CO2 that can be stored in the final product:
       (1)
Where:
− CC is the carbon content of the biogenic material;
− BC the biomass content of the finished product;
− 3.67 is the molar weight ratio between CO2 and C (Vogtländer and van der Lugt, 2015)
Consequently, as reported in Equation (2), the share of GWP from carbon uptake can be calculated by multiplying
the CO2 storage with the GWPbio index, which is a part of the total carbon storage a material reabsorbed in the land
during the storage period in a time horizon of 100 years:
   (2)
Finally, summing up the fossil CO2-eq emissions, which contribute to the GWP100y, and the CO2 uptake from biogenic
regeneration in the land (GWPbio), the final GWP value (GWPnet) was obtained according to Equation (3):
      
Where:
− ρ0 is the density of the material, in kg/m3.
Tab. 2 shows the data to compute the GWPnet values for the construction materials chosen.
2.4 The climate-neutral building assessment
The total volume of construction products used in the building was multiplied for each GWPnet value for the four BT
and the three material diets as showed in the climate-neutrality Equation (4):
 
    (4)
where:
− GWPnet,b is the specific GWPnet value calculated for each diet
− GWPnet,i is the GWPnet value of each material, expressed in kgCO2eq/m3
− vi is the volume of each building material, expressed in m3/m2RES
The total building positive GWP, based on the high and low climate intensive material emissions, has to be
neutralized by the three herbaceous insulation chosen. The volume of insulation to be installed in the envelope was
calculated according to the following Equation (5):
 
  


  (5)
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ICBBM2021
where:
- vins is the volume of insulation needed to achieve the climate neutrality in 100 years
- GWPnet,i is the GWPnet value of a generic non-insulating material, expressed in kgCO2eq/m2RES
- GWPbio,ins is the GWPbio value of the selected insulation material, expressed in kgCO2eq/kg
Tab. 2: Properties of construction materials used
Materials
ρ0
[kg/m3]
CC
[%]
BC
[%]
GWPIPCC
[kg
CO2eq/kg]
GWPbio
[kg
CO2eq/kg]
GWPnet
[kg CO2eq/m3]
Steel (reinforcement)
7850
0%
0%
0.68
0.00
5353.70
PVC Window Frame, thickness
80mm
1181.25
0%
0%
0.00
3562.50
Wood-Aluminum Window Frame,
thickness 80 mm
1042.5
0%
0%
0.00
2712.50
Waterproof membrane (polyethylene)
1000
0%
0%
2.52
0.00
2520.00
Insulated Triple Glazing,
thickness 40 mm
30
0%
0%
0.00
1670.00
Wood Window Frame, thickness
80mm
1002.5
0%
0%
0.00
1600.00
Ceramic tiles, thickness 0,009 m
2000
0%
0%
0.78
0.00
1555.56
Bamboo Cladding
1150
54%
93%
1.20
-0.48
364.92
OSB
605
50%
98%
0.61
-0.10
262.67
Gypsum plasterboard
850
0%
0%
0.29
0.00
249.05
Concrete C30/37
2300
0%
0%
0.10
0.00
227.70
Concrete C25/30
2300
0%
0%
0.07
0.00
170.20
Mineral plaster
1100
0%
0%
0.15
0.00
161.70
Cross Laminated Bamboo (CLB)
700
54%
98%
1.08
-0.48
100.63
Clay plaster
1800
0%
0%
0.02
0.00
41.40
Bamboo Flooring
700
54%
100%
0.92
-0.48
-21.88
Hemp fiber
82
45%
64%
0.14
-0.50
-32.11
Solid wood (softwood)
485
50%
100%
0.09
-0.10
-46.80
Straw
100
40%
100%
0.09
-0.50
-64.40
Solid wood (hardwood)
705
50%
100%
0.07
-0.10
-81.43
Cotton (stalks)
450
40%
90%
0.34
-0.50
-144.27
2.5 The architectural feasibility assessment
With the bio-based insulation volumes, the resulting envelope thicknesses were calculated by inserting the
insulation materials in the building envelopes, namely façade (SW ALL), roof (SR) and basements (SB), with a
constant insulation level. In this way it is possible to evaluate the architectural feasibility of having these buildings
in the urban context in terms of volume of materials that will occupy the city spaces and resulting wall thicknesses.
3 RESULTS
Tab. 1 and Tab. 3 contain all the parameters of the MEDIAM geometrical configurations statically sampled from
the TABULA/EPISCOPE database and the structural incidences. With these values and the Tab. 2 ones, it is
possible to perform the climate-neutral building assessment and obtain the volume of the three bio-based materials,
namely cotton stalks, straw and hemp fiber that are reported at the end of Tab. 3.
Tab. 3: Structural volume incidence resulting from the MATLAB code and the bio-based insulations resulting from
the climate-neutral building assessment.
MEDIAN
SFH
MFH
AB
TH
OMN
VEGT
VEGA
OMN
VEGE
VEGA
OMN
VEGE
VEGA
OMN
VEGE
VEGA
Structural material volume incidence
Steel
0.005
0.002
0.002
0.004
0.001
0.001
0.009
0.001
0.001
0.005
0.002
0.002
C25/30
0.306
0.052
0.052
0.290
0.026
0.030
0.303
0.023
0.028
0.306
0.052
0.052
C30/37
0.025
/
/
0.040
/
/
0.046
/
/
0.025
/
/
Wood OSB
/
0.074
/
/
0.047
/
/
0.047
/
/
0.047
/
CLB
/
/
0.258
/
/
0.258
/
/
0.258
/
/
0.258
Solid wood
/
0.110
/
/
0.115
/
/
0.121
/
/
0.102
/
Bio-based insulations resulting from the climate-neutral building assessment
Cotton stalks
0.861
0.308
0.468
0.826
0.230
0.384
1.04
0.216
0.381
0.843
0.298
0.445
Hemp fiber
3.87
1.38
2.10
3.71
1.03
1.72
4.66
0.971
1.71
3.78
1.34
2.00
Straw
1.93
0.690
1.05
1.85
0.51
0.860
2.32
0.484
0.854
1.89
0.668
0.998
In particular, Fig. 2 summarizes the climate-neutrality assessment results in terms of the climate negative materials
needed (three shades of green) to neutralize the emissions resulting from the high (red) and low (yellow) climate
intensive materials. The Omnivorous diets are the most volumetric-intensive ones for all the building typologies,
followed by the Vegan and concluding with the Vegetarian ones. In fact, the Vegan ones, that should be the more
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ICBBM2021
stringent due to the vast use of herbaceous materials also in the structure, still exhibit high GWPnet due to the
material transportation of bamboo from the Asiatic countries (Vogtländer and van der Lugt, 2015).
Fig. 2: Material diets showed in a positive and negative logic to reach the climate-neutrality. The quantity of
materials is expressed for the 4 BT for the three diets, namely: OMN= omnivorous diet; VEGET = vegetarian diet;
VEGAN = vegan diet
However, the material quantities resulting in each material diet are similar for the high, low climate intensive and
negative materials no matter the BT. The high climate intensive values range between 0,026 and 0,021 m3/m2RES
for the omnivorous diets, 0,01 and 0,009 m3/m2RES for the vegetarian ones and 0,032 and 0,22 m3/m2RES for the
vegan ones; while the low climate intensive values range between 0,396 and 0,377 m3/m2RES for the omnivorous
diets, 0,153 and 0,091 m3/m2RES for the vegetarian ones and 0,353 and 0,438 m3/m2RES for the vegan ones. The
climate-negative insulation volumes follow a similar correspondence. Especially, the hemp fiber required are 4,656
and 3,711 m3/m2RES in the omnivorous diets, 1,513 and 1,112 m3/m2RES for the vegetarian ones and 2,119 and 1,728
m3/m2RES for the vegan ones; the straw variates among 2,322 and 1,85 m3/m2RES in the omnivorous diets, 0,82 and
0,625 m3/m2RES for the vegetarian ones and 1,064 and 0,869 m3/m2RES for the vegan ones; while for the cotton
stalks coincide to1,036 and 0,826 m3/m2RES in the omnivorous diets, 0,438 and 0,357m3/m2RES for the vegetarian
ones and 0,483 and 0,396 m3/m2RES for the vegan ones. Furthermore, Fig. 2 highlights how the bio-based insulating
material choice can influence the volume necessary to obtain the climate-neutrality. By selecting an insulation with
the GWPnet value analogous to the hemp fiber, i.e. the worse one, a greater quantity of material is needed, whilst
preferring a solution with a GWPnet value closer to the cotton stalks one can ensure a lower quantity of insulation.
For the architectural feasibility assessment, our results (Tab. 4) demonstrate how the wall thickness vary according
to the herbaceous insulation material used. In the hemp fiber insulation cases, the thickness is the most impacting
since can reach the 4,43 m in case of the AB omnivorous diet, whereas by using the cotton stalks the wall would
be only 0,99 m wide and 2,21 m by choosing the straw. The straw values stay for most of the construction solutions
within an acceptable range for the wall thickness, smaller than 1 m. The use of cotton stalk always produces wall
thicknesses smaller than 1 m.
Tab. 4: Wall thickness for the 3 material diets for the four building typologies expressed in m3/m2 RES
SFH
MFH
AB
TH
MEDIAN
OMN
VEGT
VEGA
OMN
VEGE
VEGA
OMN
VEGE
VEGA
OMN
VEGE
VEGA
Cotton
stalks
0.861
0.308
0.468
0.826
0.230
0.384
1.04
0.216
0.381
0.843
0.298
0.445
Straw
1.93
0.690
1.05
1.85
0.515
0.860
2.32
0.484
0.854
1.89
0.668
0.998
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Hemp
fibre
3.86
1.38
2.10
3.71
1.03
1.72
4.66
0.971
1.71
3.78
1.34
2.00
4 CONCLUSION
This research shows how the material choices have a great influence on the building embodied emissions by
providing a practical approach. The use of herbaceous insulation materials that are able to neutralize the GHG
burden on the climate, shows that is possible to build climate-neutral buildings with contemporary construction
practices. In fact, current European constructions usually account for a wall thickness of 40÷50 cm in concrete or
brick buildings and even 80 cm for the strawbale buildings. At the same time, we only focused on three bio-based
insulation materials, but it could be enlarged to others according to a growing data availability on their performances
and embodied emissions. Finally, it’s important to mention that the GHG-fossil emission linked to the use of concrete
could be further reduced by implementing low-carbon concrete.
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