One of the biggest unknowns and widely debated topics for liquid-applied membranes is whether they are suitable for application on a horizontal or near horizontal wood surface, such as on a window sill, on top of a curb, or under a parapet flashing. Conventional wisdom suggests that waterproofing in horizontal applications should be impermeable to moisture, like most peel and sticks and roofing membranes. There is more than enough evidence within the window and roofing industry to demonstrate that building paper and vapour permeable sheet sheathing membranes cannot be safely used on horizontal surfaces without leading to moisture damage. In these applications, industry most commonly uses impermeable peel and stick membranes.
There are, however, several new liquid-applied flashing membranes that may be suitable for use in localized horizontal waterproofing applications, provided they are water repellent, durable enough and do not remain under ponding water for prolonged periods of time. These liquid membranes can also have other benefits in terms of ease of use, workability, and compatibility with other materials used to complete the window installation. They may also allow for some outward drying when applied to damp wood.
Material testing and evaluation has been performed on a range of available permeable to impermeable flashings, tapes and liquid membranes to help determine which characteristics would be required for a window rough opening sill applications. This paper provides the results of a series of novel test protocols to evaluate the performance of various liquid membranes in a window sill application.
This paper was presented at the 15th Canadian Conference on Building Science and Technology in Vancouver, BC.