This option was intended to create a complete vapor/air barrier by installing spray foam over the polyethylene sheeting and bottom chord of the truss (Figure 1). This required the removal of the existing system down to the structural roof deck and also a significant portion of the deck to facilitate the vacuuming of the existing blown-in insulation out of the truss space and the installation of spray foam and new blown-in insulation. New tapered insulation and roof membrane above the structural roof deck were part of this solution.
The second option required removal of the existing system down to the structural roof deck and the replacement of any wet, rotted, and/or moldy deck and blown-in insulation (Figure 2). A roof vapor/air barrier would be applied on the structural roof deck.
The application of spray foam insulation of at least 76-mm (3-in.) thick to the rim area was determined to be the most effective way in-situ to transition the polyethylene sheeting from the exterior walls to the roof vapor/air barrier. The rim area is at the top of the exterior walls at the level of the 406-mm (16-in.) deep roof trusses.
Sufficient insulation needed to be added above the structural roof deck to get the dewpoint temperature above the roof vapor/air barrier. This insulation also needed to be tapered to provide roof slope to the existing, interior, primary and overflow roof drains. The hygrothermal analysis showed a minimum of 102 mm (4 in.) of polyisocyanurate (ISO) insulation was required in order to keep the dewpoint temperature above the roof vapor/air barrier. This meant all roof drains would need to be raised to accommodate the increased insulation thickness.
This option required removal of all the existing blown-in insulation in the truss space and installing a sprinkler system to satisfy the fire code. A new roof assembly above the structural roof deck included a roof vapor/air barrier, tapered rigid board insulation, and membrane. This helped to let the dewpoint temperature occur above the roof vapor/air barrier and minimize the amount of insulation. This option also required spray foam in the rim area, as described in the second option.
The third option was quickly eliminated from further consideration because the owner decided against installing a fire sprinkler system above the top-floor ceiling due to the considerable disruption to occupants and the cost. Therefore, the blown-in insulation in the truss space needed to be maintained by selecting either the first or second option.
The second option was selected and developed into construction documents for bidding and construction (Figure 3). This was the best solution to achieve the goal of a complete vapor/air barrier. It also exposed the existing assembly to allow for the removal and remediation of wet, deteriorated, and moldy roof components. This option also maximized the reuse of the structural roof deck and blown-in insulation that was still in acceptable condition.