How do you measure the critical properties of an insulation material?

Insulation materials are subject to strict regulation. In order for architects to evaluate the best insulation products, it is important to know the different mechanical, thermal and functional properties of the material. Our goal was to measure, using industry standard methods, these properties in our material, a cellulose aerogel infused with customized proteins.

Below are the most important properties assessed in insulation materials.

Figure 1Summary of all types of test done on our aerogel

Mechanical Properties


Why do we test density?

Density is a critical property of construction materials. When building houses, architects have to be aware of the forces and the weights the building will experience when designing the foundations and support structures. In general, insulation materials have a low density, in part as a result of their key characteristic of low thermal conductivity. However it is very important to calculate their density and determine the weight they add to the structure of the house.

How is density measured?

Density is measured as an output of the mass and volume of the material. We measured these parameters in our material. To calculate the relative density, we divide the density of the material by the density of water.[1 000 kg·m-3]

Equation of Density

From these​​ measurements we calculated a Relative Density = 0.149 for our aerogel.

Equation of Relative Density

For more results on the density.


Evaluating density of the aerogel material is an easy assessment. It is not destructive allowing the use of samples for other tests. Our aerogel has a very reduced density (comparable to cork of relative density=0.150) or other insulation materials. It is ~7 times lighter than water.


Why did we test the porosity?

Our insulation solution is founded upon an aerogel. The mechanical properties of the aerogels are highly dependent upon their micro-structures in particular the size and number of pores. By measuring the porosity of our aerogel, we can verify the efficiency of our aerogel production protocol used to produce it.

Aerogel was produced by transforming a starting hydrogel by Freeze Drying to sublimate the water from the hydrogel. Testing the porosity of our aerogel would provide insight into the interior structure of the material.

How is porosity measured?

We computed the porosity of our aerogel by comparing its density to the density of the cellulose powder starting material. The following formula got us the final percentage of porosity in the aerogel (in %):1

Equation of Porosity2

From these​​ measurements we calculated a Porosity= 73.38 % for our aerogel.

For more results on the Porosity.

Young Modulus

What is the Young Modulus?

Young’s modulus describes the elasticity of a material. When force is applied on a material, the material deforms. The higher Young’s modulus, the more force is needed to deform the material. Generally, this force deformation dependence is linear at low forces, which is used to determine the Young’s modulus of the material.3

Figure 2Typical stress vs. strain diagram for a ductile material (e.g.steel)

Why did we measure Young’s Modulus?

The materials used in buildings undergo constant pressure from other structures in the building or external forces (e.g. wind, heavy interior objects). It is critical to know Young’s modulus of a material to predict deformation likelihood and insure the safety of the construction.

Moreover, we performed Young’s modulus assays on aerogel of differing cellulose concentrations to determine the ideal aerogel mixture required to safely bear the forces of a building without losing performance in thermal insulation.4

Table 1Cellulose aerogel concentrations used for further testing. *Dispersing solution is 5g of thiourea and 10g of NaOH in 85g (~85 ml) of water.
cellulose aerogel concentration cellulose [g] dispersing solution* [g]
1x 5 100
1.5x 7.5 100
1.75x 8.75 100

How do you measure Young’s Modulus?

Young’s modulus can be measured using an array of standardized tests and equipment. We decided to use a standard compression experiment. We placed our sample between two plates and used an Universal Compression Machine to apply force to the sample. The range of forces we choose were based upon real-life forces that might be experienced in a building. Based on the response of the material to applied forces, we estimated Young’s modulus of our cellulose aerogels.5,2

Figure 3Universal Compression Machine used for determining mechanical properties of the material

What are our results?

To determine Young’s Modulus of cellulose aerogel, we chose three different working concentrations of cellulose: 1x, 1.5x and 1.75x. Concentration Table. With defined concentrations we made cylindrical aerogels which were placed between metallic compression plates, and continuous increase in pressure was applied to the material until the minimum allowed material length (in our case: 2 mm). We observe that 1x aerogel behaves as other porous materials. At the initial pressure applied, we observe deformation based on the air displacement. However, after all the air is displaced, material behaves as other solids, and to further deform the material, higher force applied is needed.

Figure 4Cellulose aerogel response to compression testing.(A) Experimental setup of compression test. (B) Cellulose aerogel response is calculated as the deformation percentage to applied pressure [Pa]. The curve is obtained for 1x aerogel, 340 mm2 in area and 21 mm in length. Red arrow point towards the initial deformation of the aerogel, for which Young’s modulus approximations were done. Blue arrow points towards the solid-like behaviour of the cellulose aerogel.

For the initial deformation, there is a linear dependence of pressure applied to the deformation. This reaction range can be used for approximating Young’s modulus of the material. We did soby using 1st degree polynomial regression to obtain Young’s Modulus for aerogels of different cellulose concentrations. Summarized results are given in table 2, but for more details check out results.

Table 2Approximated Young’s Modulus based on compression tests for aerogels of different cellulose concentrations.
Material 1x aerogel 1.5x aerogel 1.75x aerogel
Young Modulus [kPa] 20.362 31.787 25.459

We can conclude that the concentration of our aerogel does not have a big influence on the value of Young’s Modulus. However, in the analysis of the results, the concentration has shown a strong impact on the elasticity of the material and its resistance to plasticity.

To fully conclude which concentration is the most resistant, we would require additional testing. Obtained results were expected due to irregular macroscopic structure that can vary its mechanical properties.The internal structure of an aerogel is therefore an important parameter to take into account.


The compression test was not difficult to perform, but requires access to a Universal Testing Machine. Thanks to the UTM, our tests are standardized and can be compared to other tests performed in other laboratories or in industry. This test however is destructive.

Thermal Properties

Thermal Conductivity

How do we define the effectiveness of an insulation material?

To define the efficacy of a material to conduct heat, engineers use the thermal conductivity index of the material. For example, metals are materials with high thermal conductivity, while air has a low thermal conductivity.

If the thermal conductivity of a material is high, it means that this material can conduct heat effectively. However, if a material needs to insulate, its thermal conductivity should be low. For insulating a house, the insulation material needs to block the heat from exiting the house when it is cold outside or, on the contrary, prevent the heat from entering the house when it is hot outside.

How is thermal conductivity measured?

To measure a property of a material, it’s important to follow the established industrial norms. We decided to test our material according to the material standards ISO 9869-1 and ASTM C1155.6

To measure the thermal conductivity of our material, we built a specialized assay using a heated chamber isolated from the outside by polystyrene.

Figure 5Home-made design for heat transfer experiment as two chamber boxes The box is isolated from the exterior with polystyrene, and the chambre separator is modifiable. Heat source is visible in the middle of one of the rooms (middle right). Heat transfer detection is done in both rooms: with (middle right) and without the heat source (middle left), and also in the separator as well (not visible).

The large chamber is to split it in two distinct sub-chambers. The separation between the chambers is modified according to the material tested. For the scope of our project, we have tested cellulose aerogel separator.

We placed two temperature sensors inside each of the two sub-chambers and in addition a heat flux sensor attached to the aerogel.

Figure 6Parts of a temperature detection: heat flux sensor Sensitivity= 12.61 [µV/(W/m^2)](A), temperature sensor (B) and heat source (C)

How to extract the results from the experience?

Figure 7 Temperature of the expanded polystyrene.(A) Heat flux was measure at the aerogel separator in the function of time. (B) Temperature was measured at three key points: chamber with heat source (red), detection chamber without the heat source (blue), and also, ambient temperature was measured for any uncontrollable heat changes.

We defined the value of the temperature in the sub-chamber with the heat source and calculated the heat flow when the system is in steady state.

During the testing phase, we measured the temperature on each side of our aerogel and the heat flux going through it. Using Fourier’s law (see below), we calculated thermal conductivity of the material based on the heat flux.

Equation of Heat conductivity

We performed a thermal conductivity test with extended polystyrene (EPS) as a control for our setup to verify if our sensors are well calibrated. We know from prior publications that EPS has a conductivity of 0.030-0.040 [W/m∙K].7

Unexpected errors in our Thermal Conductivity tests

Table 3Estimated thermal conductivity indices for 1x cellulose aerogel and expanded polystyrene
Material Aerogel 1X Expanded Polystyrene
Conductivity [W/ (m·K)] 0.0005416 0.0258

Running our positive control experiment, we were surprised to find an underestimated value of the thermal conductivity of the EPS (see table 3). We discovered the problem was due to heat losses between two chambers, due to the suboptimal room design. Further improvements in the detection design would most likely lead to previously reported values, and would allow further testing and comparison to new insulation materials.

However, when launching the experiments for the increase of the heat in the second chamber, we got comparable results between EPS and Aerogel, which led us to consider at least the material produced as an insulative material.

For more results on the Conductivity.

Further improvements in the detection design would most likely lead to previously reported values, and would allow further testing and comparison to new insulation materials.


Accurately measuring thermal conductivity was hampered by the heat loss. In sum, our experience suggests thermal conductivity needs to be measured under carefully controlled conditions with specialized equipment to achieve accurate results.


Due to the porous structure of the aerogel, it is capable of retaining air in its pores, leading to the insulation property which we wanted to take advantage of. However, cellulose aerogel as such is insufficiently protected against the environmental factors, most importantly humidity. Therefore, we designed protective protein coating of the aerogel, via silk-based biofilm. For details of this protein check out Protein design and how we coated the aerogel check out Biofilm.


At first, we tested the hydrophobicity of the silk-based biofilm on its own, by measuring water angle (Fig. 9). After concluding good hydrophobic character of the the silk-based protein, we continued with the coating of the cellulose aerogel. Unfortunately, we were unable to produce the silk-based biofilm coated aerogel, due to the lack of time. However, we did produce non-biofilm silk-based protein coated aerogel, which we tested for improved hydrophobic properties (Fig. 10).

How can we measure the Hydrophobicity?

For a specific surface, we needed to consider the contact angle of a water droplet on the material. The wettability study gave us if the material is absorbing the liquids or retracting them. We based the experiences on the below equation:8

Young Equation
Figure 8 Schematic representation of the contact angle with water

From (Fig.9), we identified clearly that the bigger the contact angle the more the water droplet keeps its spheric structure. We aimed in the tests to have an improved water angle between a control and the silk biofilm.

How to extract results from the experiments?

In the experiments we used the Keyence microscope with an angle of 70 degC. We conducted different trails that were presented in the biofilm notebook.9

Figure 9 Water droplet on the silk biofilm, view at 70 degC.(A) at the 20X magnification, (B) at the 40X magnification.

For the acquisition part we took different videos of 2 µL droplets of water deposited on the biofilm and then we used a simple image editing software to find the right angle of contact. For more results, we proved that there was an increase in the angle between the control biofilm and the silk biofilm of 27.9 degC which gave us a 50 % increase in the contact angle. We concluded that the produced silk biofilm would protect the aerogel better, even though it was far from totally hydrophobic.

Aerogel coating

Even though we were unable to produce silk-based biofil coating of cellulose aerogel, we observed that simple coating of the aerogel with the silk-based protein produced an increase in its hydrophobic property, as measured by the time the water droplet takes to get absorbed on the aerogel (Fig. 11)

Figure 10Protective coating given by the silk fusion protein.(A) Comparison between uncoated and coated with recombinant silk (01a) aerogels. (B) Boxplots of the delay time a drop of water gets absorbed inside an aerogel coated with the different proteins. Top of the boxes = mean of delay time per group; Black dots = individual delay time measurements; Red vertical line = standard error of the mean. Data are mean ± s.d., n = 3 measurements per group; ***P < 0.001, **P < 0.01; one way ANOVA test was performed followed by a Tukey HSD for performing multiple pairwise-comparisons between the means of groups.

Future improvements, such as biofilm formation on top of the cellulose aerogel are open for further exploration.


Scanning Microscope

Why did we need the Scanning Microscope?

During the project, we were interested in knowing the actual structure for our samples and we tried to assess the size of the sample's pores. In this part we used several microscopy equipments to characterize the aerogel :

  • A Keyence microscope VHX-700010

We used the following microscope that has a magnification up to x2000 magnification, to get the first shape of the manufactured porous material and to execute several 3D stitchings for measuring the depth of the samples . Some essential results were described in the results page.

Figure 11 Keyence microscope with aerogel sample 10
  • Scanning Electron Microscope (SEM)

  • We used the model Hitachi ™ 1000 as a scanning electron microscope (SEM) to get detailed images of our aerogel. Those results confirmed that the material we produced had macro-pores instead of nano pores, which means in 0.15mm-0.25mm range size (Proof of concept ). 11

    Figure 12 Scanning Electron Microscopy (SEM) of aerogel samples (A) 1.5X aerogel sample manufactured without solvent exchange. (B) 1.25 X aerogel sample manufactured with ethanol then water solvent exchange.

    From the above figure (fig. 13), we noticed that the porous material we produced did not have nano-pores but instead resembled a foam with a better homogenous structure for the sample that went through solvent exchange (fig. 13 B).


    The measurements we have taken throughout the project allowed us to assay several aspects of the properties of the novel protein coated material we have generated. In particular the key metric of hydrophobicity, permitted to assess the performance of the synthetic biology part of the project. Many additional tests would have to be carried out to have a complete analysis of this material such as additional thermal conductivity tests, thermal emissivity tests and specific heat tests.

    However we believe the measurements we have successfully made lay a strong foundation for additional characterization and improvements upon our material.


    1. Nela Buchtová, Christophe Pradille, Jean-Luc Bouvarda, Tatiana Budtova (2019)
      Mechanical properties of cellulose aerogels and cryogels
    2. Nela Buchtová, Christophe Pradille, Jean-Luc Bouvarda, Tatiana Budtova (2019)
      Mechanical properties of cellulose aerogels and cryogels
    3. J. Zhao, H.B. Lia, M.B. Wu, T.J. Li (1999)
      Dynamic uniaxial compression tests on a granite
    4. Adil Hafidi Alaoui, Thierry, Woignier, George W.Schererc, Jean Phalippou (2008)
      Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel
    5. Zwick Roell
      zwickiLine For small test loads up to 5 kN
    6. Jean-Pierre MONCHAU, Vincent FEUILLET, Laurent IBOS, Yves CANDAU
      Comparaison de méthodes de mesure de résistance thermique in-situ de parois de bâtiment : essais sur un immeuble d’habitation occupé
    7. Ivan Gnip, Sigitas Vėjelis, Saulius Vaitkus
      Thermal conductivity of expanded polystyrene (EPS) at 10 °C and its conversion to temperatures within interval from 0 to 50 °C
    8. Jiang Zhao Ph.D (2013)
      Capillary Force and Surface Wettability
    9. KEYENCE
      Digital Microscope
    10. Keyence
      Keyence Official Site: Digitalmikroskop
    11. Hitachi  (”2009”)
      Hitachi High-Technologies Announces Shipments of Tabletop Microscope TM-1000 Surpass 1,000 Units