most can lights are not airtight
A common question is whether or not it’s OK to install AtticFoil™ brand reflective insulation directly over can lights or other types of recessed lighting, when installing Radiant Barrier Directly Over Attic Insulation. These types of lights are also called “pot lights” in some areas.
The problem with can lights is they are usually NOT airtight. This means that air can easily move from inside the home into the attic directly through the can light. Usually the air inside the home is relatively warm and moist when compared to the air in the attic. In cold weather, the foil inside a vented attic is usually pretty cold. When warm moist air comes in contact with a cold surface (the radiant barrier) you get condensation. Condensation occurs when water in a vapor form converts to its liquid form. This water can turn to ice if it’s cold enough. The last thing we want is water or ice forming under your radiant barrier foil.
You must always use perforated radiant barrier foil when installing directly on top of existing attic insulation.
How Do I Prevent Condensation Under Radiant Barrier?
You have two choices: seal and make the can lights airtight, or cut a hole in the radiant barrier to allow the warm-moist air to BYPASS the foil insulation. Your first choice should be to seal the can lights. When you think about it, can lights are just 6″ holes in your home, also called the “thermal enclosure.” This is equivalent to going up to your refrigerator and drilling a bunch of 1″ holes in it. Sealing can lights is probably one of the best things you can do to help reduce air infiltration and reduce heat gain/loss into the home.
In fact, you want to seal ANY hole in the ceiling that allows a path for warm-moist air to go directly into the attic. Check areas around light fixtures, ceiling fans, smoke detectors, and air conditioning ducts and registers.
Method #1: Seal the Can Lights
Sealing can lights can be done by several methods. You can install new “airtight” lights that you can bury under insulation. These are labeled ICAT (Insulation-Contact-Air-Tight). These lights will provide both air sealing and a thermal barrier, when correctly insulated. Other options include making a foam box or enclosure (buckets, cans, etc.) to place over an older “leaky” can light and then cover it with insulation (see the photo at the top of the page). These enclosures can be sealed to the sheet rock with a can of foam. The box (or enclosure) will provide the air sealing, and the space between the light and the enclosure will create the required space between the light and any insulation to dissipate heat and keep the lights from overheating. Another option is to install an airtight trim kit. These are little cones that install from the bottom to provide a seal around the bulb and the sheet rock. Changing to CFL bulbs also significantly reduces the heat generated by can lights.
This picture illustrates using foam board to create a box that fully encapsulates the can light. The box is airtight and is sealed to the sheetrock with a can of foam. The box is several inches away from the can and will provide a space required for Non-Insulation Contact Can lights. Once the box is installed and sealed to the sheetrock, then additional insulation can be put on top.
Method #2: Cut Holes in the Radiant Barrier Over Can Lights
If in doubt, cut a hole in the foil. Unless you are SURE your can lights are airtight, you should cut a hole about 12″ round in the radiant barrier foil directly above the can lights. This will allow any air leakage with warm-moist air to bypass the foil and prevent it from getting “trapped” below the foil. For many customers this is the easiest and safest way to prevent any potential moisture problems. Don’t worry about any decrease in the effectiveness of the radiant barrier. Cutting a few holes in hundreds or thousands of square feet of radiant barrier will have only a trivial impact on its performance.
Cutting a hole is the radiant barrier foil is not ideal, but it is much better than getting condensation below the foil. If you think your can lights are leaky, cut a 12″ hole directly above them.