Another issue with fenestration systems is their transition to adjacent wall assemblies and placement within. Figure 7 illustrates the difference in condensation resistance performance when comparing two different window placements. The left image shows a thermal model with the window placed outward, which creates a misalignment of the thermal control layers, as shown with the dashed blue line. The right image aligns the thermal components of the window with the adjacent wall showing greater accommodation for higher interior RH.
Additionally, air control both within the fenestration product and at the transition between fenestration products and adjacent wall assemblies, can impact enclosure performance. Fenestration products often rely on gaskets and sealants for their air- and water-tight capabilities. These types of materials degrade over time and can create paths for air infiltration, which can cool the interior surfaces of the fenestration below dewpoint temperatures. Similarly, if air control is not achieved at the window-to-wall connection, an increase in interior RH can create conditions evidenced in Figure 8.
Exterior walls
Like roofs and fenestration, varying wall types are utilized in existing buildings. The vapor retarder, air barrier systems, and insulation, as well as the methodology and placement within the wall assembly of such components can differ greatly. Some wall types do not include any of those components, such as mass masonry walls, and can have numerous air paths through the wall. Increased interior RH can allow these paths to transport moisture-laden air into the wall assembly, potentially leading to condensation and/or deterioration of the wall.
The assembly depicted in Figure 9 is an example of another wall type, which includes insulation on the backside of the concrete block backup wall. The insulation was extruded polystyrene (XPS), which has the material properties to limit vapor and air permeance through the insulation. However, in this case, the insulation was spot adhered in place, the board joints were not sealed, and it was not sealed to adjacent enclosure components in an airtight manner. As demonstrated in Figures 10 through 13, air transfer was possible from the interior to the exterior, creating the potential for condensation.
Another common wall assembly used in existing buildings incorporates cold-formed metal framing for the structural backup wall. Some of these walls can include insulation within the stud cavity, others in the exterior air cavity, and some in both. The location of intended vapor and/or air control can impact the assembly’s condensation resistance.
A great article. There should be more articles like this.