Insulating Can Lights: The Rest of the Story

In my post of 4 February 2015, I discussed “energy leaks,” and specifically leaks that I thought were due to air flow through “can light” fixtures that penetrated the ceiling of the kitchen and floor of the attic.  I noted at the time, “The only break in the insulation envelope, is a pair of can lights above the kitchen counter, their location corresponding to the left (west) edge of the heat shadow on the roof.”  A “small and slow improvement” was made, which was basically to add more fiberglass batt insulation on top of the can lights.  The folly of this effort was soon in evidence, as after a more recent snowfall I could again see a “heat shadow” forming on the roof in the same location.  Fortunately, my friend Sam saw the post, and gave me some good advice:   “As for your fiberglass experiment over the recessed lighting. In my experience the air flow through fiberglass batts make excellent air filters and not much else.  Recessed lighting is notorious for being leaky devices that as you rightly state let the warm conditioned air of your living space into the unconditioned space of your attic.  Fiberglass loses its insulative capacity and is short circuited by air flow, so if it is not installed in a situation where there are an air barriers the R-value is decreased.  You might want to try recessed lighting insulation covers (yes, they are a thing) and then place the insulation over the top of those.  The covers allow you to seal around the light and reduce the air exchange going on with the hole in your ceiling.”  Indeed!  And thank you Sam!

Kitchen attic and fiberglass batt insulation over can lights (left), and close-up
of can lights (right)

I picked up some Insulmax CanCaps at Menards; I surely could have rigged up a homemade solution, but I was concerned about matching the “Class A fire retardant” rating (850 deg F) of the CanCaps, and at $15 each, I decided to take the surer, and easier, route.

It is marketing material to be sure, but on the label of each CanCap is the statement, “Over 2.6 Million cubic feet of air and humidity flow in and out of ONE recessed light fixture in a single year.  To give you an idea of how big that is… you could fill the entire Houston Astrodome with the air transfer of only 6 light cans in less than 3 years!”

I am not sure about you, but I would rather not be cutting that much firewood!

CanCaps, cut to fit, with special adhesive (left), and fitted in attic space
before application of adhesive (right)

In the pictures you can see that some fitting was required, and so the gaps between the caps and the floor joist and roof rafter must be filled.  Loctite PL300 Foamboard Construction Adhesive was used, which will not “eat” the foam that the CanCaps are made of.  Probably most difficult was the fitting of the CanCap nearest the eave in both pictures, between the roof rafter (2 x 12 in.) and the floor joist (2 x 6 in.), and ensuring that the joints were sealed completely with construction adhesive.  No doubt I could have used less adhesive had I been more experienced; as it was I went through the entire tube of adhesive to seal and secure these two CanCaps.

Construction adhesive “seal” in place

Remaining then was to notch the CanCap covers to allow for entry and exit of wiring, and to install the covers.  The penetrations for the wiring were also sealed using the foam board construction adhesive.  Finally, I re-installed the R-19 fiberglass batt insulation around and above the caps.  The CanCaps have a R-Value of 4.35.

There were two other light cans penetrating the attic floor, in the main section of the attic; those cans were also covered with CanCaps, although no “fitting” was required.

CanCap installation complete (left), fiberglass batt re-installed (right)

I have not performed any calculations, but it seems likely that the number of BTU’s being lost from the house through microwave vents, bathroom vents and can lights, is significant; in each instance there is in effect, an open hole between the climate-controlled interior of the house and the outdoors.  Since the future includes heating the home with wood burning stove(s) and a outdoor wood boiler (OWB), avoiding unnecessary heat loss is a top priority.

Thank you for reading and commenting on the blog.  Your comments and criticisms, your inputs and acknowledgements, are welcomed, and will help me to improve my posts.  I am learning, too.

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— John, 09 February 2015

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