How to Install Interior Moisture Barriers

Understand how moisture barriers are intended to function. Builders often use the analogy of a kitchen window pane to show how barriers work. Cooking a big meal on a winter night fills a warm room with humid air. When warm humid air hits the windowpane, which is chilled by the cold outside air, water condensation forms on the warm, inner side of the glass. While this moisture is easily wiped up or dissipated into additional warm air, the same is not true when condensation from hot/cold interaction forms on wooden, plaster or other water-permeable construction materials.

Follow general climate rules to position your moisture barrier. In northern states where houses are frequently heated, barriers need to protect as many building materials from warm humid air as possible. Putting insulation between attic rafters, then covering it all with a moisture barrier, keeps household warmth and moistness from soaking into insulation and lumber. When you go into the attic, you see the barrier, with the insulation under it. In a southern state with a warm, humid climate, the reverse applies. More moisture comes from the outside than the inside. You'd install attic insulation, therefore, after installing the moisture barrier.

Check your local building code before proceeding with a job involving moisture barriers. In many states, climate conditions are not as cut and dried as they are in the northern and southern extremes, and you cannot apply a simple indoor/outdoor rule. Further, different rules may apply for barriers applied behind bathroom tile, under laminate floors, over concrete, under siding, over or under insulation batting and in crawl spaces. At the same time, your local code may require venting as part of the project. Find out or make certain your contractor knows the local regulations.

Measure your space and allow more material than needed just to cover the dimensions. Moisture barriers cannot have gaps, holes or loose edges if they are going to do their job. Plan overlaps and generous margins for fastening barriers to rafters or studs. Overlap edges of barrier fabric and seal them with duct tape. Use tape to secure barrier fabric around electrical boxes and other sources of drafts or air escape.

Simplify good seals by wrapping barrier fabric around and stapling it to wooden furring strips. You now have a flat edge that you can secure with tacks to a stringer or stud, making a tighter seal than you can obtain by just stapling fabric to the stud.

Choose the lesser of two evils when doing small jobs that your local building code may not cover. The laundry room over a crawl space may be a challenge. Placing a barrier on the bottom of floor beams to keep ground moisture from seeping up makes good sense. With a washer, dryer and big utility sink, however, the room generates lots of humidity and dampness that needs an exit. Traditional moisture barrier installation would leave floor beams trapped between a frequently wet floor and a vapor barrier. Consider putting the barrier fabric on the ground under the crawl space and/or installing a vent-fan. One strategy may not solve it all in a room like a utility room.