John asks: Is it better to use Icynene (open-cell) or closed-cell spray foam in an unvented cathedral ceiling? We are building a new home in Atlanta, GA.
Steve Bliss, of BuildingAdvisor.com, responds: I don’t really like unvented cathedral ceilings, also called “hot roofs”, because they are very unforgiving. If the roof sheathing or rafters ever gets wet from a roof leak, or from indoor moisture, the wood will have a difficult time drying out and will decay in warm weather.
However, if you are determined to build an unvented roof, then I think that at closed-cell foam insulation is a better choice. Icynene and similar open-cell foams have an R-Value of about 3.6/in. and are highly permeable to water vapor. Closed-cell polyurethane foam has an R-value of about 7/in., and is a very effective vapor retarder.
With an unvented cathedral ceiling, it is critical to keep moist air and water vapor away from the roof sheathing where it can build up and lead to wood decay. Moisture problems in unvented or poorly vented cathedral ceilings are not uncommon, especially in cold climates where water vapor can enter the joist space and condense on the underside of the sheathing in cold weather. Atlanta has enough cold winter weather to be concerned.
While rotting cathedral ceilings have primarily been a problem in cold climates, similar problems have also been reported in unvented cathedral ceilings in areas with warm, moist climates.
While it is possible to use a permeable insulation – open-cell foam, fiberglass, or cellulose – in an unvented cathedral ceiling, it is risky. This approach relies on a near-perfect air and vapor barrier on the house side of the ceiling cavity to keep moist household air and water vapor out of the joist spaces. In a very simple ceiling without complex framing, skylights, recessed lights, or other penetrations, this may be achievable by an experienced energy-efficient builder with the help of a blower door. But, more often than not, there will be air leaks. I would not trust this detail on my own house.
Unvented cathdral ceilings. Where an unvented cathedral ceiling is required, the preferred (and most expensive) approach is closed-cell spray foam. When properly applied by an experienced installer, it will create the type of air and vapor barrier needed to protect the sheathing from moisture. This is a good approach for ceilings where good ventilation is difficult to achieve due to complicated framing, multiple skylights, or other issues. It’s also useful in renovations where achieving a perfect air barrier is especially challenging. To save money, it’s possible to spray a couple of inches of the expensive closed-cell foam against the roof sheathing and fill the rest of the cavity with a less expensive open-cell foam or fibrous (fiberglass or cellulose) insulation.
Since unvented roofs are designed to dry to the interior, you should NOT use a vapor barrier on the underside of the cathedral ceiling. Do not use polyurethane sheeting or vapor barrier paint. If a vapor retarder is required by code, one of the new “smart” (and expensive) vapor barriers such as MemBrain or Intello.
One word of a caution: With spray foam, always use a well-trained, professional applicator as these materials take a lot of expertise to install properly. An unskilled installer can make a mess and may leave a lot of voids – increasing the likelihood of future moisture problems
Vented cathedral ceilings. A less expensive alternative is the more traditional vented cathedral ceiling insulated with fiberglass, cellulose, or open-cell foam. This is best for simple cathedral ceilings that are easy to ventilate. These have straightforward framing with no skylights and with direct access to soffit and ridge vents. You will need an airtight air barrier below the ceiling insulation along with an effective vapor retarder, which could be poly, vapor barrier paint, or a smart membrane. Minimize penetrations to the ceiling and especially avoid recessed ceiling lights, which have been the cause of numerous moisture problems in cathedral ceilings — with or without vent channels.
Above the insulation, leave a 1-1/2 in. vent channel separated from the insulation by a foam, cardboard, or plywood baffle, and provide balanced venting at eaves and ridge. The vent space below the sheathing will help promote drying – whether from roof leak above or from moist air or water vapor entering from below. But do not rely on the vent channel to prevent moisture problems.
Research that has shown the roof ventilation is not very effective at preventing moisture problems in a house with a leaky air barrier and high indoor humidity. The key is an airtight air barrier below the insulation that keeps moist air out of the ceiling cavities. All penetrations need sealing with spray foam or another long-lasting sealant. A blower-door test to confirm that the ceiling is free of air leaks is always a good idea with this type of roof construction.
Effective air and vapor sealing is your primary protection. Roof ventilation is a backup that will promote drying and help prevent wood decay if a roof leak allows water to enter the roof structure.
Read more on hot roofs at Inspectapedia.com