Journal Article•DOI•

Traditional, state-of-the-art and future thermal building insulation materials and solutions – Properties, requirements and possibilities

Bjørn Petter Jelle¹, Bjørn Petter Jelle²•Institutions (2)

SINTEF¹, Norwegian University of Science and Technology²

01 Oct 2011-Energy and Buildings (Elsevier)-Vol. 43, Iss: 10, pp 2549-2563

TL;DR: In this article, the advantages and disadvantages of thermal building insulation materials and solutions have been treated and compared and various properties, requirements and possibilities have been compared and studied. But there is no single insulation material or solution capable of fulfilling all the requirements with respect to the most crucial properties.

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About: This article is published in Energy and Buildings.The article was published on 2011-10-01 and is currently open access. It has received 817 citations till now. The article focuses on the topics: Building insulation & Dynamic insulation.

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Summary (8 min read)

Jump to: [1. Introduction] – [2. Thermal background] – [3. Traditional thermal building insulation] – [3.1 Mineral wool] – [3.2 Expanded polystyrene (EPS)] – [3.3 Extruded polystyrene (XPS)] – [3.4 Cellulose] – [3.5 Cork] – [3.6 Polyurethane (PUR)] – [4. State-of-the-art thermal building insulation] – [4.1 Vacuum insulation panels (VIP)] – [4.2 Gas-filled panels (GFP)] – [4.3 Aerogels] – [4.4 Phase change materials (PCM)] – [5. Nanotechnology and thermal insulation] – [6.1 Vacuum insulation materials (VIM)] – [6.2 Gas insulation materials (GIM)] – [6.3 Nano insulation materials (NIM)] – [6.4 Dynamic insulation materials (DIM)] – [6.5 Concrete and applications of NIMs] – [6.6 NanoCon] – [6.7 Other future materials and solutions?] – [7.1 Robustness of traditional thermal insulation materials] – [7.2 Thermal conductivity of state-of-the-art thermal insulation materials] – [7.4 Thermal conductivity and other properties] – [7.5 Requirements of future thermal insulation materials and solutions] – [7.6 The potential of miscellaneous thermal insulation materials and solutions] – [7.7 Potential cost savings by applying VIPs] – [7.8 Condensation risk by applying VIPs in the building envelope] – [7.9 The cardinal weaknesses of VIPs] – [7.10 EPS encapsulated VIPs] – [7.11 VIMs and GIMs versus NIMs] – [7.12 The regulating potential of DIMs] – [7.13 The construction potential of NanoCon] – [7.14 Assessing weaknesses and strengths] – [7.15 Does the future belong to NIMs, DIMs and NanoCon?] and [8. Conclusions]

1. Introduction

As the energy use in the building sector accounts for a significant part of the world’s total energy use and greenhouse gas emissions, there is a demand to improve the energy efficiency of buildings.
Hence, in this respect, concepts like passive houses and zero emission buildings are being introduced.
Very thick building envelopes are not desirable due to several reasons, e.g. considering space issues with respect to both economy, floor area, transport volumes, architectural restrictions and other limitations, material usage and existing building techniques.
It should also be noted that recent studies (McKinsey 2009) point out that energy efficiency measures are the most costeffective ones, whereas measures like e.g. solar photovoltaics and wind energy are far less cost-effective than insulation retrofit for buildings.

2. Thermal background

The main key property of a thermal building insulation material or solution is the thermal conductivity, where the normal strategy or goal is to achieve as low thermal conductivity as possible.
A low thermal conductivity (W/(mK)) enables the application of relatively thin building envelopes with a high thermal resistance (m2K/W) and a low thermal transmittance U-value (W/(m2K)).
As the authors will see later this last coupling term is included through a factor in the expression for the gas conductivity as given in Eq.2 for the Knudsen effect.
The convection thermal conductivity λconv comes from thermal mass transport or movement of air and moisture.
The various thermal insulation materials and solutions utilize various strategies to keep these specific thermal conductivities as low as possible.

3. Traditional thermal building insulation

In the following there is given a short description of the most common traditional thermal building insulation materials of today with a relatively low thermal conductivity.
An overview of traditional thermal insulation materials may be found in the works by Al-Homoud (2005) and Papadopoulos (2005).

3.1 Mineral wool

Mineral wool covers glass wool (fibre glass) and rock wool, which normally is produced as mats and boards, but occasionally also as filling material.
Light and soft mineral wool products are applied in frame houses and other structures with cavities.
Heavier and harder mineral wool boards with high mass densities are used when the thermal insulation is intended for carrying loads, e.g. on floors or roofs.
Glass wool is produced from borosilicate glass at a temperature around 1400ºC, where the heated mass is pulled through rotating nozzles thus creating fibres.
Typical thermal conductivity values for mineral wool are between 30 to 40 mW/(mK).

3.2 Expanded polystyrene (EPS)

Expanded polystyrene (EPS) is made from small spheres of polystyrene (from crude oil) containing an expansion agent, e.g. pentane C6H12, which expand by heating with water vapour.
The expanding spheres are bond together at their contact areas.
The insulation material is casted as boards or continuously on a production line.
As an example, the thermal conductivity of EPS may increase from 36 mW/(mK) to 54 mW/(mK) with increasing moisture content from 0 vol% to 10 vol%, respectively.
EPS products may be perforated, and also cut and adjusted at the building site, without any loss of thermal resistance.

3.3 Extruded polystyrene (XPS)

Extruded polystyrene (XPS) is produced from melted polystyrene (from crude oil) by adding an expansion gas, e.g. HFC, CO2 or C6H12, where the polystyrene mass is extruded through a nozzle with pressure release causing the mass to expand.
The insulation material is produced in continuous lengths which are cut after cooling.
Typical thermal conductivity values for XPS are between 30 to 40 mW/(mK).
The thermal conductivity of XPS varies with temperature, moisture content and mass density.
XPS products may be perforated, and also cut and adjusted at the building site, without any loss of thermal resistance.

3.4 Cellulose

Cellulose (polysaccharide, (C6H10O5)n) comprises thermal insulation made from recycled paper or wood fibre mass.
The production process gives the insulation material a consistence somewhat similar to that of wool.
·8H2O) are added to improve the product properties.
Cellulose insulation is used as a filler material to fill various cavities and spaces, but cellulose insulation boards and mats are also produced.
The thermal conductivity of cellulose insulation varies with temperature, moisture content and mass density.

3.5 Cork

Cork thermal insulation is primarily made from the cork oak, and can be produced as both a filler material or as boards.
Typical thermal conductivity values for cork are between 40 to 50 mW/(mK).
Cork insulation products may be perforated, and also cut and adjusted at the building site, without any loss of thermal resistance.

3.6 Polyurethane (PUR)

Polyurethane (PUR) is formed by a reaction between isocyanates and polyols (alcohols containing multiple hydroxyl groups).
The thermal conductivity of PUR varies with temperature, moisture content and mass density.
PUR products may be perforated, and also cut and adjusted at the building site, without any loss of thermal resistance.
It should be noted that even if PUR is safe in its intended use it rises serious health concerns and hazards in case of a fire.
The HCN toxicity stems from the cyanide anion (CN-) which prevents cellular respiration.

4. State-of-the-art thermal building insulation

Below there is given a short description of the state-of-the-art thermal building insulation materials and solutions of today.
That is, the materials and solutions which are, or which are considered to be, the thermal building insulations with the lowest thermal conductivity today.

4.1 Vacuum insulation panels (VIP)

Vacuum insulation panels (VIP) consist of an open porous core of fumed silica enveloped of several metallized polymer laminate layers, see Fig.1 and Fig.2.
The VIPs represent today’s state-of-the-art thermal insulation with thermal conductivities ranging from between 3 to 4 mW/(mK) in fresh condition to typically 8 mW/(mK) after 25 years ageing due to water vapour and air diffusion through the VIP envelope and into the VIP core material which has an open pore structure.
This represents another major disadvantage of VIPs.
Thermal insulation thicknesses up to 50 cm or more in walls and roofs are not desired (see Fig.1 for a visual thickness comparison).
In addition, transport of thick building elements leads to increased costs.

4.2 Gas-filled panels (GFP)

A recent review of GFPs for building applications is given by Baetens et al. (2010c).
The GFPs apply a gas less thermal conductive than air, e.g. argon (Ar), krypton (Kr) and xenon (Xe), instead of vacuum as in the VIPs.
Low emissivity surfaces inside the GFPs decreases the radiative heat transfer.
Thermal conductivities for prototype GFPs are quite high, e.g. 40 mW/(mK), although much lower theoretical values have been calculated.
Hence, the GFPs hold many of the VIPs advantages and disadvantages.

4.3 Aerogels

Aerogels (Fig.5) represent a state-of-the-art thermal insulation solution, and maybe the most promising with the highest potential of them all at the moment, studied by Baetens et al. (2011), Hostler et al. (2008), Schultz et al. (2005) and Schultz and Jensen (2008) among several others.
Using carbon black to suppress the radiative transfer, thermal conductivities as low as 4 mW/(mK) may be reached at a pressure of 50 mbar.
The production costs of aerogels are still very high.
The tensile strength may be increased by incorporation of a carbon fibre matrix.
For aerogels to become a widespread thermal insulation material for opaque applications, the costs have to be lowered substantially.

4.4 Phase change materials (PCM)

Phase change materials (PCMs) are not really thermal insulation materials, but since they are interesting for thermal building applications, they are mentioned within this context.
PCMs change phase from solid state to liquid when heated, thus absorbing energy in the endothermic process.
Such a phase change cycle stabilizes the indoor building temperature and decreases the heating and cooling loads.
Various paraffins are typically examples of PCMs, but a low thermal conductivity (Farid et al. 2004) and a large volume change during phase transition (Hasnain 1998) limit their building application.
Corresponding melting enthalpies and melting temperatures are depicted for various groups of PCMs in the work by Dieckmann (2006).

5. Nanotechnology and thermal insulation

Shortly the authors will be seeing that nanotechnology may be applied as a scientific tool to make high performance thermal insulation materials.
The normal focus in nanotechnology is to control matter, typical particles, of dimensions between 0.1 nm and 100 nm, i.e. at an atomic and molecular scale.
For nanotechnology applied for making thermal insulation materials, the focus is shifted from particles to pores in the nano range.

6.1 Vacuum insulation materials (VIM)

A vacuum insulation material (VIM) is basically a homogeneous material with a closed small pore structure filled with vacuum with an overall thermal conductivity of less than 4 mW/(mK) in pristine condition (Fig.7).
The VIM can be cut and adapted at the building site with no loss of low thermal conductivity.
Perforating the VIM with a nail or similar would only result in a local heat bridge, i.e. no loss of low thermal conductivity.
For further details on VIMs it is referred to Jelle et al. (2010a).

6.2 Gas insulation materials (GIM)

A gas insulation material (GIM) is basically a homogeneous material with a closed small pore structure filled with a low-conductance gas, e.g. argon (Ar), krypton (Kr) or xenon (Xe), with an overall thermal conductivity of less than 4 mW/(mK) in the pristine condition.
That is, a GIM is basically the same as a VIM, except that the vacuum inside the closed pore structure is substituted with a low-conductance gas.
For further details on GIMs it is referred to Jelle et al. (2010a).

6.3 Nano insulation materials (NIM)

The development from VIPs to nano insulation materials (NIM) is depicted in Fig.8.
In the NIM the pore size within the material is decreased below a certain level, i.e. 40 nm or below for air, in order to achieve an overall thermal conductivity of less than 4 mW/(mK) in the pristine condition.
The work by Mulet et al. (2002) and Joulain et al. (2005) indicate that the large thermal radiation is only centered around a specific wavelength (or a few).
Nevertheless, these topics are currently being addressed in on-going research activities.

6.4 Dynamic insulation materials (DIM)

A dynamic insulation material (DIM) is a material where the thermal conductivity can be controlled within a desirable range.
The inner pore gas content or concentration including the mean free path of the gas molecules and the gas-surface interaction.
The solid state thermal conductivity of the lattice.

6.5 Concrete and applications of NIMs

With decreasing thermal conductivities of insulation materials, new solutions should also be sought for the load-bearing elements of the building envelope.
Using concrete as an example, one might envision to mix NIMs into the concrete, thereby decreasing the thermal conductivity of the structural construction material substantially, while maintaining most or a major part of the mechanical strength and load-bearing capabilities of concrete.
As concrete has a high thermal conductivity (1700 – 2500 mW/(mK), without and with rebars) a concrete building envelope always has to utilize various thermal insulation materials in order to achieve a satisfactory low thermal transmittance (U-value).
That is, the total thickness of the building envelope will often become unnecessary large, especially when trying to obtain passive house or zero energy building standards.
Furthermore, the large CO2 emissions connected to the production of cement, imply that concrete has a large negative environmental impact with respect to global warming due to the man-made CO2 increase in the atmosphere (McArdle and Lindstrom 2009, World Business Council for Sustainable Development 2002).

6.6 NanoCon

In principle, it is not the building material itself, i.e. if it is steel, glass, wood, mineral wool, concrete or another material, which is important.
On the contrary, it is the property requirements or functional requirements which are crucial to the performance and possibilities of a material, component, assembly or building.
Essentially, NanoCon is a NIM with construction properties matching or surpassing those of concrete.
In this respect it should be noted that carbon nanotubes have a very large thermal conductivity along the tube axis.

6.7 Other future materials and solutions?

The ultimate thermal solution will always be subject to change as time is progressing.
With other words, the thermal solution of tomorrow might be found in materials and solutions not yet thought of, which requires that the authors may have to think thoughts not yet thought of (Jelle et al. 2010a).

7.1 Robustness of traditional thermal insulation materials

The traditional insulation materials are the robust ones with respect to perforation vulnerability and flexibility issues like e.g. possible to adapt at the building site.
The traditional insulation materials have relatively high thermal conductivity values which in cold climates may require all too thick building envelopes in order to reach the goals of passive houses and zero energy or emission buildings.
In addition, the thermal conductivity increases substantially with increasing moisture content for the traditional thermal building insulation materials, i.e. a vulnerability towards moisture uptake.
The traditional insulation material with the lowest thermal conductivity is polyurethane (PUR), with values down to 20 mW/(mK) compared to the others’ values typically ranging between 30-40 mW/(mK).
The toxic gas release from PUR during a fire raises serious health hazard issues.

7.2 Thermal conductivity of state-of-the-art thermal insulation materials

The two most promising state-of-the-art insulation materials and solutions, i.e. vacuum insulation panels (VIP) and aerogels, have considerably lower thermal conductivity values than the traditional ones.
Comparing the miscellaneous traditional and state-of-the-art thermal insulation materials and solutions, the VIP solution has definitively the lowest thermal conductivity value of them all, i.e. typical around 4 mW/(mK) in the pristine non-aged condition, whereas the typical low value for aerogel is 13 mW/(mK).
The aerogel conductivity on the other hand is not considered to be increasing substantially with time, and perforations represent no problem.
Both VIPs and aerogels are very expensive, but it has been demonstrated that VIPs may be cost-effective (ch. 7.7), and aerogels in their transparent or translucent state offer application areas where one may be willing to accept higher costs.

7.4 Thermal conductivity and other properties

Furthermore, the thermal conductivity should not increase substantially over a 100 year or more lifetime span.
The VIP solution with an envelope barrier around an open pore structure supposed to maintain a vacuum does not satisfy this specific requirement, as cutting a VIP will result in a total loss of vacuum and an increase of thermal conductivity up to typically 20 mW/(mK).
Several other properties also have to be addressed.

7.5 Requirements of future thermal insulation materials and solutions

The thermal insulation materials and solutions of tomorrow have to satisfy several crucial requirements.
Table 1 summarizes the various properties with their proposed requirements.
As it can be seen, the proposed thermal conductivity requirement in the pristine condition is a conductivity less than 4 mW/(mK), which is the typical value for non-aged VIPs.
Naturally, the thermal conductivity after a certain period of time or service life, is of vital importance.

7.6 The potential of miscellaneous thermal insulation materials and solutions

In addition to being a summary, Table 2 may be utilized to initiate a chain of thoughts of how to proceed beyond today’s state-of-the-art thermal solutions.
Also note that Table 2 expresses the current status for the state-of-the-art solutions of today and the foreseen status for the beyond state-of-the-art solutions, where certain items in the table might be subject both to discussion and change.
Currently, the NIM solution seems to represent the best high performance low conductivity thermal solution for the foreseeable future.
DIMs and NanoCon represent two solutions with a huge potential, with thermal insulation regulating and load-bearing capabilities, respectively.

7.7 Potential cost savings by applying VIPs

Today’s vacuum insulation panels (VIP) are relatively costly.
If one uses less than 330 EUR/(m2 living area) for the actual VIP purchase, the difference will be a clean-cut profit in favour of VIPs.
The wall thickness reduction could have been even lower with respect to the thermal resistance of the VIPs, but a minimum wall thickness of 15 cm is chosen due to construction reasons including load-bearing properties.
It should be noted that the calculations are simplified and do not consider all conditions.

7.8 Condensation risk by applying VIPs in the building envelope

A very low water permeability close to zero, e.g. for VIPs, may represent a condensation risk depending on the actual construction and the indoor and outdoor climate conditions, for both new buildings and thermal retrofitting of existing buildings.
These issues have been addressed by Sveipe et al. (2011) through experimental and numerical investigations of temperature and humidity conditions in building envelopes during thermal retrofitting of timber frame walls with VIPs.

7.9 The cardinal weaknesses of VIPs

Vacuum insulation panels (VIP) have the lowest pristine thermal conductivity of all thermal insulation materials, with typical values around 4 mW/(mK) in the non-aged condition.
VIPs may be handled and stored with great care, but nevertheless will many panels suddenly and unexpectedly lose all their vacuum for no evident reason.
This vulnerability may imply miscellaneous restrictions to the applications of VIPs, e.g. other building envelope techniques or various restrictions to perforate the building envelopes.
That is, you may not put up your pictures everywhere on the all the walls in the same way as you have done before.
The increasing thermal conductivity during time represents yet another cardinal weakness of VIPs.

7.10 EPS encapsulated VIPs

Thermal insulation solutions with VIPs wrapped in expanded polystyrene (EPS) have been proposed and are also in actual use.
These EPS-VIPs have an extra protection due to the EPS covering, and enables some adjustments at the building site as parts of the EPS perimeter may be cut away.
Applying EPS encapsulated VIPs in two layers, ensuring there is always at least one VIP layer in the EPS joints, reduces the (EPS) heat bridges.
These EPS covered VIPs have also some drawbacks.
Firstly, the loss of vacuum in VIPs totally covered in EPS is not so easy to detect and increases thus the probability of installing VIPs with no vacuum.

7.11 VIMs and GIMs versus NIMs

A vacuum insulation material (VIM) or a gas insulation material (GIM) needs to maintain a vacuum or a gas inside a closed pore structure, respectively.
The VIM grid structure has to be strong enough to withstand the vacuum inside its pores, and the air and water vapour diffusion through the grid structure and into the vacuum pores have to be so small enough that the VIMs will maintain their low thermal conductivity below a certain value for at least 100 years.
That is, the most challenging task after the VIMs have been manufactured, as to make VIMs is a highly challenging task in itself.
Hence, compared to VIMs and GIMs, the NIMs are regarded to have the largest potential.

7.12 The regulating potential of DIMs

To be able to understand thermal conductance so well that one may tailor-make materials where one can dynamically regulate the thermal conductivity of a material from a very low to a very high conductivity has a tremendous potential.
Limiting ourselves to solid state thermal conduction, changes in the thermal conductivity involve changes in the chemical bonds between atoms.
Analogies might also be drawn towards the field of electrical superconductivity, i.e. the ability to transport electrical current without resistance and hence without loss of energy.
There is still much to be discovered and understood within the realms of atoms, quantum mechanics and the matter that surrounds us.
Clearly, both theoretical and practical, the DIMs represent a tremendous potential for applications in buildings and several other areas.

7.13 The construction potential of NanoCon

The idea of making a construction material which also possesses the properties of a high performance thermal insulating material has a huge potential.
NanoCon represents an ultimate solution in this respect.
NanoCon may be envisioned both with and without the use of steel rebars.
Today the corrosion of rebars in concrete constructions represents a very large worldwide problem.

7.14 Assessing weaknesses and strengths

When assessing which thermal building insulation material or solution to choose for a specific application or building, all the important properties should be evaluated with regard to their weaknesses and strengths.
As of today, no single thermal insulation material or solution exists which is superior or best in all respects, i.e. the evaluation of all the pros and cons for the different properties and applications is crucial when determining which thermal insulation material or solution to choose.

7.15 Does the future belong to NIMs, DIMs and NanoCon?

The large challenges associated with inventing and developing thermal insulation materials like nano insulation materials (NIMs), dynamic insulation materials (DIMs) and NanoCon or similar concepts should neither be concealed nor understated.
Research efforts will be needed, both along known theoretical principles within physics, chemistry and material science, but maybe also along more unknown paths within these areas.
Theoretical investigations have to be followed with concurrent experimental examinations and explorations.
The efforts put into these research paths will for certain pay off sooner or later, thus leading to the development of new thermal building insulation materials and solutions.
Hence, the authors may conclude that NIMs, DIMs and NanoCon may take the step or leap from the conceptual stage to being part of the high performance thermal insulation materials of beyond tomorrow.

8. Conclusions

Essential properties and issues raised are among others thermal conductivity, perforation vulnerability, building site adaptability and cuttability, mechanical strength, fire protection, fume emission during fire, robustness, climate ageing durability, resistance towards freezing/thawing cycles, water resistance, costs and environmental impact.
It is concluded that currently there exist no single thermal building insulation material or solution which satisfies all the requirements with respect to the most crucial properties.

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Figures (17)

Figure 1. (left) Typical VIP structure showing the main components (Simmler et al. 2005c) and (right) a comparison of equivalent thermal resistance thickness of traditional thermal insulation and VIP (Zwerger and Klein 2005).

Figure 8. The development from VIPs to NIMs (Jelle et al. 2010a).

Figure 11. NanoCon is essentially a NIM with construction properties matching or surpassing those of concrete. (Jelle et al. 2010b).

Figure 2. Cross-sections of typical envelope materials for VIPs: (a) Metal film, (b) Single layer metallized film and (c,d) Three layer metallized films. The four foil types are commonly named (a) AF, (b) MF1, (c) MF2 and (d) MF3 in literature (Willems and Schild 2006). Note that different types of foil with the same name are used in literature (Heineman et al. 2005, Brunner et al. 2006).

Figure 6. Nanotechnology and its application on high performance thermal insulation materials.

Figure 7. The development from VIPs to VIMs (Jelle et al. 2010a).

Figure 10. NIMs as thermal insulation for concrete, i.e. (top) NIM as outdoor and/or indoor retrofitting of concrete,(middle) NIM applied in the midst of concrete and (bottom) NIM mixed together with concrete.

Table 3. Available means for VIP purchase as function of living area market value and wall thickness reduction. An interior floor to ceiling height of 2.5 m is assumed. Mineral wool costs of 15, 18, 20, 24 and 26 EUR/m2 are used for mineral wool thicknesses of 25, 30, 35, 40 and 45 cm, respectively, for a corresponding wall thickness reduction of 10, 15, 20, 25 and 30 cm by application of VIPs.

Figure 3. Centre-of-panel thermal conductivity for VIPs with a fumed silica core as function of elapsed time. For two different panel sizes of 50 cm x 50 cm x 1 cm and 100 cm x 100 cm x 2 cm, and for three different foil types AF, MF1 and MF2 (Baetens et al. 2010a).

Table 4. Profit in EUR/(m2 living area) by application of VIPs as function of living area market value where the wall thickness reduction is 20 cm for an example building of 10 m x 10 m. An interior floor to ceiling height of 2.5 m is assumed. See Fig.12.

Table 5. Profit in EUR/(100 m2 living area) by application of VIPs as function of living area market value where the wall thickness reduction is 20 cm for an example building of 10 m x 10 m. An interior floor to ceiling height of 2.5 m is assumed. See Fig.12.

Table 1. Selected and proposed requirements of the future high performance thermal insulation materials and solutions.

Table 2. The potential of the traditional, state-of-the-art and possible future thermal building insulation materials and solutions of tomorrow.

Figure 9. Gas thermal conductivity and a (left) 2D-plot depicting the effect of pore diameter for air, argon, krypton and xenon and a (right) 3D-plot depicting the effect of pore diameter and gas pressure in pores for air (Jelle et al. 2010a).

Figure 4. Barrier foil and baffle structure inside a GFP (LBNL 2008).

Figure 5. (left) Peter Tsou from NASA with a translucent aerogel sample developed for space missions (NASA 2010), (middle) matches on top of aerogel are protected from the flame underneath (NASA 2010) and (right) an example of aerogel as a high performance thermal insulation material (Aspen Aerogels 2009).

Figure 12. Profit in (left) EUR/(m2 living area) and (right) EUR/(100 m2 living area) by application of VIPs as function of living area market value where the wall thickness reduction is 20 cm for an example building of 10 m x 10 m. An interior floor to ceiling height of 2.5 m is assumed. See Table 4 and Table 5.

Frequently Asked Questions (2)

Q1. What are the future works in this paper?

As an endnote, one may conclude that future research may beneficially be conducted along three paths, i. e. ( i ) improving the existing traditional thermal insulation, ( ii ) improving the existing state-of-the-art thermal insulation, and ( iii ) exploring the possibilities of discovering and developing novel high performance thermal insulation materials and solutions with properties surpassing all of today ’ s existing materials and solutions.

Q2. What are the contributions in this paper?

Various properties, requirements and possibilities have been compared and studied.