Dealing Effectively With Heat Under The Roof
Figure 1. Assumed flow of external
temperature & temperature
beneath roof covering
- If the intense heat under the roof is not taken into consideration at design stage the internal climate will be subjected to oppressive thermal Summer heat
- This can lead to stifling internal living conditions & create and uncomfortable living environment
- Dealing with the problem of the thermal heat generated under the roof should be considered at the commencement of designing & planning of any project
Facts
- Insulation is not just insulation material to minimise heat costs in Winter. It also needs to be able to provide thermal protection from the oppressive internal temperatures in the Summer
- Correct planning, design & material selection can solve the problem of oppressive internal heat in the Summer Designing to cater for (thermal protection) the ‘heat under the roof’ also allows for all the advantages of low energy consumption & ecologically friendly living
- At planning, overheating effects can be reduced by controlling the gains of solar energy
- Intelligent design means that considerably less heat reaches the interior of the house in a relatively short period of time
- As with thermal & moisture protection during the Winter, the airtightness of the construction is also essential
- Construction materials suitable for Summer thermal protection should insulate effectively & also have a high bulk density & high specific heat storage capacity
Thermal Protection in Summer
Planning
At the planning stage, overheating effects can be reduced by controlling the gains of solar energy, in summer. A larger roof overhang ensures that the windows are kept shaded from the sun in summer, when it is high in the sky. In winter, when the suns’ latitude is 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
Figure 2. Phase displacement
& amplitude suppression
Configuration
In relation to thermal & moisture protection during the winter, the air-tightness of components and their construction is of great significance:
If in summer the hot air from outside enters via the construction component, the effectiveness of that component with regard to thermal protection in summer will be significantly reduced, even with good planning design & using suitable construction materials.
Amplitude suppression. Phase displacement A key function of any insulation material is to provide thermal insulation in Winter but it must also provide thermal protection in Summer. The critical parameters in this case are the amplitude suppression & the 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 & 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. (see Figure 4)
The phase displacement is the time span between the highest external temperature & the highest interior temperature in this example 12 hours (between 14.00 and 2.00). (see Figure 4)
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 internal 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 & the interior side is known as amplitude suppression. Depending on the construction, usage & exposition, a minimum amplitude suppression of 10 to 15 is desirable.
Figure 3 (Above). Exemplary temperature flow in direct sunlight
Figure 4 (Above). Roof construction with an amplitude supression of factor 10 & a phase displacement of 12 hours
Materials for roofs & walls
Construction materials suitable for summer thermal protection are materials that guarantee a very slow temperature permeation, i.e. as low a thermal conductivity as possible. These are materials that insulate well, but which, alongside their low thermal conductivity, also have a high bulk density & high specific heat 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 & timber based materials, followed by wood fibre or cellulose sheets & plasterboard. With these construction materials, which are used in modern timber frame constructions, correct planning & configuration makes it possible to easily combine low energy standards with effective Summer thermal protection.
Figure 5. Calculation of the coefficient of thermal conductivity of building materials
Especially critical: Roof surfaces
In the loft space the significance of construction materials is somewhat different: The pitch of the roof means that it absorbs more heat than the walls. Drawing off heat from the airspace beneath the roof covering does not function as well as with a ventilated facade.
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.
Here it is particularly important that amplitude suppression & phase displacement are achieved with an insulating material with a low thermal conductivity.
Cellulose & wood fibre insulation products
Measurements in a roof construction by the TNO Delft in Holland in summer ’97 show clear differences in the behaviour of roofs fitted with different insulation materials.
At the same k-value, the roof insulated with cellulose sheeting displays a significantly slower temperature conductivity than the one insulated with mineral insulating material. The roof insulated with wood fibre insulation performs even better.
The observed build up of room temperature over a period of days is avoided by the ventilation of the rooms. In the trials in Holland the rooms were not ventilated.
The findings of the TNO indicate clearly that simulation calculation & temperature behaviour of the roof are in practice comparable, & confirm the advantages of wood fibre and cellulose insulation sheeting for summer thermal protection.
The natural construction materials timber, timber based materials, wood fibre & cellulose fibre insulation batts, together with plasterboard, provide the opportunity in modern timber frame construction to employ reduced component cross sections to create low energy standards & guarantee a comfortable, balanced living climate in summer.