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Sealing Can (Recessed) Lights

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.

Duct & Air Sealing

leaky ducts are a huge energy waster!

At we would LOVE to sell you some radiant barrier foil. However, our FIRST priority is to help you save money and make your home more comfortable! If you have an older home check the ducts for leakage before you ever think about buying radiant barrier or more attic insulation. If your air ducts are leaky, several things can be happening. Leaks on the SUPPLY side push hot or cold air INTO the attic instead of your home. This is money wasted and causes outside air to be pulled into the home due to negative pressure. Leaks on the RETURN side cause hot, humid dirty attic air to be drawn directly into your air conditioner. This is like running a hair dryer in your refrigerator and will cause the home to be hot, humid and inefficient.


The key to preventing ANY moisture problems is to seal, seal, seal holes in the ceiling under the foil. If you can prevent the water vapor from condensing then there is usually never an issue to deal with. Oftentimes customers lean on the conservative side and cut holes in the radiant barrier above potential moisture spots to allow extra drying potential (ability for water vapor to pass through).


Older ducts, especially metal ducts, are notorious for leaking. Studies show that there is generally a direct correlation between the age of the duct system and the percentage of duct leakage. Additionally, wrapping a duct with insulation does virtually nothing to reduce duct leakage; it’s like wrapping a leaky pipe with a rag – even if it slows down the flow, it still will not completely stop a water leak.


The duct collar should be sealed with duct mastic to prevent air leakage and make an air tight seal.

Old ductwork separated from the plenum box can be seen in this photo.

The dark line on the insulation is caused from years of air being pushed out of the metal seam.

Energy Saving Tips:

Check for leaky ducts. This is probably the most overlooked area on most homes. If the air in your ducts were water, your attic would probably fill up within a few minutes. In California, they require duct testing/sealing to be performed with the installation of a new A/C unit – read about it here. Many homes (even newer homes) have duct leakage of over 15%; in older homes with metal duct work, the leakage is usually OVER 50%! Don’t waste your time on radiant barrier or insulation until your ducts are tight. Here are some industry study results on duct leakage and other air conditioning problems.


Find possible sites for air infiltration/holes in your home. Once again, think of your home like a refrigerator. It does not matter how much insulation you put on it, until you close the door it’s going to run all the time. Can lights, attic doors, windows, doors, etc., anyplace air can get in needs to be inspected. Seal them up!


Inspect areas of thermal bypass. Thermal bypass is is a term for places where heat is entering your home; these are essentially bald spots in the homes insulation or thermal envelope. You can reference the Thermal Bypass Checklist at the EPA/EnergyStar website. If you have a 2 story home please read section 4: Walls Adjoining Unconditioned Spaces.


Check attic ventilation. Radiant barrier will reduce the heat getting in, but you still need good airflow to help carry the heat out. Attic ventilation is very simple: holes in the top of your attic (i.e. wind turbines, static vents or attic fans – which consequently we do not recommend) and holes in the bottom of your attic (usually in the form of soffit vents) that are clear and open. You must have both! Air cannot move out unless it can move in from the bottom. Electric attic fans usually just suck the cold air out of your home, which is why we typically recommend passive systems for attic ventilation.


Windows that catch sunlight. We hate to tell you this, but windows, and especially west-facing windows, are a brutal source of heat gain. As a rule, a home’s heating/cooling bill is always directly proportional to the percentage of window surface area. The best windows have an R-Value of about 3 to 4. Even low-e double pane windows are a gap in the thermal envelope that allow heat to enter or escape. What can you do? If a window catches direct sunlight, then we recommend you invest in some solar screens or really good professional-grade window film (nano-ceramic window film) as it will make a huge difference.