House wraps are lightweight sheets that act as air barriers. House wraps stop the air movement through walls, and are mostly applied over the outer walls beneath the siding. Materials that are used for house wrap allow the water vapor to escape to the outside but at the same time keep out wind. (Wagner J. D. , etal, 2005, pg 168) House wraps are typically spun-binded polyolefin or polypropylene fabrics. They act as wind breakers. Unlike polythene film, house wraps are typically installed on the exterior of the building, on the outside of the sheathing.
House wraps come in rolls, so that the entire height of the floor level can be covered without seams. Some insulating sheathings include in-house wrap bonded to the surface of the insulation. The insulating sheathings are put up and the joints are taped, to create an air barrier. House wraps could be placed on the inside of the wall near the polyethylene films, but it’s easier to install them on the outside because there are typically fewer wall penetrations. This reduces the worries concerning the wall to wall, wall to floor, or the wall to ceiling intersections.
There are also fewer electrical and plumbing penetrations in the exterior wall assembly. All the aforementioned show that it is easier to install the house wraps continuously on the outside rather than the inside of the building. An exteriorly installed house wrap also reduces wind washing. The wind blowing through a wall can reduce the insulating value, for example, of fiberglass. A building paper is less effective an air barrier because it is installed in three-foot-wide sheets with loose overlaps. This makes it easier for the water to move easily through the building paper.
House wraps stop the air leakage better than the building papers. (Air barriers, 2008) On the other hand, since a building paper is vapor permeable, it’s able to stop the water leakage better than the house wrap. The house wrap is water repellant and replaces the function of building paper as well. Experts say that it does a better job because it comes in larger rolls and there are fewer seams. It’s important to install the house wrap carefully because it will not serve its purpose when wrongly installed.
In many part of North America, house wraps have effectively replaced the building papers, as their long term performance has been preferred over that of the building paper. Like most plastics, house wraps can not endure long term exposure to the sun, because it causes them deterioration. This is why siding should be installed soon after the house wrap has been installed. (Carson Dunlop etal, 2003, pg 55-56) Purpose of house wraps The house wraps act as dual purpose weather barriers, by minimizing the flow of air that moves in and out of houses, while at the same time stopping liquid water and acting as a drainage plane.
House wraps are not the same as water reterders. They have a unique characteristic of allowing water vapor to pass through it as it blocks the water penetration. This means that it will be difficult for the rain to penetrate in to the house, and that it will permit the humid air in the houses from escaping to the outside. It is important to use house wrap because almost all the finishes at the exterior of the buildings allow water to penetrate through. A continual soaking of the wall sheathing on the frames results to growth of mould on the interior of the house, or a dry rot.
The house wraps should also be used because they stop the water that passes through the sidings, allowing it to drain through the structural members. In places where the air is always humid, house wraps are recommended because they prevent the humid from damaging the framing. At places where the weather condition is dry most of the time, the walls are not destroyed since there is little or no moisture flowing in to the house from the exterior. However, it is important to use it too because it serves as an insulator. Properties of weather resistive barriers
House wraps are manufactured in a variety of materials, and they can either be perforated or not. The perforations are important in that they fasten the outflow of the moisture trapped inside the building, while they reduce the liquid transfer through the walls. On the other hand, the building papers can either be made with or without perforations, and are not as effective as the house wraps. a) Resistance to water penetration House wraps are designed to resist or to eliminate the absorption and the penetration of water, while the building papers resist penetration only temporarily.
The moisture that passes through the wood can cause extraction of chemicals and in time, this can cause the water to pass through the house wrap. Such an occurrence can be prevented by coating all the sides of the wood siding with clear water repellant wood preservative, priming and finish painting with two coats. (Weather resistive barriers, 2000) b) Vapor permeability Vapor permeability is the amount of water vapor that can pass through a surface; the higher the permeability, the greater the vapor flow.
All weather resistive barriers must be rated at five perms or higher. A house located at a highly humid region requires a house wrap that will prevent the moisture from flowing inside effectively, hence the reason the higher perm house wraps are preferred as they speed the escape of the trapped moisture. (Weather resistive barriers, 2000) c) Durability House wraps vary in their resistance to radiation, ultraviolet (UV) radiation, moisture tolerance and resistance to ripping. The tear resistance should be good in order to resist ripping when being installed.
Unlike the building papers, house wraps are highly resistant to tearing. During the construction and the installation process, the UV rays attack the building materials. It’s important to check the UV resistance of the wrap paper before purchasing, as the resistance varies widely from one manufacturer to another. It is also important to check the moisture tolerance of the house wrap. Many house wraps tolerate repeated wetness because plastics do not absorb moisture. (Weather resistive barriers, 2000) d) Air resistance
House wraps that are water resistant make effective air retarders, when all the penetration and seams are sealed fully with an appropriate tape or sealant. Most of the house wraps air leakage rates fall between 0. 03 and 0. 08 CFM/ft2 . The higher the air leakage rates, the greater the airflow in and out of the building. (Weather resistive barriers, 2000) Examples of weather resistive barriers a) Reflective insulation Reflective aluminium insulation is used as a radiant barrier. A reflective insulation is made of two layers, either of plastic bubbles or of foil with foam.
A radiant barrier reflects heat instead of allowing it to pass through or absorbing it. Every unit of the radiant heat energy reelected away from the house during the hot seasons, or the heat energy reflected back during cold seasons may be cost saving because the heating conditioning and heating systems will be rarely used, reducing the utility costs. The reflective insulation performs all the other purposes that other kinds of house wraps perform. Being water proof, it is able to protect the penetration of water from outside which may cause damage to the insulation, framing or the sheathing.
When it is properly installed, the reflective insulation is able to prevent the any air that may pass through cracks, which helps to keep moisture out thus ensuring proper air conditioning. The major advantage of a reflective house wrap is that it can reflect a minimum of 97% of the sun’s rays falling on the house. A reflective insulation can either be combined with other types of house insulation or it can be used by its self. b) Building paper as a house wrap This is felt material or paper sheet that is impregnated with ashphalt.
The impregnation helps it to increase resistance from the penetrating water. Building papers can be perforated to increase the rate of moisture evaporation. Its disadvantage is that it does not provide insulation and does not act as a radiant barrier. c) Spun plastic sheet materials These have been commonly used as house wraps. Spun plastic sheet materials are normally wrapped around the house to protect a building from penetrating moisture when in the process of being constructed. They help to keeps out water while allowing the evaporation of moisture.
Just as the bulding paper, they offer no insulation or reflection of the radiant energy. d) Rigid foam board insulation The rigid foam board insulation is a natural resistance to water penetration. In some wall systems, it can be used as a weather resistive barrier and as a combined insulation. If used, it eliminates the need to use a building paper or a house wrap. (Weather resistive barriers, October 2000) Installing a house wrap A house wrap may be attached directly to studs, on top of the wall sheathing, or over rigid foam.
It should not be installed by one person; this is a two or three men job because the modern house wraps come in huge twelve feet rolls. Tools required for the installation of the house wrap include a staple gun and a sharp utility knife. Step I: Cover the inside and the outside corners This has been described as an optional step, but which should not be avoided to provide extra protection especially if the siding will be installed directly over the studs. In this step, fold a 2 ft wide wall-high length of wrap in half and staple it vertically over the outside corners and into the inside corners.
(Haun Larry etal, 2002, pg 154) Step II: Wrap the house from corner to corner If the 12 ft roll is too tall for the house being worked on, the roll should be cut roughly to the required length with a circular saw. The free end of the roll should be stapled near the corner of the house and then unrolled to the entire side of the house. One person should unroll the house wrap; while the other staples it as it unrolls. The windows and the door openings should be covered. When finished with one side, roll the paper to the next side, until all the walls are covered.
(Arnold Rick, 2007) Step III: Seal the joints and repair the tears if it is necessary If it is not possible to create a continuous wrap around the house, you should make sure you overlap the wrap by at least 16 in, or one stud space. The seam should be sealed with sticky, self adhering tape recommended for this purpose. House wrap tears should be repaired by applying a patch from the top of the wall down over the tear, so that water will drain properly. The tear should then be sealed with a tape. (Arnold Rick, 2007) Step IV: Cutting and wrapping the window and door openings
To finish the installation, cut an “X”-slice in the house wrap, cover each window and door opening, then pull the cut ends inside the house and staple them around the trimmers, header and rough sill. Staple the flaps right away so they don’t get caught by the wind and torn. (Haun Larry etal, 2002, pg 154) References 1) Wagner John D. , DeKorne Clayton, Barns, Sheds and Outbuildings: Plan, Design, Build, Creative Homeowner, 2005, pg 168 2) Carson Dunlop and Associates Staff, Carson Dunlop, Dearborn Home Inspection (Firm), Principles of Home Inspection: Insulation, Dearborn Real Estate, 2003, pg 55-56)
3) Weather resistive barriers; technology fact sheet. October 2000, retrieved on 3/20/2009 from http://www. ornl. gov/sci/roofs+walls/insulation/fact%20sheets/weather%20resistive. pdf 4) Haun Larry, Laurence Vincent, Fuller Millard, Snyder Tim, Habitat for Humanity How to Build a House, Taunton Press, 2002 5) Air barriers, U. S Department of energy, 30th Dec 2008, retrieved on 3/20/2009 from http://apps1. eere. energy. gov/consumer/your_home/insulation_airsealing/index. cfm/mytopic=11300 6) House insulation, Wrap your house with reflective insulation, 2009, retrieved on 3/20/2009 from http://www.
house-insulation. com/ 7) Harley, Bruce, Insulate and Weatherize: Expert Advice from Start to Finish, Taunton Press, 2002, pg 21 8) Arnold Rick, Guertin Mike, Installing House wrap, Fine home building, Feb/March 2007, No. 107, retrieved on 3/20/2009 form http://www. taunton. com/finehomebuilding/PDF/Free/021107044. pdf 9) House wrap, Technical fact sheet, no. 23,, FEMA 499/August 2005 10) Weather resistive barriers, Technology fact sheet; retrieved on 3/19/2009 from http://www. toolbase. org/PDF/DesignGuides/weatherresistantbarriers. pdf