Should we use open cell or closed cell SPF insulation on the underside of our attic roof in southeast Louisiana?
Open cell has been recommmended by our contractor to use in the unvented attic of the home we are building so that any leaks can eventually be identified/located, but this could allow moisture into the attic space. Most manufacturers of foam products recommend using closed cell because it will prevent the water vapor from coming through the roof. I know we can use enough open cell foam to get an equivalent R factor to the closed cell, but should we have any concerns about the moisture coming through? I want to be energy efficient, but I also do not want to worry about damage from water.
This is a question that many people in the building industry wish could be answered quickly and succinctly without discussion.
The first and foremost factor in choosing the correct solution is climate. Since you mention that you are building in southeast Louisiana, that would place you into a hot-humid climate zone. There are four ways moisture can enter a structure. Typically, the lowest on the priority list is vapor diffusion. However, given the fact that you live in a humid climate, your concern is valid.
Closed cell foam will typically provide you with a vapor barrier that will prevent water vapor from moving into your attic enclosure. The downside is that if for some reason water vapor gets into the assembly, it cannot leave unless it is mechanically removed. Open cell foam will allow the movement of water vapor into and out of the enclosure.
To your question: Which one should I choose? The body of knowledge on the topic suggests a vapor retarder should be installed at the walls and roof plane and mechanical ventilation should focus on drying the interior. For a deeper understanding of this recommendation, I suggest reading this paper from Building Science Corporation.
The product that you choose has to meet the requirements of a vapor retarder. The 2000 IRC Building Code defines a vapor retarder as having a perm rating of 1 or less. A product becomes impermeable at a perm rating of 0.1 or less and the term used in this case is a vapor barrier. There is much confusion on the interchangeability of these two terms. In addition, you should review your local building code to ensure they have adopted the 2000 IRC definition.
At the end of the day, the cost to install insulation to meet R-value and perm rating should dictate your decision. Open cell or closed cell foam will work as long as it meets the R-value criteria and suggested perm rating of 1 or less but greater than 0.1.
Now you are empowered to review manufacturer specifications for each product -- rather than suggestions made to a large audience without regard for climate -- and make the right decision.
Two final suggestions as you move toward the construction phase of your new home:
Do not oversize your cooling equipment. Ask your HVAC contractor to provide Manual J calculations to ensure you do not install a system that runs too infrequently during the cooling season. An oversized system that runs infrequently will not properly control interior moisture during the cooling season in a hot-humid climate.
Consider purchasing a Builders' Guide specific to your climate. These reference manuals are invaluable in getting best-practice suggestions and the building science understanding for each decision you make on your project. I prefer the guides from Building Science Corporation, and they have one for a hot-humid climate.
For more information:
You should read architect Chris Benedict's Q&A "Which foam insulation is best for the south coast Atlantic region: closed cell or open cell?"
Also, the U.S. EPA has recently published comprehensive safety information for installers and building occupants.