Green Building Library
Weatherization Tips
How to Baffle Heat Producing Fixtures
- Some electrical devices located in attics, generate lots of heat when operating. If you pile insulation on top of a heat producing fixture (HPF), it could get hot enough to start a fire. Before blowing insulation into an attic, place baffles around all HPFs.
Heat Producing Fixtures
Heat producing fixtures include most recessed lights and heaters, doorbell transformers, electrical wiring connections not in junction boxes, metal flues and knob & tube wiring. Heaters with an Underwriter's Laboratory label that says "Heater" or "Air Heater" don't need to be baffled. The same is true of newer recessed lights that are rated "IC," which means they can be covered with insulation. They must have an identifying label. The label could also be on the outside of the housing, visible from the attic. If it's inside the housing, you'll have to remove the light bulb and maybe the trim piece that fits inside the housing to find the label. If you don't see any of these labels, then you must baffle the fixture.
Baffles
Baffles should be made from a solid, flame resistant material. Wide metal flashing makes a great baffle (except for use around wiring). It's easy to bend, can be fastened with staples, and cuts easily with a utility knife. In some areas, fiberglass insulation batts can be used as baffles. Treated cardboard, sometimes used to baffle attic vents, is not acceptable for baffling HPFs. Baffles must be be firmly attached to something in the ceiling structure, such as the ceiling joists, rafters, or roof sheathing.
Clearance Between HPFs and Insulation
Baffles prevent overheating by maintaining the proper clearance between the fixture and the insulation. The "Three Four Rule" is an easy way to remember the proper clearance. Don't create too much clearance, because the additional uninsulated area will increase heat loss.
3" |
At least 3 in.—but not more than 4 in.—clearance between the HPF and the baffle. |
4" |
Baffles should have an open top that extends 4 in. above the final level of insulation. This makes it easier to keep insulation from falling into the baffle. |
HPFs Between Joists
Staple one end of the flashing to a ceiling joist. Wrap the free end around the HPF and staple the other end to the same joist. A round baffle is easiest, but sometimes that would make the clearance greater than 4 in. One way to get the right clearance is to make the baffle in the shape of a comma (,) as shown in the drawing. Some installers find it easier to make the baffle from two pieces of flashing.
HPFs Over Joists
If the HPF is close to a joist, the baffle must go around the joist to maintain the proper clearance. Cut slots in the flashing with a utility knife. Drop the baffle over the joists and staple the tab to the top of the joist. Be sure the bottom of the baffle rests on the ceiling, so insulation can't sneak under the baffle.
HPFs Near Eaves
You will often find HPFs near eaves with very little space above them. In this case, staple flashing to the roof sheathing and framing. It may be easier to use several smaller pieces instead of one big piece. Try to leave an opening at the top that will be 4 in. above the finished insulation level.
© Copyright 1993 Iris Communications, Inc.
