Make sure your can lights are sealed, Go around and foam all light boxes from the top.I would use cellulose or foam. fiberglass will not do anything to stop air.

Any recessed lights in an insulated ceiling should be in air-sealed enclosures which shouldn't need to be sealed further but it does often reduce the maximum lamp size so you need to pick your fixtures carefully or perhaps use low voltage models.

The air barrier should be the drywall ceiling rather than the insulation unless you want double protection.

I always put the insulation between the rafters and avoid the problems associated with insulated ceilings and vented attics, especially when HVAC ducts or equipment are in the attic.

watch the rating on the can lights. Some are 0 clearence and some are not. That rating will determine how you insulate.

Look at your load calculations and see what R rating was used. If you don't meet that rating the equipment could fall short of capacity.

Recessed ceiling "downlights" for new construction fit into "housings" that are essentially metal boxes that are put into the ceiling before the ceiling finish is put up and the "trim kit" is installed into it later.

They have 3 basic classifications:
-for non-insulated ceilings (no insulation can be closer than 2" to housing and none on top)
-for insulated ceilings (insulation can touch housing)
-for insulated ceilings below roofs (insulation can touch housing and the enclosure is air tight with a seal ring at the ceiling opening.

These categories are shown in the housing model numbers so they are easy to check. Any "air-seal" housing should meet MEC & WSEC requirements.

There should be no reason to further seal an air-seal housing or be concerned about air movement through the insulation. NEVER ATTEMPT TO SEAL A UNIT NOT RATED FOR AIR-SEAL INSTALLATION.

The maximum recommended lamp size in an air-sealed unit is sometimes reduced for obvious reasons. Some are so small you have to wonder what use they would be in a home. If a larger lamp is installed later, the fixture might overheat and turn off, then turn on again after cooling. Eventually the fixture will fail and have be replaced.

This is not a do it yourself activity.

tom

i had blown cellulose installed in attic spaces. I had them blow in 11 inches of cellulose which provided an R38 barrier - min code is R30. In my last house we had R30 blown fiberglass and I did not like it at all - too many cold spots. I really like the cellulose - and it did not cost much for the extra few inches to get to the R38.

One thing I noticed aftre the install was that some of the IC recessed cans could not take the max bulb as stated above. it was generally the 4 inch IC cans that had the issue - the lights would turn off after an hour or so (heat sensor kicked in). I wound up backing off the wattage. If you see this happen - back off 10 to 15 watts on the bulb size. I used halogen bulbs so that i could back off the wattage but keep the lumens up.

even though we have insulation around the cans - it doesn't completely stop air infiltration. i think the only product that could do that would be foam but you have to look at the extra cost for that. I don't think a small trickle of fresh air coming in to replace the stale air is such a bad thing either. if you completely seal the house with foam - you will need an air exchange system to bring in freash air, especially if you are burning any kind of fuel for the HVAC system and that is another cost you have to consider.

Seal the house as best you can with exterior wrap, interior caulking of all stud seams and at the floor and ceiling, use foam in small voids, and then use the cellulose. also - make sure you get good windows and doors that provide tight seals since this is where you will have a higher potential for air infiltration then the recessed cans. This approach worked well for me and was cost effective.

I got to test this recently when we lost power for 26 hours in an ice strom and we had no heat (power lost at 1230 in the afternoon and wasn't turned back on until 230pm the next day). In 26 hours, the house temp only dropped from 65 (where we normally keep it) to 52. I was amazed that in the freezing cold that it wasn't more than that and there were no drafts. maybe an all foam package would have done better - but i am very happy with results for the measures that i took.

IC-rated air tight cans are the best choice.

The other thing to think about is other bypasses in the envelope. Things like:

-wiring and plumbing penetrations
-HVAC penetrations
-drywall-to-top-plate joints
-dropped soffits

If you have foam or cellulose, these will take care of these other bypasses (the soffit maybe as the exception). Otherwise, you'll want to go at them with canned foam or caulk to prevent every outlet and connection down lower from being a pathway for air to make it into your attic.

My personal preference is foam- I've found using it in my old house that it's best for the big gaps you'll find, and it's tenaciously sticky. It's not as flexible as caulk, but I haven't seen any problems from movement where I've used it (I think because of relatively small movement and good adherence).

To the several who mentioned it,
All the can lights are ICAT (in-contact air-tight) rated.
All the top plate penetrations are caulked.

To rabadger,
The load calculations were done by a highly-trained professional. Whether celulose or fiberglass (I see that most here prefer celulose), we will insulate to the R rating that was used in the calculations.

For those who mentioned foam,
Are you talking about the cans I could buy and put in myself (Great Stuff is one brand name) or are you talking about the foam a professional insulation company would use?

To everyone,
Thank you!

Insulation around the fixtures should not be expected to stop air infiltration; the seal should be at the ceiling plane. The fixtures should be rated as air-sealed or air-tight in addition to being rated for contact with insulation.