Insulation slows heat as it moves through solid material. Put enough of it between a warm space and a cold one and heat crosses more slowly. That is the entire mechanism, and it works well when heat has to actually travel through the material. Air sealing does something different: it closes the openings that let air move from one space to another, carrying its heat and moisture with it, without ever passing through insulation at all.
The practical consequence is that a house can be well insulated and still lose a great deal of heat, because air is bypassing the insulation instead of moving through it. That is why homeowners who add attic insulation and see little change are frequently not wrong about the product. They bought the right material for a problem they did not have, or they bought it before the problem it could not solve was addressed.
Air movement in a house is mostly vertical. Warm air rises and looks for a way out at the top, and as it leaves, air gets pulled in low to replace it. The top exits are in your ceiling plane: the wall cavities open at the top plate, the chases around plumbing stacks and flues, the holes cut for wiring, the recessed light housings, the bath fan boxes, and the attic hatch. The low intake is at the foundation, along the rim joist and sill.
None of these are visible from inside the house. You experience them as drafts near the floor, dust patterns around ceiling fixtures, rooms that give up their temperature quickly, or an attic that smells like the inside of your home. Insulation laid over the openings at the top does not stop the movement. Loose material in particular has air pass through it readily, which is exactly what it was designed to be.
Sealing has to happen at the ceiling plane, meaning at the attic floor. Insulation gets installed on top of that plane. If insulation goes down first, then anyone who comes back to seal has to move it, work under and around it, and put it back, or remove it entirely. That is labor you already paid for once, and it is why a contractor who does both will want the sequence in the right order.
This is also the clearest test of whether a bid was put together thoughtfully. A quote that names only depth and square footage, with no mention of the penetrations, has skipped the step that determines how well the rest performs. Ask for the sealing scope listed separately, with the openings it covers described. Two contractors quoting the same number can be selling very different jobs, and the sealing line is usually where the difference lives.
Ask what specifically will be sealed and where: top plates, chases, the hatch, can lights, fan housings, wiring and plumbing penetrations. Ask how recessed lights are being handled, since some can be covered and some cannot. Ask whether the bath fans terminate outside the house or into the attic, because a fan dumping into the attic is a moisture problem being fed by every shower, and no amount of insulation addresses it.
Then ask what happens at the eaves. Insulation blown to full depth around the perimeter can bury the soffit vents an attic needs for airflow, so ask whether baffles or blocking are included. A contractor who answers all of that easily is thinking about the attic as a system. One who redirects to depth alone is selling material, and material by itself is only part of what makes an attic work.
Sealing is frequently the less expensive of the two and occasionally the more noticeable. Drafts, cold floors near exterior walls, and rooms that lose temperature quickly are often air movement rather than material thickness, and closing the big openings can change how the house feels without touching the insulation at all. That is not always the case, but it happens often enough that it deserves consideration before a full attic job.
It is also worth saying plainly what neither job does. Sealing and insulating will not fix ducts that leak into a crawl space, a system that cannot keep up, or a window that has failed. If your house has one of those, addressing it is the purchase that matters, and an insulation contractor who tells you so is worth more to you than one who quietly writes the attic job anyway.
Sealing, in almost every case. It is what makes the insulation perform, it is harder and more expensive to do after insulation is in place, and it is often the smaller line item. A contractor who wants to seal first is not upselling you. They are keeping you from paying to move new material later.
Some homeowners do the accessible parts, like the hatch. The rest means working in low clearance over ceiling framing, around wiring and light housings, with dust and material in the way. Falling through a ceiling is a real risk, and some fixtures must not be covered. It is reasonable to at least have the attic assessed before deciding what to take on.
Yes, in the same way. Air moving between your house and the attic does not care about the season. In summer the attic is the hot side, and the openings that let heat out in winter let hot attic air in the rest of the year. The work is the same work regardless of when you have it done.
Cold rooms, drafts, ice at the eaves, or an attic you have never been in. Any of it is a place to start.