How to Build a Box Around an Attic Furnace to Prevent Ice Dams

Installing a furnace in your attic can save space in other parts of your home and give the heat more direct access to the home's ventilation system. However, it may also increase the risk of ice dams. An ice dam forms when heat escapes through the roof, melting snow and ice, which then freezes once temperatures drop. Water pools up behind the ice dam and may enter the home through the roof, damaging roofing materials, which may necessitate expensive repairs. Building a tight box around an attic furnace will likely violate building codes, which often call for several feet of free space around an attic furnace. However, you can construct an insulated box to partially conceal the furnace's heat as long as you have enough room to do so.

Tape measure
1/2-inch plywood sheets
Circular saw
Safety glasses
2 sawhorses
Tubes of foam adhesive
Rigid foam insulation panels
Utility knife
Box of 2-inch nails
Claw tooth hammer
Tubes of silicone caulk

Instructions

- 1
  Measure the space around your attic with a tape measure, including the distance between roof trusses and the height of the trusses above the floor. Determine the dimensions for a three-sided box that attaches to the trusses and provides two sides and a top that will conceal the portion of the attic where the furnace is located.
- 2
  Transfer your measurements to sheets of 1/2-inch plywood. Lay the plywood over a pair of sawhorses and cut out the sides of your box with a circular saw.
- 3
  Cut rigid foam insulation panels so they're slightly smaller than the plywood sides for your box. Cut several pieces of insulation to cover each piece of plywood, if necessary.
- 4
  Apply beads of foam adhesive to the interior surface of each piece of your box. Place the beads several inches apart and within 2 inches of the edges of the plywood.
- 5
  Attach the foam insulation panels by pressing them firmly against the plywood, twisting gently to spread out the adhesive. Allow the adhesive to dry according to the manufacturer's recommended timeline before proceeding.
- 6
  Position the plywood panel that serves as the top of your box against the roof trusses directly above the furnace. Nail it to the trusses using 2-inch nails and a hammer. Nail directly through the insulation and plywood, into the trusses. Place the nails about 6 inches apart along each truss that the panel touches.
- 7
  Position one side panel so it abuts the top panel of your box, with the insulated side facing inward, toward the furnace. Nail the top edge of the panel to the roof truss using the same nails and spacing as you did with the top panel. Repeat the procedure for the other side panel.
- 8
  Apply beads of silicone caulk to the corner seams where the side panels of your box meet the top panel.

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