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Proper layering for cathedralized ceiling insulation

UDD8Wwz4kg | Posted in Green Building Techniques on

Hi – I’ve just put new attic roof trusses on my house and I am looking to maximize the interior conditioned space available (behind the knee-wall), while keeping the costs down. I reside in southern Ontario.
I would like to do 2lb closed cell spray foam, but, the cost is prohibitive for me.
Can someone please advise if the following is an acceptable layering (from the finished exterior roof looking in towards the new attic space):
– metal roof (with underlayment affixed directly to plywood sheeting)
– plywood roof sheeting
– 1″ air gap
– standard rafter vents (to provide 1″ air gap)
– 5.5″ R20 batt insulation (to fill the truss cavity)
– 2″ spray foam insulation 2lb closed cell to give ~R10-12 (to be sprayed onto and adhered-to the fibreglass batts)
– drywall

Many thanks!

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  1. user-1012653 | | #1

    Others can chime in, as I am not completely sure of the code in your area, but you should be able to spray the foam directly onto the underside of the plywood, eliminating the vent space and turning it into much needed insulated space. I would then recommend netting and blowing cellulose or fiberglass instead of batts. What is the depth of your rafters?
    Keep in mind your r-30-35 ceiling is ideally much lower then it should be in your climate, and would be much lower then code in my area as well. (zone 6)
    I do not know what your headroom is below the rafter, but one option is to use plywood strapping and 2x4s to hang from your structural rafters, cheaply increasing the depth (I will assume a 2x10) from 9.25" to whatever you want, ideally closer to 14-15" total depth. This would allow you to hit close to an r-60 with a nearly completely thermally broken ceiling.

  2. GBA Editor
    Martin Holladay | | #2

    I don't think you will find any spray-foam contractors who will agree with your plan.

    When insulation contractors use the flash-and-batt method, the insulation is usually installed on the exterior side of the batt, not the interior side.

    I don't know the details of the building code in Canada, but in the U.S., the code allows a house in climate zone 6 to use closed-cell spray foam plus fiberglass batts in an UNVENTED cathedral ceiling, as long as the spray foam that is installed on the underside of the roof sheathing has a minimum R-value of R-25. That means your closed-cell foam would have to be at least 4 inches thick. More information here: Creating a Conditioned Attic.

    It's possible to use only 2 inches of spray foam, as long as you install ventilation channels. However, you can't use cheap polystyrene "Proper-Vents." You need to use a stiffer product like AccuVent, or you need to make site-built baffles out of 1x1 sticks and thin plywood. If you follow this plan, the spray foam goes in first -- then you install the batts.

  3. user-966377 | | #3

    Hi Mike,

    I work with an architect in Toronto, and we routinely have to try to decide what the best method of insulating attics is in our climate. I think we can agree though, that insulating and making the attic a conditioned space is best for any services running in the attic, and to get some additional floor space.
    First, I would stay away from fibreglass batts in the first place. They promote mold, get wet and rot, and are a haven for vermin. Try Roxul batts, or blown cellulose if you want.
    Sprayfoam is a great way to insulate, and from what I am told it does work well if you plan on venting the assembly as well. AccuVent, as Martin recommnds, is a good idea; and then sparyfoam as required.
    Many jurisdictions in southern Ontario still have issues with Sprayfoam insulation: case in point, in Toronto and surrounding municipalities, they still require a vapour barrier installed on the warm side (even though the foam itself can act as the vapour barrier.) At bit of a waste, but it's not doing any harm in the end.

    Since you are concerned about the cost of sprayfoam, I would stick to a vented assembly with Roxul batts, vapour barrier on the inside and your finish material. The 7-1/4" thickness will get you the required R-28 for cathedral ceilings in the OBC.

  4. UDD8Wwz4kg | | #4

    @ Everyone - thanks!
    @ Jesse - This part of the truss is 2x8.
    @ Martin - If the foam is in first (against roof sheeting), and, if it is acting as a vapor barrier, then, is there not a problem to put more insulation on the warm side? I figured that the vapor barrier needed to be directly on the warm side.
    @ Donald - R-28 is one of 3 different R values provided by 3 different spray-foam installers. Is this still accurate per OBC for cathedral ceilings?
    Thanks! Mike

  5. GBA Editor
    Martin Holladay | | #5

    If you are installing spray foam directly against the roof sheathing, it has to be thick enough to ensure that the indoor side of the cured foam stays above the dew point. If you do that, there won't be any condensation or moisture accumulation, because the foam will always be warm. That's why the International codes require such foam to be R-25 or more in climate zone 6.

    To learn more about the theory behind exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing. Although the article refers to walls, not roofs, the building science principles are the same.

  6. UDD8Wwz4kg | | #6

    Martin - thanks so much for your response. You may have already answered this, but I found another thread: "Regarding the November 2011 article on Synthetic Roofing Underlayments"
    I have a metal roof w/ underlayment (non-breathable) installed. The mfg installation instructions states that the underlayment needs to be installed over ventilated space.

    Does usage of this underlayment mean that I cannot have an unventilated roof - period - or, would the R-25 method you describe in #2 above be an appropriate 'hot-roof' style installation and overrule the mfg's statement regarding installing over ventilated space. Thanks!

  7. GBA Editor
    Martin Holladay | | #7

    If you install spray polyurethane foam on the underside of the roof sheathing, you are creating an unventilated roof assembly.

    Most manufacturers of synthetic roofing underlayment require their products to be used only on ventilated roof assemblies. That means either:

    1. Above a ventilated, unconditioned attic, or

    2. Above a cathedral ceiling that includes a ventilation channel directly under the roof sheathing.

  8. user-901114 | | #8

    What type of underlayment are you using exactly? Has it gone on yet? I have just built a house in zone 6 with the same attic (24oc 8in)trusses and a steel roof. But we used a breatheable underlayment. I think I'm going to make 1in vents out of cut foam board + 2in polyiso foil cut into 24ish inch x 8 foot pieces (can foamed edges) + r21 kraft faced glass batts. This will keep my roof vented. I'm also going with r38 cellulose under the attic floor because I got a bunch for free.

  9. UDD8Wwz4kg | | #9

    Hi Stephen - it is interwrap titanium udl-30, already installed. Mike

  10. user-901114 | | #10

    Mike, another idea would be to build one inch plywood vents from eave to peak out of thin plywood. Then put your sheetrock ceiling up under your attic floor trusses and sheetrock your attic walls. Furring out that one pesky spot that would only have 7in of angled ceiling. Then blow cellulose from the peak, filling the entire cavity behind the kneewalls and up to just below that mini attic peak area. You could put roxul batts up there for checking the settling access later. Do you have "energy trusses"? What are the dimensions of the house?

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