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I live in Southwest Florida. Winter heating is not an issue, A/C is. Since heat rises, how important is it to heavily insulate the ceiling in a rancher? How much of an R-value should I try to get? Thanks.
Heat not only rises, but is conducted from the warmer to the cooler side, that means your baked in the hot Florida sun overheated attic to your living room that you are paying the electric company to keep cool.
I don't know what is recommended in Fla., but I wouldn't skimp. It also pays to keep the heat from building up in the attic by ventilating. I don't think 12" R38 would be overkill. Check with local building department & see what is required by code for new construction.
good point ed21. thanks.
Heat does not rise, that is a common misconception. Hot air rises, but heat always goes to cold. Heat move three ways, conduction, convection and radiation. Radiation and conduction are the primary movers in your situation.
The sun radiates heat to your roofs top surface. The radiation law of reciprocity says that the characteristics of a material to absorb heat is the same as to radiate it. So the roof absorbs the sun's rays and gets hot. The heat is conducted to both surfaces. Both sides radiate heat, the underside of the roof radiating into the attic. The attic air gets hot and the heat conducts to the top of your ceiling. The ceiling conducts the heat to both surfaces, where the underside radiates this heat into your house.
Insulation on the top side of the ceiling will slow down this conduction to the ceiling. A light colored top roof surface will help to keep down the absorption of heat.