per·me·a·ble [pur-mee-uh-buh l]
adj. That can be permeated or penetrated, especially by liquids or gases: permeable membranes; rock that is permeable by water.
Permeability, with regards to Exterior Insulating Finishing Systems, refers to the ability of water vapour to pass through it. Consider materials such as glass or thick plastic and metal: all are relatively solid and neither are permeable - water vapour, gases and liquids cannot pass through them. Now consider materials such wood, carpet or foam: all of which are permeable - water vapour, liquids and gasses can pass through them with ease, and they are "breathable". There are also varying degrees of permeability, as sometimes water vapour can pass through a material easily and sometimes it can only partially permeate a material, allowing any leftover water vapour, gas or liquid to remain behind.
Higher-permeability EIFS materials:
- Expanded polystyrene foam (EPS)
- Some acrylic coatings
- Fiberglass mesh
- Cementitious base coats
Lower-permeability and non-permeable EIFS materials:
- Epoxy coatings
- Some acrylic coatings
- High-resin base coats
ASTM, or the American Society for Testing Materials, determines international standards for materials worldwide. Because the permeability of EIFS is determined by the permeability of each of its components, which can differ by which material is used as shown above, a numerical value can be given to each material to make it easier.
The empirical standards for measuring the permeability of a material is grain per hour per square foot per inch of mercury, which breaks down as follows:
- Grain = 1/7000 of a pound
- Inch of Mercury = the amount of vapour in material that is not present in dry material.
The permeability of a material is also dependent not just on its thickness or composition, but on external factors as well, such as heat and humidity on each side. This can be explained by thinking of a glass window on a cold day. When the humid, warm indoor air hits the window, it turns to liquid water droplets in the form of condensation. This happens because the water that was previously a gas (water vapour) found in the warm indoor air has hit the cold window and cooled to its liquid form. If the window was not made of glass, but rather was made of a permeable material, the water would pass through it and evaporate on the other side rather than hang around on the inside of the glass.
Barrier EIFS vs. Drainage EIFS and water vapour
When water vapour comes from within the substrate and is faced with barrier EIFS, it has nowhere else to go but leave slowly through the EIFS. When the same water vapour is faced with drainage EIFS, it can easily and quickly escape through the drainage cavity. A similar situation is seen when there are joints within the barrier EIFS, they provide another “easy way out” for the water vapour.
When drainage EIFS is used, a barrier is placed between the substrate and drainage cavity. This barrier is called a weather or water resistive barrier, or WRB, and they can come in sheet-form or be liquid applied. Typically, liquid-applied is preferable as one can imagine the issues that may arise if a WRB is applied as separate sheets that have seams. The permeability of these liquid-applied WRBs can be high, which is important for any water vapour to escape so that it does not hang around within the wall and rot any wooden structural components.
Weather and permeability
Knowing the conditions of which the EIFS will be subjected to with regards to the weather, heat and humidity are important for determining how easy it will be for water to leave the EIFS. When water hangs around within the EIFS, problems begin to arise.
The hot summers of Ontario allow for the quick drying of EIFS, but this quick drying capability is significantly lessened on the more humid days. The dry Ontario winters allow for quick drying, but the increased risk of freezing temperatures pose problems for any condensation remaining within the wall, which will freeze. Ontario receives a well-proportioned mix of colder temperatures, warmer temperatures, quick freezing and thawing, high humidity and dry air. These are all factors that must be taken into consideration with respect to the materials chosen when constructing an EIFS wall.