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Method to Insulate a Cathedral Ceiling

user-1147664 | Posted in Energy Efficiency and Durability on

I am currently in the design process of our future home to be built in the summer of 2013. One of the details I am getting hung up on is how to insulate our vaulted/cathedral ceilings. I’ve read up on several unvented methods most of which appear expensive and difficult to construct. I also cannot vent my ceiling as there are several dormers on the south face that would prevent continuous air flow. I intend on performing much of the work myself so ease of constructability is equally as important as cost.

My preferred method would be to fill the rafter bays with spray foam insulation, but I’m concerned about cost as I cannot perform this task myself. Another option would be to install rigid insulation atop the sheathing, but with several valleys at the dormers, this approach seems difficult to construct.

I was wondering if the following approach is acceptable? I would cut rigid insulation to fit tight in my rafter bays below the sheathing and seal all joins extremely well with sealant. I’d seal between the rafters and the plywood as well as between the rigid and the timbers and at abutting insulation joints. I’d likely install 2 layers of 3″ of rigid and fill the remainder of the bay with batt. Does this approach spell disaster from a vapor management standpoint?

You advice is very much appreciated!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brian,
    From a building science standpoint, your approach will probably work. Some installers worry, however, that lumber shrinkage, temperature changes, and moisture variations will eventually break the seal between the rigid foam and the rafters or trusses, allowing humid air to contact cold sheathing.

    Moreover, your suggested approach fails to address thermal bridging through the rafters. If you want to use rigid foam, the best place to install it is above the sheathing, not below the sheathing.

    Your suggested insulation method is fussy and very time-consuming to install.

    Finally, there are code-compliance issues; the code isn't clear on whether your approach is acceptable. Talk to your local code official on that point.

    If you haven't finalized your plans yet, consider a roof design without any dormers. More information here: Martin’s Ten Rules of Roof Design.

  2. jwyman | | #2

    Martin,

    In this case wouldn't upsizing the rafters to 2 x 12's, providing a continuous vent channel under the sheathing and applying taped polyisocyanurate rigid to the underside of the rafters work? strap over the rigid and apply your interior finishes. This addresses thermal bridging, vents the roof and is much simpler to construct. At the dormers, use roof to wall vents and if you have valleys, drop the valley rafters to continue your venting.

  3. GBA Editor
    Martin Holladay | | #3

    Jon,
    Your suggested method is very commonly used.

    However, I don't recommend that people specify vented roof assemblies if the roof has dormers and valleys. If it's too late to change the configuration of that type of roof, I think it's better to bite the bullet and design an unvented roof assembly.

  4. user-1147664 | | #4

    Thank you very much gentlemen. I'd prefer to stick with an unvented assembly if not for any reason by to gain 2" of insulation.

    With regard to the thermal bridging, I intend on either strapping the underside for rafters to create a deeper insulation channel or attached additional rigid insulation to the underside of the rafters.

    I will see what I can do to eliminate the some if not all of the dormers but it might be a tough sell to my wife

  5. wjrobinson | | #5

    I have seen my share of rot next to rigid foam. Plant filled homes, moist homes, trouble. Leaky rigid foam, trouble. I would never cut foam and install it in framing ever. You want foam? Northeast Sprayfoam. Call them, and pay the price to do foam right. Rigid foam needs to be continuous and still I have seen failures that would end your thoughts of using it as a DIYer.

  6. user-1147664 | | #6

    Genlemen,

    Your comments are much appreciated. I've decided to re-detail the dormers to be shed roof dormers such that the rafters of the dormer are parallel with the main gable roof rafters. My thought is that by using this strategy I can provide a 2" vent space that will be continuous from eave to ridge. Ofcourse the air space will change angle slightly where it transitions from shed dormer roof to the main roof, is this a problem. I intend on using 2" rigid foam as my baffle to form the 2" air space and followed by 10" of fiberglass (14" deep rafters). Do you see any issues with this approach?

    Thanks!

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