Housewrap defines all synthetic replacement materials for sheathing tar paper

Tar paper

Tar paper is a heavy-duty paper used in construction. Tar paper is made by impregnating paper with tar, producing a waterproof material useful for roof construction. It can be distinguished from Roofing felt:Asphalt-saturated felt. Roofing felt has been in use for over a hundred years...

. These materials are all lighter in weight and usually wider than asphalt designs, so contractors can apply the material much faster to a house shell. Housewrap functions as a weather-resistant barrier, preventing rain from getting into the wall assembly while allowing water vapor

Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 to pass to the exterior. If moisture from either direction is allowed to build up within stud or cavity walls, mold and rot can set in and fiberglass or cellulose insulation

Cellulose insulation

The word cellulose comes from the French word for a living cellule and glucose, which is sugar. Building insulation is low-thermal-conductivity material used to separate the internal climate and sounds of a building from external climate and sounds...

 will lose its R-value

R-value (insulation)

The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it or R = \Delta T/\dot Q_A.The R-value being discussed is the unit thermal resistance...

 due to heat-conducting moisture.

House wrap may also serve as an air barrier

Air barrier

Air barriers control air leakage into and out of the building envelope. Air barrier products may take several forms:*Mechanically-attached membranes, also known as housewraps, usually a polyethylene-fiber or spun-bonded polyolefin , such as Tyvek is a generally accepted moisture barrier and an air...

 if it is sealed carefully at seams.

Major types of housewrap

• asphalt-impregnated paper or fiberglass (the original material, still widely used, also known as Asphalt Saturated Felt)
• micro-perforated, cross-lapped films
• films laminated to spunbond nonwovens
• films laminated or coated to polypropylene
  Polypropylene
  Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
  
   wovens
• supercalendered, wetlaid polyethylene
  Polyethylene
  Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
  
   fibril nonwoven ("Tyvek
  Tyvek
  Tyvek is a brand of flashspun high-density polyethylene fibers, a synthetic material; the name is a registered trademark of DuPont. The material is very strong; it is difficult to tear but can easily be cut with scissors or a knife...
  
  ")

Installation

Housewrap must be both waterproof and have a high Moisture vapor transmission rate

Moisture Vapor Transmission Rate

Moisture vapor transmission rate , also water vapor transmission rate , is a measure of the passage of water vapor through a substance....

 (MVTR) to be effective. It must also take handling abuse during installation and be resistant to UV. Housewrap is often left exposed for some time after construction, awaiting exterior siding installation. The original asphalt

Asphalt

Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

 paper design, while heavy and slow to install, is still a contender. It can be nailed and abused during installation and still function. Some new designs must be installed carefully or they will slightly rip or tear during installation, possibly allowing for water infiltration at the damaged areas. Most newer designs do not "seal" well against nails or staples like asphalt products.

Housewrap is installed over the sheathing and behind the exterior siding. Siding can be vinyl, wood clapboard, cedar shingles or brick facade. In all cases, the housewrap is the last line of defense in stopping incoming water or exterior water condensation from getting into the wooden stud wall.

Properties

• typical MVTR ~200 grams/100 square-inches/24hours (or greater, i.e., Tyvek
  Tyvek
  Tyvek is a brand of flashspun high-density polyethylene fibers, a synthetic material; the name is a registered trademark of DuPont. The material is very strong; it is difficult to tear but can easily be cut with scissors or a knife...
  
  is ~400))
• typical 2 ounces/square-yard (varies greatly with manufacturer)
• typical width 90" on a 3" core