Q: We are contemplating building a new home and have talked with a local builder. I mentioned to him that I have noticed their company's use of OSB instead of plywood and the fact that they were not using an exterior building wrap on their homes. He said that the industry now prefers OSB over plywood and that there wasn't a need for building wrap. I am concerned about mold and allergies penetrating the house. Are my concerns about these two issues warranted? If so, what is the preferred methods and materials for stick-built homes or what precautions should I be aware of?

A: OSB or oriented stand board has a checkered history. When originally introduced, the product had an adhesive problem that caused it to fail when exposed to moisture. Over a half billion dollars have been paid out as a result of one class action suite for a product used as exterior sheathing. Since then, the manufacturers have reportedly addressed the problem and the OSB is stated by them to do well with moisture. I have seen it used in new construction without a problem but remain somewhat skeptical. Perhaps in a few more years my comfort level will be better. When I was growing up, houses used solid 1" x 8" boards for sheathing and plywood was being introduced. My father was skeptical at that time. The pressure to keep costs low tends to keep building manufacturers moving away from the original trees. The further you get away from the tree, the more you lose the natural mold resistant properties of wood (terpene based chemicals).

Building wraps are water repelling barriers and should not be vapor membranes. The wrap should allow vapors to pass (including water vapor) but not liquid water. The most important factor is that the wall should "breath" and that liquid water should be kept outside the exterior sheathing. The worst scenario is when vapor barriers are on the exterior and interior sides of a wall thus trapping moisture inside the wall (the problem is greatest in warm humid climates where the air conditioning causes condensation of water vapor behind the vinyl wallcovering). Vinyl wallcovering on the perimeter walls of a building is one of the most well known example of a vapor barrier on the inside of a wall (resulting in major litigation in Florida). Some vinyl wallcovering manufacturers have created micro punched products which they say will allow the wall interior to pass water vapor out of the wall cavity.

The use of rain screens on the exterior of the building is a relatively new way to minimize moisture entry into a wall. Rain screens create an air space gap (plastic mesh or strapping) between the siding and the exterior sheathing that allows moisture to drain out safely behind shingles, clapboards and other siding without entering the building. The drainage plane behind bricks and the weep holes at the bottom of the wall is the same concept. Although not used extensively yet in residential construction (in my experience), I feel the added costs are outweighed by the long term advantages. The other important factor is for the contractor to use flashing around windows doors and other building envelope penetrations effectively. Poor seals around a house' windows are a common cause of water entry.

Mark Goldman, CMC
Senior Air Quality Scientist
EFI Global, Inc.

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