Housing...Inside and Out

June 2, 2000
How To Avoid Moisture Problems Inside Walls

Condensation problems in wall cavities develop when moisture from the warm, moist inside air moves into the wall cavity in the form of vapor. It condenses on the insulation near the outside surface or on any other cold surface.

Moisture in wall cavities is avoided in nearly all newly constructed homes by installing a vapor barrier on the warm side (the inside) of the wall insulation. This vapor seal is built into most batte type insulation.

A newer type of batte insulation has no vapor seal or any covering at all. It is cut in blocks and fitted to completely fill the cavity between the studding. The vapor barrier is provided by stapling a clear polyethylene sheet on the inside of the studding after the insulation has been installed. Install electrical wiring, plumbing and heat ducts before the vapor barrier so wallboard or paneling can be applied immediately to protect the vapor barrier from damage.

When remodeling an older house, often insulation is blown into the wall cavities. Applying a vapor barrier on the warm side of the insulation may be difficult in these cases. If the inside of the wall finish is smooth plaster or wallboard, a good vapor barrier can be provided by two coats of aluminum paint covered with two coats of decorative paint.

However, if the plaster is rough and covered with paper, applying paint as a vapor barrier is not practical or effective. In some cases, an effective vapor barrier can be installed by placing a polyethylene sheet over the papered wall and applying wallboard or paneling on the inside walls of the home.

A more difficult and expensive but effective method of applying a vapor barrier is to remove the plaster and lathe, applying insulation with a vapor barrier and then finish the outside walls with wallboard or paneling.

Usually the siding of older homes is not tightly constructed and, therefore, is in a condition that the wall "breathes." This helps avoid condensation trouble in the stud space because it assists in evaporating and carrying away any condensation that occurs on the insulation.


Written by Dr. Francis Graham (Retired)

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