That image above is of a recessed light in a house built in 1994. The blower door was 5750 CFM50, which unless one were testing the Astrodome, is quite high. The outside temperature was around 10F and you can see that inside the can is 27F. Bad bad bad.
One hates to overstate how problematic recessed lights can be, but…they sure are a pain in the energy auditor butt. There are worse problems (wet basements), more extensive ones (insulating a complicated roof line) and more frustrating ones (the cross-purposes of energy evaluations and homeowner desires). But few elements of the house combine all three in as tidy a package as recessed light cans.
Recessed lights can be an problem. A large, incredibly leaky problem. Of the 10 worst blower door numbers I’ve seen in newer houses, all 10 involved recessed light installations. A lousy blower door test means loads of conditioned air escaping the house and uncomfortable homeowners.
Fixing the recessed can air leaks can either be an enormous project or a an actually impossible project. You would either replace all the cans with air tight ones or build drywall boxes from overhead. Oh, wait? You don’t have access to the recessed cans? And good luck if they’ve been installed in a cathedral ceiling.
The final hurdle is that there’s a good chance that your customer likes them. You’ve tromped around for three hours, waved your thermal camera about and calmly declared that the homeowner’s sleek modern recessed lights are a big problem. It will always be a difficult conversation.
Recessed Lights – The Problems
Why all the venom? The building enclosure is a continuous layer separating the interior from the exterior. The closer the building enclosure is to a uninterrupted whole, the more efficient and comfortable the space will be.
The recessed lights replace ugly ceiling mounts with the trade-off being that you’ve punched a hole through the building enclosure. Sure, you’ve got a neat, uncluttered interior look. However, you’ve trade a substantial amount of conditioned air leakage for that appearance. The heat is escaping like crazy.
Where a homeowner sees neatness and order, I see an atrocity to building science. They penetrate the air barrier and almost always make the house less efficient.
What to Do?
You can check out this article for more details on how to air seal existing recessed light cans. But here are a few thoughts:
First, exorcise recessed light cans from your mental vocabulary. There are loads of ways to light your house other than gouging a hole through the air barrier. Once you move away from the recessed cans, you may improve on other recessed can issues (like that they are fixed in one spot, pointing straight down).
If you do plan on using recessed lights, consider building a soffit box. This will keep the lighting out of sight without punching holes into your attic. And if one insists on punching a hole into your attic, make sure the recessed can is a non-perforated ‘sealed’ can.Warning: Tech Talk!
I like to say materials are tested by very smart guys in lab coats. Recessed light cans are tested by the ASTM (American Society for Testing and Materials) using the ASTM E283 test procedure for air leakage. Super sexy.
Non-perforated recessed lights limit air leakage to 2.0 CFM (cubic feet per minute) at 75 pascals of pressure difference. That’s pretty tight; not air-tight but better than not. And they are always rated for insulation contact.
End of Warning: Tech Talk
Recessed lights don’t need to be the enemy. But too often they are installed with little care toward their impact on the building enclosure. Look at other options and if you still go recessed, make sure they’re not trashing the house’s air barrier.