REN EWABLE EN ERGY PERGAMON zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Renewable Energy IS (1998) 377-382 DYNAMIC THE APPLICATION OF INSULATION IN BUILDINGS B J TAYLOR’ and M S IME.4BI of Architecture, The Robert Gordon Umversity, Aberdeen, ABlO 7QB, U.K. ’ Scott Sutherland School zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ’ Department of Engineering, Kings College, The University of Aberdeen, Aberdeen, AB24 3UE, U.K. ABSTRACT Dynanx insulation, a form of ‘Breathing Wall’ construction which allows the movement of air and moisture through the external walls of a building, was seen as one possible method for reducing building envelope heat A research investigation was conducted fo provide a firm losses and achieving high indoor air quality. scientific understanding of dynamic msulation. An important outcome of the work will be the development of building envelope designs which effectively and economically employ dynamic insulation in cold climates. This paper presents some general conclusions, confmnmg that the energy saving produced by dynamic insulation alone 1s small relaave to that obtained in conjunction with conventional air heat recovery methods. 0 1998 Elsevier Science Ltd. All rights reserved. KEYWORDS Breathing wall; buildings; dynamic Insulation; PROPERTIES OF DYNAMIC heat recovery; U-value; ventilation. INSULATION Modem buildings have attempted to reduce their energy requirements by improvmg the air tightness of the envelope and increasing the thickness of insulation. This trend has developed simultaneously with increased use of synthetic materials in construction, furnishings and decorations, which give off volatile organic compounds. Increasing living standards have also resulted in higher indoor temperature and moisture generation rates wtthin homes. The outcome has been a reduction in indoor air quality which chrectly affects occupant health, and increasing problems of dampness in homes, particularly for the poor. Dynamic Insulation, a form of ‘Breathing Wall’ construction which allows the movement of air and moisture through the external walls of a building. was seen as one possible method for reducing ventilation and building envelope heat losses and achieving high indoor air quality. Heat Transfer Physical insight into design can be gained dynamic U-value for be mcorporated into savings. This simple the heat and mass transfer processes from a simple 1-D analytical model. the envelope and the mass transport an energy and air flow balance for analysis which can be camed out on 0960-148!/98/$-see front matter PII: SO960-1481(98)00190-6 0 In dynamic insulation for any proposed envelope The model can be used to predict the effecnve or rate for any gas species. The dynamic U-value can the whole building to estimate the overall energy a spreadsheet is ideal for the conceptual design of 1998 Elsevier Science Ltd. All rights reserved WREC 378 1998 buildings. However, for the detailed design of the air permeable envelope, 2-D models of the heat air and moisture transport should be used to assess (I) air bypassing the insulation through defects or construction details, (ii) buoyancy effects in the porous insulation, defects and cavities, and (iii) increased heat losses and vapour transport due to the above. There are a number of such models in existence but they tend to be research tools and not available for use by practitioners. Hens (1996) p rovides an excellent review of heat air and moisture transport modelling in general and specific computer programs in particular. This paper will describe the practical results of the 1-D analytical model. It was shown (Taylor et., 1996) that the dynamic U-value for a multi-layer envelope can readily be calculated from the total thermal resistance of the wall (R_) and the air flow through the wall (v) “, =R,(expg.E:R,,-1, The dimensionless group of variables resemblance to the P&let number that controls the behaviour of dynamic (1) insulation has a formal c L \‘p Pe = -.-AA-k (4 Unlike boundary layer analysis where it is the fluid physical properties that are employed, the thermal conductivity, k, in this case refers to the porous material. The density, r, and specific heat, c; are that of the air. Table 1 illustrates how the material thermal conductivity and the air flow combine to determine the dynamic U-value for two envelopes, one comprising 200 mm of cellulose insulation and the other 200 mm thrck porous masonry block such as Pumalite. Table 1: Dynamic U-Value versus thermal conductivity Cellulose (k = 0.035 W/mK) and air flow rate. Pumalite (k = 0.3 W/mK) 1 10 1 10 1.91 19.1 0.224 2.24 U, / US 0.33 9.5 E-8 0.89 0.27 U, (W/m% 0.058 1.7 E-8 1.34 0.4 v (m/hr) Pe --_ The masonry wall requires an air flow approximately ten times that of cellulose to achieve a comparable improvement (U,/UJ in U-value. However, to achieve the same insulation value the air flow through a PumaJite wall would have to be about 100 times that for cellulose. Consideration of the pressure drop across the wall (280 Pa) at a flow rate of 100 m/h leads to the conclusion that this would not be practical. Thus dynamic insulation works best with materials that are inherently good insulators. However, the thermal capacity of the masonry can be combined with the insulating properties of the cellulose to produce a composite permeable wall with a low U-value and high thermal capacity. Another reason why this is the case is that the analytical theory assumes that the air and the solid matrix of the porous insulation are in local thermal equilibrium. This assumption is valid for low air flows. Calculating the air flow at which the equilibrium theory is not applicable in terms of the physical properties of the porous medium is one of the useful results to be obtained from a non-equilibrium theory of dynamic insulation which is under development. It is sometimes suggested that with dynamic insulation less insulation material may be used in the wall. From Table 2 it can be seen that to get a significant insulation high air flows are again required. reduction in U-value for a wall with only 40 mm of WREC Table 2: Dynamic U-Value versus insulation Cellulose (L= 200 mm) v (m/hr) 1 10 319 1998 thickness. -_- Cellulose (L= 40 mm) ..--..-_-._.-_-..10 1 Pe 1.91 19.1 0.382 3.82 U, 1 U, 0.33 9.5 E-8 0.82 0.085 U, (W/m% 0.058 1.7 E-8 1.23 0.13 . Another feature of dynamic insulation is that as the air flow increases the inner surface temperature decreases (Taylor and Imbabi, 1997). This is because more heat has to be put into the inner surface of the wall to heat the increasing amount of air which in turn increases the ‘temperature drop across the air film thermal resistance. The temperature drop is about 0.5 “C for a flow of 1 m/h through a wall with 200 mm of cellulose Even at low air flows this temperature depression will insulation increasing to over 5 “C at 10 m/h. significantly alter the radiant heat exchange within a room. Mass Transfer Diffusive insulation is a special case of dynamic insulation where the air flow is zero. In other words its thermal behaviour is no different from a conventional wall. Indeed diffusive insulation is merely a wall which suc h a s polythene or metal foil. Such wall does not include a vapour retarder with a high vapour resistance zyxwvutsrqponmlkjihgfedcbaZYXWVU constructions are acceptable in certain circumstances and BS 5250 (BSI, 1989) quotes a useful but not infallible rule of thumb that the vapour resistance on the warm side of the insulation be at least five times greater than that on the cold side. Diffusion can be stopped if the air is flowing in the opposite direcnon to the diffusion process. The critical air velocity V~ required to do this is dependent only on the ratio of the concentrations of the gas (inner concentration C, assumed to be greater than the outer concentration C,) and the total diffusion resistance of the multi-layer wall R,, (Taylor d., 1996): This explains how dynamic insulation can act as a vapour barrier. If the air velocity is greater than v, then water vapour will be carried from outside to inside despite there being a higher water vapour concentration on the inside. For a typical timber frame insulated wall construction with total thermal resistance of 6.434 m’K/W (200 mm cellulose insulation) and the indoor and outdoor temperature and humidity conditions of 15 “C, 85% RH and 5 “C, 95% RH respectively as specified in BS 5250, this critical air velocity is very low at 0.0063 m’/m’h. This is very much lower than the recommended air flows of 0.5 to 1.5 m’/m’h (Dalehaug, 1993). The partial vapour pressure difference corresponding to the standard internal and external conditions, stated above, is 621 Pa. The authors have measured the air permeability of a variety of commonly used insulating materials. The air permeance of 200 mm of cellulose is found to be 1.5 m’/m’hPa. and that for 12 mm thick fibreboard was 0.116 m’/m*hPa. The controlling resistance to air flow in a wall construction compnsing of wood wool board (air permeance too high to measure), 200 mm cellulose, and 12 mm fibreboard is that of the fibreboard. The pressure drop across this wall at the critical air flow corresponds to a difference in air pressure of only 0.054 Pa Thus water vapour cannot flow from inside to out through a wall operating in contra-flux @eat and mass flow in opposite direction) mode. There LS then a conflict between the air flow requirements to minimise heat iosses and that necessary to maximise the removal of water vapour or other indoor pollutants. On the other hand provided one can ensure that air is flowing inwards through the envelope at all time then there should, in general, be no problem of interstitial condensation. However, if the outer wall cladding is saturated by wind-driven rain 380 WREC followed by heating by the sun then the and condensation could occur in the completed IEA Annex 24 on Heat Air on air permeability, vapour permeability 1998 temperature and relative humidity in the cavity could rise very quickly A useful outcome of the recently still relatively cool insulation. and Moisture transport in buildings has been the compilation of data and hygroscopicity for many building materials (Kumaran, 1996). SYSTEMS ANALYSIS Equation (1) can be readily incorporated into an air tlow and energy balance for a whole house to calculate the heat loss through the air permeable parts of the envelope (Taylor and Imbabi, 1996). A fact that is often overlooked by the proponents of dynamic insulation is that whilst the heat loss to the outside is reduced more heat needs to be put into the interior surface of the wall in order to warm the incoming air than would be the case without air flow. Therefore, if the air coming through the wall is merely vented to atmosphere without heat recovery little is gained. With an air-to-air heat recovery scheme as shown in Figure 1 the ventilation requirements are supphed partially through the wall, my and partially through the heat exchanger, m,. The model also allows for air leakage through doors and windows, m,. The heat input to the building Q (Partly supplied by incidental gains) compensates for the heat lost through the porous envelope Q,, the non-porous part of the envelope, Q” and the ventilation loss. The only way a building can be reliably de-pressunsed in the mild and variable UK climate is by using fans. In northern Scandinavia with a 40 “C temperature difference between indoors and out in winter a reliable and significant stack affect may be obtained. The de-pressurisation must be no greater than 5 to 10 Pa otherwise the occupants will have difficulty opening doors and windows (BSI, 1989). This restriction on depressurisation could be relaxed if the opening and closing of windows and doors were mechanically assisted. Since the pressure drop through an air-to-air heat exchanger and associated ductwork is in the region of 50 to 100 Pa both a supply and an extract fan are required. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED mL Fig. 1: hlodel of a dynamically insulated house with air to air heat recovery The results of analysis of such a scheme are shown in Figure 2. The ordinate plots the reduction in energy consumption over a conventional envelope construction of the same static U-value for the same air change rate to maintain an indoor temperature of 20 “C when it is 0 ‘C outside. The curves show how a dynamically insulated building and conventional envelope compare when both use air-to-air heat recovery. At low air change rates the conventional building performs better than the dynamically insulated building. The bigger 2nd better the heat exchanger the higher IS the air change rate before it becomes worth while to think about dynamic insulation. Both schemes show a maximum saving at around 1.5 to 2 sch. This level of ventilation in a conventional house could be achieved merely by opening the windows. W REC‘ 1998 Reduction Fig. 2: Variation m Power in power consumption with convenoonal and dynamic envelopes ,4n air-to-water heat exchanger operating at 2.0 ach would require a connnuous steady flow of water of the order of 1550 kg/day in a home. This flow rate is much larger than the domestic requirements for bathing, showermg and laundering. Also, the temperature constraints imposed by the exhaust air flow and the heat exchanger mean the water temperature will nse only from about 5 ‘C to 15 “C. If the warm air were used instead to melt snow the water it would provide at, say, 10 “C is a more manageable 170 kg/day. However, snow is not a r&able heat smk in most areas of the UK. With the simple tools developed so far the designer can explore, for example, how the proportion of nonpermeable surfaces (such as glazing) to permeable surfaces and how the size of the building affect thermal To make the most effective use of dynamic performance. The results are much as one might ekpect. insulation as great a proportion of the external envelope as is practical should be air permeable. This has obvious implications for the use of incident solar radiation for lighting and heating. It also means that a detached house IS a more suitable candidate for dynamic insulation than a small apartment with only one or at most two external surfaces. As the volume of the building increases, the ratio of volume to surface area increases and so the relative importance of the ventilation heat loss to envelope loss increases. In general, where energy conservation is the main objective, dynamic insulation would appear to be appropriate only for small detached buildings. FURTHER ASPECTS OF DYNAMIC INSULATION It has been theoretically established that a dynamically insulated wall will inherently act as a tilter (Taylor et., 1997). Studies of porous ceilings in barns where the ventilaaon rate can be as high as 80 m’/m’h have shown that over a span of 20 years the pressure increase due to dust accumulating in mineral wool insulation is insignificant (Wlvik, 1989). In homes, the ventilation rate will be an order of magnitude smaller and the rate of dust accumulation in the walls will be correspondingly slower. However, insulation materials such as cellulose and mineral wool will not remove chemical pollutants in the way that activated charcoal filters would. Cellulose insulation fibre is treated with borax to prevent fungal growth and infestation by insects and rodents. Bacteria cannot survive in the air on their own: they require dust particles to sustain small colonies. When such dust (and other) particles are trapped in the insulation, bacteria hving on them may multiply unaffected by the borax in the cellulose. The microbes and or toxins they produce could then subsequently be disseminated into the living space. It is also known that certain types of bacteria provide the nutrients requrred by moulds and fungi to grow (Singh, 1994). This potential health hazard requires investigauon in order to identify the circumstances under which dynamic insulation may act as an amplifier and disseminator for bacteria, fungal spores and viruses. In view of the risks attached to mechanical ventilation systems, which may be overcome by proper maintenance, the hybrid scheme (Fig 2) offers no health advantages over a purely mechanical ventilation 382 W REC 1998 system. Contaminants released within the building, such as Volatile Organic Compounds (V’OC’s), body odours, cooking smells, spores from moulds are best dealt with by extracting them at source and venting directly to outside. Dynamic insulation could be able to contribute to their dilution and removal by permitting higher ventilation rates or preventing their spread by plug tlow of fresh air from a wall or ceiling. THE FUTURE OF DYNAMIC INSULATION RESEARCH Dynamic insulation ~111, as demonstrated, reduce conductive heat loss through the wall. Operating in contraflux mode, it will also prevent water vapour getting into the wall from the interior. This had not previously been fully appreciated. However, these benefits are not easy to achieve given that (i) the rest of the building needs to be exceptionally air tight and air flow through the walls needs to be reasonably uniform and (ii) it is difficult to ensure air flows inward through the wall under all internal and external climate conditions. Furthermore, (iii) energy must be recovered from the ventilation air, (iv) under certain conditions (e.g. sun shining on wet timber cladding) one may get interstitial condensation with air tlowing inwards, and (1.) the building occupants need to understand how the building works and to behave accordingly. In short, the design effort and quality control during manufacmre and erection of a dynamically insulated building is greater than that required for a building with conventional air tight, well insulated envelopes in order to ensure acceptable hygro-thermal performance. The acceptance of dynamic insulation will ultimately hinge on the development of designs that are safe for humans, durable and cost effective. The filtration properties of dynamic insulation, as a means of controlling the harmful effects of particulate air pollution within the built environment could have a profound effect on future building design, and will be fully investigated. ACKNOWLEDGEMENTS This study was funded by the Engineering and Physical Sciences Research Council (EPSRC), Grant Reference GR/K23461. The authors’ are grateful to Mr C Weidermann of Camphill Architects, Beildside, Aberdeen for supplying drawings and data for their dynamically insulated house. REFERENCES BSI (1989). BS 5250: Control of Condensation in Buildings. British Standards Institution, London. Pomus Insrhtion in fYuL!r. Research Repoti No 53. Hokkaido Prefectural Cold Region Dalehaug, A. (1993). Housing and Urban Research Institute. Hens, H. (1996). LEA- Annex 24 on Heat, Air andM oistureTmnsportin H&b4 Insuhted Envelopes.Final Report L’ol zyxwvutsr 1 Task I: M ode&g. Leuven. Kumaran, M K. (1996). lEA- Annex 24 on Heat, Air and M oisturnTranq~oriin High4 huhted Envelopes, FinaL Report Vol3 Task 3: M aterial Properties.Leuven. Sallvik, K. (1988). The Influence of Clogging on the Air Penetrability in Porous Materials used for Air Inlets. Bundesanrtaltfur A~en~n~scbe zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA L_andw itiscb&Gun@ enstein,Inining. Austria, 18-19 October. Singh, J. ed. (1994). Bui&g M ycology:M anageRtentofDecay and Health in Buitihgs. E & F N Spon, London. Taylor, B. J., Imbabi, M. S. (1996). Dynamic Insulation - A Systems Approach. 4” Syp@osium Bu&?trg P&h in the Nordic Countries VoI2. Espoo, Finland, 9-10 September. Taylor, B. J., Cawthome, D. A., Imbabi, M. S. (1996), Analytical Investigation of the Steady-State Behaviour of Dynamic and Diffusive Envelopes. Suiting andtintimnntenr, 31, 519-525. Taylor, B J., and Imbabi M. S. (1997). The Effect of Air Film Thermal Resistance on the Behaviour of Buildingand Envimnment32,5,397- 404. Dynamic Insulation. Taylor, B. J., Webster, R. and Imbabi, M. S. (1997). The Building Envelope as an Air Filter”, under muiew.