01 May 1999
hot asphalt or solvents are used in construction of the membrane, the insulation must resist
degradation by these agents.
2/ Common materials for adhered systems include faced polyisocyanurate foam
(ASTM C1289), perlite (ASTM C728), rigid glass fiber boards (ASTM C726), wood fiber (ASTM
C208), and cellular glass (ASTM C552). Verify that the combination of membrane and roof
insulation/facer are Fire and Wind Rated.
3/ For mechanically fastened and ballasted systems expanded and extruded
polystyrene (ASTM C578) are added to the above list. (They could melt or dissolve if placed in
contact with hot bitumen or solvent-based adhesives).
4/ Some decay of R value is observed with HCFC blown foams (urethanes and
isoboards) due to diffusion of air and moisture into the cells of the foam. Manufacturers publish
aged R values to reflect this decay. Always used aged R-values for these materials. Since
diffusion starts at the surface of the foam, thicker foams are more thermally stable.
5/ Wood fiber boards should be manufactured to use as roof insulation (meeting
ASTM C208), not sheathing boards, and should be limited to 1220 mm (4 ft.) in length and width.
6/ When mechanical fasteners are used to secure thermal insulation, it is
recommended that the nail-one, mop-one technique be used. This minimizes a thermal bridging
and avoids nail-popping of the membrane. When multiple layers of insulation are used, vertical
joints should be offset to reduce heat losses. Straight-through joints can take away 10% of a
roof's insulating ability.
(d) Protected membrane systems (figure 2-17).
1/ In PMR systems only extruded polystyrene is suitable in the sometimes wet
environment above the completed roof membrane. This insulation protects the membrane from
thermal stress and abuse.
2/ Ballast and filter fabric is needed to hold the loose-laid insulation in place.
g. Energy and Solar Radiation. The ratio of roof area on a low-rise commercial building is
high relative to wall area. Such roofs can provide a great opportunity for energy conservation.
This can be accomplished by using well insulated, high thermal mass structures to reduce
summer cooling loads, garden roofs, or high albedo roof coatings.
(1) Heat Gain. For roofs in hot or temperate climates, light colored roof surfaces reduce
heat gain. For membrane roof systems, light colored aggregate (gravel surfaced roofs) or mineral
granules (capsheets [ASTM D249, D371, D3909] and MB capsheets [ASTM D6162, D6163,
D6164, D6222, D6223]) will reduce heat maximum surface temperatures by up to 20C (35F)
over black membranes. Aluminum coatings (ASTM D2824) are approximately the same, while
white coatings (ASTM D6083) have been observed to reduce temperatures by more than 23C
(45F) (provided that the roof stays clean). The use of pavers and heavy ballast reduce heat gain
through thermal lag; that is, the high heat capacity stores some of the gained heat delaying the
startup of the building's air conditioning system. The ASHRAE Handbook of Fundamentals uses
the term Equivalent Temperature Difference (ETD) in energy calculations. The smaller the ETD,
the better the system is at reducing peak solar loads.