Peter L. Pfeiffer, an architect and energy consultant in Austin, Texas, replies: Whether or not the house is air conditioned, in hot climates it is important to create a self-ventilating roof system. A ventilated roof diverts heat away from the living space below by exhausting heat through a continuous ridge vent and drawing replacement air through a continuous soffit vent.
To keep a cooler interior space, the air should flow beneath a radiant barrier incorporated into the metal-roof system. The radiant barrier, a foil-faced sheathing material installed with the shiny side facing down, reduces the emission of heat toward the living space. The radiant barrier thwarts radiant-heat transfer in much the same way that low-E glass does. The radiant barrier is placed on top of the rafters and beneath the roofing purlins. This reflective sheathing can be any number of materials that come with a highly reflective coating (Thermax or Tuff-R by Celotex Corp., 4010 W. Boyscout Blvd., Tampa, Fla. 33607; 813-873-1700). Radiantbarrier foils such as Denny Foil (Denny Products, 3500 Gateway Drive, Pompano Beach, Fla. 33069; 305-971-3100) also work; however, they’re more difficult to install and work around because they’re not rigid material.
If you decide to install air conditioning in the house, it makes sense to build a tight interior envelope, which usually is accomplished with a well-sealed drywall system. (Do not use a vapor barrier in this climate; it causes internal wall rot.) Although the primary purpose of insulation is to keep heat in the home, insulation is also effective for maintaining a cool interior. So in the rafter bays, I’d install batt insulation, which is less expensive than rigid insulation. Hold the batts at least 1-1/2 in. below the radiant-barrier reflective sheathing to create the soffit-to-ridge airspace.
The quantity or thickness of insulation in a cooling-load-dominated climate, such as Puerto Rico’s, is not too important. I recommend no more than R-19 in the ceiling system. Then you can frame with 2x8 rafters and still have room for the continuous airspace.
If the roof system contains a radiant barrier and incorporates continuous soffit-to-ridge venting, the color of the metal roofing isn’t critical. Still, it’s not a bad idea to use a white or reflective material, such as galvanized or galvalume metal. I have had good results with 26-ga. or 29-ga. galvalume Channel Drain metal roofing (Wheeling Corrugating Co., 1134 Market St., Wheeling, W. V. 26003; 304-234-2352). I would advise you to consider the effects of salt mist on any metal-roofing product, and I also advise you to consult the manufacturer accordingly.