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Justin Robert Milliet Rodrigues Taylor
Project Manager, CPHD, LEED GA
Asked a question 2 years ago

Green roof design question (for NYC) - for a low slope, extensive green roof, is there a potential for condensation inside the insulation layer if the airtight layer is in the middle of the insulation layer? In this scenario (I'm listing the layers from inside to outside), you would have blown cellulose in between the joists, air tight layer/substrate, roofing membrane, XPS insulation, and then the green roof system (drainage layer, root barrier, and green roof). This is for a renovation project, and we're trying not to raise to height of the roof too much, which is why we're insulating in between the joists, though insulating completely on the outside of the air barrier isn't out of the question. Would love to know what the community has to say! Thank you!!

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Ed May
Partner, Building-Type LLC

Hey Justin, 

If you are using air-permeable insulation like cellulose I think you'd want to comply with the IBC's 1202.3.5.149 in that case, right?:

Insulation shall be located in accordance with the following:
5.1: Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
5.1.1: Where only air-impermeable insulation78 is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.
5.1.2: Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Item 5.1.1. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table 1202.328for condensation control.
5.1.4: Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.

I don't think (?) that the green roof changes the vapor-permeability of the roof does it? Any roof membrane would be impermeable, and so would the green roof once all the control layers are in place. So you need to be sure you a) ensure against condensate formation as per 5.1.4 and then b) ensure it can dry to the inside. Are you doing a smart-vapor retarder on the underside of the joists?

When we do that 5.1.4 calc for NYC, I think we use 0C (average of the coldest 3 months) if I remember right.


I agree with Ed May‘s response.  Section 5.1.2 applies to your scenario.  However, I think you are also asking about the assembly order, to IRMA or not to IRMA, and who installs the XPS?

My opinion is that the XPS should be installed beneath the roofing membrane.  Most any building material performs better and lasts longer when it is warm and dry, and XPS is no exception.  

The drainage layer in the green roof assembly will not keep XPS placed above the roofing membrane dry.   Moisture that reaches the XPS layer will eventually find its way to the roof membrane, particularly during freeze-thaw cycles. The question is: will moisture that migrates between the XPS layer and the roofing membrane adversely impact the effective R-value of the XPS layer?  I don’t know the answer to that question but, my hunch is that it could.  If it does, that impacts how much rigid insulation you need above the sheathing to satisfy 5.1.2, insuring that the temperature of the underside of the sheathing stays above dew-point.

In the interest of both space and long-term durability, I think that you are better off placing the insulation beneath your roof membrane.  Of course, that may mean that your roofer (or someone else) may need to install the rigid insulation instead of your green roof installer. 

Devon Basher
Director of Eco Energy Options, NJ, USA

Hi Justin,

Hope you are well!  Maybe this one is in the books. My thinking with a flat/low slope roof as described (and many assemblies with no or low outward drying capacity and hygroscopic inboard insulation) is that a taped interior vapor-variable membrane like SiGa Majrex, Pro-Clima Intello, ect (some are pretty different) can provide enhanced building durability security.    

Kind regards, 


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