Rigid Insulating Materials for Low-Slope Roofs
An insulating material for low-slope roofs should have high thermal resistance, adequate resistance to compression, denting, gouging, moisture decay, and fi re, and, if part of a hot applied system, high resistance to melting or dissolving when hot bitumens are mopped onto it. No single material has all these virtues. Some rigid insulating materials commonly used on low-slope roofs in North America , along with a summary of their advantages and disadvantages. The best choice is often a combination of materials, or a composite board that combines two or more materials into one product, to exploit the best qualities of each. A composite insulating board for installation under a built-up bituminous roof membrane might include, for example, a bottom layer of poly isocyanurate foam with high insulating value and a top layer of perlitic board resistant to hot bitumens.
If rigid insulating boards are located below the roof membrane, they may be adhered to the deck with hot asphalt or adhesives, or fastened to the deck mechanically with screws or any of a variety of fasteners made especially for the purpose. Mechanical fasteners are favored by insurance companies because they
are more secure against wind uplift.Lightweight insulating concrete is an economical insulating material that also creates a nail able roof deck. Formulated with lightweight aggregates or foaming, air entraining agents, this material has densities ranging from 20 to 40 lb/ft3 (320 to 640 kg/ m3) compared to 145 lb/ft3 (2320 kg/m3) for conventional concrete. Lightweight concrete may be applied directly to corrugated steel decking or over rough concrete decks and can easily be tapered during installation to slope toward points of roof drainage.
Thermal resistance per inch is not as high for this material as for most other types of roof insulation. However, plastic foam boards may be embedded in the insulating concrete to achieve higher insulating values within reasonable thicknesses. Lightweight concrete fill insulation contains large amounts of free water at the time it is placed. It must be cured and dried as thoroughly as possible before application of the membrane, and some form of venting to allow the escape of moisture vapor from the insulation during the life of the roof, via either topside vents or bottom side slotted metal roof decking, is usually advisable. Poured-in-place gypsum, another decking material popular in the past for forming lightweight, nail able sloping roof decks, is no longer used in new construction.
Vapor Retarders for Low-Slope Roofs:
The membrane in a protected membrane roof serves also as the vapor retarder. However, when insulation is located below the roof membrane, a separate vapor retarder is recommended in cold climates or when enclosing high-humidity interior spaces. The most common type of vapor retarder for a low-slope roof consists
of two layers of asphalt-saturated roofing felt bonded together and adhered to the roof deck with hot asphalt. Vapor retarder sheets made from factory-manufactured self-adhering bituminous membranes are also common.
Polyethylene sheeting, used as a vapor retarder in many other types of construction, is seldom used in low-slope roofs because it melts at the application temperature of the hot bitumen used in many roof membranes, and it does not stand up well to the rigors of foot traffic and other construction activities that occur during roofing installation. When a vapor retarder is included in a low-slope roof assembly, it must be located within the assembly such that it will always be warmer than the dew point of the interior air under common conditions of use.
This usually means putting the vapor retarder below the insulation. However, a vapor retarder should not be installed directly over a corrugated steel deck, where it would have to bridge across the corrugations and would be vulnerable to damage until it was covered by insulation. In such cases, substrate board (thin panels of wood, gypsum board, or foam insulation) is first laid over the deck, followed by the vapor retarder and then the insulation boards. When some portion of the roof insulation is located below the vapor retarder, the designer must carefully calculate the dew point location in this assembly to be sure that the vapor retarder lies below it.
Membranes for Low-Slope Roofs:
The membranes used for low-slope roofing fall into three categories: bituminous roof membranes, single-ply roof membranes, and fluid-applied roof membranes.
Bituminous Roof Membranes :
Bituminous roof membranes are of two types, built-up or modified bitumen. A built-up roof (BUR) membrane is assembled in place from multiple layers of asphalt-impregnated roofing felt bedded in additional layers of bitumen (Figures 16.13–16.15). The felt, made from cellulose, glass, or synthetic fibers, is saturated with asphalt at the factory and delivered to the site in rolls.
The bitumen is usually asphalt derived from the distillation of petroleum, but for dead-level or very low slope roofs, coal tar pitch is used instead because of its greater resistance to standing water. Poly mermodified as phalts, as described below for modified bitumen roofs, may also be used. Both asphalt and coal tar pitch are applied hot in order to merge with the saturant bitumens in the felt and form a unified, multi-ply membrane. The felt is laminated in overlapping layers (plies) to form a membrane that is two to four plies thick.
The more plies used, the more durable the roof. To protect the membrane from sunlight and physical wear, a layer of crushed stone or other mineral granule aggregate is embedded in the top surface. Less commonly, a built-up roof may be made from felt plies bedded in cold applied mastics (solvent-based as phalts), that is, compounds of asphalt and other substances applied by spray or brush at ambient temperatures and then cured through solvent evaporation A modified bitumen roof membrane is made from factory-manufactured sheets of polymer-modified bitumens.
Modified bitumens are asphalt materials to which compounds such as at actic polypropylene (APP) or styrene-butadiene- styrene (SBS) have been added in order to increase the material’s flexibility, cohesion, toughness, and resistance to flow. Modified bitumen roof membrane sheets are also reinforced with plastic or glass fibers or fibrous mats. Sheet thickness typically ranges from 0.040 to 0.160 inch (1.0– 4.0 mm)