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Moisture Control | Roof
Decks | Rigid Insulation | Roof
Membranes
Rigid Board Insulation
Criteria for Installing Rigid Insulation Boards
The designer shall incorporate the following criteria into roof assemblies
containing rigid insulation boards:
1. Rigid insulation boards shall be installed in multiple layers for energy
efficient roof systems, and all joints shall be offset between each layer.
The end and side joints of each layer of insulation shall be staggered
and installed so as to provide moderate contact with each adjacent board.
The long dimension of the insulation boards shall be installed perpendicular
to the flow of water.
2. A minimum of two layers of insulation boards limits heat transfer which
reduces the thermal loss that occurs through the joints between single
layers of insulation boards. Also, the gaps between single layer insulation
boards are a more direct conduit for hot and cold air transmission as
well as for condensation. Double layer insulation systems: 1) Eliminate
the thermal shorts at board joints; 2) Better protect the roof system
membrane from exposure to moisture vapor from within the building; and
3) Reduces possible buckling that can occur with some types of insulation.
In addition, two or more layers of insulation minimizes thermal transmission
through the fasteners since only the first layer is mechanically attached.
Mechanical fasteners are direct conduits for thermal transmission.
3. For roof systems installed over steel and nailable roof decks, the
first layer of insulation shall be mechanically fastened, and shall have
an insulating value greater than or equal to the second layer. The second
layer of insulation shall be completely adhered into the specified Fields
Asphalt. The following requirements are for steel decks:
a. The direction of the steel deck flutes, the orientation of the insulation
board, and the layout of the roof system membrane shall be designed so
as to accommodate the correct attachment of the roof system, and so as
to facilitate installation of the roof system membrane as well as daily
tie-offs and water cut-off installations.
b· Insulation boards shall be firmly supported by the bearing surfaces
of the steel deck metal flanges. The rigid insulation boards shall not
cantilever over the steel deck ribs, because the insulation and the roof
system membrane will not be sufficiently supported.
4. For installations of three or more layers of insulation, the first
layer may be set in place, and then secured at the time of mechanically
fastening the second layer of insulation.
Tapered Insulation Roof Systems
Tapered insulation roof systems shall be installed so as to comply with
the requirement for positive drainage where the roof assembly does not
provide such drainage.
Deck Slope & Tapered Insulation for Positive Drainage
Fields requires positive drainage for all roofs. The correct roof slope
shall be achieved by either sloping the structural deck or by installing
tapered installation over the structural roof deck so as to achieve the
necessary slope. A combination of deck slope and tapered insulation may
also be specified in order to achieve positive drainage. The designer
shall specify the design that is best for the long term performance of
the roof assembly. Roof drains, scuppers, and gutters shall also be specified
into the roof assembly as part of the overall drainage plan.