If we take a look at the sustainable solutions for heating a house we find a wide variety of solutions for heating: solar panels, trombe wall, biomass… however, most of the options available to cool the house in a sustainable way are passive: natural ventilation, thermal inertia; they have a high cost such as geothermal energy; or they are limited by the humidity content of the air such as evaporative cooling.
Normally, building envelopes play a passive role in the air conditioning of dwellings:
- In some cases they provide insulation by limiting the amount of heat that can enter the house from the outside.
- In other cases they act by thermal inertia, i.e. they are massive walls that ‘store cold’ during the night and slowly release it during the day.
Building regulation approaches only view green roofs from this passive point of view. Turning the green roof into a substrate with thermal inertia and vegetation that contributes to insulation. From this point of view, vegetation on roofs should have as little or no irrigation as possible, in order to minimise maintenance and water consumption. Not all green roofs are actively cooled.
In this article we are going to give you a new point of view on how to use a green roof, we are going to show you how to use irrigation water to convert each litre into frigories, creating an active cooling system for homes. Goodbye air conditioning.
How do green roofs provide active cooling? The numbers
Each litre of water needs an energy of 539.4 kcal (frigories) to evaporate, this is the energy of change of state of water.
1 litre of water = 539,4 frigories.
This means that if we have a 100m2 roof that evaporates 5l/m2/day, 20.83l of water/hour in 100m2, we have a total cooling power of 11,235.70 frigories for 100m2.
To put this figure in context. A commercial air conditioner recommended to cool 100m2 has a power of 5,848 frigories. In other words, with a green roof we have twice the cooling capacity we would need if we were to use a conventional air conditioner.
100m2 of green roof = 11.236 frigories.
At this point there are several points and clarifications: not all of this power is directly transferred to the cooling of the house, but we can design the roof to maximise its use. The following points should be taken into account:
- Cooling of the outside air. The roof is on the outside, so part of this energy is dedicated to cooling the air around the house. If we have a garden, our green roof will contribute to create a microclimate around our house.
- Thermal transmittance. Let’s suppose that the air in our house is at 30ºC and the active green roof is at 22ºC, if we have placed thermal insulation between our green roof and our house, the active cooling will remain outside the house. Do not insulate.
- The amount of water available. If we have made a green roof, for example, with succulent plants and we do not water it in summer. At the hottest times of the day and with no water available, the plants in the roof close their stomata to conserve water, so we have no evapotranspiration, without evaporation there is no cooling. Evapotranspiration must be maximised.
Criteria for a green roof with active cooling
The first thing we must do to create a green roof with active cooling is to think about maximising the water to produce evaporation, that is to say, to think exactly the opposite of how a traditional green roof would be created.
In a traditional vegetation cover we try to use species that have the minimum water consumption and irrigation, if it exists, would be done at night or at hours of minimum sunlight incidence, to minimise water evaporation.
In an active cover we try to maximise evaporation, so we can use any plant species and irrigation in summer will be carried out during the hours of greatest sunshine in order to maximise water evaporation and cooling.
Someone might say, aren’t we wasting water? The answer is no. The justification is simple. The justification is simple.
Spending electricity for cooling is almost 6 times more expensive and 6 times less sustainable than spending water.
Economically 1Kwh of cooling with conventional air conditioning costs 0,92€ of electricity; 1Kwh of cooling with an active vegetation cover costs 0,16€ of water (taking as a reference the most expensive water price in Spain).
From an ecological point of view, the carbon footprint produced to obtain the electrical energy we need to cool with conventional air conditioning is greater than the carbon footprint produced to obtain the water we use in the green roof with the same cooling capacity at the same ratio of 6 to 1.
Construction recommendations for an active green roof
The keys to designing an active green roof are the regulation of the irrigation system, the properties of the substrate, maximising cold transmission and the choice of vegetation.
Irrigation. We need to keep the canopy moist at the hottest times of the day, this entails several dangers: if we water too much we can lose water through runoff or we can flood the canopy causing diseases for the plants due to excess humidity, to avoid these problems in our installations we use drip irrigation with short times and very low flow drippers. If we were to use drippers with high flow rates we would lose a lot of water through run-off or have excess water on the canopy.
The substrate. The substrate of the active plant cover will be permanently damp in summer at high temperatures, which can cause problems with fungi and diseases, so it is best to use inert substrates with high aeration.
Transmission of cold. It is advisable not to use insulation between the roof and the house to maximise the transmission of active cooling.
Choice of vegetation. Any type of vegetation can be used, but the more evapotranspiration capacity it has, the greater the efficiency of our roof. By using the right vegetation we can evaporate up to 10l/m2/day on the hottest days. Multiplying by two the figures given in this article.
