Rigid Insulation Assemblies

Rigid Insulation Assemblies are the most popular way to insulate and slope a flat roof.  Most building codes in the state of Washington require an R-value minimum of R-38 for all new construction and remodels.  This can be achieved above the plywood decking of a roof by using two layers of 3.3″ Polyiso insulation or by an engineered tapered EPS system.  Here are some systems that are most commonly seen in building specs of a flat roof assembly.

• Polyiso Rigid Insulation: Carries a higher R-Value than other types of insulation, but is also more expensive.  It has a high absorption rate and has a low tolerance of moisture. It is the most common board for achieving an R-30, R-38 or R-49 factor.
• EPS (Extruded Polystyrene or Styrofoam) has less R-value than Polyiso, but a higher R-value over time and a consistent R-value.  EPS is very tolerant to moisture and has a low absorption rate.  EPS is generally engineered to the project and can be engineered to have the R-value and the taper in one panel, where polyiso requires several layers to achieve the same result.
• XPS (Expanded Polystyrene) is commonly used for its ultra rigidity or PSI factor and its zero absorption of moisture, which can allow it to be installed above a single-ply membrane and act as a cover board for ballasted roof systems. It is commonly called Foamular, which is Owens Corning brand name for its XPS.
• DensDeck or SecuRock:  are two types of high-density fiberglass mat boards, often used in an assembly to achieve a high-density cover board to protect the underlying assembly from being crushed by the weight of the equipment or overburden (rock, green roof, pavers) that are going over the top.  These cover boards also help a roof achieve a Class A fire rating.
• Tapered Insulation Systems: Typical taper systems will be designed to provide enough slope to alleviate pooling water on a flat roof.  These tapered insulation systems can be either Polyiso or EPS.  The most common taper size is 1/4″ per foot, which means at the drain or gutter it will be 1/4″ and it will gain 1/4″ for every foot to the other end of the slope.  An example would be if you had a wall that was where you wanted the slope to begin and it was 30′ from the drain, then you would calculate that as 30*.25=7.5″.  In this scenario you would need at least 7.5″ of clearance to reach a true 1/4″ taper.  Tapers can be as low as 1/8″ to 1″ per foot.