Which foam insulation is best for the south coast Atlantic region: closed cell or open cell?
When we talked to various installers of foam spray-on insulation they referred to "closed cell" and "open cell." Because of the pros and cons with regard to moisture levels as well as the difference in cost, I need advice on which would be preferred for the south coast Atlantic region.
While there are distinct differences in the properties of closed cell and open cell foams, both can be used in a southern coastal climate.
Here’s how to determine which foam to use:
R-value (a rating of a material’s resistance to heat flow): Closed cell foam has a higher R-value per inch and may work better in cases where there is less room to insulate.
Weight: Closed cell foam weighs more than open cell and may cause a problem in some applications.
Cost: Closed cell foam appears to cost more than open cell.
Vapor permeability: Closed cell foam will form a vapor retarder that inhibits drying of the wall on the side of the wall to which it is applied. Open cell foam is vapor permeable and will not inhibit drying of the wall. Understanding and planning for how a specific wall assembly will dry when it gets wet will help inform this choice.
Liquid water management: Closed cell foams resist liquid water. Open cell foams may absorb minimal amounts of water.
Air barrier: Both foams can be used as air barriers if installed properly in a planned air barrier system.
The most important property of both types of foam is their ability stop the movement of air if installed properly in a planned air barrier system. Water vapor carried by leaks in building enclosures to places where the water vapor can condense poses a larger threat to the health and durability of a building than water vapor moved by diffusion through vapor-permeable building materials.
In the South, one is engaged in a battle to keep warm, humid air from condensing on cool air-conditioned surfaces, especially if the cool surface is inside a sheetrock wall where liquid water will cause mold to grow. Closed cell foam can work well in this battle if it is installed on the outside of the house and also serves as an air barrier. When the installation is done correctly, warm, humid air never hits the cool interior surfaces.
Regarding vapor: An additional vapor barrier in the wall can cause more problems than it solves in the South. It can be a condensing surface if it is on the cool side of the insulation and it can prevent a wall from drying.
For more information:
You should read Danny Kelly's Q&A "I'm building a brick home in a humid climate. I'm confused as to which foam insulation to use for walls/attic: closed or open cell?" Also, Richard Goyette answered the question "Should we use open cell or closed cell SPF insulation on the underside of our attic roof in southeast Louisiana?"
The U.S. EPA has recently published comprehensive safety information for installers and building occupants.