There is an ongoing debate as to the use of a closed cell foam or an open cell foam to seal the rim joist area of a basement or a crawl space. It is Emecole’s position that an open cell polyurethane spray foam works well in this application.
The major difference in the properties of a closed cell vs. an open cell spray foam is that a closed cell foam is an air barrier as well as a vapor barrier, while an open cell spray foam is an air barrier but is breathable (it is not a vapor barrier). As a consequence, a rim joist area which is being exposed to moisture coming from the concrete foundation can have the moisture escape through the open cell foam while it would be trapped by a closed cell spray foam which can lead to rotting and mold formation. Further below, you will find articles which lead us to the conclusion that an open cell spray foam is the preferred product to seal rim joist areas. Your comments are welcome.
Due to its superior insulating values, many contractors naturally select a closed cell foam. However, in a basement or crawl space you have to deal with the phenomena of moisture being sucked up the walls via the porous concrete foundation by capillary action and onto the rim joist areas.
Most basement water leakage results from either bulk moisture leaks or capillary action. Bulk moisture is the flow of water through holes, cracks, and other discontinuities into the home’s basement walls. Capillary action occurs when water wicks into the cracks and pores of porous building materials, such as masonry blocks, concrete, or wood. These tiny cracks and pores can absorb water in any direction—even upward.
This moisture entry into the basement through the foundation travels to the wood joists which, if sealed with the closed cell foam acting as a vapor barrier, does not let the moisture or water vapor to escape. If instead you use an open cell foam as your sealer, it is breathable or capable of allowing the vapor to pass through and not giving the wood a chance to feed off of the moisture and rot. Not only is the open cell foam able to allow water vapor to escape through it, it does this quite efficiently and without becoming brittle with time and possibly breaking away from the wall as often is found with rigid closed cell foams. As stated, a closed cell foam is both a vapor barrier and barrier to air flow. An open cell foam acts as a barrier to air but allows water vapor to travel through it and away from wood sitting on the basement foundation.
In addition to open cell foam the contractor should consider using Emecole’s concrete sealer, Pene-Seal-Crete, on the walls of a basement to reduce and even eliminate the porosity of the walls so that little if any water vapor passes through the concrete to the rim joint area.
- Why the Use of Open-Cell Foam in Sealing Rim Joists Saves Contractors from Making Costly Error
- Inside Story: Open-Cell Foam & What Every Contractor Should Know