Your home’s roof structure is similar to your skin. The roof’s surface is heated by the sun’s rays. Through conduction, the entire bulk of the roof (roofing, roof paper, nails, sheathing, and rafters) warms up. The structural mass of the roof will soon get so heated that it will radiate heat on its own (like the sun). This radiant heat travels through the attic space and strikes the material on the ceiling structure’s surface (insulation, wood joists, drywall, ducting etc.). The entire mass of the ceiling structure suddenly transforms into a massive heat radiator. This heat returns to the roof, where it will continue to emit heat long after the sun has set. Much of this heat will radiate downward into your home if your ceiling is not airtight and exceptionally well insulated.
Air in the attic that comes into contact with the hot frame surfaces will be heated by conduction. A heated air sandwich is generated between the roof and ceiling structures as a result of this. The attic interior becomes substantially hotter than the outside temperature due to the heat of all these surfaces.
Some roofing materials would be affected by trapped air and heat in the attic. It could also result in condensation, which could lead to mold problems. This is why ventilation must be included into the building of attic areas. The lower eave portions and the top roof ridge or gable end have air holes. Fresher, colder outside air circulates naturally through convection up from the eaves and out through the gable or ridge vents through these apertures.
Any HVAC equipment located in an overheated attic will be under constant stress and may not last as long due to the overtaxing of its systems.
If your home has a heated attic and ceilings in the summer, a system, not just a powered attic exhaust fan or ventilator, is the answer. A design that includes radiant barriers and sufficient ventilation should be considered to eliminate a hot attic. This is always best done when the house is being built. It may be difficult, if not impossible, to adapt this construction once it has been built.
What you need to do is keep the heat from escaping into the house. A completely sealed ceiling, a very thick layer of insulation, radiant barriers (reflective foil layers) above the insulation (preferably between the rafters) to block the radiation and isolate your hot attic from your cool house, additional ventilation openings, and possibly a powered attic fan to remove warm air from the attic at the proper exchange rate are all likely requirements for your plan.
Consider the conditioned-space attic, which is an even better option. Using foam insulation placed to the pitched roof, create a conditioned space in your attic. This resulted in a perfectly cool attic, lower electricity bills, and easier temperature control in the second-floor rooms. Despite the fact that this layout was a little more expensive, it provided a completely cool and comfortable second level.