Best Approach to Hot-Climate Cathedral Ceiling

Q: I have an unfinished space above our garage that we are converting into a flex space.  It has a cathedral ceiling.  We are in Houston, TX  so I am concerned about the heat and humidity in the space.  The west side of the space is sloping roof with soffit and gable vents along the entire length, so creating a baffle out of rigid foam is simple.  We have knee walls with storage space behind to create a couple of walls along the space. The east side is attached to the remainder of the house.  It has ridge vents, but no access to the soffit, so there is no air flow there (see photo).  Can I install insulation directly against the roof even though there is a ridge vent there?

I am planning on using Rockwool or a similar product. We’ve been told we can remove the ridge vent and place the insulation against the roof sheathing in the entire attic, but the west side gets painfully hot and it seems like we should keep the air gap.

I don’t want to risk any chance for mold to grow and I also don’t want to age my roof any quicker.  It’s only three years old.  Any advice would be grateful appreciated! — CF

A: With a complex cathedral ceiling like yours – part vented, part unvented, knee walls, and one or more dormers, spray foam is usually the least complicated approach, but usually the most expensive as well.

In principle, you could treat some sections of the roof as vented and others as unvented, but getting this right can be very tricky. Whatever method you choose, you will want the following:

• A good air seal between the interior space and the insulated roof cavities.
• Drying potential to the interior or the exterior, preferably both. That usually means no plastic, foil, or other vapor barrier on the interior side of the insulation unless you have roof underlayment and roofing material that is permeable to moisture.
• Keep all ductwork within the conditioned space if possible
• Insulate the roof from eaves to ridge, not the knee walls. This keeps everything inside the thermal shell and simplifies air sealing.

The least complicated, but usually the most expensive, approach is sprayed closed-cell foam on the underside of the roof sheathing. This creates as air-sealed, unvented “hot roof ” that would meet the energy code minimum or R-30 or R-38 typical in your region. Read more about Hot Roofs.

To save on foam, and dollars, you could spray about an inch of closed-cell foam against the underside of sheathing as an air seal and thermal barrier, and then fill the rest of the cavity with fiberglass or rockwool (I would avoid cellulose in your climate). With 2×6 rafters, that would get you up to about R-22. For additional insulation, you could frame or fur down from the rafters or add a layer of permeable foam, such as EPS (expanded polystyrene) or XPS (extruded polystyrene). EPS is two to three more times as permeable as XPS. You can read about combining foam and fiber insulation here.

If you want to minimize the use of spray foam, you could go with a vented cathedral ceiling in the areas with soffit and ridge vents, and treat the other areas as unvented roofs, as described above.

One last option is an unvented roof with fiberglass or rockwool – using “vapor diffusion ports” to allow water vapor to vent at the ridge. This a fairly new approach, but has been researched and tested for use in Climate Zones 1-3. It is allowed in these zones under the latest version of the IRC.

If you use this approach, you need to seal the soffit vents and replace the ridge vents with diffusion ports.

The simplest way to create a vapor-diffusion port is to cover the opening at the ridge with a durable air-tight, but vapor-permeable layer and then install a standard roof vent on top. This is done at the ridge as well as and hips or dormer ridges. The vapor permeable material can be flexible, such as vapor-permeable roof underlayment (many are not permeable) or solid, such as Georgia Pacific DensGlass or gypsum sheathing. The material should be sealed to the underlayment or roof sheathing with a high-temperature flashing tape or high-temperature sealant for an airtight and watertight seal.

Since this is a pretty new approach, you may have trouble finding a contractor (or building inspector) with the necessary experience or confidence to use and approve this approach. Also, I have not used this approach myself (I live in a cold climate), but the research and field reports look very promising. Read more on Vapor Diffusion Ports.

Finally, you ask if an unvented roof will shorten the life of your shingles — I am assuming you are using asphalt shingles. Some manufacturers have reduce warranty provisions for unvented roofs. And research has shown a small  effect on the lifespan of asphalt shingles. However other factors such as color and orientation, and quality of the shingles, have much greater effects and the energy savings from a well insulated roof more than compensate for any harm to the roof. The reality is that roof warranties are usually  not worth much. They are prorated, usually materials only, and often not honored because of other warranty details. So buy light-colored, high-quality shingles and you should be fine.

— Steve Bliss, Editor, BuildingAdvisor.com