Attics are a great place to reclaim living space without the expense of an addition. If you have the headroom, you can gain at least one extra room by finishing your attic.
But with energy codes requiring more and more insulation, it can be difficult to pack all of that R-value into the skinny little rafters that are common in older houses.
One solution is to add layers of rigid foam under the rafters, but this cuts in to living space. A better solution is to add layers of foam to the outside of the roof. This not only boosts the roof’s R-value, but it breaks the thermal bridge of the rafters and increases the air-tightness of the whole roof assembly.
1. The higher the R-value, the better, and where you put the insulation matters.
2. Exterior insulation solves a lot of thermal nosebleeds, but the amount of foam you put on the outside needs to be proportional with the amount of insulation on the inside, within the rafter cavity. You need enough exterior foam to prevent condensation on the inside of the roof sheathing by keeping that plywood warm.
3. Is it a good idea to make an unvented roof with a lot of foam on the outside? Yes. Roofs are like walls that are severely out of plumb, and an unvented wall can work.
Just make sure that you don’t do it willy-nilly: if you are going to ask more of your buildings, you need to ask more of your details.
For more roof design details, browse the detail library in the roof/wall connections section.