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Spacesmiths

I agree, ensocar. And indeed, that's what the story says: insulating the house while absorbing water. It’s cooling too, since it doesn’t have a surface that reflects the sun’s rays. Most of the examples here do not require irrigation or human intervention once they are established. But, like all new gardens may need some helping until the plants are mature. Some of the detailed studies above may help you calculate the balance between all the factors as technology, costs and benefits are evolving quickly.

   
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ensocar

Spacesmiths - that may be what you meant, but the sentence "It’s cooling too, since it doesn’t have a surface that reflects the sun’s rays." actually says that it's the reflection of the rays that cools the house. Since plants and dirt don't reflect the sun's rays the statement is incorrect. I did notice some of the examples don't use irrigation, but I'm making the point that people thinking about this as a green idea need to make sure their choice doesn't unintentionally cause a problem elsewhere. Which is often the case with such choices.

   
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spam_is_evil

1. Creating a "dead zone" around a habitat is one of the greener, more earth-friendly ways to keep pests from entering our homes and buildings. Green roofs might force us to use chemicals that might either be inappropriate for the green part (especially if edibles are being grown), or inappropriate for the roof part (applying agricultural pesticides on structures is often a no-no). What happens to pest-control chemicals that exit the roof with the storm water runoff?

2. Technically there is a difference between green roofs and "insulation". And thermal performance may depend a lot on keeping the medium moist. End result is reduced heat gain from a variety of effects, one of which is shading the rooftop from the solar radiation, and evaporative cooling from the moist soil and the green leaves. Evaporative cooling is significantly impaired though in humid climates. And water is actually thermally conductive, which is kind of the opposite of insulation, so an insulation layer will probably still be important, especially in the winter. Ideally you would cover the roof with deciduous plants that shed their leaves in the winter, but this might not be practical. The depth of the material does play a factor, as it takes time to heat up material. This is one reason why green roofs are cooler during the day but actually warmer at night - the heat that does get absorbed into the thick material is slowly released after the sun goes down. During the winter this thick material also slows down the rate at which heat escapes the building, as it takes time to heat up the material from inside the building and the material is also heated by the sun during the winter but with less evaporative cooling during the winter (less heat = less evaporation). It's a very dynamic system as opposed to the very static effect of simply increasing thermal insulation and perhaps reflective thermal barriers to slow down heat transfer.

  1. Green roof vs. solar panels. Both rely heavily on industrialization (petrochemical plastics for green roof and silicon crystals grown for a week or a month in an industrial furnace). Depends a lot on the climate where the building is located, what building materials are available from the local market, etc. If we take the dollars saved and put them into building a house bigger than we need then we aren't doing much good. Producing renewable energy and increasing efficiency cannot undo the damage from excessive consumption.

4. Appropriateness. The green roofs are going to have to be maintained, inspected, etc. Doesn't seem like such an easy or safe task when the roofs are on an incline or have no railing. Just walking on the roof is going to damage the vegetation and risk disturbing the growing medium as well. A modified approach for many sites might be to have a roof deck that can be a living space with a vine trellis for shade and an opportunity to harvest edible fruits and herbs.

5. Affordability and Accessibility. I am firm believer that housing could be much more accessible if individuals had more freedom to build their own homes without shelling out hundreds of dollars in permit fees and spending a fortune on materials that conform to building codes. In densely-packed urban areas this isn't always feasible, but I live in rural Hunt County, Texas, an hour commute from where I work in Dallas. Out here there are no permits, no inspectors, no building codes for owner-builders, except for the septic system. It would be a great place to build a tiny house and use a pond liner to construct a green roof.

   

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