We’re building a prefab home in Puerto Rico. How can we keep it cool without central AC?
Your project sounds commendable, and Aguadilla is a beautiful setting for a passively ventilated sustainable home. The questions you raise are of utmost importance for the initial design phase. Considering orientation, shading, and ventilation strategies early in the process creates the best results in the end.
Your home will be slightly rectangular in form, ideally with the long walls facing north and south and the shorter ends facing east and west. Minimizing the eastern and western exposure limits the most extreme heat radiation from the sun in the morning and afternoon. Facing your kitchen east will provide early morning lighting for breakfast. If you enjoy sleeping late in the morning and reading in the evening, orienting your bedrooms to the west will provide dark mornings and light evenings.
I would recommend planting shade trees along the southern and western exposures as soon as possible so the trees can grow and start providing shade by the time you move in. You should also include window overhangs in your design—larger overhangs are better in a hot climate, especially in Puerto Rico, where there are zero "heating degree days." (Heating degree days reflect the demand for energy to heat a home over the course of a year.) When planning your window shading, put the priority on exterior shading devices such as louvers or awnings. These are more effective at blocking heat from the sun than interior blinds or shades.
I would determine which direction the prevailing cool breezes (ocean breezes in your case) are coming from and consider this as you decide how to orient the home. You will want to align window and door openings so they are facing the breezes, allowing airflow to help cool the house. Ideally you would have at least two windows in each room on different walls. Placing windows on opposite walls allows you to take advantage of cross-ventilation.
Cork flooring is a great flooring option made from a renewable resource. But for your home in Puerto Rico, since you will be living without central air-conditioning, I would suggest a light-colored material and a surface that remains cooler, such as tile or terrazzo. A hard surface would also be more durable and resistant to water damage. Tile and terrazzo can be heavy, and therefore expensive and energy-intensive to transport, so try to use materials that are manufactured locally.
I highly recommend that you use a white or light-colored roofing product which will reflect much of the heat that hits the roof surface. Installing sufficient insulation in the roof system will protect your home's interior against the remaining heat absorbed; I would recommend R-38 insulation for your region. (Use this insulation calculator to search for recommended R-values by U.S. zip-code.)
Since your house won't have central AC, I suggest you install a whole-house fan, located centrally at the highest point in the attic, to draw the heat out of the home.