How to Choose a Proper House Wrap For Yourself
Today, there are many products of HOUSE WRAP.Most are non-woven or woven fabric products, the main component is polyethylene or polypropylene.Some are microporous membranes that let water vapor escape from the micropores;Some allow water vapor to pass through the fibers.How to choose the right one?We can start by some ICC-ES (International Standards Council Assessment Service) - which mainly evaluates building products,including house wraps and provides opinions on code compliance.For HOUSE WRAP, ICC-ES evaluated a number of key characteristics:
1. Water resistance: it is the most basic characteristic of waterproof membrane.ICC-ES mentioned three different testing methods.
1) “Boat Test”: paper and felt are commonly tested by this method, the least stringent standard.It is overly sensitive to humidity and vapor transfer; therefore, it’s unreliable for plastic house wrap.
2)“Water Ponding”: place the house wrap in a 1-inch water for 2 hours.
3) Hydrostatic test: the most stringent test.Place a column of water 55cm or higher on the house wrap.
The qualified house wrap can pass all the above tests.
The WATERPROOF MEMBRANE must be able to withstand all the forces encountered during installation, and the water resistance should not be affected.Tear strength and tensile strength are the best available indicator to express this characteristic.UV resistance and low temperature resistance index are also important.The UV resistance index usually refers to the period of time after the installation of the WATERPROOF MEMBRANE and before the installation of cladding.Some manufacturers of waterproof membrane will stipulate that the UV resistance time is 3-6 months, but in most cases, the waterproof membrane should be covered by the cladding within one month.Low temperature resistance index refers to the lowest temperature that the product will not crack.
3. Air permeability:
Permeability refers to the amount of water vapor allowed to pass through a BREATHABLE MEMBRANE within a certain period of time. A good breathable membrane can discharge the moisture inside the wall insulation as much as possible, preventing the wall structure from mildew and decay. The unit we use to describe air permeability is “perm”.The higher the Perm value is, the better the air permeability is.
But, the condition in the lab is not much like the real using condition on the building.If we want to ensure the permeability of breathable membrane, a good installation method is very important.There must be an air layer between the breathable membrane and the cladding board.Just a wooden strip can solve all the problems.The strips can be installed on the all the breathable membrane laps.The air layer can promote the drying of condensed water discharged from the cavity wall. This is really a good method which can solve several problems in one time.
4. Windproof performance:
Good wind resistance can prevent discomfort caused by air drafts indoors.Most air barriers have tested this item, but wind resistance is not required by ICC-ES. Most time, a good installation will determine the wind resistance performance.The method mentioned above can not only enhance the breathable performance but also the windproof performance, which is the best installation method we have found till now.
5. Drainage capacity:
No matter what kind of MOISTURE BARRIER, there will be a certain amount of water absorption performance in it.Because the water vapor inside the structure will condense on the surface of MOISTURE BARRIER when it meets the external cold air, and the porosity of the membrane means the MOISTURE BARRIER will contain a certain amount of water. If these moisture cannot be discharged in time, especially on the shade side of the house, it will lead to serious moisture problems.Therefore, it is particularly important to have an air layer between the MOISTURE BARRIER and the wall cladding.It's still the method we mentioned above.A good way to do this is to use long insulation screw to fix the strip and moisture barrier to the sheath wall, and then attach the wall panel above the strips.