Understanding what house wrap is designed to do and not do will help understand the control layers in a wall. House wrap is a water resistive (not waterproof) barrier (WRB) and is used to prevent liquid water from entering the wall cavity from the outside elements/atmosphere. This means, WRB is permeable to water vapor and acts as a water plane. Rain and moist air enter a wall assembly from outside as well as indoor moist air can enter the wall assembly from inside the building. House wrap allows for the transfer of liquid water to the outside by allowing it to drain down and out of the wall running down the wrap as a water plane behind the siding which prevents the building materials from getting wet. House wrap, WRB is not an air barrier except a few special types of WRB manufactured specifically for that purpose.
Since WRB is installed over sheathing and behind the siding or cladding and since it is not an air barrier, house wrap or WRB alone will not reduce air flow, which translates into increased heating/cooling costs. There are too many areas where air can move across the wrap due to unsealed seams, top/bottom edges not properly or completely sealed, rough edges where it is cut leaving openings that must be sealed, numerous tears occur during installation and, siding installers usually have to make cuts or slits in the wrap on inside corners and these are difficult to seal.
House wrap does however, help reduce air leaking or filtering into the house through natural pressure changes. When the air pressure outside is greater than inside, air wants to move to the lower pressure thus pulling the wrap into seams, gaps, penetrations and helps prevent the flow of air into the home. When WRB or house wrap is installed with expertise, it helps reduce air infiltration but does not block air and it has zero insulation properties. It must be permeable to prevent liquid water from entering the wall.
An air barrier is not house wrap or WRB. A home is truly a structural framework with control layers. These control air infiltration which, when not controlled can reduce R-values up to 60% which is a bit frightening and disheartening.
Can I use Radiant Barrier as House Wrap?
Radiant Barrier is an air barrier and reflective barrier and it is perforated either on one side or both to help with the vapor flow. The placement of air and vapor barriers is CLIMATE dependent because, one size does not fit all our climate zones. Radiant Barrier is an air barrier and secondary drainage plane, so it acts a little like house wrap and with perforated RB, it is vapor permeable. In essence, RB is a secondary house wrap with the added job of reflecting up to 97% of radiant heat coming into the wall which, helps the insulation perform or reach its true R-value. Humidity escapes the wall assembly while liquid water drains down the house wrap and the Radiant Barrier. House wrap and Radiant Barrier together provide a drainage plane an air barrier and permeable vapor barrier protecting sensitive material from direct rain exposure while allowing the wall and siding to dry.
For standard wall assemblies, house wrap must be vapor permeable and the higher the Ng (permeability rating) or Perm Rating, the better at moisture evaporation.
Radiant Barrier should be installed as an air barrier and secondary drainage plane and will be installed directly behind cladding WITH appropriate air space for the Radiant Barrier to function (using furring strips). The foil should be closest to the heat source on a one-sided foil barrier. The wall must assemble as follows: siding, air gap, Radiant Barrier, House Wrap, studs, Interior wall cavity then insulation etc. Radiant Barrier as house wrap should be perforated, pin-sized holes so it will be permeable to water vapor while small enough to act as an air barrier and act as a draining plane for liquid water.