Roof and walls design by climatic zone (mass, insulation, solar protection) (Claudio Del Pero)

In this lecture we will go through the roof
and walls design by climatic zone. Decisions about shape, colour and composition of the roof and walls of a building are crucial, because they determine its overall performance. Roof design in single storey buildings
is especially critical, while the decisions about the walls are equally critical
in both low and high rise buildings. The roof is the part of a building that receives
the greatest solar radiation. Thermal performance depends to a great extent
on its shape, its construction and the materials used. The most widespread types of roof are: flat roof,
mono-pitched roof, double-pitched roof, hipped roof, vault and dome. The flat roof is practical in areas with limited rain.
It is also a good reflector and ri-radiates heat efficiently. Pitched and hipped roofs are most common
in hot-humid climates, having the capability to drain rainwater
and to provide shade to the walls. Curved roofs are suitable for areas with intense
total solar radiation and low diffuse radiation, for example hot-arid regions. The first key-element for the roof is a highly reflective surface, to minimise the amount of solar energy absorbed. Polished metal sheets and light-coloured finishes
are the most common technological solutions. More advanced and so-called “cool roofs”
are also available on the market. The most suitable material for the upper roof layer
is aluminium sheeting. However, this material has some drawbacks, such as the glare from dazzling sunlight and the noise from rain, wind or other materials striking it. In addition, condensation may occur because of its cooling down during the night, through ri-radiation. For such reason, the most effective roof-type for all East African Community is a ventilated double skin. In ventilated double skin roofs the heat between
the two skins is removed by the airflow crossing the roof space through openings facing the prevailing winds. The outlet opening should be larger than the opening for the inlet; they should also be placed at different heights in order to obtain air movement by the stack-effect when the wind is not blowing. The heat load is reduced by ventilation in the daytime and rapid cooling is allowed at night. In ventilated roofs like these in the figure, slopes
should be oriented towards the prevailing breeze. In both types of ventilated roof, any element
which would interrupt the airflow next to the surface of the roof should be avoided. In the figure some solutions for roof ventilation
are shown. The ventilation outlet can have different designs and should be possibly equipped with grating
to protect the cavity. A reflective surface in the cavity is highly
recommended since it reduces the radiant heat-transfer by reflecting the long-wave radiation emitted
by the hot upper layer. This foil (called a radiant barrier) should be applied
to the inner surface of the roof. In this way, radiant heat is prevented, and
convective heat is removed by ventilation. A simple and effective solution in hot-humid
conditions is a flat roof shaded by an aluminium screen. The performance of the screen can be improved if the lower surface is covered with a low-emission layer, or the upper surface of the flat roof
is covered with a reflective layer. Another appropriate solution is a sloping roof
with wall-shading overhangs, and a well-ventilated space between roof and ceiling,
provided that the ceiling is massive (for example a concrete layer with a minimum
of 10 – 15 cm thickness, covered with 5 cm insulation). If, instead of aluminium, galvanised corrugated
sheets are used, insulation thickness has to be increased
by at least 3 centimeters. In both cases a reflective surface on the insulation layer or in the lower surface of the roof would improve the performance. The flat roof is practical in areas where
it seldom rains. It is also a good reflector and ri-radiates heat efficiently, especially if it is made of a solid, white painted material. High and solid parapet-walls along the edge
of the roof can provide daytime shade and privacy, but can also have the disadvantage
of creating an undesired stagnant pool of hot air. Therefore, the construction and placement
of parapet-walls should be carefully evaluated. The performance of a flat roof can be improved
by separating roof and ceiling with a ventilated cavity. If this technique is used, the material of the roof should be light and the ceiling material should be massive. Aluminium foil between the cavities is recommended. As regards to building walls, the thickness
and material can be varied to control heat gain. The resistance to heat flow through the exposed
walls may be increased in the following ways: increase the thickness of the wall;
adopt cavity-wall construction; use walls made of suitable heat-insulating materials; fix heat insulating material on the inside
or outside of the exposed wall; use radiant barriers; apply light-coloured whitewash on the exposed
side of the wall. An efficient, although expensive, solution
to the problem of reducing radiant heat is a ventilated and reflective outer skin. Heat dissipation at night is more efficient
than with a structure using outside insulation. A way of reducing the radiant heat-transfer between
the two skins is the use of a low emission surface on the inside of the outer skin, thus realizing
a radiant barrier. Bright aluminium foil can be used. As regards to solar protection,
east and west-facing walls especially, should be shaded or protected by reflective surfaces, for example by pergolas, louvers or vegetation. Another important topic to pay attention to
is the thermal mass of the construction. The thermal mass influences the amount
of excess heat accumulated during the day (for example solar radiation), which is stored
in the mass and then released during the night. In conclusion, some rules-of-thumb can be identified
in each climatic context: In hot-arid conditions, walls of daytime living areas should be made of heat-storing materials; walls of nighttime areas should have
low heat-storage-capacity. Medium weight or insulated roofs are suggested. In upland climate, solar radiation can be used to improve comfort and reduce, or even eliminate, any need for a heating system. The amount of excess heat accumulated during
the hours of sunshine can be used in the other periods, by means of medium weight structures. In hot-humid conditions, walls should be as light
as possible with minimal heat storage capacity. Roof should be lightweight and ventilated.

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