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

Venting and insulation for cathedral ceilings

Cem Zafir | Posted in Energy Efficiency and Durability on

We’re in Hamilton, Ontario, trying to insulate our attic which has cathedral ceilings. Previous owners had more or less shoved R12-R20 fiberglass insulation and vapour-barrier, with no airflow. We recently had our roof done, shingles straight on the original, nearly century old boards. Our roofer cut out a ridge vent for us, but later on we came to the realization, that there is parged or plastered brick all around the perimeter of the attic floor, probably there for “insulation”, old school… we’re not sure how to get the airflow to happen, just in case the removal of the brick may affect the structural integrity of the roof. What is the best way for the air to flow from soffits to ridge vent?

Also, the rafters are only 4.5″ (by true two inches thick) and we will probably use Roxul R24, having to extend/deepen the rafters by a couple of inches. We were going to use R40 fiberglass, but 11″ is too much to manage and too much space loss. We don’t want to use spray foam, due to cost and misinformation. Most say no air-flow needed, some say it is needed.

AAAAnd, we found old knob & tube wiring mixed with really shoddy electrical… absolute fire hazard! Truly, we don’t know what we’re dealing with here…this awkward, but beautiful 20x30x10 foot high attic space…

There are no cross-braces. So, do we need to worry about some sort of support, if we were to remove the brick?

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. Cem Zafir | | #1

    Thank you for your time, in advance, and apologies, if I'm using inaccurate terminology... very green!

  2. GBA Editor
    Martin Holladay | | #2

    First of all, it's a shame that you already installed new asphalt shingles. By far the easiest and best way to insulate this attic would have been to install new rigid foam on top of the existing roof sheathing. But it's a little late for that approach.

    In order to insulate these rafters from the interior, you're going to lose interior space. There is no free lunch. If you wanted to maintain your head room, you should have insulated above your roof sheathing.

    At this point, you should probably install spray foam and forget about venting.

    For more information on all of your insulation options, see How to Build an Insulated Cathedral Ceiling.

  3. Cem Zafir | | #3

    Thank you, Martin!
    Yes... I don't know much, but at the age of 46, I should know to listen to my instincts. I wanted to install insulation and do the roof concurrently, just wasn't sure how to approach it. I was told by pros to not worry about it and we did it on the roofer's schedule. Ah well...

    A few questions:
    What do you know about the brick extension technique used here? Have you seen it before? Is it for the purpose of insulation or structural or both?

    The quotes we got for 4.25" spray foam hovered around 5K... steep! Not only that, but a few in the know expressed concern for air-flow for spray foam even... that it could result in a saggy roof down the road. Is this true?

    I have further ecological and structural concerns for spray foam. What if the wood rots and we have to toss it all out?

    Also, if we were to install R-24 or less Roxul by extending/deepening the rafters, could we remove some wood from the eaves for air-flow to happen from the soffits? Or does it make more sense to rebuild and reseal the small brick knee-wall and install some vents from the lower part of the steep roof? Or, do we really not have those options?

    Thank you so much for your insight and the fantastic forum here!

    one more pic for clarity...

  4. GBA Editor
    Martin Holladay | | #4

    If this is a wood-framed house -- not a house with brick walls -- I would assume that the bricks and mortar were installed as an air-sealing measure. It is probable that the bricks can safely be removed. However, you'll want to be sure that you air seal any leaks in this area that may allow interior air to rise into your attic. That air sealing work may require spray foam.

    It's very hard to determine whether there is any way to retrofit good soffit venting without a site visit. Almost anything is possible if you have a large enough budget -- but again, this is an example of work that would have been easier to perform before you got new roofing.

    If you choose a competent spray-foam contractor, there is no reason to think that spray foam will rot your roof sheathing. But the installer needs to know what he or she is doing, and you need to be sure that the insulation is thick enough to meet minimum code requirements.

    All of the various options available to you are explained in the article that I linked to in my last answer.

  5. George Cushing | | #5

    I would stay away from anything that results in a warm roof. A warm roof is one that has no ventilation between the insulation and the roofing system. Simply spraying foam over the underside of your sheathing will create a warm roof.

    I built my current home 25 years ago in a climate similar to yours. It is a heavy timber frame sheathed in Winter R-34 panels ( The panels are part of the roofing structure. They are principally wafer board/4.5” polyurethane/1/2” sheetrock sandwiches. The soffet overhangs are wafer/polyurethane/wafer. The roofing is asphalt over building paper. Pitch is 10/12.

    This is a warm roof. In spite of the heavy insulation, the exterior of the roof is always going to be a bit warmer than the outdoor environment, if it is not vented. Oh, it will hold snow, but as the outdoor temperatures moderate the snow will melt at the snow roofing interface. This cyclic melting wets the roof, rusts the fasteners and rots the wafer board sheathing. I feel that this insulation system should only be installed with a ventilation space between the panels and the sheathing or with a vented metal roof.

    In your situation, I would go with standard roof venting (openings in the soffets and space above the insulation). Go with 3.5” glass in the cavities and 2” of foil faced polyurethane sheathing fastened over the faces of the rafters. I used this system in my barn. I hung the polyurethane sheathing with ring shank nails through furring into the sheathing and then the rafters. As the barn is not living space, I’m finishing with boards fastened to the furring. In living space your code will probably require some sort of fire break. The ring shanks should bear the weight of the rock. They will pull through the furring rather than be extracted.

    This system will get you a tight R-30 roof without the problems associated with a warm roof. Don’t let anyone tell you sprayed foam roofing insulation under the sheathing or foam panels on top of them doesn’t need venting until they’ve seen roofing slide off after ten years and spent a couple of weeks tearing up and replacing in rotten sheathing.

  6. GBA Editor
    Martin Holladay | | #6

    To avoid problems with insulated sloped roofs, you definitely want to be sure that you have enough insulation. If you skimp on insulation, you can definitely have problems.

    The DOE climate zone map doesn't include Ontario, so I don't know what climate zone Cem is in. But the 2009 IRC (a residential building code) requires a minimum of R-49 roof insulation in Climate Zone 6, an area that includes Vermont and southern Wisconsin.

    George, your SIP roof didn't have much R-value -- only R-29. Moreover, the insulation method you recommend (3.5 inches fiberglass plus 2 inches of rigid foam) will only give you R-24 or R-25.

    I strongly suggest to you and to Cem that you should aim for higher R-values -- at least meeting minimum code requirements. That will go a long ways toward keeping you out of trouble.

    It's also important to make sure that your ceiling is airtight.

    Finally, George, I agree with you that ventilation above your insulation layer reduces the risk of ice dam problems. But it's also important to have enough insulation in your roof.

  7. Cem Zafir | | #7

    Thank you very much, gents!
    Yes, we would like as high an R value as we can get in there (& afford).

    Our first plan was to install R-40 FG (11"), but that would require adding about 8" to the rafters... unreasonable. We like Roxul better.

    We are near Buffalo, NY, on Lake Ontario, so zone 5A or 6A. We should be good with close to R30.

    The issue is what to do about airflow... especially the brick knee wall and its function.
    If I hear you right, Martin, we may have to bite the bullet and just go with spray foam and would be ok with no added ventilation... (?)
    btw, Martin, it is a brick building. That little extension appears to be a later addition.

    Either way, do we reseal that brick knee wall or remove it... is there a way we can access the existing soffit vents to create a continuos airflow to the ridge vent? btw, roof slopes about 11/12.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |