I look the attic over before anybody seals a thing, because a few problems have to be fixed first, and I've listed those in the questions below. Then our crew works across the attic floor and closes the gaps between your living space and the attic, around recessed lights, plumbing pipes, wiring holes, bath fans, the attic hatch and the tops of the inside walls, using caulk, foam or rigid blocking depending on how big the hole is. Where old insulation covers a spot, we pull it back, seal underneath and lay it back down instead of leaving it in a heap. We leave the soffit, ridge and gable vents alone, because your attic needs to breathe, and closing those just trades a draft problem for a moisture problem. If you've got a gas furnace or water heater venting up a chimney, we'll talk about getting it tested for backdrafting, which is when exhaust comes back into the house instead of going up the flue, since sealing changes how air moves through your house.
Sealing often ties into other work. If your house has a basement, the top of the foundation wall is a common leak, so we usually seal it as part of basement insulation. When your ductwork runs through the attic, closed-cell foam insulation, a dense foam that also blocks moisture, sprayed under the roof seals and insulates in one go and brings those ducts inside. It costs more, though, and when the ducts aren't up there, sealing the gaps and topping up the loose fill is usually the better buy.
Sealing the attic floor on its own
This suits a house that already has a decent layer of insulation up there but still has drafty rooms or a hot upstairs. We pull back what's in the way, seal underneath and lay it back, so you aren't paying again for material you already own.
Sealing first, then a fresh layer on top
This suits an attic with thin, packed-down insulation, and anything soaked or full of droppings gets insulation removal instead of being buried. We seal while the floor is open, then add blown-in insulation, loose fill blown through a hose, evenly all the way out to the edges.