If the house has an accessible attic, insulating and air sealing can be done at the ceiling level, from the attic side. If there are cathedral ceilings or sections of attic that are not accessible, then the only good solution is to insulate on top of the roof.
If insulating the attic at the ceiling level, be sure to fully air seal the ceiling/attic plane using spray foam. Chimneys require two inches of clearance from combustibles, so seal around the chimney with metal that is caulked to the chimney using fire rated caulk. However, the building side of the metal can be sealed to the structure using any type of caulk. The two inch space should be insulated with Rocksilk or other nonflammable insulation. Do not eliminate the air sealing as the Rocksilk insulation is air open.
Existing insulation will have to be removed to allow air sealing of all of the penetrations into the attic from below. Items like wires and pipes have to be sealed where they breach the ceiling. The spaces between the top wall plates and the adjacent sheetrock are closed using caulk or spray foam. Anything that goes through the ceiling and into the attic has to be sealed on the attic side.
Recessed lights that protrude into the attic need to be rated for insulation and have a thermal switch that automatically turns the light off if the assembly becomes too hot. Replacing the incandescent bulbs with LED bulbs is also recommended. Build insulation boxes, with the seams taped closed, and leave one side open. The open side of the box is set over the light assembly and caulked to the attic side of the ceiling. This creates an airtight box while keeping the insulation from coming in direct contact with the light fixture.
Once all of the holes have been sealed, the previously removed insulation can be replaced provided it is still in good shape. There should not be any gaps in the reinstalled insulation. Fiberglass insulation needs to be air sealed on all six sides to function because it is air open. Once the air sealing is complete and any reused insulation is in place, cellulose insulation should be blown in. It can go on top of any fiberglass to fill in the gaps and seal the top to restore the insulating function of the fiberglass. The cellulose should be at least 12” deep. More is always good due to the low cost of the cellulose.
If the attic does not fully encompass the ceiling, or there are inaccessible attic areas, cathedral ceilings and other barriers which do not allow access to fully air seal and insulate, or if the homeowners want to use the attic for storage, then the insulation and air sealing needs to be done from the outside, on top of the roof’s sheathing. Begin by stripping off the existing roofing and cutting the roof overhangs back to the exterior edge of the existing wall.The first layer on top of the sheathing needs to be air and water tight. A self-adhering ice and water shield is the product to use here. This will prevent air and moisture from penetrating the insulation from below and will also keep any moisture that does get into the insulation from moving down into the structure. Think of this as an inner roof that is never exposed to the elements and thus will remain functional for the life of the building. Layers of two inch isocyanurate insulation will then be placed over the ice and water shield. The seams of each layer need to be offset at least twelve inches from the layer below, and sealed to eliminate any micro channels. These channels allow movement of heat and moisture that can lead to rot and mold issues. You only need to use enough fasteners to hold the insulation in place while the next layer is being positioned. Once the insulation is thick enough to meet or exceed the code requirement, secure all of the layers to the roof using 5/4” x 3” furring strips with screws long enough to go into the rafters at least 1 ½”. As with the walls, these screws need to be spline drive, or the screw heads are likely to strip. Next, a layer of sheathing needs to be secured with screws to the furring strips. The roofing will be applied to this sheathing appropriate to the material selected. The peak should be vented using one of the many products made for this purpose. This provides a fully vented roof that is also insulated. This will prevent ice dams from occurring because the roof surface will be cold. Any moisture that gets under the top roof will not become trapped under the sheathing because of the venting at the peak.