Bill writes: We are having an issue with our 13-year-old house where it appears that the flashing membrane material has leaked out along the edges of our windows. This leakage appears black and we cannot figure out what else it could be. Since we are planning on replacing all the front windows of the house that face the summer sun, we are looking for other materials to use so we do not have this issue in the future. Hope you can tell us if our thoughts are correct.
Steve Bliss, of BuildingAdvisor.com, responds: The problem you are facing is not uncommon, especially in hot climates, and is usually associated with the more common rubberized-asphalt (modified-bitumen) membranes, but can also occur with the more expensive butyl-based membranes. The two main causes of black stains are high temperatures and incompatibility with adjacent materials – especially sealants and flexible vinyl flashing.
Every tape formulation is a little different, so the only safe approach is to get the recommendations from the manufacturer of the flashing tape you plan to use and follow them. Unless you can identify the specific product that failed on your house, it may be difficult to determine the exact cause.
In general, standard flashing membranes should not be used in very hot climates as most begin to soften and flow at around 180ºF and some are rated to only 120ºF. Special high-temperature products can tolerate temperatures over 200°F but may be difficult to locate and are primarily marketed for commercial applications. Butyl tapes are more expensive but tend to be more stable at high temperatures. However, it is important to check the specifications for the specific product you are using as they vary considerably.
Compatibility with soft vinyl and sealants is mainly a problem with rubberized-asphalt membranes. The plasticizer that keeps the vinyl soft can migrate out and dissolve the asphalt. Solvents used in many sealants, including silicones and polyurethanes, can do the same. This is generally not a problem with butyl membranes, which remain chemically stable unless in contact with asphalt-based products like roofing cement. However, this is rarely an issue with window flashing.
If the windows with the greatest problems were facing full sun exposure on the south or west, then high temperatures were most likely to blame. To avoid these problems in the future, you should select a butyl-based flashing rated for at least 180 degrees. In a worst-case scenario, a dark-colored aluminum cladding in a hot climate facing full sun exposure, you might want to consider special high temperature flashing tape.