Vapor Permeability

To tape or not to tape?  Because the installation method of our windows was not typical in this part of the world, we did run into some differing opinions on how to best seal the window assembly. The big issue here is to avoid trapping moisture – whether coming from rain infiltration, diffusion from building materials, or air leakage – inside the wall assembly. Standard windows with flanged frames are typically taped and/or caulked from the outside, generally allowing moisture buildup in the gap between wall and window to escape toward the inside. However, for inset windows such as ours that are not attached to the exterior surface, it was recommended that we place the primary air seal (silicone or tape) on the interior side of the window frame. This is to pressure moderate the cavity, and to prevent warm, moist interior air from getting into that gap where it would condense as it cools.  The bigger point of contention regarding the assembly, however, was what to do on the outside of the window frame. Do we seal the exterior perimeter with tape?

RDH Group, our local building science experts who have given us much invaluable advice on our foundation, wall, and roof assemblies, weighed in on this issue.  Their concern with installing seals on both sides of the frame was that this would create an un-drained pocket between the window and the surround, a condition they typically discourage.  A seal on the outboard side of the window, in addition to the interior seal, would create the potential danger that if the outside seal fails, water may become trapped between the two seals.

They felt that our interior seal plus spray foam already gave us two air seals and a thermal barrier around the window, and recommended that the exterior side be protected solely by the cladding or other “shedding” surface, allowing the cavity to weep out excess moisture. And If any seal was applied to the exterior, at least the seal at the sill of the window should remain open to drain any water that might get into the space.

Window installation spray foamed but not sealed from the exterior

Our window installers from Internorm had a slightly different approach.  Rather than using non-permeable tape or silicone for the interior edge of the window frame, they provided us with a vapor-permeable fabric tape, to be used along with spray foam, as I described in an earlier blog entry.  Trusting in their expertise in high-performance building materials, and understanding that vapor permeability is almost always a good thing in a wall assembly, we accepted this change. They then urged us to also seal the entire outside edge of the frame with vapor permeable tape.  This provides weather protection from the outside but allows moisture to escape out of the cavity in either direction. Since we felt that the window manufacturer’s recommendations were thoroughly tested and well established, we followed their advice as closely as possible.  As a part of our Passive House certification, we are closely following the airtightness of our envelope.  This issue also had an effect on our decision as the additional layer of tape gave us an extra air-tight layer.

vapor-permeable airtight seal on the interior side of the window frame

Finding a vapor-permeable tape to seal up our window installation from the outside was not easy.  Unfortunately, even though the use of vapor barriers is becoming more controversial in modern construction as the importance of vapor-open construction is becoming better understood, there are still not many products on the market that have both good vapor permeance AND good adhesion.  Typically, the best adhesives are either SBS or butyl based, neither of which is vapor permeable.  Most vapor-permeable adhesives use something similar to certain 3M tapes, which are not good in external conditions.  Our Austrian window manufacturers pointed us to several products which were not able to find in the US (Illbruck/Tremco Duo Membrane HD, and Würth Dichtband Aussen). RDH ended up pointing us to a vapor-permeable air barrier membrane manufactured by Henry called Blueskin Breather which we ended up using along with their corresponding primer to make sure we got good adhesion.