Should desert communities promote the use of greenroofs, i.e. planting sort of a garden on their roof? How beautiful the appearance of greenery on your roof might be, plants and shrubs need water, none more so than in a desert climate. But water can be hard to come by. Is the cost of the water of irrigating the greenroof a limiting factor, or are the benefits of a greenroof tangible enough to be worth the water cost?

In desert communities, greenroofs can greatly enhance the residents' life quality due to their benefits:

• greenroofs provide thermal insulation, so as to reduce cooling loads (and avoid a few of the downsides of heavy AC use, like Legionnaire's disease and thermophilic actinomycetes)
• greenroofs provide acoustic insulation,
• greenroofs reduce heat island effect,
• greenroofs create wildlife habitat,
• greenroofs absorb occurring rain water, saving drains from intermittent overloading,
• greenroofs clean ambient air by absorbing CO2 and producing oxygen,
• greenroofs increase the roof lifespan.

Hotel Remota, Brazil

Residential roof, Waite, UK

Starbucks, Austin, USA

But greenroofs cost money.

• Roof structures sometimes need reinforcing to hold the extra weight
• There is the initial investment in rooftop landscaping.
• Depending on the owner's preferences & choices, there are maintenance needs or not (green fingers not required, rather optional)
• Greenroofs would require water to survive in desert climates.

The build-up of a typical greenroof

The questions is: do the benefits outweigh the costs? Is it worth promoting greenroofs in desert climates?

What information do we need to analyse this:

1. How much extra thermal insulation does a greenroof provide exactly? How does this translate to a lower use of AC? How much electrical power would be saved per year by using less AC?
2. Can we quantify the other benefits, which are basically qualitative? Can we add them to the equation?
3. How much water does a greenroof need to survive in a desert climate?
4. How much electrical power would be required to provide that water, with desalination from sea water?

Answers

1. How much extra thermal insulation does a greenroof provide exactly?

According to an article from the Environmental News Network, a 3- to 7-degree temperature drop translates to a 10% reduction in air conditioning requirements. For a one-story structure with a green rooftop, cooling costs can be cut by 20 to 30%.

According to the Coachella Valley Water District, proper landscaping with irrigated shrubs can reduce cooling requirements up to 24 percent, and a mature wide canopy shade tree can cut cooling costs by 42 percent.

Adding up the sub-optimal energy savings above we get a total ac power savings of 42% for a one-story structure.

The Association of Home Appliance Manufacturers states that residential AC systems range from 5KW to 20 KW, depending on load. Desert areas qualify as a high load. With an average use of 7 hours per day, year round, this makes for 2.555KWh of total electricity consumption per year.

A savings of 42% on 2.555KWh = 1.073KWh, or about 1MWh saved per year by home with a greenroof

HSBC bank, Mexico DF

2. Can we quantify the basically qualitative other benefits?

Do we need to? Let's see.

3. How much water does a greenroof need to survive in a desert climate?

According to the California Department of Water Resources, replenishing water lost to plant evapotranspiration in desert areas amounts up to a minimum of 54 irrigation days per year of 10 gallons average per 10m²,

---> 54 gallons, or 194 litres, per year per m²

Thus, a model residential building of 150m² would need 29.100 litres per year for irrigation. Is this the water quantity we need to have covered by desalinating sea water?

NO.

Residential buildings have 2 streams of waste water due to domestic use:

• Greywater: this is the waste water from cooking, dish washing, showers, baths, etc.
• Blackwater: from toilets

Greywater and blackwater can be largely recycled by using soft or natural water cleaning systems:

• a natural reetbed filtering system,
• a wetpark system,
• a living wall system,

Or there is also the hard and direct system of distillation, which should be easily provided by the abundant solar energy. Paradoxically many plants & shrubs prefer cleaned greywater to tap water because of the nutrients it contains.

A typical small reetbed filtering system

So what is the average use of domestic water in a typical 150m² home in a desert area?

According to the Pacific Institute for Studies in Development, Environment and Security, this is the rank for per capita water withdrawals

1. New Zealand
2. Armenia
3. Barbados
4. Cuba
5. United Arab Emirates
6. United States

According to Nichole Abrachinsky's research, University of Wisconsin, a typical American person uses 50-70 gallons per day for domestic use. Let us take the high range for our desert community here. This means a 4 person family will consume about 368.000 litres of water per year.

Compared to the almost 30.000 litres of water needed for the greenroof, we would only need 8% of our greywater.

In conclusion

The only severe downsides of having greenroofs in a desert climate are cost of installing it, and then the water use for irrigation. But as we've seen, the water needed for irrigation is more than covered by available greywater. So the only trade-off to make is between the investment cost of having a greenroof with all necessary amenities, and the savings on energy bills due to their extra insulation.

The other benefits a greenroof provide for: a beautiful area, with an abundance of birds and other wildlife, right on top of your home, comes at an extra, totally for free.

Too good to be true? Maybe, but then so is living in a desert climate. No cold sores, no wind chill, no miserable drizzle for weeks on end, no runny nose.