Energy Codes and Roof Drains

What level of insulation should you have at roof drains? Do you need the minimum above-deck insulation level as prescribed by International Energy Conservation Code (IECC)? The answer is yes – unless you're using tapered insulation.

When using the IECC, you can reference Table C402.1.3 to identify the minimum above-deck insulation for your project's climate zone (in 2018 IECC, CZ 2 or 3 is R-25, CZ 4 or 5 or 6 is R-30, etc.) Roof assembly exceptions section C402.2.2 states, "Where tapered insulation is used with insulation entirely above deck, the R-value, where the insulation thickness varies 1 inch or less from the minimum thickness of tapered insulation, shall comply with the R-value specified in Table C402.1.3."

This is allowing the flat stock insulation below the 1” of tapered to be reduced by 1" for minimum R-value compliance at a roof drain. As an example, a typical assembly for Climate Zone 2 or 3 requires an R-value of 25. For roof insulation that is entirely above deck, that would consist of two layers of 2.2"-thick Polyiso flat stock for a total thickness of 4.4”. (The energy code states you should use two layers of insulation with staggered joints to minimize energy loss through the board gaps.)

But as we saw with the exception, we want to taper or slope to the roof drain. A minimum 1/4" per foot slope (2% slope) would allow a taper to start at the roof drain and grow to 4' as it reached the 1" threshold. This is allowing for the creation of a sump area at a drain, or an area of 8’ x 8’, which is sloping directly to the drain at 1/4” per foot. You may decide to design with a steeper slope because you want more positive drainage – maybe you choose 1/2” per foot slope (4% slope). This would allow the distance from the drain to be 2’, or an area of 4’ x 4’ around the drain.

Going back to our example, instead of using two layers of 2.2”-thick Polyiso flat stock (4.4” Polyiso, R-25 design), we're designing with one layer of 2.4”-thick Polyiso flat stock and one layer of a prefabricated Polyiso target sump board (8’ x 8’). A target sump is tapered Polyiso insulation which starts at the edge of the board at 2”-thick and tapers down to the center at 1/4” per foot. The center has a final thickness of 1”, which meets the minimum R-value while using the exception. This means the Polyiso is a minimum of 3.4”-thick at the drain, instead of 4.4". If we chose to use the 1/2” per foot sloped prefabricated target sump (4’ x 4’), it would start at the edge of the board at 3”-thick and taper down to the center at 1/2” per foot for a final thickness of 1”. This meets the minimum R-value while using the exception as well. And again, this means the Polyiso is a minimum of 3.4”-thick at the drain instead of 4.4”, but the sump area is reduced from 8’ x 8’ to 4’ x 4’.

This example also highlights how much time contractors can save by using prefabricated target sumps. Prefabricated sumps allow the installer to skip the labor-intensive step of cutting sumps out of tapered panels on the jobsite. The tapered insulation exception can also be employed at through-wall scuppers, as you would design a sump area here to increase the flow of water off the roof.

For more information about tapered target sumps, click here.  

For more information about tapered insulation, click here.  

If you have any questions, please contact Craig Tyler.