Barriers
In the context of construction, barriers are most often strips of material used to protect the home from wear by the elements. They are especially used to ward off sunlight and water, protection which is achieved by means of radiant barriers and weather-resistive barriers respectively.
Radiant Barriers
Radiant barriers are reflective sheets placed on the floor of the attic which protects the vulnerable top portions of the home from overheating as a result of exposure to sunlight. Where standard insulation tends to absorb incoming solar energy, radiant barriers bounce it back, reducing by up to 97% the radiant heat received from the sun and thus cooling the home considerably.
Radiant barriers are usually coated with aluminum or a similar reflective material, usually on one side -- the upside, but occasionally on both. Because radiant barriers can also cause desirable heat to dissipate in the winter, they are sometimes used in conjunction with normal insulation. This provides a thermal balance in the home that neither allows heat to leak in nor seep out, but instead keeps it at a comfortable median.
Weather-Resistive Barriers
Weather-resistive barriers resemble radiant barriers and serve to protect the home from moisture. Such barriers consist of impermeable materials made from tar or felt, called building papers, or various synthetic films, called housewraps. These are placed beneath the siding and prevent the seepage of water into the housing material. In addition to protecting against the direct threat of rain, weather-resistive barriers are highly resistant to moisture, air flow, and ultraviolet light. They are strongly built and seldom tear or break. They can occasionally produce a negative reaction when used with wood siding that reduces their effectiveness, but this can be forestalled by use of a wood preservative on the home's exterior. In general, building papers tend to be more effective at holding back water, but housewraps tend to be more durable and longer-lasting.
Advantages of Barriers
Barriers can dramatically improve the durability and comfort of a home. A radiant barrier can provide a homeowner with significant energy savings (as much as 10%, according to some studies) in the summer when the reduced heat in the house allows air conditioner use to be kept to a minimum, and so is both cost-efficient and environmentally sound. A weather-resistive barrier helps reduce dampness in the home and prevents damage to the building's frame which may require costly renovations. Weather-resistive barriers are frequently used by businesses and offices, which are often required to provide such insulation by law.