Housewrap, Building Paper, or Felt

Just researching online...and found this from the department of energy which for those who need a 101, this is a good start:

" Housewrap, building paper, or impregnated felt should be part of the exterior wall system that protects the building from water penetration. None of the materials are waterproof, but are intended to shed rainwater that penetrates exterior cladding. The surface formed by these materials is called a drainage plane, house membrane, or rain barrier. They are used to shed liquid water that may penetrate siding or roofing and to prevent liquid water from wicking through them, while remaining sufficiently vapor permeable (“breathable”) for outward drying (Straube 2001). By helping to keep building materials dry, these membranes improve building durability, decrease maintenance costs, and reduce the risk of moisture-related problems such as pests, mold, and rot.

Building Paper is a Kraft paper sheet impregnated with asphalt to increase its strength and resistance to water penetration. It is primarily employed as a drainage layer. It is graded according to a test of the amount of time required for a water-sensitive chemical to change color when a boat-shaped sample is floated on water. Common grades include 10, 20, 30, and 60 minutes. The larger the number, the more resistant the paper is to water.

Building Felts have been in use over a hundred years. Originally made from rags, today’s felts are made of recycled paper products and sawdust. The base felt is impregnated with asphalt. Ratings for felt harken back to the traditional weight of the material before the oil crisis of the 1970s. At that time 100 square feet of the material (1 square) weighed about
15 pounds. Modern #15 felt can weigh from 7.5 to 12.5 pounds per square depending on the manufacturer.

Housewrap typically refers to plastic sheet materials. Housewrap comes in a variety of materials and can be perforated or non-perforated. If joints and connections are sealed, housewraps can serve as air retarders to reduce air leakage. Housewraps are highly resistant to tearing, unlike building paper. Non-perforated wraps tend to have higher liquid water resistance because the holes between plastic fibers are very small.

IMPERMEABLE COVERINGS
Never install impermeable coverings inside the house.

BACKPRIMING
Backprime all wood cladding to avoid water saturation.

AIR SPACE
Creating an air space between the exterior finish and the drainage plane effectively increases the durability of both components. Information on housewrap and building paper performance behind brick and stucco can be found at
www.buildingscience.com/resources/walls/brick_stucco_housewraps.pdf

TWO LAYERS OF BUILDING PAPER OR FELT
If building paper or felt is used in areas prone to severe rain, install two layers. The use of two layers was once common and provides better than twice the performance of one layer. By providing a double drainage plane, they offer increased resistance to leakage at fasteners and allow for more flexible installation.

CAREFUL INSTALLATION
Installation is key for all types of housewraps. The sheets must be lapped, shingle-style, especially over and around windows, doors, and other penetrations (and their flashing systems). Use manufacturer-specified fasteners and space them closely
enough to provide required support.