High performance thermal insulation materials for buildings
Abstract:
Recent developments in high performance thermal insulators based on nanotechnology have enabled a strong drop in the effective thermal conductivity of insulation materials, down to 0.004–0.014 W/(mK). The reductions are achieved using the Knudsen effect, which describes the effect of pore size distributions and partial gas pressure in materials on the gaseous heat transfer. The resulting thermal insulation materials have specific properties of importance for the building industry, which should be considered in each project. Further exploitation and a similar approach to solid conduction may result in the next-generation high performance thermal insulators.
9.1 Introduction
The true origins of thermal insulation are difficult to identify. Prehistoric humans clothed themselves with wool and animal skin and built homes of wood, stone and earth, whereas the Romans as well as early inhabitants of Spain already used cork as an insulating material for roofs. Mineral fibers from volcanic deposits were first used by the Hawaiian natives to blanket their huts, but it was not until the first industrial revolution that commercial application of thermal insulation became common with Cabot’s Quilt in 1891 as the earliest example. Since then, rock wool, fiberglass and extruded polystyrene have appeared commercially as thermal insulators in commercial and residential buildings (Close, 1947; Jester, 1995).
Recent progress in the development of high performance thermal insulators is due to progress in nanotechonolgy and material sciences, allowing the adaptation of known theoretical principles of thermal physics in practice. High performance thermal insulators (HPTIs) strongly differ from traditional insulators on base principles considering heat transfer. Traditional thermal isolators are distinguished by how they trap a gaseous material, i.e., in a fibrous material, in a cellular material, or in a granular material. These insulators have a thermal conductivity in the range of 0.025–0.040 W/(mK) and show a lower limit for their thermal conductivity close to the thermal conductivity of the trapped gas. As such, high performance thermal insulators are generally defined as an insulator with a thermal conductivity below 0.02 W/(mK). Similarly to the distinction in traditional thermal insulators, current high performance thermal insulators are distinguished by how they achieve rarefaction of the gas, i.e., by a nanoporous solid structure, or by application of a partial vacuum, whereas the best results are achieved with a combination of both. The exploitation of the divergent physical theory of heat transfer in nanoporous materials is the reason why nanotechnology has produced a breakthrough in thermal insulators the last decade, as summarized in Fig. 9.1.
Fig 9.1 Nanotechnology and its application to high performance thermal insulation materials. (Jelle, 2011).
9.2 Heat transfer in thermal insulators
9.2.1 Macroscopic heat transfer
The macroscopic heat transfer in a material is described by Fourier’s law relating the energy flux ø to the temperature gradient by its thermal conductivity k as
The effective heat transfer ø for a porous insulator is a combination of four heat transfer mechanisms and can be approximated by their sum
where φcd,s denotes heat conduction of the solid skeleton, φrd heat transfer by longwave radiation, φcd,g heat conduction of encapsulated gas molecules and φcv heat transfer by gas convection. The coupling term φk describing possible reciprocity between the different processes is generally neglected. Traditional thermal isolators use the inherently low conductivity kcd,g of a gas at standard temperature and pressure to achieve a low overall heat transfer. Convection in the gaseous material is avoided by reducing the pore sizes, and the impact of solid conduction φcd,s is reduced by means of a high porosity.
9.2.2 Rarefied gas regimes in current HPTIs
The traditional Fourier law is no longer valid if the materials characteristic time scale θ or length scale Λ has the same order of size as the natural time scale τ or length scale l of the physical problem, meaning that the physical process of heat transfer occurs at the same scale as the scale at which the basic solid properties of the materials are defined. Recent progress in the development of high performance insulators is based on this principle for the gaseous component φcd,g of the overall heat transfer. Here, the natural time or length scale denotes the distance traveled or time between two collisions of gaseous molecules, whereas the characteristic material scale denotes the size of pores in which the collisions occur.
The gas properties are described by definition of the velocity distribution function f(r, v) defining the number of gas molecule dn having a velocity in the range [v, v + dv] in the volume element r + dr. The resulting conductive heat transfer φcd,g is expressed as the transport of the particles kinetic energy (Volz, 2007), as:
where m is the particle mass and 〈v⟩ the local average velocity. The change of this one-particle distribution function f is described by the Boltzmann equation (Boltzmann, 1884) as:
where F is an applied external force field. The right-hand side describes the effect of collisions between particles and the walls of a container determined by the molecular chaos assumption depicting a binary collision of particles. The collision term is often approximated as –[f – f0] / τ under the assumption that collisions in a gas from a non-equilibrium state will tend to an equilibrium state, with τ(v) the relaxation time to return to equilibrium within dr and f0 the local equilibrium distribution. The resulting Boltzmann equation can be expressed by means of dimensionless parameters as:
where the accented parameters are dimensionless. One can notice that the changes in f are driven by vτ / Λ, i.e., l / Λ in a stationary system, or by τ / θ in a homogeneous non-stationary system. This ratio expresses the ratio of the natural scale of the physical problem to that of the material, and is known as the Knudsen number Kn. Depending on this ratio, we can define two distinct regimes, i.e., a collisional regime with Kn < < 1 as considered in the macroscopic laws with a system state close to f0, and a rarefied regime with Kn > > 1 where inter-particle collisions are negligible and heat transfer is ruled by collisions with the walls. Here, the velocity distribution function evolves according to the Boltzmann equation with the collisional term equal to zero.
In contrast to the collisional regime, the heat transfer in a rarefied gas proceeds for the major part from the momentum exchange by collision between the gas molecules and the pore walls instead of by collision between gas molecules. The resulting conductivity drop is described by the so-called Knudsen effect (Kennard, 1938). The heat transfer in rarefied gas regimes is generally expressed as derived from parallel surfaces by normalizing the heat transfer φcd,g by the heat transfer 
denoting the apparent macroscopic heat transfer of the gas in the collisional regime. The resulting ratio 
depends only on the Knudsen number and a term β(α, γ) describing the heat transfer by collision between the gas molecule and the pore wall, depending on the thermal accommodation coefficient α and heat capacity ratio γ, expressed as:
and as shown graphically in Fig. 9.2,, whereas the β is generally simplified as constant. The required Knudsen number can be determined based on the kinetic theory for an ideal gas as:
9.2 The thermal conductivity of air as a function of the air pressure and the average pore diameter of the medium.
where σ is the collisional cross-sectional area equal to πd2 with d the particle diameter, kB the Boltzmann constant, T the average gas temperature, and p the total gas pressure.
9.2.3 Rarefied photon and phonon regimes for next-generation HPTIs
The Knudsen effect has shown that macroscopic laws of heat transport no longer apply at nanoscale. Convective heat transport at this level is well understood, but the background of solid conduction and radiative heat transfer at this characteristic length is a rather unexplored field for non-metallic materials.
Whereas the gas molecules are the energy carriers for gaseous conduction, the energy carriers of solid conduction and radiation are respectively photons and phonons. The behavior of phonons and photons is similar to gas molecules in several ways: they are treated as classical particles beyond a certain length scale, i.e., the coherence length for phonons and the wavelength for photons, and their propagation is described by a Boltzmann equation. As such, similar to strong reduction of the gaseous heat transfer φcd,g in the ballistic gas regime due to the so-called Knudsen effect, a reduction of the heat transfer by solid conduction φcd,s and radiative heat transfer φrd can be denoted in the ballistic regimes of their energy carriers.
Radiative heat transfer is described by definition of the specific intensity Lv(u, r) in a frequency band [v, v + dv] depending on the direction u and the considered point r. This intensity can be interpreted as the product of the number of photons per unit volume nv(u, r) with the energy per photon hv and the speed of propagation vv. The resulting radiative heat transfer φrd is expressed as:
in the solid angle dΩ. The transport of specific intensity Lv is described by the radiative transfer equation (Chandrasekhar, 1960) as:
stating the radiative energy balance of a beam, with negative terms from extinction by absorption and scattering, and positive terms from scattering and thermal emission, and where μv is the monochromatic absorption coefficient, σv is the scattering coefficient, n is the real part of the refraction index of the medium, T is the local temperature and pv is the fraction of the energy flux in direction u that is scattered in direction u'.
Analogous to radiative heat transfer, solid conduction can be described by definition of the phonon radiative intensity Iω(u, r), interpreted as the product of the number of modes per unit volume dΩ with the number of phonons per unit volume nω(u, r), with the energy per photon ηω and the group velocity vω. The resulting (solid) conductive heat transfer φcd,s is expressed as:
in the solid angle dΩ. Also the transport of phonon radiative intensity Iω is described by a phonon radiative transfer equation (Srivastava, 1990) as:
stating the radiative phonon energy balance, with creation and destruction of phonons during collision, and where uω is the reciprocal of the mean free phonon path Λω equal to vωτω.
Both the radiative transfer equation and the phonon radiative transfer equation denote that also radiative and conductive heat transfer has a ballistic regime with a characteristic length lext equal to respectively (μυ + συ)− 1 and μv− 1. The characteristic length of radiation and solid conduction is, however, lower than the typical values for gaseous conduction and can be found in the range of 1–0.01 nm. This means that an equally strong reduction on φcd,s and φrd can be achieved as depicted by the Knudsen effect. Solid layers or structures with a thickness below this lext show a lower k due to ballistic phonon and photon transport. Here, the phonon radiative transfer equation is reduced to boundary scattering, whereas the radiative transfer equation for the ballistic regime can be obtained by inserting the decomposition of the specific intensity Lv as Lv,bal · δ(u – u0) into the RTE, resulting in a ballistic component equal to
denoting a strongly reduced φrd and resulting k-value. The reduction of both the solid conductive and radiative heat transfer component in φ can be used in two ways. Firstly, it could be used to reduced the effective thermal conductivity k of current state-of-the-art thermal insulators to values below 0.014 W/(mK) at ambient conditions. Secondly, it could be employed to develop more robust high performance thermal insulators with equal thermal properties to current state-of-the-art materials but without their drawbacks by reducing the importance of the gaseous conductive component φcd,g into the overal low heat transfer.
9.3 State-of-the-art insulators
As traditional thermal isolators are distinguished by how they trap a gaseous material, current high performance thermal insulators are distinguished by how they achieve rarefaction of the gaseous material and its resulting high Knudsen number, i.e., by a nanoporous solid structure with a low pore size λ, such as fumed silica or aerogels (Baetens et al, 2011; Dorchech & Abbasi, 2008; Hüsing & Schubert, 1998; Jelle et al., 2010; Richter, 1995; Wang et al., 2007), or by application of a partial vacuum such as for vacuum insulation panels (Alam et al., 2011; Baetens et al., 2010a; Jelle, 2011; Jelle et al., 2010) with a low gas pressure p. The best results are achieved with a combination of both.
9.3.1 Nanoporous thermal insulators
As most thermal insulators that apply a partial vacuum use a material with nanoscale pores, nanoporous thermal insulators will be treated first. Within this context, the main nanoporous thermal insulators are aerogels (see Fig. 9.3) and microporous silica. Aerogels are used as a main thermal insulator on its own, whereas microporous silica is generally only used as core material for partial vacuum thermal insulators and will be treated later. Aerogels are essentially the solid framework of a gel isolated from its liquid medium and were discovered in the early 1930s by S.S. Kistler (Kistler, 1931). The main type of aerogels as high performance thermal insulators are silica aerogels, SiO2. Aerogels have an extremely high porosity and a pore size distribution f(Λ) below the mean free path for air molecules at standard temperature and pressure, making them a high performance thermal insulator.
9.3 Typical VIP structure showing the main components and an example of aerogel as a high performance thermal insulation material for building applications (Baetens et al., 2011; Jelle, 2011).
The unique properties of aerogels are determined by its synthesis, which can be divided into three general steps: gel preparation, aging of the gel, and drying of the gel. A detailed comprehensive review on the synthesis of silica aerogels has recently been written by Dorcheh and Abbasi (2008) and the author would like to refer to this work for a more extensive analysis of the aerogel synthesis.
Gel preparation
Gel preparation happens by means of a sol–gel process (Brinker & Scherer, 1990; Dorchech & Abbasi, 2008), i.e., a process in which solid nanoparticles dispersed in a liquid agglomerate together to form a continuous three-dimensional network extending throughout the liquid. The main precursors for silica aerogels are silicon alkoxides. A simplified reaction based on tetramethoxysilane may be presented as
The sol becomes a gel when the dispersed solid particles stick together during collision to form a network of particles spanning the entire liquid. Nanoparticles containing reactive surface groups stick by bonding or electrostatic forces, whereas others may require an additive. In general, acid hydrolysis and condensation results in weakly branched chains and microporous structures in silica sols and resulting long gelation times, whereas uniform particles are easily formed in base catalysis, leading to a broader distribution of larger pores which is less favorable for thermal insulation materials.
Aging of the gel
Aging of the gel in its mother solution is required to prevent the gel from shrinking during drying (Haereid et al., 1996). The silica spine of the gel contains a significant number of unreacted alkoxide groups, and hydrolysis and condensation must continue a sufficient time for strengthening of the silica network. During this aging period, material transports to the spine neck region and small particles dissolve into larger ones. Common aging procedures involve ethanolsiloxane mixtures, adding new monomers to the solid SiO network, and increasing the degree of cross-linking. After aging, all water remaining within the pores must be removed before drying: any water left in the gel leads to an opaque and very dense aerogel.
Drying of the gel
Drying of the gel is the most critical stem in the production process and happens under special conditions to prevent the gel structure from collapsing due to shrinkage and possible capillary tension in the small pore sizes. If a liquid is held under pressure always greater than the vapor pressure, and the temperature is raised, it will be transformed at the critical temperature into a gas without two phases having been present at any time (Kistler, 1932). As such, high temperature supercritical drying and low temperature supercritical drying from carbon dioxide is mostly used as drying process.
The unique structure of aerogels result in exceptional material properties: a bulk density typically of 70–150 kg/cm3 due to a porosity of 85–95%, a specific surface area of 600–1000 m2/g, a particle size below 5 nm, a f(Λ) averaging 20 nm with maximum pore sizes of 100 nm close to the mean free path of 70 nm for air molecules at standard temperature and pressure, resulting in an effective thermal conductivity of 0.014 W/(mK) at atmospheric conditions. Also translucent aerogels can be achieved depending on water removal before the drying process, resulting in a transmittance between 0.8 and 0.95 in the visible and (near-)infrared spectrum for a layer of 1 cm and a low refraction index of around 1.0.
9.3.2 Partial vacuum thermal insulators
Vacuum insulation panels (VIP) (see Fig. 9.3) are defined as an evacuated foil-encapsulated open-porous material as thermal insulator. As such the insulator is no typical thermal insulation material, but consists of a system of three parts, each with their own specific purpose(s), i.e., the open-porous core material, the foil envelope and the applied vacuum. A perfect vacuum is the most effective reduction of the gas thermal conduction φcd,g,achieving its limit value of ‘zero’. This perfect vacuum is pure theoretically, but a low pressure Pg has a positive influence on the gaseous heat transfer. As such the core has a dual purpose, i.e., to withstand the pressure of the applied partial vacuum, and preferably to strengthen the thermal effect of this vacuum by its pore size distribution f(Λ). This core material is typically a traditional open-porous thermal insulator, or a nanoporous high performance thermal insulator. The foil envelope only serves to maintain the applied partial vacuum in the core material and generally consists of an aluminum layer. Due to the relatively high thermal conductivity of such an envelope, the heat flux increases at the edges and corners. Furthermore, the foil envelope is not able to keep the applied vacuum constant at the pristine pressure due to gas and moisture mitigation through the foil and foil seams. The envelope choice is generally a compromise between the allowed pressure drop through time and the allowed thermal bridging at the panel edges.
Current state-of-the-art vacuum insulation panels consist of a core material of fumed silica with a bulk density of 160–220 kg/m3, a specific surface area of 100–400 m2/g and a f(Λ) with maximum pore sizes around 300 nm close to the mean free path of 70 nm for air molecules at standard temperature and pressure, resulting in a effective thermal conductivity of 0.020 W/(mK) at atmospheric conditions. The applied partial vacuum is around 5 mbar in pristine conditions, lowering the effective thermal conductivity of the fumed silica core material to 0.004 W/(mK). The multilayer films usable for VIP envelopes consist of different layers with an overall thickness of 100–200 μm. Two different film types are mainly being used for VIP envelopes: metal foils consisting of a central aluminum barrier layer laminated between an outer scratch-resistant polyethylene terephthalate layer and an inner polyethylene sealing layer, and metalized films made from up to three layers of aluminum-coated polyethylene terephthalate films and an inner polyethylene sealing layer.
9.4 Applications
Each of the mentioned materials has their specific application properties as thermal insulators in the building sector as summarized in Tables 9.1 and 9.2, and will be discussed below.
Table 9.1
Summary of thermal properties for vacuum insulation and aerogel insulation (Baetens et al., 2010b; Tenpierik & Cauberg, 2007)
Table 9.2
Summary of advantages and disadvantages for vacuum insulation panels and aerogel insulation
Thermal insulators for building constructions require a specific set of main functional requirements. The first requirement is, naturally, its resistance to heat transfer expressed in his low thermal conductivity k. Second, it must withstand degradation through time of its thermal properties, e, generally its resistance to radiation, its resistance to degradation by moisture requiring a hydrophobic material, and its resistance to mechanical impact. The third requirement of importance is the possible ease of installation and its economic feasibility.
9.4.1 Nanoporous thermal insulators
The areas of application for aerogel insulation are strongly linked to their physical properties, and so these will be discussed first.
Properties
The main benefit of aerogel insulation is its low thermal conductivity at ambient conditions, while the material also shows exceptional properties concerning optical transmittance, sound absorption, and fire retardation.
Aerogel insulators have an overall thermal conductivity at ambient pressure down to 0.012 W/(mK) at ambient pressure and to 0.004 W/(mK) at a pressure of 50 mbar or less, whereas commercial aerogel thermal insulators for building purposes have a thermal conductivity of around 0.014 W/(mK) at ambient temperature and are very little affected up to a temperature of 200 °C.
Silica aerogels have a high transmittance of radiation within the range of visible light, i.e., radiation with a wavelength between 380 and 780 nm. Monolith translucent silica aerogel in a 10 mm thick packed bed has a solar transmittance Tsol of 0.88 and a possible high transparency in the infrared spectrum Tir of 0.85. Light reflected by silica aerogels appears bluish and transmitted light appears slightly reddened. This light scattering can be explained by λ–4-type Rayleigh scattering caused by the interaction with inhomogeneities, and becomes more effective when the size of the particles is similar to the wavelength of the incident light. The presence of pores within this range f(λ) acts as scattering centers, and the efficiency of scattering will depend on the size of the scattering centers as well as the wavelength λ. Heat treatment of aerogels can increase their transparency, and the optical properties can be influenced further by selecting optimal synthesis parameters in the sol-gel process.
The sound absorption of a material increases with an increasing surface area facing the sound. As aerogels have a high porosity and a high specific surface area, sound waves are strongly absorbed and attenuated: monolith silica aerogels have a lower speed of sound than air. Sound velocities down to 40 m/s have been measured, whereas non-monolith commercial products claim to have a sound velocity of 100 m/s through the structure. Granular aerogels, on the other hand, are exceptional reflectors of audible sound, making excellent barrier materials. By combining multiple layers with different granular sizes, average attenuations of − 60 dB have been found for a total thickness of only 0.07 m.
In contrast to combustible organic foam insulation that emits deadly fumes and smoke when burning, silica aerogel materials are non-combustible due to their non-organic SiO2 structure and withstand heat up to 1400 °C.
Due to their physical solid structure, aerogels show a low tensile strength and a brittle nature, whereas its porous structures make it very sensible to moisture due to high surface tensions.
Aerogel insulation sheets suffer from dust production. As most of the commercial aerogel insulation products consist of complete amorphous (and thus 0% crystalline) silica, exposure limits in the range of 5 mg/m3 for respirable dust are set in the US by the Occupational Safety and Health Administration (OSHA). However, the International Agency for Research on Cancer (IARC) considers synthetic amorphous silica to be not classifiable as to its carcinogenicity to humans (i.e., belongs in group 3). No evidence of silicosis has been found from epidemiological studies of workers with long-term exposure to synthetic silica, whereas studies of various animal species show that amorphous silica can be completely cleared from the lungs (Merget et al., 2002; Warheit, 2001).
Areas of application
Aerogel insulation panels have only recently been introduced to the market in small-scale production. The main building applications may be divided into two groups, i.e., as traditional thermal insulators by means of aerogel blankets, and in translucent form for high performance glazing.
Commercial manufacture of aerogel blankets began around the year 2000 and has been developed to meet various demands. An aerogel blanket is a composite of a silica aerogel and fibrous reinforcement, turning the brittle structure of a silica aerogel into a durable, flexible, and hydrophobic material. The resulting blankets have a thermal conductivity k between 0.013 and 0.014 W/(mK). The aerogel insulation material consists of amorphous silica instead of crystalline silica, reducing possible health risks at exposure. Aerogel blankets can be used in the entire building industry similar to the use of traditional thermal insulators. However, their current high economic cost means that they are only used where limited space is available and where the use of vacuum insulation panels is not possible due to their drawbacks.
Aerogel is especially very interesting as a translucent or transparent insulation material because of its combination of a low thermal conductivity and a high transmittance of daylight and solar energy. At present, there are two commercial types of such aerogel-based daylight systems, i.e., Scoba-lit and Okagel windows. The aerogel product has a thermal conductivity of 0.018 W/(mK) and the fabricator offers skylights with a heat transmittance coefficient between 0.6 and 0.3 W/(m2K) for layers of 30 and 60 mm Okagel respectively. The visible light transmission Tvis is 0.40 and the sound reduction is 52 dB. Research has been conducted in the last decade on the development of highly insulating windows based on both granular aerogel and monolithic aerogel. Two types of granular aerogel are used in prototype windows: semi-transparent spheres with a solar transmittance Tsol of 0.53 for a 10 mm packed bed and highly translucent granulates with a Tsol of 0.88. The granular aerogel is stacked in a polymethylmethacrylate double skin-sheet, between two gaps and glass panes. Increasing the aerogel thickness to 20.0 mm will lower the U-value further to approximately 0.5 W/(m2K), while the solar transmittance will still stay above 0.75.
9.4.2 Partial vacuum thermal insulators
The areas of application of vacuum insulation panels are strongly linked to their physical properties and so these will be discussed first.
Properties
The main benefit of vacuum insulation panels is the reduction of the required thickness of the insulation layers. With a pristine center-of-panel thermal conductivity k of 0.004–0.005 W/(mK), equal thermal resistances are achieved within a thickness 5–8 times lower than traditional thermal insulators. The way this low thermal conductivity is achieved determines at the same time its main drawbacks, i.e., degradation through time of the thermal conductivity, thermal bridging at the panel edges, and strong limitations for installation and its resulting areas of application.
Degradation through time of the thermal resistance occurs mainly due to air and moisture intake through the panel envelope, and depends on the environmental conditions and the foil resistance for moisture and air transport. The intake results in an increase of the inner gas pressure δpg(Γ,κ), moisture pressure δpwv(Γ,κ) and water content δuw(Γ, κ), resulting in an increasing thermal conductivity δk(p, u) through time strongly depending on the panel dimensions Γ, the permeability κ for air and moisture of the envelope material, and the environmental properties in the domain of application. Knowledge of the long-term thermal performance of vacuum insulation panels is still limited: predictions are based on calculations or short-term on-site measurements. General values for the thermal conductivity are 0.007 and 0.010 W/(mK) for large vacuum insulation panels after respectively 25 and 100 years, whereas higher values of 0.009 and 0.015 W/(mK) are depicted for small panels as shown in Table 9.1. The main differences between large and small panels is due to the envelope-to-volume and edge-to-volume ratio.
Thermal bridging at the panel edges occurs due to the metalized panel envelope. The edges reduce the effective overall thermal resistance of the insulation panel with a linear thermal transmittance coefficient ψe(ke, k), depending on the equivalent thermal conductivity ke of the panel envelope and the center-of-panel thermal conductivity. In case of undamaged panels, the linear thermal transmittance measures 0.01 W/(mK) for metalized films and 0.04 W/(mK) for metal foils. Resulting from both degradation through time as well as thermal bridging of the panel edges, an equivalent thermal conductivity keq is generally used of 0.008 W/(mK), twice the center-of-panel thermal conductivity under pristine conditions.
Limitations of installation are due to the panel envelope which serves to maintain the inner vacuum of the panel, as this envelope may not be damaged. Vacuum insulation panels cannot be cut on-site into the required form, and much attention goes into the careful placing of the panels during construction and protection of the panels against mechanical damage during service life. When the vacuum is not maintained due to damage of the panel envelope, the thermal conductivity k increases to that of the core material under standard pressure conditions, i.e., 0.020 W/(mK) for a fumed silica core.
Areas of application
Vacuum insulation panels have already been introduced to the market in large-scale production, but manufacturing is still mainly hand-labor. In recent years, several building applications for VIPs have been proposed and/or tested, and a large-scale study has been carried out on the possibilities of vacuum insulation panels in insulated building envelopes. The main building applications may be divided into several groups, i.e., for building envelope retrofitting, as main building envelope insulator, in sandwich elements, and in domestic appliances.
Retrofit insulation of the existing building envelope is an obvious application of vacuum insulation panels as this is the domain where its main advantage has most value, i.e., its highly limited thickness. As installing additional insulation on the inside results in a great loss of floor space, vacuum insulation panels are of great interest for renovations. However, special attention has to be paid to low surface temperatures and possible condensation damage at connections to surrounding compounds.
The use of vacuum insulation panels as main building envelope insulation for new buildings differs from retrofit solutions as the thickness is of less importance, i.e., the structure can be adapted for thicker insulation layers without loss of space. In such applications, it is always studied whether or not the complete envelope building components should be pre-assembled in advance, to ensure the proper handling of the vacuum insulation panels. As such, sandwich elements applying vacuum insulation panels as vacuum insulated sandwich elements in door frames, window frames, curtain walls (see Fig. 9.4) and in non-load bearing walls form a major part of this application.
9.4 Four edge spacer construction types for vacuum insulated sandwich elements: (a) aluminum spacer of double glazing, (b) folded edge construction, (c) thermoplastic spacer and (d) reinforced non-metallic tape (Tenpierik et al., 2008).
Thermal insulation of household appliances, such as pipe insulation, insulation of thermal storage tank, insulation for underfloor heating, but also refrigerators form a last area of application. The strong reduction in required thickness is the main benefit with these cases, where the restriction of possible envelope damage forms a lower risk due to the possible protection of the panels.
9.5 Future trends
For further development and application of high performance thermal insulators based on nanotechnology, further progress is required in two domains: product and application development of the current products toward more durable solutions, and further exploitation of the (theoretical) physics of inhibited heat transfer, including radiation and solid conduction.
The reduced thermal conductivity k in the Knudsen regime is exploited in the nanoporous thermal insulators based on its f(Λ) as well as in partial vacuum thermal insulators based on their reduced p. Neither silica aerogels nor vacuum insulation panels, however, form a durable solution for high performance thermal insulators (based on the Knudsen effect) due to their drawbacks, i.e., the very low pristine thermal conductivity k of 0.004 W/(mK) of vacuum insulation panels must be weighed against their strong aging through time and limited application possibilities, whereas the combination of a rather low thermal conductivity k of 0.014 W/(mK) with a possible high solar and visible transmittance of silica aerogels must be weighed against their fragile, brittle nature.
Aerogel insulation and vacuum insulation panels are today’s best high performance thermal insulators. However, whereas traditional thermal insulators are ‘cheap’ and available in bulk, their specific properties and drawbacks compared to traditional thermal insulators require a shift in the way thermal insulators are applied in constructions.
A proper best practice of vacuum insulation panels in new constructions requires a shift of application toward prefabricated constructions, i.e., where the complete building process can be carried out under controlled circumstances, and to sandwich elements for curtain walls. Proper detailing of these constructions protects the panels from puncture during the period of use and may also slow down aging of the panels. Also aerogels require protection due to their intrinsic vulnerability, i.e., low tensile strength. At material level this is solved by fibrous reinforcement in aerogel blankets, but this increases the effective thermal conductivity and results in non-translucent aerogel solutions. Also here, a proper best practice for aerogel insulation in new constructions requires a shift of application toward prefabricated construction elements, i.e., window fabrication, sandwich elements or as core material of vacuum insulation panels. Finally, due to their very low thermal conductivity k and as they are generally applied in thin layers, thermal bridging in constructions insulated with aerogel or vacuum insulation panels becomes more important.
Besides using current state-of-the-art thermal insulators the best possible way, one can envision the development of a thermal insulator combining the positive properties of aerogels and vacuum insulation panels but solving their specific drawbacks.
The basic required material properties could be stated based on the properties of the known nanoporous thermal insulators and the partial vacuum thermal insulators. First, a pore size distribution f(Λ) completely below the mean free path of air at ambient conditions, i.e., 70 nm, which can be achieved based on aerogel synthesis technology. Secondly, an inner vacuum is desired without the need for a material envelope. As such, the thermal conductivity k of the material can be reduced further to the solid and radiative conductivity of 0.004 W/(mK) without the restriction of not being able to cut the material on site and without possible damage by puncture. This could be achieved by a closed porous structure instead of the classic open porous structure. However, the open porous solid structure is required in current vacuum insulation panels to enable a vacuum inside the material. As such, the vacuum pore structure must be created during the synthesis process of the material. One way to accomplish this is to envision a solid state material blowing itself up from within during the formation and subsequent expansion of an inner pore structure, or to create a grid structure which will efficiently and completely absorb the pore gas molecules, e.g., by a chemical reaction process.
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