Vented vs. Non-vented cathedral roof – Zone 1
jnarchitects
| Posted in Energy Efficiency and Durability on
Hi
I’m working on a project in Zone 1 and looking for some thoughts/ perspectives on venting vs. not venting a cathedral roof. All of the second flr spaces are under cathedral ceilings.
Rafters are only 2×6’s. I have been considering filling the bays with cellulose or fiberglass and adding 2″ of rigid to the roof deck.
Or spraying full depth medium density foam and no rigid. But I’m wondering if I should be trying to vent the roof. Are there measurable thermal benefits in a hot climate to venting the roof?
The house may not ever be air conditioned, so it might be a moot point.
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Replies
Chris,
The building code calls for a minimum of R-30 ceiling insulation in your climate zone. As long as you reach that minimum, there are many approaches that can work.
Clearly, there is less of a need for venting in hot climates than cold climates. Especially in hot, humid climates, there is a strong argument to be made that you should never invite the exterior air into your house.
It's also true that venting makes the most sense if your roof has no valleys, skylights, or dormers. If your roof has any of those features, an unvented roof makes more sense.
Here's an article you may wish to read: How to Build an Insulated Cathedral Ceiling.
Thanks Martin,
I was more curious to know if in a hot climate are there benefits to keeping the roof cooler by venting it. Does venting the roof potentially reduce the heat gain through the roof plane or improve the performance of the insulation by reducing the temperature on the exterior side of the insulation.
Chris,
Q. "In a hot climate are there benefits to keeping the roof cooler by venting it?"
A. No.
More information can be found in these two documents:
How Important is Attic Ventilation?: "A third reason for ventilation is that roofs will get hotter and damper without ventilation, shortening the life of shingles. This argument has convinced shingle manufacturers who warranty their products only on ventilated roofs. However, a study by [William] Rose found that ventilation had minimum impact on shingle temperature. In ventilated attics, the temperature of the sheathing directly beneath the shingles was roughly 10 degrees Fahrenheit lower than it was for the sheathing above unventilated attics. Sunlight and wind were the primary predictors of shingle temperature. The final argument for ventilation is that it keeps a house cooler in summer. However, the research that [Anton] Ten Wolde and Rose reviewed make it clear that such ventilation does not have much impact on the temperature of the house below..."
Another document: Roof and Attic Ventilation Issues in Hot-Humid Climates:
"Our simulations and measurements have shown that asphalt shingles applied over vented roofs in hot-dry climates operate warmer than the same asphalt shingles applied over unvented roofs in hot-humid climates. To our knowledge, there have been no shingle warranty adjustments for Las Vegas versus Orlando, and that difference in location is more significant in regards to shingle temperature than vented versus unvented."
Great...Thanks for the sources.
Another somewhat comprehensive source relevant to climate zone 1 roof venting issues:
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf