Hi Bob. Hope all's well.
To seal your attic, firstly I would determine if you have a vapor barrier in your attic. If no and if your insulation is old and in poor shape, you may want to remove it and re-use any if it is salvageable and start over with a bare attic.
Go out and buy yourself a good foam gun, cans of foam - + the can of cleaner or you'll be sorry ! - thick rubber gloves or you'll never get the foam residue off your fingers, a good respirator to protect your lungs, coveralls ... etc.
Your goal is to seal the attic floor tightly throughout to prevent or at least slow the transfer of moisture laden air from your warm zone to the cold zone in your attic. Some say it is more important than laying a poly vapor barrier.
- Check all the small foamable gaps, voids, holes, openings that you can find ... check around for old wiring holes and new ones and foam all the holes.
- Check around the bases of any plumbing or vent stacks ... stuff all gaps with a filler - pieces of scrap insulation - and then spray foam on top until it has a tight seal.
- If you have a chimney, check around the base of it for gaps, voids ... etc. DO NOT USE FOAM HERE; IT IS FLAMMABLE. Use a non-flammable mineral wool product to place in the gaps or you can cover them over with a galvanized metal sheeting and seal them on the edges with high temperature chimney caulking.
- Check all the top plates of perimeter walls, interior walls, closet walls, divider walls ... etc. You should be able to see the outlines on those top plates. As the house settles they tend to shrink and create gaps where warm air can escape into the attic. Spray foam the entire top plate or cover it with pieces of pink or blue board and spray foam the edges so it is sealed.
- For larger holes, you may want to frame them in and cover them with plywood and then a piece of pink/blue board and spray foam the edges ...
- Check at any new, renovation additions as there may be gaps there or at pony walls ... etc.
SEAL ALL THE HOLES AND GAPS IN THE ATTIC THAT YOU CAN SEE !!!
Consult with a professional insulator if you need a 6 mil poly vapor barrier. Some say it is a must. Some say don't add a poly barrier to an old house. But I do know that sealing your attic is your best defence.
Remember that attic ventilation's important too ... check if you have enough and if it is working ... this is another topic !
Good luck Bob.