发泄vs. hot roof for addition with steel roof
Patrick Krekelberg| Posted inGreen Building Techniqueson
We are adding onto an existing 1.5 story bungalow (10:12 roof / cathedral ceiling, 4:12 dormer) which as far as I can tell is a hot roof. I see no roof vents and I don’t believe there is a ridge vent.
We are making an addition which will have a full second story, all 4:12 pitch with steel roof throughout. This will use trusses so we will get as much cellulose as we can in there, at least R60.
What would be the pros and cons doing a hot roof on the addition vs. venting to the ridge?
Zone 7.
Cheers!
Patrick
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Replies
Hi Patrick,
I've not dug into the pros and cons of each approach, specifically, but most of the designers and builders of high-performance homes that I have talked to over the years build unvented, insulated roofs in two situations--when the mechanicals are going to be located in the attic or when the design calls for a cathedral ceiling in a finished space.
Of course the cathedral ceiling could still be vented, but that means less room for insulation. Otherwise, using raised heel trusses, an insulated attic floor, and roof venting is often more cost-effective and a less risky assembly. In either case, success is about getting the details right and air sealing is as important as R-value.
If you can achieve a relatively air-tight ceiling along with your R60 insulation you have more latitude in your method and amount of ventilation. I'm not fond of the various ventilated ridge details for metal roofs; they're tough to build and tough to keep the elements out. If you have a location in gable end(s) for a louver(s) or elsewhere, you may be able to get by with those and soffit vents. Ditto Brian's recommendation for energy heel trusses (and proper ventilation baffles).
Patrick, this is a question that comes up regularly on the projects I design. I agree with Brian's points. Will your addition have a flat ceiling? I assume so because you say trusses, but trusses can also be made for cathedral ceilings. In most cases, venting the roof is less expensive, more forgiving of potential moisture issues, and means that you don't need to use foam with its environmental and health-related issues. If your ceiling are sloped, you can still do an R-60 vented roof; you just need to plan for a 1- to 2-inch vent space. (More is better.)
yep, I should have mentioned that - the ceiling will be flat.
What would you say would be the best method for venting? I'm not sure there would be space for louvers on the gable end on the East side, and on the West side of the addition there is the union with the existing building.
I think the best approach is the IRC recommendation of venting 1/300th of the floor area, with 50% at the eaves and 50% at the ridge. Metal roof ridge vents are a bit clunky looking but I have never had a problem with them, but I also tend to use unvented roofs in high-wind locations. Most readily available venting systems are designed for this requirement, for average-sized homes.
If you're in a high-wind area I would seriously consider an unvented roof, or make a custom ridge vent--like a small roof sitting on top of the actual roof.