Why Is There Roofing Paper on Top of My Insulation Under the House?

Vapor Barriers

• During cold weather, the moisture in the air can condense on the insulation and structure of unheated basements and crawlspaces. If moisture is allowed to condense on the insulation or wooden structure of your foundation over a period of weeks, it will promote rot and the growth of mold. Vapor barriers separate your insulation and the air in the basement, causing moisture to condense on the barrier instead of the insulation. In heated basements, the insulation is installed on the walls instead of the ceiling. In this case, the vapor barrier is installed on the interior of the basement, since warm air from the interior of your home often contains significant amounts of moisture.

Replacement

• If the the vapor barrier is damaged, this can allow moisture to condense on your insulation. If there are tears in the roofing paper over your insulation, you should consider replacing or patching the vapor barrier. You should also inspect the edges of the roofing paper to make sure that the paper is securely attached to the floor and the ceiling.

Installation

• To install a vapor barrier, measure the existing barrier and cut new pieces of roofing paper to fit over the insulation. Make sure that you allow for extra material so that the pieces of roofing paper are large enough to completely cover your insulation while allowing the pieces to overlap. If the roofing paper is intact but there is moldy or damp insulation behind it, you should locate and eliminate the source of the moisture before replacing the roofing paper. Coating your foundation walls with a concrete sealer will often prevent moisture from penetrating the walls.

Alternatives

• If the roofing paper used as a moisture barrier in your basement is damaged, there are several ways you can replace it. The most common form of moisture barrier is a sheet of plastic stretched over the insulation and held in place with staples. This type of vapor barrier is less durable than roofing paper, but it is typically cheaper and easier to work with. If you need to replace the vapor barrier and the insulation behind it in a finished basement, you can install foam board insulation with a vapor retardant backing over the new insulation. This will prevent moisture from the heated interior of your basement from condensing on the cold walls of the foundation.