Dynamic insulation

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.

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.

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.

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.

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.

[3] Equation (1) has an analytical solution [4] For the boundary conditions: T(x) = To at x = 0 T(x) = TL at x = L where the parameter A, with dimensions of length, is defined by:

[5] 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.

This implies heat is flowing into the wall at a greater rate than for conventional insulation (air speed = 0 mm/s).

Specifically a space heating system six time larger than that for a conventionally insulated house would be needed.

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).

On the cold side of the insulation the temperature gradient gets steeper with increasing air outward flow.

In pro-flux mode heat is flowing out of the wall at a greater rate than the case for conventional insulation.

In a dynamically insulated wall it is necessary to ensure the air flow is inward at all points of the building under all wind speeds and directions.

For a wind speed of 5.7 m/s at ridge height (taken as 8m) there is zero pressure difference across the side walls when the building is depressurised to -10 Pa.

These changes from contra-flux to pro-flux mode could be delayed by depressurising the building below -10 Pa. By locating this building in a particular geographical location then wind speed data for this site may be used to estimate the proportion of the year in which one or more of the walls will be operating in the risky and high heat loss pro-flux mode.

[8] This is the total time during an average year in which a building with dynamically insulated walls has significant heat losses.

If, by way of example, the building in Fig 8 were located in Footdee, Aberdeen, the Ordnance Survey Land Ranger grid reference is NJ955065.

Nevertheless, the above calculations indicate that a square plan building of 2 storeys located in Footdee, Aberdeen could have one or more of the walls operating in the risky and high heat loss pro-flux mode for about a quarter of the year.

[8] Thus a building with a dynamically insulated ceiling would offer a consistent performance independent of a varying wind speed and direction.

[10] Dalehaug also recommended that the pressure difference through the construction at the design minimum air flow (> 0.5 m3/m2h) should be about 5 Pa.

[12] Table 3: Measured Air Permeability of Building Materials [13] (1) Pressure drop calculated at flow rate of 1 m3/m2h The application of the theory of dynamic insulation is best explained by way of an example.

Putting dynamic insulation in the ceiling effectively limits the house to a single storey.

For a 12mm thick sheet of fibreboard this gives, for the maximum pressure difference of 10 Pa, an air flow rate of 1.12 m3/h per m2 of ceiling.