Forward
Jelle et al. [
1] in the paper currently in print entitled “the path to the high performance thermal building insulation materials and solutions of tomorrow” made a thorough review of future of thermal insulations. In the abstract, they wrote:
“The objective is to go beyond VIPs and other current state of the art technologies. New concepts are introduced, i.e. advanced insulation materials (AIMs) as vacuum insulation materials (VIMs), gas insulation materials (GIMs), nano insulation materials (NIMs) and dynamic insulation materials (DIMs). These materials may have closed pore structures (VIMs and GIMs) or either open or closed pore structures (NIMs). The DIMs aim at controlling the material insulation properties, e.g. solid state thermal conductivity, emissivity and pore gas content. Fundamental theoretical studies aimed at developing an understanding of the basics of thermal conductance in solid state matter at an elementary and atomic level have been addressed. The ultimate goal of these studies is to develop tailor made novel high performance thermal insulation materials and dynamic insulation materials, the latter one enabling to control and regulate the thermal conductivity in the materials themselves, i.e. from highly insulating to highly conducting. Furthermore, requirements of the future high performance thermal insulation materials and solutions have been proposed. At the moment, the NIM solution seems to represent the best high performance low conductivity thermal solution for the foreseeable future. If robust and practical DIMs can be manufactured, they have great potential due to their thermal insulation regulating abilities.”
The paper of Jelle et al. [
1] gives an excellent review of the key function of thermal insulations, namely, to control flow of thermal energy. Yet, modern thermal insulation systems deal with control of heat air and moisture. The following paper will, therefore, expand the scope of the class of dynamic insulation materials (DIM). Following a pioneering work of Häupl et al. [
2] who introduced calcium silicate as a capillary active material, we will call it capillary active dynamic insulation material (CADIM).
Concept of capillary active interior insulation
Figure 1 presents the concept of capillary active insulation, as presented by Häupl [
2]. The layer shown on the interior side of the masonry wall is shown to have a high capillarity and redistributes moisture condensed on the back side of this thermal insulation through the larger volume of the material. In this way, it can perform both the functions of thermal and moisture protection.
One can ask the following question: why does the capillary active insulation must be open for diffusion? To answer this question, we need to go back to the principles of moisture transfer in capillary porous materials. Figures 2 and 3 show a technique developed by Bomberg and Shirtliffe [
3] that uses a specimen with moisture applied on the warm side of the specimen that is immediately sealed in a polyethylene bag and placed in a heat flow meter apparatus to be exposed to the specified thermal gradient.
Thermal gradient drives moisture in vapor phase toward the cold side. This process goes fast in the glass fiber insulation (Fig. 3(a)), moisture is carried to cold plate of the equipment already in 3 to 4 h and the final dynamic equilibrium develops in the span of 9 to 10 h.
The process of redistribution goes much slower in the cellulose fiber insulation (Fig. 3(b)). At the period 3 to 4 h, a significant fraction of moisture is still on the warm side of the specimen, and the final dynamic equilibrium develops in the period of 17 to 18 h. We use the term “dynamic equilibrium” because it is a combination of two conflicting transport processes. Water vapor is driven toward the cold side, and condensing there comes to a very high level of relative humidity at which liquid flow develops. As liquid viscosity is not strongly depended on temperature, the liquid flow tries to equalize moisture content, i.e., drives moisture backward. How far backward it can be pushed depends on the amount of moisture and liquid flow component in the total moisture conductivity expressed as a function of moisture content.
Figure 3 shows the moisture content redistribution in the same type of experiment performed by Kumaran <FootNote>
Kumaran M K. Private communication of his presentation to International Council of Research and Innovation in Buildings, 1986
</FootNote> on the glass fiber and sprayed cellulose fiber specimens.
The technique shown in Fig. 2 is one of the most powerful tools used for verification of material characteristics used as input data to hygrothermal models [
5]. It can also be used to the assessment of the hygrothermal properties used for capillary active materials.
Figure 3 shows, however, why a high water vapor permeance is required for a good candidate of capillary active material. The residual moisture content on the warm side of the cellulose fiber is high, and this material can dry inwards, even though the bulk of moisture is transported to the cold side.
The concept of capillary active insulations was applied to thermal rehabilitation of masonry buildings. The critical consideration was the elimination of mold. After successful demonstration for several German buildings, this technique was also used in a few world-known monuments such as the church of Our Lady in Dresden, some cultural properties in Japan and Rijksmuseum (Rembrandt) in Amsterdam. In this work, we will use cellulose-based materials because, as shown in Fig. 4, it shows all properties needed for capillary active medium. Of course, those cellulose or wood fibers are mixed with reinforcing fibers and bonded together.
Building blocks for development of CADIM
In science, the transition from the old to the new concepts is often difficult to recognize. In the case discussed here, we use some advantages of a double-wall concept, but we do not use double walls; we use the concept of capillary active materials, but the selected material does not have a strong capillarity but a high vapor permeability and a large storage for hygroscopic moisture; we talk about moisture buffer, but we do not place material on the inner surface of the wall. We also talk about integration with heating, cooling and ventilation, air conditioning (HVAC) and low grade geothermal and solar energy, but the detailed solution is not presented here because they depend on the climatic and service conditions. To better understand the composite of advantages and disadvantages of the proposed system, we need to analyze a number of these aspects, one at a time, before listing elements of research needed to develop for successful implementation of these concepts.
Concepts of moisture buffer and permeable walls
Simonson et al. [
6-
8] showed that the moisture transfer between indoor air and the building envelope can significantly impact the indoor humidity, comfort, and air quality [
9-
10]. Yet, Simonson et al. [
6] analyzed to what extent moisture from indoor air can be stored in the interior finishing and thereby reduce the peak indoor humidity. On the basis of field measurements and numerical results, the authors demonstrated that the vapor-permeable envelopes with and without hygroscopic insulation in a cold climate may reduce the peak indoor air humidity. Simonson [
8] stated:
“Even though this moisture transfer can improve the indoor climate and IAQ, it can be detrimental to the building envelope if it is uncontrolled. A vapor resistant layer on the inside of an insulated envelope in cold climates is needed to prevent the excessive diffusion of water vapor from indoor air into the building envelope. However, the required magnitude of this vapor resistance has been debated in recent years [
11]. In addition, recent research on summer condensation has peaked interest in vapor-permeable building envelopes to enhance drying to the inside of the building.” [
12-
13].
“There is no official definition for a vapor-permeable building envelope, but in this concept, it means that there is such moisture flow between indoor air and the structures that it can have a significant effect on the peak values of the diurnal variation of the indoor air relative humidity. This does not necessarily mean that the vapor permeance of the inside sheathing should be very high, because the vapor resistance layer could be assembled also at the exterior side of the vapor open, hygroscopic interior sheathing layer. It is important to distinguish between vapor permeability and air permeability. Low air permeability of the building envelope can (and should) be achieved also when the building envelope is vapor permeable.”
This Finnish research encouraged the International Energy Agency (IEA) to examine the whole house hygrothermal performance with a focus on development of better test methodology. The official Journal of the International Building Physics Association published several papers on this topic [
14-
16]. Perhaps the most important lesson from this activity was bridging the gap between the transient measurements and calculations.
A concept of double walls
Increased requirements for thermal insulation motivated many builders and architects to consider the double wall as a practical solution. Indeed, as late as 2010, in scientific conferences and seminars, one or other form of staggered studs or systems with continuous air cavity appears in the proceedings. From the building physics point of view, one may introduce a classification of these different technical solutions, namely:
1) glazed wall capturing solar energy on the exterior of the insulation layer;
2) two structural systems separated with an insulated cavity;
3) frame wall with cavity used for ventilation and no interior insulation;
4) walls with ventilated interior insulation systems.
Typically, type 1 can be a modification of the so-called Trombe wall, where solar heat gains are used as preheat of ventilation air [
17]. As discussed by Saelens and Hens [
18], this can also be utilized for natural ventilation in high-rise buildings.
Typically, type 2 is used only for introducing continuous interior thermal insulation and does not include air cavity or other hygrothermal considerations.
Type 3 is an extension of the current technology and used when space is available at low cost. Such a construction uses thermal insulation on the exterior side of the frame and interior finish without interior insulation. Yet, in the Northern climates where moisture condensation risks are very high, this system is used because it reduces the condensation potential.
Type 4, by definition, is not a double wall but functions as such because two leafs of the wall are thermally separated by energy source or sink in the form of the air cavity. Figure 4 shows temperature profile in the wall in summer for wall without ventilated cavity (dashed line) and wall with air conditioned air in the cavity.
Figure 4 shows that if the temperature of ventilating air is lower in the summer (or higher in winter) in comparison to the temperature that this air would have without preconditioning, i.e., if the space was sealed and thickness of the cavity was smaller than one causing the onset of convection, the values of incoming and outgoing heat flux can differ.
For clarity of the reasoning, we may assume that the thermal gradient is presented not in geometrical scale, but that the horizontal axis represents thermal resistance of the wall parts on both sides of the air cavity. How long can we maintain this situation that incoming and outgoing heat fluxes differ? It depends on the thermal mass of the wall and on the efficiency of heating or cooling of the ventilating air.
Changes in construction introduced by the technology progress and sick buildings syndrome
The first energy crisis in 1970s started a process of changes that, in some respects, continued, and others were restarted by the recent increase in the price of fossil fuels. The continuing part of the process relates to the increase in house airtightness. Figure 5 shows the Canadian statistics from 1980s, and what is characteristic in addition to the trend toward the tighter homes is also the reduction of the scatter of the measured values. One can argue that this is a natural process; the builder strives to make a house better, and introduction of new materials such as air-vapor barriers in form of a polyethylene film in cold climates made its impact.
The second source of changes that are accelerated by energy cost is an increase of the level of thermal insulation. Depending where this insulation is placed, it may have a positive effect (exterior insulation) or negative effect (in the cavity of the wood frame) on the condensation risk, and following it introduces potential durability problems. In North America, the second pattern was prevailing. Not only typical cavity was increased from 89 to 140 mm, but even several builders try the double wall system, i.e., placing the one part of the wall in high moisture risk in both cold and hot climates unless the moisture design paradigm is changed [
19].
Bomberg [
20] in a plenary paper at Nanjing conference highlighted how a wood-frame wall with a high level of moisture tolerance lost its resilience became prone to damage. In this process, the following major changes in wall design have taken place:
1) increased levels of thermal insulation;
2) increased level of water vapor resistance;
3) increased air tightness of the walls;
4) reduced moisture buffer capability;
5) introduction of more moisture sensitive materials.
Each of these changes and all of them reduce the moisture tolerance of the wood-frame wall. One must change the moisture design paradigm, and the solution includes airtight but permeable for moisture walls that allow drying to indoor and outdoor.
In China, the high-rise construction is typically made in concrete. Yet, the progress in the interior finishes has also been negative. The traditional lime stucco used for interior finishes has been replaced by polymer modified (acrylic) cement stucco. The surface has higher resistance to impact and is better with respect to its ability for washing but resulted in reduction of its moisture buffer - i.e., worse indoor climate conditions forcing people to buy often unnecessary air conditioning. Furthermore, typical air conditioning equipment is noisy and inconvenient, while today’s technology has other means of achieving the comfort level.
In summary, we can say that while airtightness reduces energy consumption, it also brings the need for mechanical ventilation to avoid the sick building syndrome.
Demand-based and night ventilation plus other means of improving indoor air quality
When residential housing became so tight that mechanical ventilation became necessary, several new concepts used in office buildings became also available to buildings with multiple dwellings. One of them is night ventilation that, in addition to removing pollutants, can be used to interact with thermal mass in elimination of summer- day overheating; the other is demand controlled ventilation or sometimes also called “personal ventilation.” Taking into consideration the public preferences, we consider hybrid ventilation as the best model.
Yet, of use mechanical ventilation may also be beneficial if it is integrated with improvement of indoor air quality and utilization of energy sources that otherwise cannot be used. It may be geothermal cooling in summer because except for permafrost regions, for the most of the continent temperature of a soil on 6-8 feet level, it is only slightly affected by the season. Assuming that we have basement, crawl space or mechanical room that can be built tightly and function as the air mixing chamber, we can use air led through an underground pathway.
Similarly, we can use another heat exchanger that would transfer heat collected from roof or wall solar collectors or other devices such as pipes located near the surface of the exterior envelope to preheat water used for the instantaneous water heater.
Concept of CADIM integrated with a wall structure
Extensive research on dynamic walls was carried out in 1980s. The conclusion of this research was that even when using air filtration through a glass fiber insulation, there was only marginal saving in comparison to a standard wall [
21]. So we really do not expect any difference in the total energy, yet the purpose of our exercise is different - we want to eliminate energy peaks in summer days and valleys in winter nights.
Materials and system
The material that will be used in this project is a bio-fiber thermal insulation. It is either wood or cellulosic fiber with admixture of other fibers bonded together to form water resistant but permeable board. Wood fibers have much higher specific heat (2100 J/(kg · K)) than inorganic thermal insulation materials and fairly good thermal conductivity coefficient (λ = 0.038 W/(m2·K)), i.e., 3.75 (ft2·oF·hr)/(Btu·in), making them a preferred candidate for a combination of thermal mass and capillary active materials.
Materials on the other side of the cavity differ; it can be a concrete of the structural construction or oriented strand board (OSB) with integral water-resistive barrier (WRB) if wood or steel frame wall is used. In principle, this wall system can be considered as a heat exchanger and depending on the climate and prevailing thermal loads, the air circulating in the wall can either be a return air from the ventilation system that is removed from the building or air pulled out from the basement in summer or attics in winter in quantities equivalent to the fresh make-up air delivered to the space.
The quantity of air to be exhausted during the day will be small but increased in the night. For the night ventilation in the summer time, we may not use dehumidifiers in the mechanical room so that moisture can be absorbed into the bio-fiber insulation. During the summer day, however, when needed, fresh air will be dehumidified so that the same bio-fiber material will undergo a wetting and drying cycle contributing to the thermal mass in addition to the phase change agents incorporated in the bio-fiber product.
Scope of the research project
As discussed elsewhere, hygrothermal models currently available in public domain are not suitable for real-time simultaneously occurring heat, air and moisture transfer in construction assemblies. The heat and moisture transfer calculations have been verified in endless cases, and providing that material characteristics are independently verified [
5] can be used for integrated thermal modeling and testing work [
22-
23]. Yet, measurements and modeling of air flow effects on thermal performance and interaction between air and moisture present many unresolved problems.
Therefore, the following program should include a few PhD projects:
1) Expand and verify the 2-D HT model to include intermittent wetting and drying under the condition of air movement on the material surface. In particular, this project would include characterization of the boundary conditions and developing methodology of integrated testing and model application to improve precision of material characteristics [
20].
2) Verify the predicted by model contribution of latent heat transfer on series of wetting and drying experiments with mid-scale CADIM mockup. If possible, expand this series to include the bio-fiber with phase changing waxes. Expand the analysis to quantify these effects on improvement of indoor air quality.
3) Evaluate durability of CADIM under repeated wetting and drying conditions. Provide guidelines for the range of air flows and moisture loads acceptable. Expand the analysis to quantify service life of these systems in the field situation.
In addition to these PhD projects, we need to have industrial support on material property modification and development of construction details for incorporation of the CADIM into the wall assembly and integration with temperature controlled air. At this stage, it is uncertain if the system can perform in a dynamic fashion only in one season either cooling or heating and in both seasons. We hope that between Southeast, Tongji and Syracuse Universities with support of manufacturing industry, this project will be performed, as improvement of indoor climate in all high-rise concrete buildings is of high interest to China.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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