To Vent or Not to Vent?
GBA Editor
| Posted in Energy Efficiency and Durability on
I will be building a new home in Vermont this summer and I was planning on installing a vented cathedral ceiling and insulating it with dense pack cellulose. I recently spoke with an insulation sub-contractor and he explained to me that if the R-value is above 60, then there is no need to vent the roof. In fact, his company guarantees my roof performance. I was under the impression that venting was the only way to go. Which method is better?
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Replies
Graham,
There is no single answer to the question, "which is better: a vented or an unvented roof"?
In general, venting is cheap insurance that may, under some circumstances, lessen the chance of ice damming. However, the configuration of your roof plays a big role in your decision.
While simple gable roofs are generally easy to vent, many other roof types are difficult or impossible to vent well: hipped roofs, roofs with many valleys and dormers, or the "chopped up" roofs that are becoming increasingly popular with some architects. In general, such roofs are best designed as unvented roofs.
If you're using cellulose and decide to go the no-vent route, remember that your roof sheathing will be at an increased risk of moisture accumulation. That's why building scientists recommend that such roofs need enough rigid foam insulation on top of the roof sheathing to keep the roof sheathing above the dew point during the winter. Once you've done that, the use of cellulose is low-risk.
Graham,
You'd have to install at least 16" of cellulose to achieve R-60. Do you have that much space in the cathedral framing? If you do, you may be better off using 1½" of that space for venting, since roof venting eliminates ice dams, reduces summer radiant heat gain, extends the life of composite roofing materials, allows roof sheathing to dry if the roof ever leaks, and reduces moisture accumulation in the insulation. You would, however, need continuous vent channels from eaves to ridge, continuous soffit vents and a ridge vent with exterior wind baffles (such as AirVent Shingle Vent II or Lomanco OR-4).
IRC requires that conditioned unvented attics mut be insulated with an air-impermeable insulation. If you use cellulose you may need to vent the roof or check with your code official to see if installing an air barrier in conjunction with the cellulose will qualify and as air-impermeable insulation.
The 2007 IRC allows either air impermeable or air permeable insulation in an unvented cathedral ceiling or cathedralized attic, as long as the condensing surface (interior of foam or roof sheathing) remains above 45° for average monthly winter temperatures.
Exterior foam board that meets the R-value requirements of Table R806.4 will allow permeable insulation and meet this code standard. http://resourcecenter.pnl.gov/cocoon/morf/ResourceCenter/article/1520
However, the IRC recognizes and promotes the value of venting attic and ceiling assemblies, but allows this variance for difficult to vent rooflines or attics with HVAC.