Replies

1. 
   jollygreenshortguy | Jun 21, 2023 12:06pm | #1

   Four years back I designed a 3-unit apartment building in Portland, OR, with a building massing that was carefully kept to the scale of the neighborhood, and with low-slope roofs that were a combination of decks and green roof, mostly green roof. The planning department nixed the idea and insisted we go with sloped roofs that roughly matched the roofs of some surrounding Craftsman homes.

   C'est la vie.

   Properly designed green roofs offer many of the benefits of cool roofs as well as better environmental conditions for water control, plant and animal life.

1. Expert Member
   MALCOLM TAYLOR | Jun 21, 2023 12:17pm | #2

   jgsg,

   Joe Lstiburek disagrees: https://buildingscience.com/documents/insights/bsi-052-seeing-red-over-green-roofs

1. Expert Member
   BILL WICHERS | Jun 21, 2023 12:48pm | #3

   You've gotta love his footnotes though!

   Downsides I can see to green roofs:
   You need a whole lot more structural support to hold all that stuff up.
   You ensure the top of your roof is moist ALL the time
   You have to water it during dry times to avoid it becoming a "brown roof".

   If you're in the middle of a city, the "park on the roof" idea has some merit, but for most other areas, I don't think a green roof has any real practical benefits. If you want to control runoff, put in a retention pond and landscape it.

   I have one customer that uses their retention pond to double as an evaporative cooling "tower" for their building too, which is something of a win-win. Surprisingly though, very few people ever ask how they can run their fountain in that pond even in the winter (the building houses a datacenter, so there is always waste heat to keep the pond from freezing).

   Bill

2. Expert Member
   DCcontrarian | Jun 21, 2023 06:27pm | #4

   I have a green roof on my own home. In fact, the whole roof area is either green roof, roof deck or solar panels.

   A couple of comments. First, the purpose of a green roof is not to provide insulation. Dirt is a lousy insulator, about R0.3 per inch. In terms of weight it's even worse. The purpose of the green roof -- or the reflective roofs touted in the article -- is not to keep the sun's heat out of your house, that can also be done more cheaply and effectively with conventional insulation.

   There are three main benefits to a green roof. The first is that in Washington, DC, where I live, stormwater management is a huge deal, the city is spending billions of dollars to improve the storm sewer system and it's not enough. All new single-family homes have to manage their stormwater on-site, you have to capture it all into a dispersion tank that works -- and costs -- like a septic system. Having a green roof allows you to have a smaller dispersion tank.

   Second, the green roof keeps the roof from getting so hot, which makes the adjoining roof deck more pleasant. We had a long discussion a few months back about why cities are hotter than rural areas. The science is clear; it's not the albedo, it's not "thermal mass," it's that cities have less vegetation. The photosynthesis process works best between about 70F and 90F, plants have evolved a method of keeping their leaves in that zone in the direct sunlight by soaking up water with their roots and letting it evaporate through their leaves when the temperature gets too hot. This evaporative cooling cools the leaves, and also the environment around. If you've ever been near a highly-reflective roof it's not a pleasant place to be on a sunny day.

   DC has also embarked on an aggressive tree-planting and tree-preservation program.

   The final benefit of the green roof is what Dr. Joe alludes to, it's attractive and makes the house more pleasant.

   I would say the Post article gets about a B- for scientific rigor. First, it omits the fact that solar gain is only a net negative in places that are cooling-dominant; in heating-dominant climates it's a positive. But the big thing it misses is that in buildings constructed to modern standards solar gain through the roof is going to be pretty minimal.