GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulating a 2/12 roof above the roof deck

bary_g | Posted in GBA Pro Help on

My first house, purchased a couple of years ago, needs a new roof. We recently had a major storm in the area and all roofers seem to be so busy that it is difficult to have someone over to discuss my case. So far I spoke with 3 roofers, each with a different idea, so I am doing my own research to see my options.

I have done some research but none of discussed roof setups was identical to mine.
– Zone 5 (Chicago), cold winters and hot summers,
– Basic gable roof, two slopes only,
– 1 story ranch on slab,
– 2/12 roof pitch,
– Unvented roof,
– No attic
– No ceiling insulation,
– Existing (old) natural fiberboard insulation installed above the roof deck, most likely ¾” thick, maybe 1 ½”,
– Roof deck consist of 2×6 tongue and grove lumber exposed on the interior side/ cathedral ceiling
– Roof deck extends into 24” overhung , no ventilation in soffit,
– Built-up roof,
– Roof penetrations: furnace vent, bathroom vent, oven hood exhaust, plumbing stack,
– AC duct entirely in slab,
– 2 electrical boxes installed on the roof deck for kitchen light.

Since the old insulation got wet and it’s insulating properties are poor anyway I am considering installation of the new insulation. Remember, insulation entirely above the deck.
ICC requirements most likely will not be met because the amount of insulation on the roof would be around 10”, creating odd thick roof. I am comfortable with up to 5” extra on rooftop…
I am trying to find out if I need venting in my new roof. Throughout my search I found contradicting opinions in that subject.
I am aiming to achieve up to R-20 which will be significant improvement comparing to my current condition. Seems like Cornell ThermaCal1 (SIP) vented panels on roof deck would be my best option.

I am aware that installation of the shingles on a 2/12 roof is questionable, however I came across GAF and ARMA guidelines for installing the shingles on low-slope roof. Besides, half of the houses on my street with the same roofs are shingled. Membrane roofing is not out of the question, it’s just the shingles are my preference…

I would appreciate your 2 cents on my choices. Your comments/suggestions would help me to get my roof before the winter!

Many Thanks + Regards,

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    You can create either a vented roof or an unvented roof; it's your choice.

    Cornell ThermaCal is a product called nailbase (rigid foam with OSB adhered to one side). There are many brands of nailbase. Using nailbase is a perfectly acceptable approach for your roof.

    It's possible to buy unvented nailbase or vented nailbase. Vented nailbase is somewhat more resistant to ice dams than unvented nailbase.

    For more information, see these articles:

    How to Install Rigid Foam On Top of Roof Sheathing

    Insulating Low-Slope Residential Roofs

  2. Expert Member
    Dana Dorsett | | #2

    You won't need 10" of foam to hit IRC performance levels on a U_factor basis. It really only takes 7" of polyiso at most, even without the 2x6 and fibergboard. In your case you'd make it at 6".

    To hit IRC requirements it need to duck under U0.026, or above R38.5 "whole-assembly".

    You have ~R1.8 of 2x6 wood decking...

    ...another ~R0.9 (combined) of air films...

    ... at least R2 of fiberboard...

    ...all adding up to ~R4.7.

    At a derated R5.5/inch adding 6" of polyiso would give you another R33, bringing it up to R37.7.

    You'd get another R0.5 out of a half-inch OSB nailer deck, for R38.3

    ...and the roofing materials will nudge it over the top.

    That's adding 6.5" of total thickness- a bit more than 5", but quite a bit less than 10".

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |