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The temperature experienced in any space is a combination of the ambient air temperature and the radiant heat exchange with the surroundings. If the surroundings are warmer than you, you gain heat; if colder, you lose heat - this is why sitting next to a window feels cold in the winter in N Europe. So the trick with passive designs is to pay attention to the temperature of the building fabric, as reducing the air temperature takes some active measures.
The first principle of passive temperature control is thermal mass. A large thermal mass reduces the swings between night and day temperatures, giving you a cooler temperature during the day. The simplest thermal mass to add is through walls. (You can also increase effective thermal mass by part burying a structure, but this can bring lots of flooding issues). A wall thickness of 1m or so is a good rule of thumb without calculation - what you are trying to achieve is a 12 hour delay between external and internal temperature peaks. (At night the air will be cold, so it does not matter if the internal wall surface is warmest then). Cheapest way to build walls is probably a brick or block skin with an infill.
Second feature is limiting thermal gain by the walls in the first place by using an overhanging roof to shade the walls. You can limit indirect gains by planting shrubs around the building (outside the overhang) to limit radiant gain from the surrounding ground.
Third feature is limiting isolation (sun shining through the windows) and radiant heating from the ground outside. This means using small windows (typically narrow slits that are just enough to provide lighting inside.
Fourth: To limit thermal gain from the roof you need two things:
4.1 A ceiling. This should be as high as possible (so that you get less heat gain from the warm air trapped under the roof effectively radiates less energy downwards), and ventilated (to allow warm air to pass through).
4.2 A ventilated roof space. Spaces under thatch roofs can be quite cool (as they are porous), but thatch brings a whole lot of maintenance issues. Mixing thatch and CGT sheets requires a complicated roof structure, unless directly overlaid with all the corrosion problems that that brings.
Options for ventilated CG roofs are using a "Mozambique" roof where the two parts of the roof have a vertical gap of 0.3m or so at the ridge that is fitted with a ventilator all along it, or fitting ridge ventilation all the way along the roof. Monopitch roofs are not recommended because of solar from the wall. You need to have fairly generously sized vents to encourage a good air flow.
As this is a drug store you will need to screen vents to prevent bat and rodent entry.
You can also fit lower level vents in the walls to encourage cooling through the stack effect, but the danger is of drawing in the very hot air from just off the ground, so you may need to put them a bit higher than you might otherwise.