How are vapor barriers, moisture barriers, and house wrap different?
Vapor Barriers
Vapor barriers prevent moisture in the air from moving between locations. This is important when considering the building envelope in a home.
Since humans produce a lot of moisture through daily activity and bodily functions, humidity tends to be higher indoors than out during the heating season. If this moisture migrates out through the walls and ceiling when it's cold outside, it will condense into liquid water or frost where it encounters the dew point temperature. This is bad for some obvious reasons. Rot, mold, and pests can result.
To prevent these problems, vapor barrier is usually installed toward the conditioned space in a home--typically over framing and just behind drywall. This keeps airborne moisture inside the heated space where it's less likely to condense.
Vapor barrier installed in homes is most commonly polyethylene sheeting in 4 mil thickness, though thicker sheet may be used where fire retardance is needed, for example, or where higher durability is important.
Moisture Barriers
This term is often synonymous with vapor barrier but is sometimes used to contrast the permeable traits of house wrap with the water-blocking nature of impermeable sheeting. Common uses for moisture barriers are shower walls, roofs, and backyard ponds. EPDM and PVC are materials used in these cases.
House Wrap
House wrap, such as Tyvek™, is air-permeable membrane used under siding to reduce airflow, thus saving energy otherwise used to heat or cool a home while allowing moisture in the air to escape. It also prevents liquid moisture from passing into the wall due to leaks in siding.
House wrap is not a vapor barrier. In fact, serious problems can arise from using solid material like polyethylene sheeting as house wrap when a similar vapor barrier is installed to the interior. Moisture can become trapped between those barriers, leading to mold, decay, and other damage.