Green roof insulation value

Energy modeling of green roofs is pretty complicated business. A lot of the conventional energy saving strategies for roofs -- e.g. insulation values -- don't adequately address the real energy benefits of the green roof. In other words, it's not really about insulation. Depending on who you ask, the R-value per inch of dry soil is not very much. Other relevant issues include things like heat flux direction, peak temperature delay, evapotranspiration rates and other factors that can lead to reduction in rooftop cooling unit demand and other energy efficiencies. My understanding is that savvy engineers can incorporate these issues into their energy models.

I've made this more complicated without actually answering your question; sorry. An engineer could probably tell you more specifics. Also, I've heard that there's a green roof module for Energy Plus.

This has kind of been a problem for me for some time now. There are some fundimental concepts here that do not support some of the claims made about the energy performance of vegitative roofs. Dirt has insulation quality, but is not much. The overall R value is highly dependent on moisture content, and that is a huge variable.

Do not get me wrong, there are many, many great reasons for vegitative roofs, but I do not believe energy is one of them.

Here is what we did on a project to at least get an idea of the possible savings...run a model, then determine the total load in cooling (the primary claim made) that is attributed to the roof, ie as if there was no conduction or radiation losses from the roof. That is the theoretical maximum benefit a roof can have. You will see that the overall affect is very small on most buildings, because most buildings are not dominated by roof loses.

There seems to be significant research on green roofs, and I see all sorts of data on stormwater retention and treatment, but very little on energy.

If anyone has data to the contrary, I would be the first one to read it!

They main point here is that the variables involved with green roofs and energy use are highly variable by project and circumstance. I have several green roofs on my current project, and after talking through various factors with the engineers, we decided not to tweak the energy model at all, and that the green roof had no impact. My project is a 24/7 hospital in which the roof has minimal impact, which is common for large commercial buildings, as Scott mentioned. An opposite example might be a big box store that has night flushing.

I heard a great presentation on green roof energy research by Kirstin Weeks, a building science and green roof specialist at Arup. I don't think the work has been published yet, but I will post a link when it is. (I am itching to share the punch line, but have to hold my tongue.)