Building a cathedral-style ceiling in a room with a roof without ventilation

Building a cathedral-style ceiling in a room with a roof without ventilation

Building a cathedral-style ceiling in a room with a roof without ventilation

Q. I’m planning a room addition to our home and would like to make the ceiling a sloping, cathedral one. In the past few months I’ve been hearing about people building roofs like this without any ventilation. Is this allowed? And, if so, how do you go about it? What are the pros and cons?

D.B. Kalamazoo.

A. Unvented roofs have some advantages and also some liabilities. If you’re looking into doing this, you’ll need to consult with your local building department during the design phase. It is true that some «ventless» roof assemblies were sanctioned by the 2006 version of the International Residential Code. However, whether or not this practice is allowed in your area is up to your local building department, which has the autonomy to adopt or reject portions of the IRC. Here are a few things you should know:

One item you’ll discover early on in your investigation of ventless roofs is that the methods of work need to be just about perfect for the idea to have a successful conclusion. That’s because this type of assembly uses foam insulation to both insulate and seal the space between the underside of the roof sheathing and the backside of the drywall on the ceiling.

Foam insulation has many admirable qualities. It is an air barrier, and closed cell polyurethane foam is a vapor retarder. Spray foam sticks to building components very well, insulates better — per inch of thickness — than any other common insulative material, and is quick (though messy) to apply in even oddly shaped cavities. Newer versions are derived from agricultural materials, and are therefore more «green» than were previous incarnations. However, unlike other insulations, closed cell foam does not readily allow moisture to evaporate out when it is trapped behind the material.

Because of this latter property, the application of the insulation has to be flaw-free. Any gaps or holes left in the foam will allow water vapor to pass through the insulation barrier and condense on the cold underside of the roof sheathing. From there the condensate can spread, soaking the sheathing. And, with no air movement to dry things out, the sheathing can rot. The same liability would be present, of course, with a roof leak from the top. Foam in ventless roofs can trap water, while a conventional vented roof has some leeway with regard to disposing of «accidental» water intrusion and accumulation.

Another thing to check on is a ventless roof’s appropriateness with the type of shingles you’re planning to put on top of the roof. In the past, manufacturers voided the warranty if their shingles were installed over an unvented attic or cathedral roof assembly. The thinking was that a ventless roof would become hotter than one where a supply of ventilation air is cooling the underside of the sheathing. However, some shingle manufacturers have recently changed their stance with regard to this issue and now grant full warranties for applications over unvented roofs. But check with the manufacturer of the shingles you intend to use to make sure what you’re doing will meet with their requirements.

Finally, if you’re planning on installing recessed ceiling canister lights in the ceiling, you’ll face several challenges. The first has to do with keeping the insulation behind the lamp housing airtight. Another is figuring out how to insulate behind the lamp housing without creating a situation that would allow the housing to overheat. Even IC (insulation contact rated) lights need some space between foam and the housing.

Zolton Cohen is a former ASHI-certified home inspector based in Kalamazoo. Write to Zolton Cohen, Around the House, P.O. Box 2007, Kalamazoo, MI 49003, or contact him through Michigan Live at www.mlive.com/forums/homeimprovement/.


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