Designing a Roof Garden - Insulation

Designing a Roof Garden - A Designers Guide

Many people assume that roof gardens contribute to the thermal insulation of a building. To determine the extent of this benefit, a number of resistance thermometers were placed in the garden overlying Gateway House in Basingstoke, England, and connected to a chart recorder. Two sections of the resultant trace are reproduced below (fig 5). Analysis of the construction at Gateway House, showed that whereas the uncovered portion of the roof had a U-value of 0.931 W/m C.

The areas which were covered by garden have a U-value of 0.585 W/m C, which correlates with the readings shown in fig 5. This building does not have the level of insulation that is required today, but the roof garden did increase the building's thermal efficiency. It was not a cost effective solution since the value of the garden was considerably more than any insulation material.

It is not possible to make any definitive assertions on the basis of this information because it was collected over a one week period, but it is apparent that the roof garden does buffer the roof membrane against extremes of climate and so must extend its life. Another good example of this effect is Derry & Toms' roof garden in London, England, where the mastic asphalt layer is in almost perfect condition after 70 years.

The buffering effect of a garden can also reduce the need for certain structural elements, which in turn can produce a saving. For example, it has been claimed that the Willis Faber & Dumas building in Ipswich, England received its roof garden at no extra cost because it was possible to omit a single movement joint.

As a general rule, plants only start into growth when the temperature of their root system rises over 10C. Above this threshold growth rates accelerate as root temperatures rise. Preliminary readings would seem to indicate that it is common for the soil of a roof garden, built on a heated structure, to be at least 5C warmer than a conventional landscape.

The elevated temperature of the soil brings the plants into growth earlier in Spring and sustains their growth in the Autumn. In the case of unheated structures, such as multi storey car parks, the converse is true. In colder climates insulation is needed to reduce the speed at which, and the degree to which, the substrate cools. Fortunately the effect of frost on both the root system of plants and growing mediums would appear to be minimal.

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