Dynamic insulation is a form of insulation where cool outside air flowing through the thermal insulation in the envelope of a building will pick up heat from the insulation fibres. Buildings can be designed to exploit this to reduce the transmission heat loss (U-value) and to provide pre-warmed, draft free air to interior spaces. This is known as dynamic insulation since the U-value is no longer constant for a given wall or roof construction but varies with the speed of the air flowing through the insulation (climate adaptive building shell). Dynamic insulation is different from breathing walls. The positive aspects of dynamic insulation need to be weighed against the more conventional approach to building design which is to create an airtight envelope and provide appropriate ventilation using either natural ventilation or mechanical ventilation with heat recovery. The air-tight approach to building envelope design, unlike dynamic insulation, results in a building envelope that provides a consistent performance in terms of heat loss and risk of interstitial condensation that is independent of wind speed and direction. Under certain wind conditions a dynamically insulated building can have a higher heat transmission loss than an air-tight building with the same thickness of insulation. Often the air enters at about 15 °C.
Introduction The primary function of the walls and roof of a building is to be wind and watertight. Depending on the function of the building there will be also a requirement to maintain the inside within a suitable temperature range in a way that minimises both the use of energy and the associated carbon dioxide emissions.
Dynamic insulation is normally implemented in timber frame walls and in ceilings. It turns on its head the long accepted wisdom of building designers and building services engineers to "build tight and ventilate right". It requires air permeable walls and/or roof/ceiling so that when the building is depressurised air can flow from outside to inside through the insulation in the wall or roof or ceiling (Figs 1 and 2). The following explanation of dynamic insulation will, for simplicity, be set in the context of temperate or cold climates where the main energy use is for heating rather than cooling the building. In hot climates it may have application in increasing the heat loss from the building.
As air flows inwards through the insulation it picks up, via the insulation fibres, the heat that is being conducted to the outside. Dynamic insulation is thus able to achieve the dual function of reducing the heat loss through the walls and/or roof whilst at the same time supplying pre-warmed air to the indoor spaces. Dynamic insulation would appear, therefore, to overcome the major disadvantage of airtight envelopes which is that the quality of the indoor air will deteriorate unless there is natural or mechanical ventilation. However, dynamic insulation also requires mechanical ventilation with heat recovery (MVHR) in order to recover the heat in the exhaust air.
For the air to be continually drawn through the walls and/or roof/ceiling, a fan is needed to hold the building at a pressure of 5 to 10 Pascals below the ambient pressure. The air that is being continuously drawn through the wall or roof needs to be continuously vented to outside. This represents a heat loss which must be recovered. An air-to-air heat exchanger (Fig 2) is the simplest way to do this.
Annotation for Air Tight Timber Frame Construction Element Description 1 brick cladding 2 ventilated cavity 3 sheathing board with breathing membrane on exterior surface 4 insulation 5 plasterboard (vapour control layer optional)
Annotation for Air Permeable Wall Construction Element Description 1 brick cladding 2 ventilated cavity 3 sheathing board (air permeable) 4 insulation (air permeable) 5 air control layer 6 ventilated cavity 7 plasterboard
Science of dynamic insulation All the main features of dynamic insulation can be understood by considering the ideal case of one-dimensional steady state heat conduction and air flow through a uniform sample of air permeable insulation. Equation (), which determines the temperature T at a distance × measured from the cold side of the insulation, is derived from the total net flow of conduction and convective heat across a small element of insulation being constant.
where
uair speed through the insulation (m/s)
caspecific heat of air (J/kg K)
ρadensity of air (kg/m3)
λathermal conductivity of the insulation(W/m K)
For two- and three-dimensional geometries computational fluid dynamics (CFD) tools are required to solve simultaneously the fluid flow and heat transfer equations through porous media. The idealised 1D model of dynamic insulation provide a great deal of physical insight into the conductive and convective heat transfer processes which provides a means of testing the validity of the results of CFD calculations. Furthermore, just as simple 1D steady state heat flow is assumed in the calculation of the heat transmission coefficients (U-values) that are used in the design, approval and building energy performance rating of buildings so the simple 1D steady state model of dynamic insulation is adequate for designing and assessing the performance of a dynamically insulated building or element of the building.
Insulations such as polyurethane (PUR) boards, which due to their micro-structure, are not air permeable are not suitable for dynamic insulation. Insulations such as rock wool, glass wool, sheep's wool, cellulose are all air permeable and so can be used in a dynamically insulated envelope. In equation () the air speed through the insulation, u is taken as positive when the air flow is in the opposite direction to the conductive heat flow (contra-flux). Equation () also applies to steady state heat flow in multi-layered walls.
Equation () has an analytical solution
For the boundary conditions:
T(x) = To at × = 0
T(x) = TL at × = L
where the parameter A, with dimensions of length, is defined by:
The temperature profile as calculated using equation () for air flowing through a slab of cellulose insulation 0.2 m thick in which one side is at a temperature of 20 °C and the other is at 0 °C is shown in Fig 3. The thermal conductivity of cellulose insulation was taken to be 0.04 W/m2K.
Contra-flux Fig 3 shows the typical behaviour of the temperature profile through dynamic insulation where the air flows in the opposite direction to the heat flux. As the air flow increases from zero, the temperature profile becomes increasingly more curved. On the cold side of the insulation (x/L = 0) the temperature gradient becomes increasingly horizontal. As the conduction heat flow is proportional to the temperature gradient, the slope of the temperature profile on the cold side is a direct indication of the conduction heat loss through a wall or roof. On the cold side of the insulation the temperature gradient is close zero which is the basis for the claim often made that dynamic insulation can achieve a U-value of zero W/m2K.
On the warm side of the insulation the temperature gradient gets steeper with increasing air flow. This implies heat is flowing into the wall at a greater rate than for conventional insulation (air speed = 0 mm/s). For the case shown of air flowing through the insulation at 1mm/s the temperature gradient on the warm side of the insulation x/L = 1) is 621 °C/m which compares with only 100 °C/m for the conventional insulation. This implies that with an air flow of 1mm/s the inner surface is absorbing 6 times as much heat as that for conventional insulation.
A consequence of this is that considerably more heat has to be put into the wall if there is air flowing through from outside. Specifically a space heating system six time larger than that for a conventionally insulated house would be needed . It is frequently stated that in dynamic insulation the outside air is being warmed up by heat that would be lost in any case. The implication being that the outside air is being warmed by "free" heat. The fact that the heat flow into the wall increases with air speed is evidenced by the decreasing temperature of the inner surface (Table 2 and Fig 4 below). A dynamically insulated house requires also an air-to-air heat exchanger as does an airtight house. The latter has the further advantage that if it is well insulated it will require only a minimal space heating system.
The temperature gradient at point in dynamic insulation can be obtained by differentiating equation ()
From this the temperature gradient on the cold side of the insulation (x = 0) is given by
and the temperature gradient on the warm side of the insulation (x = L) is given by
From the temperature gradient on the cold side of the insulation (equation ()) a transmission heat loss or U-value for a dynamically insulated wall, Udyn can be calculated (Table 1)
This definition of dynamic U-value would appear to be consistent with Wallenten's definition.
The ratio of the dynamic U-value to the static U-value (u=0 m/s) is
Table 1 Dynamic U-value Air speed u, (mm/s) Temp gradient at x/L=0 (°C/m) Conductive heat loss (W/m2) Udyn (W/m2 K) 0 100 4 0.2 0.25 41.8 1.672 0.084 0.5 14.6 0.584 0.029 0.75 4.49 0.1796 0.009 1.0 1.26 0.0504 0.003
With this definition, the U-value of the dynamic wall decreases exponentially with increasing air speed.
As stated above the conductive heat flow into the insulation on the warm side is very much greater than that leaving the cold side. In this case it is 6.21 X 4 / 0.0504 = 493 times for an air speed of 1 mm/s (Table 1). This imbalance in conductive heat flow is raising the temperature of the incoming air.
This large heat flow into the wall has a further consequence. At the surface of a wall, floor or ceiling there is thermal resistance which takes account of the convective and radiant heat transfer at these surfaces. For a vertical internal surface this thermal resistance has a value of 0.13 m2 K/W. In a dynamically insulated wall, as the conduction heat flow into the wall increases then so does the temperature drop across this internal thermal resistance increase. The wall surface temperature will become increasingly colder (Table 2). The temperature profiles through dynamic insulation taking into account the decrease in surface temperature with increasing air flow is shown in Fig 4.
Table 2 Temperature drop across air film thermal resistance Air speed u,(mm/s) Temperature drop across the air film (°C) 0 0.52 0.25 1.02 0.5 1.69 0.75 2.44 1.0 3.23
As the operative temperature of a room is a combination of the air temperature and the mean temperature of all the surfaces in the room this implies that people will feel increasingly cooler as the air flow through the wall increases. Occupants may be tempted to turn up the room thermostat to compensate and thereby increasing the heat loss.
Pro-flux
Fig 5 shows the typical behaviour of the dynamic insulation temperature profile when the air flows in the same direction to the conductive heat flow (pro-flux). As air at room temperature flows outwards with increasing speed the temperature profile becomes increasingly more curved. On the warm side of the insulation the temperature gradient becomes increasingly horizontal as the warm air prevents the insulation cooling down in the linear way that would occur with no air flow. The conductive heat loss into the wall is very much less than that for conventional insulation. This does not mean that the transmission heat loss for the insulation is very low.