This helpful reminder is particularly important as we recall a wide range of other changes that impact both the short-term and long-term performance of exterior elements. Our drive to tighter, more water-resistant, longer-lasting exterior elements has encouraged a wide range of technologies. There are more than 50 different housewraps on the market, with vapor permeance ratings ranging from at least 5 to 50 perms. These are applied over a range of insulated and non-insulated sheathings that have much greater resistance to liquid water flow and range in permeance from 0.1 to 10 perms. Combine these with synthetic trim products that have excellent water resistance (because of very low absorption rates), and exterior finishes show remarkable progress in long-term moisture performance. However, with the speed of production, the tighter material installation tolerances over highly variable weather conditions means that any water or water vapor trapped either in or between the exterior finish and the weather-resistant barrier can expand very quickly during hot sun exposure. The effects of this micro-moisture environment has resulted, for example, in some isolated incidents of freshly applied sealants bubbling or blistering.
In its research of this phenomenon, sealant manufacturer Henkel has found that there is a critical window of time in the cure process of each individual type of sealant and that in this time period the combination of solar radiation and water vapor movement through and around exterior finishes results in vapor pressures that can cause a bubbling of sealants.
Henkel’s Henry Ashton reports that there are numerous key parameters that can influence this effect under certain sets of conditions and in certain combinations. For example, it has been known that certain backer rods can, in combination with some sealants, bring about undesired bubbling. The key ingredients are always moisture and temperature. Henkel has been able to use UV lamps and climate chambers to simulate real-world conditions and then experiment with the varying common installation practices for exterior finishes and sealants. Ashton notes that priming of siding edges resulted in more significant bubbling of sealants than if the edges were not primed.
Other variables being studied include the depth of the air space behind exterior finishes; the vapor pressures of other chemicals used in either the exterior finish, the WRB, or the sheathing; and even the differential in temperatures over the cure time. Henkel expects to have more complete answers as they delve into this further in the coming months. While there is more work being done on this phenomenon, Ashton notes it is fair to say that taking a total-system view is beneficial.