Beyond SRI
Tile cool roof performance goes beyond reflectance and emittance. It is important to understand that, even with the highest-reflectance materials, some IR energy is not reflected or re-emitted from the surface as radiation. That energy either:
- transfers as heat to the air above it;
- penetrates beyond the surface and builds up in the mass of the tile;
- emits as radiation downward to the building; or
- transfers as heat through the tile to the layer below.
All four of these things happen to different degrees. Tile roofs limit the heat transferring into the building through several mechanisms.
Thermal mass is what allows a material to store energy in the form of heat. Both concrete and clay tile have high thermal mass, so they store heat like a sponge that can soak up a certain amount of water before any starts to drip out. This buildup delays the shedding of heat through the bottom of the tile and toward the building. During daylight, roof tile heats up slowly, and does not begin to transfer heat toward the building until well into the day. After the sun goes down, the heat continues to transfer out of the roof tile, but cooler nighttime ambient temperatures compensate for this. This delay factor reduces both cooling and heating demands.
Insulation is a material’s ability to prevent or reduce heat transfer. Clay and concrete roof tile systems use one of the world’s best insulators as the layer directly below the tile—air. As both clay and concrete tile are hard, structurally robust objects, they can be molded so the body of the tile stands off from the underlayment and roof deck below.
Structural contact (and therefore thermal bridging) is limited to a few points on the underside of the tile. This forms an air channel under the tile assembly with two important effects:
1. It acts as an insulating layer, limiting heat conduction.
2. It creates passive ventilation, using convection to carry heat away. As air under the tile is heated, it rises up the roof’s pitch through the channel and escapes at the ridge, carrying heat with it. This ventilating effect has been shown to play a significant role in reducing heat transference through the roof system.
Arched tiles, such as classic clay barrel tile and S-mission tile, have sizeable air channels under their arches. This insulating/ventilating layer has been cooling roofs for millennia, since the time when tiles were molded on the tile-maker’s thigh.
Flat tiles, including those shaped to emulate traditional materials such as slates or wood shakes, can also be provided with air channels. Tile is normally fastened to battens, nailing strips attached to the roof deck parallel to the roof ridgeline. The tile’s upper edge is nailed to the batten to create a downward pitch from tile to tile, so each tile sheds water to the tile below it. A variant on this method involves counter-battens fastened to the deck running perpendicular to the ridge, and then parallel battens atop those. This creates an air channel under the battens. The ORNL study found slate-shaped concrete tile fastened in this manner caused substantial cooling.
With flat tile, the same effect can be achieved at lower installation expense using specialized, raised battens, which eliminates the need for counter-battens. Air channels are cut through the raised batten’s underside, creating the cooling effect.
The ORNL/LBNL study confirmed the combination of thermal mass and sub-tile air channel substantially reduced heat transfer. It compared a roof with dark concrete slate-shaped tile to a roof with asphalt shingle that had similar solar reflectance and thermal emittance. Temperature was measured both at the roof deck and in the conditioned space beneath the ceiling. The study found at peak solar loading, “deck venting caused a significant 50 percent reduction in the heat penetrating the conditioned space compared to the direct-nailed asphalt shingle roof that is in direct contact with the roof deck.” The data suggested tile with air-channel venting reduced heat transfer equivalent to having a 24-point higher solar reflectance.
Highly reflective roofs, because they limit heating from the sun, cause a penalty during winter—the heating season—when the sun’s warmth would be a welcome addition. However, the ORNL study also found tile roofs mitigated this penalty. A tile roof improves interior heat retention because of the air channel’s insulating effect, while the thermal mass effect adds heat during the night.
Additionally, the roof’s underside may be provided with a radiant barrier, and thermal insulation may be installed below the roof and above the conditioned space, both of which improve thermal performance even further. A radiant barrier—usually an aluminum foil layer—stops penetration of the IR that is being emitted downward by the tile. Insulation prevents heat transfer by convection and conduction into the conditioned space.
Design options
The tile cool roof system’s thermal performance allows design professionals to achieve sustainability goals and still have a broad range of aesthetic options. There is no need to compromise architectural vision to hit a cool roof target. Unlike a white membrane roof, which is usually hidden from street view by a parapet screening off its unappealing aesthetics, a tile cool roof can make an architectural contribution. It can create a connection to traditional architecture, or express entirely contemporary, breakaway ideas.
Pitched roofs on low-rise commercial buildings often convey a ‘home-like’ quality with a safe and welcoming sensibility—an effective strategy for any hotel, restaurant, public library, or other building seeking to encourage public access. Steeply pitched mansard roofs on high-rise buildings reference late 19th-century grandeur.
Cool roof tiles are available in both traditional and contemporary styles. Barrel tile is the classic clay style, dating back thousands of years. Concrete versions in this style are also widely available. Both clay and concrete can be molded to resemble other traditional materials, such as slate tiles, wood shingles, and wood shakes.
Cool roof tiles can also support contemporary designs. Some generally resemble traditional shapes such as slates, but do not try to emulate traditional colors or textures. Instead, they may feature smooth textures, and colors not associated with wood, clay, or stone.
Many linear and geometric effects can be created by staggering the tile placement. Combined with the possibilities of using multiple colors—distributed randomly or arranged in patterns—a tile roof can truly become a canvas for architectural expression.
