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 sprayfoamed but not sealed from the exterior