Which Roof Do I Insulate?

We just moved into a 47 year old home which was built in sections. The original home had a peaked roof and as they added on to it they put on flat roofs. The house looked like it had a hat on, anyway, the flat roof leaked and we had a new peaked roof put on over the entire old one. We actully have two roofs, now, do I insulate the old roof so the insulation is close to the ceiling or do we insulate the new roof which would be over the old one. Thank you. Pat

 

Place the insulation as close to the ceiling as possible. How much insulation will that give you? (It will depend on how tall your joists are that this flat roof was covering. If it has 2X6's, you are limited to an R-19 insulation. 2X8's would allow you to use R-25.) Is that amount enough for your area? If not, and you need to add more, add it on top of the flat roof. Would it be possible to remove most (or all) of the flat roof at this point? Sure would make insulating the attic easier.

 

Put the insulation as close to the living space as possible. Insulation stops the transfer of heat/cold from one side to the other side of the insulation. If you insulate the roof rafters of your attic and not the floor of your attic, you attic becomes heated/cooled by the rest of the house giving up its heat/cool to the attic. Even if you don't intend to heat/cool the attic, if you do not insulate between the living space and the attic, the attic becomes heated or cooled because there is nothing to keep the heat/cool in the living space. Whatever temperature is inside the living space, will travel until it reaches the insulation.

The goal for most attics is a minimum of R19 (about 6" fiberglass batt insulation) but preferably up to R38. The larger the R value, the less temperatures will travel between the two sides of the insulation.

Don't forget to provide adequate ventilation in the uninsulated portions so that you do not develop mildew and rot. Ridge vents, gable vents, soffit vents all work in concert to bring fresh air in at the bottom of the roof structure and move it up and out the top.

 

I would like to thank you for your reply. The roof was put on in the summer and we have no way of stripping the old roof now. There are vents at each end of the gable and most of the old shingles were torn off before the new roof was constructed. We were thinking of having insulation blown in, is there any benefits in using the insulation you described as opposed to blown insulation?