While what I am about to say seems ludicrous—even to me—but fiberglass insulation is pretty close to being an oxymoron. Sure, it is certainly better than nothing, but how much better than nothing is the real question.
Since the 1970’s I have railed that fiberglass batts are terrible and over and over again one can find installations where it is acting more like a filter than an insulator. That coupled with the ability of air to move around poorly fitted edges or at difficult angles, results in the all too familiar ghosting patterns that can be seen at interior ceilings and uneven melt patterns in frost at the exterior.
I personally think that the fiberglass insulation industry has some serious explaining to do and have likely perpetrated a fraud on a population convinced we need to save energy both personally and as a nation.
At a recent inspection, in the attic I noticed something I have seen before, but this time was able to get some pictures that I think convey what I will attempt to describe in words. First of all there was approximately 15 inches of insulation installed as indicated by the depth markers present—equivalent to about R-42. This quantity of insulation should certainly be sufficient to keep any house warm or cool assuming that it is acting like “insulation.” There were a large number of can lights present in the ceiling and all were energy efficient air-tight type cans rated for direct burial in insulation.
In this first picture the red arrow points to some tell-tail dirty insulation that was my first clue.
Moving some of the surface away even more of the dirty insulation became apparent.
In this picture, with the insulation removed down to the top of the can light even more of the sooting of the insulation becomes visible.
Because the can lights were likely not installed properly, every location gave itself away by a pattern of black sooting indicative of air movement around the cans and up through the 15” of insulation. Most of the year, 24/7 air is going to be moving into the attic around these can lights—through the insulation—due to “stack effect.”
Stack effect is the tendency for air to rise in buildings where it is pulled out of the building under negative pressure on the building’s exterior (wind going by) and/or positive pressure within the building. Two and three story houses accentuate this effect. This particular house had air intakes at all the windows in the house, consistent with Washington State energy code requirements for the time the home was built, for providing make-up air for the whole house ventilation fan and other ventilation fans. The problem is, that these stack-effect-assisted- can-lights are now vents all their own and result in air movement into the home 24/7 if the window vents are open and to some degree all the time. If the lights are turned on the venting is increased.
If all the window vents were closed, the whole house ventilation fans and other fans could even result in all these can lights becoming the source of make-up air for the fans. In other words air could move both ways through the insulation. This movement of air, regardless of direction, is what is causing the sooting visible in the insulation.
Can lights in general are a bad idea, and should never be used in insulated ceilings in my opinion. They can become a huge violation of the building envelope. While there are ways to seal these cans and adequately insulate over them, fiberglass insulation obviously is NOT the answer.
A can light that means energy code requirements has a gasket that is supposed to seal the gap between the can and the drywall. Until recently the gasket was always part of the trim kit and was meant to seal the gap between the metal can and the drywall surface. While the gap all the way around the can does not seem very big, even with the best fit it will always add up to at least 1-1/2 square inches of opening. For this gasket to function the can light has to be flush with the drywall surface. If the can light is recessed at all, or sticks down below the drywall, the gasket will not properly seal the opening. Of course caulking/sealing the gap around the can either above the ceiling or below the ceiling and not relying on the gasket is a good idea, but I can tell you this rarely gets done.
Perhaps as opposed to “can lights” we should call them “can vents”—as they certainly can vent.
Modern can lights have the gasket on the exterior side of the drywall with the idea that the drywall will seal against the gasket. However, even with this type of can light if the guts of the can light are not installed properly level with the bottom of the joists/rafters there will once again be no proper seal and air can by-pass in and out of the roof structure/attic.
Aside from the obvious issue with the improper installation of the can light, if air can move through this insulation so easily, what is to prevent heat from moving through it in areas where there is no actual air movement? While air is not all that good of a conductor, heat still rises and heat transfer can and will occur through the insulation. With this type of insulation, the air “pockets” are not so much pockets as passage ways. The best insulators have tiny air pockets that are not really connected together. That is why foam type insulation makes the best insulation—although it has its issues as well. Cellulose fiber in sufficient depths—certainly at the depths the above fiberglass insulation was installed to—is a very good insulator allowing for very little air movement under normal house pressure differentials.
Here is a drawing of what the poorly installed can light installation looks like and how air moves around it—regardless of where gasketing is installed.
The drawing shows the can installed too high for either the orange gasket on the trim or on the can above the ceiling to make contact sufficient to make a good seal. While the inner liner (the blue line in the drawing) is adjustable, and would help with the gasket on the trim if it is slid down enough to make contact, but is of no help (or minimal help) if the gasket is above the ceiling. The following picture is of one of the actual can light installations.
In the actual installation the gasket on the trim is visible and clearly would not seal against the edge of the metal housing and the housing has clearly been installed too high. Proper installation of the can light is obviously essential.
However, even proper installation of the can light will not fix the overall problem of the poor quality of the fiberglass insulation.
Charles Buell, Real Estate Inspections in Seattle