_____ are intended to reduce the summer's heat gain and the winter's heat loss.

Radiant-heat barriers are specifically designed to reflect radiant heat away from living spaces during the summer months and to prevent heat loss during the winter. These barriers work by using materials that have high reflectivity and low emissivity, effectively keeping heat out in warmer conditions and retaining it in cooler conditions.

In contrast, insulation materials primarily function to slow down heat flow by adding thermal resistance, but they do not reflect heat. Natural ventilation provides fresh air and helps regulate temperature through air movement, rather than specifically addressing heat gain or loss. Mechanical systems, like HVAC, actively manage temperature but do not address the challenges of radiant heat flow directly. Thus, radiant-heat barriers are uniquely effective for controlling heat transfer due to their reflective properties, making them the correct choice in this context.