GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

I want to add my roof insulation to the top of my 4/12 cathedral roof

user-7395838 | Posted in Green Building Techniques on

I want to add my roof insulation to the top of my 4/12 cathedral roof, but to get my R-value, I have 15” I want to secure.

I have it at a great price, but will it be feasible? Or how do I do this?

It’s a 4/12 roof and I hope that putting the foam on top which is already sealed with a peel and stick then the foam which we plan in staggering and taping seams, then attach with screws.  We are still costing everything but the foam is inexpensive and the screws aren’t.

Yet is this feasible to even do before I start planning and go further? We hope to have R-60 which won’t be possible without 10” of spray foam at we believe will be twice the price.

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

    First of all, can you tell us your name? (I'm Martin.)

    You mention "roof insulation," but you forgot to tell us what type. I'm guessing rigid foam -- either EPS, or polyiso, or XPS -- but unless you tell us, we can't be sure.

    What is your geographical location or climate zone?

    If you are trying to install 15 inches of continuous rigid foam on the exterior side of your roof sheathing, that's a lot of foam -- it's not easy to do. Here is a link to a relevant article: "How to Install Rigid Foam On Top of Roof Sheathing."

    1. user-7395838 | | #4

      The foam is eps. And the pricing is more economical than blow in The information I have found in Green builder has not been helpful in the limited discussions related to depth of external roofing and securing even for cold climates. It has been a great starting point. Our new building codes will require r50 for our coastal BC Canada region. It is a lot of foam yet the challenge of fitting and then sealing into cathedral ceiling IJoists is tough. I know less is expected from cathedral ceilings but does it need to? The cost of spray foam would be twice the cost.
      There is not enough room for blow in insulation to meet the code

  2. gusfhb | | #2

    Call a roofing supply house [ABC roofing for instance]

    Roofing screws sold by the bucket are super cheap, and my new favorite fastener

    they only get up to about 11 inches that I can see

    1. user-7395838 | | #6

      Thx I am in contact with one and have access to 20” if need be but the cost is high at that length. It seems 7” is a cost point.

  3. Aedi | | #3

    I had a similar question on the feasibility of thick layers of foam a few months ago. See Is there a maximum thickness for exterior rigid foam?

    In short, the only practical limit on foam thickness is the availability of long screws, but thicker layers of foam become increasingly difficult to attach.

    Like you, I had heard that R60 is a good target for roof insulation. However, that target is not really applicable to a roof insulated with just foam. For one, the oft-cited R60 is not really meant to be a "whole assembly" R-value for the roof, but a rough center of cavity estimate. Since insulating with just foam means there is no thermal bridging, the target can be revised down significantly. Further, that R60 figure is usually associated with blown-in attic insulation, which is very cheap and has little labor associated with it. That means piling in more comes at little cost compared to thicker wall insulation, and so it makes economical sense to utilize more of it (lower payback period). Finally, if you have a well-sealed house there is no performance reason to insulate a roof more than a wall. A tight house will experience little temperature stratification, and so the conventional wisdom behind having higher roof insulation levels of "hot air rises" does not apply.

    All that means that there is little reason to aim for R60 in your cathedral ceiling unless you live in an arctic climate. Instead, it is more sensible to aim for the recommended wall insulation levels, usually somewhere around R40. That means you can get away with a much more manageable ~8" of foam, which in turn can use more commonly available 10" screws.

    On the off chance you do live in an extreme cold climate, Joe Lstiburek has a helpful guide here: BSI-031: Building in Extreme Cold. Even in this situation, he only recommends 10" of foam (~R50).

    1. user-7395838 | | #5

      Thx it was helpful. Long screws are highly costly as well.
      The blow in insulation just would not provide the r factor needed. The foam is at an excellent price as it is reused and excellent shape.
      The depth is more with EPS yet it performs more consistently in the cold and with time has been a consideration.
      I do not fully understand that r40 would suffice because the I understand the r60to be the factored amount. If there is a way to learn more that would be great.
      I need to think more in this. Or if you have more direction it would be appreciated

  4. Aedi | | #7

    >Thx it was helpful. Long screws are highly costly as well.

    Unfortunately, that is true. Thankfully, 10" screws are much more affordable than the ~18" screws you would likely need for 15" of insulation. As a point of comparison, I have seen these 10" screws recommended for the purpose of attaching up to 8" of foam (though that cuts it very close), whereas these similar 16" screws cost twice as much and would still be insufficient for 15" of insulation.

    >The blow in insulation just would not provide the r factor needed. The foam is at an excellent price as it is reused and excellent shape.

    Yes, reclaimed foam is an excellent value proposition as you have mentioned. But there are additional costs with the foam that make it more expensive to make a thick layer. There are the screws, as you mentioned, but also the labor. Labor costs increase linearly (or faster) with rigid foam, as each layer of foam requires additional man hours and becomes increasingly difficult. It is also worth keeping in mind that most contractors likely lack experience with attaching more than 6" or so of foam, and may charge even more as a result. Comparatively, blowing insulation into an attic has a relatively constant labor cost, as it takes about the same amount of effort to blow in 12" as it does to blow in 6" -- it's just a guy standing there with a hose either way.

    This isn't to say that you should use blown-in insulation, as it does not really make sense for cathedral ceilings. The point is that it does not make sense to have the same insulation target as you would with blown-in, as all that additional effort is better placed elsewhere.

    >I do not fully understand that r40 would suffice because the I understand the r60to be the factored amount. If there is a way to learn more that would be great.

    In the post I linked earlier, Dana linked to this resource on high R-value enclosure recommendations, taking things like cost effectiveness and opportunity cost into account. The article is nearly a decade old, but a useful reference point. Take a look at Table 2 on page 10:
    Your category would be "compact roof", for which they recommend R50 for your climate zone.
    Now, it is important not take those numbers at face value. For one, a typical compact roof uses a combination of fluffy and foam insulation, reducing labor and fastener cost. Additionally, the economics have changed significantly since then. Rooftop solar has become much less expensive, and as a general rule it is now better to put money into generating additional energy via solar than it is to save energy with extreme insulation levels.
    Taking these into account, I wouldn't aim for more than 6-8" of foam -- if you want to add additional R-Value on top of that, it is likely adding batts between the rafter bays will be a much cheaper way to do so, even accounting for the deal you are getting on the foam.

    Keep in mind the R50 code requirement likely has a whole assembly alternative minimum, which would be easy to meet with foam.

    Also worth noting is that this British Columbia resource on wall design does not include screw specifications for foam thicknesses of more than 8" (see page 17):
    It seems likely that you will encounter resistance from builders going any thicker.

  5. gusfhb | | #8

    Lay down 7 inches of foam,
    Lay down 2x4 on the flat on 24 inch center, screw through them into the foam
    infill with 1 1/2 foam
    lay down 6 or 7 more inches of foam, screw into the 2x4

    Now you save money on screws, but have you saved money, I wonder

    Screwing down 4x8 sheets there are less than , what 1 screw per square foot, so 1000 to 2000 screws for the average size house

    The long fasteners shown are much more expensive style than foam screws

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |