Walls and Ceilings - Insulation dilemma

I have an old 2 story home(bottom floor is cut into the top of a hill, almost a basement) that gets very hot in the summer, especially the top floor. I am thinking about replacing the upstairs ceiling and in the process putting in some thick insulation(there is none at the present) to keep some of the heat from coming through from the roof. The problem is that in the winter, this same heat comes in and warms up the house during the day and at night as the heat is slowly released. This is in S. Calif, up in the Santa Monica Mountains, rarely get frost, low temperatures usually in the upper 30's.

If I insulate the attic and block off some of the heat coming from the roof, then in the winter this will probably result in increased heating bills because the house won't warm up as much during the day. And that's my dilemma, the summer heat is miserable and I want to do anything I can to alleviate it, but I'm not sure how to approach this problem. I'd welcome any suggestions.

John

I don't think you are looking at the complete picture. Maybe the heat from the attic does provide some help in the winter daytime but what about all the heat you lose at night. Personally I can't imagine insulation not helping.

I'm not sure what the whole picture is...but at 9pm last night the indoor temperature was 68deg and outdoor 57deg. That's 3 1/2 hrs after the sun went down, and the temperature reached about 86 deg upstairs during the day. It was warmer inside the house than outside, so I have to assume that the warmth came through the roof and was trapped inside, accumulating. Because the downstairs is about 50% cement walls I'm guessing that they absorbed some of this heat during the day and released it as the temperatures cooled down.

Thus my question, if there was really good insulation in the attic, how much less heat would get into the house and how is that balanced by the insulation helping the house retain what heat does get through. If the attic insulation lowers the daytime temperature by 10 deg, will there be enough heat left to keep the house at 68 deg 3 1/2 hours after the sun goes down?

This sounds like a question for an engineer, doesn't it.

John

You are missing one very important factor since you are concentrating on the roof.

You home is sitting on and partially in a huge temperature moderator. It warms you when it is cold and it cools you when it is hot. Energy specialits would call it a heat sink since it moderates the temperature of the air around it.

Having lived in the area, I understand how warm it can get, depending on the wind direction. Insulating the roof will provide benefits by maintaining a more moderate temperature inside during all seasons. With you climate try to concentrate on ventilation to improve the comfort level.

Dick