Breather Membrane

What is a breather membrane?  It is a membrane installed on a house to prevent moisture from leaking into the house while still allowing water vapor to escape, primarily to protect thermal insulation from sustaining water damage.  It is generally multiple layers of aluminum mesh bonded together.  The mesh is fine enough to prevent water from entering but also porous enough to allow water vapor to escape.  It can be used on roofs, walls, and floors.  It is also known as house wrap, home wrap or building wrap.  Most are designed to be installed under exterior finishes (roofing, cladding, etc) but some are also designed to be exposed to the elements.  There are many manufacturers of breathing membrane.  They include Tyvek® breather membrane, Glidevale breather membrane, and Kingspan Nilvent® breather membrane.

It has several uses for roofing.  If the roof loft is a vent-less system (usually only used for wood and tile roofs), the breather membrane allows for moisture from the loft to escape through the roofing while preventing water to leak into the loft, such as is possible during a rain.  It is generally installed horizontally over the rafters with the roofing installed on top of it.  It can also be used with asphalt or composition shingles.  It is generally installed between the shingles and the plywood decking in place of tar-based underlayment paper. Tyvek® Supro breathing membranes and Kingspan Nilvent® are products designed for roofing.

Building wrap used on walls is designed to be installed on the exterior walls between the framing and the cladding.  It is designed to keep moisture from getting into the wall-cavity insulation if the cladding leaks or comes off while letting water vapor from the interior of the home escape out.  Tyvek® Housewrap, Tyvek® Homewrap and Glidevale Protect T200 Thermo are among the many products designed to be used as wrap.

If the building wrap is going to be the outer layer, it needs to be designed to withstand the elements such as rain, wind, and the sun.  Most wraps designed for this purpose will have a UV protection listed on it just like the UV protection number listed on sun tan lotion.  UV-15 breather membrane is generally the lowest rating that is recommended.  

When used for flooring, breather membrane is generally used to prevent moisture from leaking up onto wood or tile flooring but still allowing any water vapor to escape underneath the flooring.  It can also be used to protect basement insulation in the ceiling from releasing water vapor into the floor overhead and making the basement airtight.  Tyvek® SD2 can be used for this purpose.

Breather membranes are an important part of a building.  They prevent moisture from leaking in while allowing water vapor to escape.  This helps prevent water damage, rot, deterioration, mold, fungus, and various other damages related to long-term water exposure from damaging timber and metal framing.