Dear Jim: I opened the attic access panel in my closet to get a box and the heat was staggering. We have plenty attic insulation so is this increasing our cooling costs? If so, what can we do to reduce the heat?

- Michael N.

Dear Michael: Yes, that hot attic can increase your electric bills for air-conditioning. Most people are not aware how hot their attic gets in the afternoon sun. That heat does not just stay in the attic. It comes through the insulation to make the ceiling hot and your entire house warmer.

The form of heat that comes from a hot roof and attic is called radiant heat. This is a different form of heat transfer than normal conductive heat which thermal insulation is used to block. Radiant heat is the method the sun heats the Earth and why you feel warm in the sun on a cold day.

As your attic starts to warm up in the morning, the thermal (fiberglass, rock wool, etc.) insulation on the attic floor blocks the heat from the warm attic air. As the underside of the roof gets hotter in the sun, it gives off more radiant heat which can easily pass through the attic floor insulation.