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Sealing the Attic Floor

Wires, Light Cans, and More

by: Paul Kando

Most Maine homes could use additional insulation in the attic and in attics not used for storage this is an easy task. However, before any insulation is added, the leaks and penetrations in the attic floor should be sealed. Often there are tell-tale signs of where the leaks are: the existing insulation is soiled with black dust, and there maybe a concentration of cobwebs (spiders build their webs where the dinner is likely to float by on a convenient air-stream). However it is best to systematically inspect the attic floor by lifting the insulation section by section.

recessed light
Recessed Light 1
picture credit: Efficiency Vermont

Commonly, leaks are found where wires or stacks penetrate the attic floor, at electrical boxes, and at the cans of recessed lights sticking up into the attic. Let's take these one at a time. Wire penetrations are easy to fix with a dab of caulk or expanding spray-foam (like Great Stuff®) if the hole is large. Seal around plumbing stacks using spray foam; this hole is usually sizeable.

Seal around electrical boxes with caulk. Also caulk all the extra holes in the metal box from above, but do not squeeze more caulk into these than necessary to close them. Electric wires inside the boxes don't like to be caulked.

recessed light in cage
Recessed Light in cage
picture credit: Efficiency Vermont

Recessed lights are a special problem. They are major causes of heat loss because they function like chimneys, especially when the lights are on. Also the light-cans must be kept about 2" away from any insulation to avoid a fire hazard. The most energy efficient solution is to get rid of recessed lights. Or they can be replaced by new zero-clearance units. If neither of these options is feasible, the next best thing is to carefully seal any gap between the can and the attic floor (i.e. the ceiling below) with high temperature caulk. (Do not use ordinary caulk.) Next build a circular dam around the can to keep the insulation away. The dam should be as tall as the total amount of insulation on the attic floor. 1/2" x 1/2" mesh (wire) hardware cloth is a good product to make this dam out of. You can see a neat installation on Photograph 1. Still another option is to build a metal box around the light can, at least 2" larger on all sides and on top. This box will take the place of the wire mesh dam and may be round, square or multi-sided, as long as it is large enough. This idea is illustrated on Drawing 2.

sealing around the chimney
Sealing around the chimney
picture credit: Efficiency Vermont

Drawing 3. shows how to air-seal around a masonry chimney, using sections of metal flashing nailed to the surrounding framing so that its opposite edge fits snugly against the brickwork. After the metal has been installed on all four sides, the seams between the flashing pieces and between the chimney and the flashing are caulked with high temperature caulk. Round chimneys are dealt with similarly, except that the flashing must be cut to match the circumference of the chimney. If you don't know the radius of the curve you need to cut, measure the circumference with a tape or string, divide that measurement by 3.14, then divide the result by 2. The result will be the radius of the chimney stack. Draw a half circle with this radius on two suitably wide pieces of metal flashing, making sure they overlap when combined around snugly the chimney, nailing the pieces to the framing that surround the chimney. Seal as above. All chimneys must also have a dam around them to keep insulation 2" away on all sides.

Having done all this, you still have several major leaks to seal: wall cavities open to the attic and between the joists under any knee-walls. Those tasks will be covered in a future column.