Decrement Delay

Two identical houses were insulated to the same 0.21 U-value in Delft in Holland with insulation materials of different density (Fibreglass 20kg/m2 and Cellulose 70kg/m2). Both houses had the heating turned off and had similar orientation to the South. The external temperature fluctuated by 30 degrees. Measurements by the TNO University in Delft in summer ’97 clearly show differences in the behaviour of both houses. The house insulated with Cellulose had a temperature fluctuation of 3 degrees and the house insulated with fibreglass had a temperature fluctuation of 13 degrees. I've been in Passive houses that had temperature fluctuations of 7 degrees and were insulated to 0.15 U-value with lightweight insulations like fibreglass and polysterene. So it is possible to have one house insulated to 0.25 U-value with heavy woodfibre (250kg/m3) that has the same heatloss and temperature fluctuation as a house insulated to 0.15 with polysterene (20kgs/m3). But if you insulate to 0.15 with heavy insulation materials like we do you get even better results.

At the same U-value, the house insulated with cellulose insulation displays a significantly slower temperature conductivity (slower heat loss) than the one insulated with fiberglass. A roof insulated with Wood fibre or Paroc performs even better because they are both much heavier than Cellulose.

House A		House B
Construction from interior to exterior		Construction from interior to exterior
Plasterboard or gypsum fibre boards	12,5mm	Plasterboard or gypsum fibre boards	12,5mm
Mineral fibre insulation WLG040 20kg/m3	40,0mm	Cellulose insulation batts WLG040 70kg/m3	40,0mm
Air-proofing strip		Air-proofing strip
Mineral fibre insulation WLG040 20kg/m3	160,0mm	Cellulose insulation batts WLG040 70kg/m3	160,0mm
Open diffusion sub-strip		Open diffusion sub-strip
Back ventilation level		Back ventilation level
Roof covering		Roof covering
u-value	0,21 W/(m2K)	u-value	0,21 W/(m2K)
Amplitude suppression	5	Amplitude suppression	12
Phase displacement	6 hours	Phase displacement	11 hours
Expected temperature flow on the underside of roof (simulation calculation according to Haindl)		Expected temperature flow on the underside of roof (simulation calculation according to Haindl)
Click on the image to enlarge		Click on the image to enlarge

Here you can see the weight and performance of various construction materials

The findings of the TNO indicate clearly that simulation calculation and temperature behaviour of the roof are in practice comparable, and confirm the advantages of dense insulation for summer and winter thermal protection.

Natural construction materials like timber, timber based materials, wood fibre, Paroc and cellulose fibre insulation, together with plasterboard, provide the opportunity in modern timber frame construction to employ reduced component cross sections to create low energy standards and guarantee a comfortable, balanced living climate in summer.

THERMAL PROTECTION IN SUMMER

Planning Even in the planning stage, overheating effects can be reduced by minimising the gains of solar energy, in summer. A larger roof overhang means that the windows are kept shaded from the sun in summer, when it is high in the sky, and that in winter, where the sun sits low in the sky, its rays can enter the house interior directly. A similar effect can also be achieved with shading from roller blinds or shutters, awnings or climbing plants , which do not shade the window area in winter. The use of heat retaining interior construction components also reduces the level of overheating.

Design Intelligent design means that considerably less heat reaches the interior of the house. Facades that stand out from the house or are ventilated behind draw off so much heat that the interior wall surfaces remain markedly cooler than, for example, a composite insulation system with the same kvalue.

Configuration

Amplitude suppression
Phase displacement

In the case of the roof, the external temperature is taken as the temperature below the roof covering, which in summer may well reach up to 70°C.

For example, if the external temperature variation is 30°C and the interior temperature variation 3°C, the value of the amplitude suppression is 10 (30°C/3°C)

In other words:
The temperature variation is suppressed by the construction component on its way from the exterior to interior to one tenth.

The phase displacement is the time span between the highest external temperature and the highest interior temperature – in the above example 12 hours (between 14.00 and 2.00).

One aim of thermal protection in summer is to retard temperature penetration of a roof or a wall to such an extent that the highest temperature of the day only reaches the room side when the outside temperature is so low that the heat can be driven out by ventilation. The target here is a phase displacement of 10 to 12 hours. A portion of the heat stored in the construction components is then returned to the exterior of the house.

This means that the temperature on the interior side of the building does not reach that on the exterior. The relationship between the maximum temperature difference occurring on the exterior side and the interior side is known as amplitude suppression.
Depending on the construction, usage and exposition, a minimum amplitude suppression of 10 to 15 is desirable.

Materials for roof and walls

Construction materials suitable for summer thermal protection are materials that guarantee a very slow temperature permeation, i.e. as low a temperature conductivity as possible. These are materials that insulate well, but which, alongside their low thermal conductivity, also have a high bulk density and high specific thermal storage capacity. With many materials, e.g. steel, high density stands in contrast to low thermal conductivity. Materials with a high density are generally bad insulators.

Ideal construction materials from the point of view of slow temperature permeation are timber and timber –based materials, followed by wood fibre and cellulose sheets and plasterboard. With these construction materials, which are used in modern timber frame construction, correct planning and configuration makes it possible to easily combine low energy standards with good summer thermal protection. Wall constructions from timber frame, timber materials and plasterboard with ventilated facades usually have the values necessary for slow temperature conductivity. Windows and their shading also play a further important role here.

Especially critical:
roof surfaces

For this reason, temperatures under the roof covering may reach up to 80°C. In addition, the roof surface, which conducts heat, is awkwardly large in relation to the space contained beneath it. With the exception of plasterboard on the interior side, a roof – seen from the airspace below the roof covering – consists largely of insulating material. There is hardly any storage mass.

It is particularly important that a Decrement Delay of 12 hours is achieved with an insulating material with a low temperature conductivity.