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

Cathedral ceiling insulation

thermalpro | Posted in Energy Efficiency and Durability on

I am looking at insulating a house that has a simple gable end roof with a 10/12 pitch. It has 24″ deep open web trusses 24″ OC . It is going to have a metal roof but currently just has the underlayment.

The general contractor had originally thought 2-3″ of closed cell directly to the bottom of the roof deck then extra fiberglass batts. For the same money I could deliver about twice the R- value with open cell but should also spray the foam [skin still on since no trimming necessary] with a vapor retarder paint.

Looks like it could be vented with continuous soffit to ridge vents which obviously is the best way but the peak is over 35′ above the floor and the contractor did really want to go there. The ceiling is going to be tongue and groove wood perforated with openings for can lights speakers, etc. so not really any air barrier.

The orientation is such that both sides of the roof will see good sunlight during the winter. At just under 5000 sq.ft. of roof with most all of it way up there staging and safety is an issue.

I would prefer to install vapor permeable vent chutes then a minimum of R-40 open cell. I have used open cell foam for similar applications for over 20 years with out a recall being careful about the north sides of steeper roofs. I have also used a lot of closed cell but the only concern is the potential for a roof leak going undetected and the cost.

I know there are differences between the brands of open cell foam in terms of water absorption,etc. My feeling is that it would be fine without venting and the vapor retarding paint on the foam skin but would prefer to have a vented roof. The extra time and hassle it would take is nothing to what it would take if we have problems down the road due to high humidity or whatever. If I can not talk him into a vented assembly what is the better approach?

Not cost effective for total r value with closed cell alone. Also don’t really want to go over the whole area a bunch of times. Climate zone 5b mixed hot and humid summers, can be pretty damn cold and windy in the winter.


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. brendanalbano | | #1

    If you can't talk the contractor into a vented approach, an unvented assembly with an appropriate thickness of closed-cell foam for your climate zone and batts/open-cell foam to make up the rest of the desire R-value is the probably what you want to be looking at. BSC says the closed-cell should be 40% of the total R-value in zone 5:

    Unvented with open-cell foam in climate zone 5b sounds like a bad idea. It also may not be code compliant, depending on the details of your local codes. I couldn't track down the original source, but Martin references BSC giving vapor retarders sprayed onto open-cell foam a thumbs-down in this thread:

    Vented with open-cell sounds reasonable (with the caveat that the open-cell foam becomes your air barrier in this case due to the T+G ceiling not being an air barrier, and some folks are hesitant about the long-term durability of spray-foam-based air barriers)

    Unvented with closed-cell sounds reasonable. The closed-cell layer thickness is determined by your climate zone. The remaining r-value could be provided by open-cell foam, fiberglass batts, etc. A high quality batt installation between trusses 35' off the ground might be difficult, and the more robust approach might be a closed-cell + open-cell combo.

    1. thermalpro | | #2

      Thanks Brendan for the input. I understand how the vapor retarder paint wouldn't work on open cell foam that has been trimmed but we have applied both DC-315 thermal barrier and latex paint to open cell foam that has not been trimmed and still has the skin. Perhaps Martin can have some input on this. That being said I think your thoughts are well put and sound. I do not think the addition of fiberglass under the closed cell would be of much value due to the open web trusses creating gaps and the leaky lid. I think you are correct that the open after the closed cell would be better and not leaving a gap between the two helps.


      1. GBA Editor
        Martin Holladay | | #3

        As far as I know, vapor-retarder paint is ineffective when applied to open-cell spray foam, whether or not the foam has been trimmed.

        I don't recommend the use of open-cell spray foam on the underside of roof sheathing, unless you are planning to install drywall and vapor-retarder paint on the interior side of the spray foam, or unless you have a ventilation channel between the spray foam and the underside of the roof sheathing.

        Closed-cell spray foam is safer. For more information on this issue, see these two articles:

        "Open-Cell Spray Foam and Damp Roof Sheathing"

        "High Humidity in Unvented Conditioned Attics"

        1. thermalpro | | #5

          Thank you for the advice, I appreciate your expertice. I bid it with Closed cell.
          Always to be on the safer side of alternatives. What do you recommend about minimizing potential damage to roof sheathing from a roofing leak going undetected and not able to dry out very well? I see that it is not advised to use a impermeable underlayment with metal roofing when CC is under the deck so it would seem to me it would be best to use 30lb felt and then sleepers under the metal. How about a typical shingled roof?
          Thanks again,

  2. DavidfromPNW | | #4

    I have the same exact scenario. For that portion of the roof I am going unvented, using closed cell foam directly to the interior side of the roof sheathing then filling the rest with fiberglass. I am in 4c so I believe code requires a min of 2" however, I am going to three as I have the room.

    I tried everything I could to stay away from it, but the heavy beams for the site built part of our roof just wouldn't allow us to vent that portion of the roof. good luck

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |