Warm air rises, cool air sinks. Because walls and windows are usually cooler than the middle of a room, they spur convective loops that can feel like a draft. A similar thing can happen inside a wall cavity.
When air is heated, it expands, and therefore becomes less dense, so it rises. The rising warm air displaces cooler air, which sinks. When the motion is constant, it’s called a convective loop.
Woodstoves and windows cause convective loops by heating or cooling (respectively) the air closest to them.
Even in homes with airtight walls and ceilings, convective loops can feel like a cool draft and be uncomfortable to the people in the room.
Convective loops can occur inside poorly insulated wall cavities, too, degrading the performance of the insulation.
Heat flows through materials by conduction
Wood is a better insulator than nothing at all. Snow melts off the uninsulated rafter bays more quickly than directly above the wooden trusses, which have an R-valuearound 1.1 per inch, or R-4 for a 2×4 top chord.
Conduction is the flow of heat energy by direct contact, through a single material or through materials that are touching.
Substances that conduct heat readily are called conductors, while substances that don’t conduct heat readily are called insulators. Metal is a good conductor; foam is a good insulator. Wood falls somewhere in between.
Radiation heats objects, not air
Radiation is the transfer of heat by electromagnetic waves that travel through a vacuum (like space) or air.
Radiation cannot pass through a solid object like plywood roof sheathing. When the sun shines on asphalt shingles, heat is transferred to the plywood sheathing by conduction. After the plywood has been warmed by conduction, it can radiate heat into the attic.
Radiant barriers are materials (for example, aluminum foil) with a low-emissivity (low-e) surface. Although radiant barriers have a few applications in residential construction—they are sometimes integrated with roof sheathing—they are rarely cost-effective when compared to conventional insulation options.