Phius Senior Scientist Graham Wright weighs in on an interesting proposal for a green/blue roofing system and its feasibility for use on a Phius project.
The Question: “…The design team is considering a Green/Blue roofing system. Some of these systems / designs show rainwater being stored underneath the continuous insulation on the roof. We wanted to run these design concepts by you to understand what questions we should be asking and what information we should be gathering in order to model this, whether you have encountered this and have thoughts on how to model / approach this, and/or whether we should steer away from any of these designs altogether.”
The Answer: As far as I can tell, Green roofs and high insulation are not compatible, or, this is a research area.
The concept shown has only a thin layer of insulation. The Opti-Green system in the WUFI database is about R-3 overall. This research paper from 2012 looked at an R-22 roof.
So, first thought: you probably could not do a large area of this and hope to meet the energy targets. It might be OK to experiment with it in a small area. They should ask if what is being proposed has any track record. Has this ever actually been built before in this climate?
Second thought: There is also clearly a tradeoff with the insulation positioned where it is. On the one hand, placing it above the water helps keep the water from freezing. On the other hand, how does the water get up through the insulation to the plants? If there are perforations, then the “fastener correction” calc should be done to derate the insulation. This becomes more troublesome the thicker the insulation is. Also, water flowing and draining away beneath the insulation will defeat its winter performance. This will happen whenever it rains enough during the heating season, and there should be another derating for that.
Third thought: I think the idea of these is there is an evaporative cooling benefit in the summer. So it might make sense for a cooling-dominated building in the right kind of climate — e.g. one with warm summers but not too dry summers — so you get free rain water and don’t have to pump water up for irrigation. In terms of both energy savings and heat island mitigation, I think a foam-insulated and cool-membrane roof would compete very well with this concept and would be a lot lighter. If they are thinking of doing a whole roof this way, I would suggest doing a comparison to such a baseline case on both cost and simulated performance by WUFI Pro.
The article about green roof modeling mentioned in the WUFI help is here
Energy and Buildings
Volume 145, 15 June 2017, Pages 79-91
Energy and Buildings
A hygrothermal green roof model to simulate moisture and energy performance of building components
D.Zirkelbach S.-R.Mehra K.-P.Sedlbauer H.-M.Künzel B.Stöckla