Attic Penetration Sealing

If you want to improve the energy performance of an older house, one of the first steps is to plug your attic air leaks.

There are four basic steps to sealing attic air leaks:

  • Inspecting your attic;
  • Patching the big holes;
  • Sealing the cracks and small holes;
  • Weatherstripping the access hatch.

Once this air sealing work is done, you may want to add more insulation to your attic floor. If you want to add insulation, remember that air leaks have to be sealed first.

Inspecting your attic

The easiest way to find air leaks is with a blower door. In some cases, a theatrical fog machine is also very useful.

If you don’t have a blower door, you’ll have to find your attic air leaks using your eyes and your powers of deduction. You’ll also need a powerful flashlight. If you don’t like balancing on joists, bring a couple of 2 ft. by 3 ft. pieces of plywood to step on while inspecting your attic. Needless to say, you don’t want to step between the floor joists and punch a hole in the ceiling.

If your attic has no vermiculite, the first step is to get your bearings and look around the entire attic. You may be surprised at what you discover.

If the attic floor is insulated with fiberglass batts, look for stained or dirty insulation. The most common cause of dirty fiberglass batts is air leakage; if the batts are very dirty, it means that dusty indoor air has been rushing through the batts for years. The batts strain out the dust from the flowing air, just like a furnace filter. If you lift the dirty batts, you’ll probably find a crack or a gaping hole.

Look for sunken batts. These are often a clue that there is a soffit or suspended (dropped) ceiling under the insulation. In most cases, these soffits and dropped ceilings lack any ceiling drywall — and therefore lack an air barrier.

Look for plumbing vent pipes, ducts, and exhaust fans. Areas near these items are frequent leak locations.

Think about the layout of the floor directly under the attic. If you know of any dropped ceilings or soffits, try to locate those areas in your attic. You’ll probably want to lift the existing batts (or redistribute the blown-in insulation) to find out what’s underneath.

If you take your time and you’re thorough, you should be able to find all of the big holes in your attic using these techniques.

Patching the big holes

Big holes in your attic floor — holes above soffits, dropped ceilings, and utility chases — can be patched with gypsum drywall, plywood, or OSB.

Whatever type of sheet material you use to seal your large holes, cut a piece of material so that it covers the hole, and secure it in place with nails or screws. The perimeter of each piece of material can be sealed with caulk, non-hardening acoustical sealant, or canned spray foam.

Gaps around brick chimneys are dealt with differently than holes above soffits. Because chimneys can be hot, these gaps should be covered with sheet metal, not rigid foam. After nailing four pieces of sheet metal in place — one on each side of the chimney — the seams where the metal pieces overlap and the gaps between the metal and the chimney can be sealed with high-temperature silicone caulk.

Gaps around metal chimneys are sealed with techniques that are similar to those used for brick chimneys. The easiest way to seal around a metal chimney is with two overlapping pieces of sheet metal.

Finally, it’s worth checking whether chimneys are in use before you begin your air sealing work. Unused chimneys represent a thermal bridge as well as an air-leakage path, so all unused chimneys — both brick chimneys and metal chimneys — should be removed. At the very least, the top section of an unused chimney should be demolished down to a level that is lower than the ceiling air barrier, so that the penetration through the attic floor can be patched.

Sealing cracks and small holes

Here are some of the cracks and small holes that need to be sealed in a typical attic:

  • Cracks near recessed can lights;
  • Cracks around ceiling-mounted duct boots;
  • Cracks around bath exhaust fans;
  • Cracks around plumbing vent pipes;
  • Cracks between partition top plates and partition drywall;
  • Cracks at ceiling electrical boxes;
  • Holes drilled in top plates for electrical cable.

Cracks near recessed can lights.

Most are responsible for very significant air leaks. The best solution to the can light problem is to permanently remove the can lights and replace them with surface-mounted fixtures. If you aren’t willing to do that, you may be able to install airtight covers on the attic side of the recessed cans to reduce air leakage.

Cracks around ceiling-mounted duct boots.

If your house has ceiling-mounted HVAC diffusers or grilles, then you’ll need to seal the crack between the galvanized duct boots and the ceiling drywall. It’s usually easier to seal these cracks from below than from above.

Cracks around bath exhaust fans.

Most ceiling-mounted bath fans have a removable plastic grille. Remove the grille from below and caulk the crack between the fan housing and the drywall.

Cracks around plumbing vent pipes.

These cracks need to be sealed with caulk or acoustical sealant; some builders prefer to use European air-sealing tapes.

Leaks at ceiling electrical boxes.

These leaks are fairly straightforward to seal. Using caulk, seal the crack between the electrical box and the ceiling drywall. Then seal around the knockouts at the sides and back of the box, as well as any location where electrical cables enter the box.

Holes drilled in top plates for electrical cable.

Again, these leaks are fairly straightforward. Seal them with caulk or canned spray foam.

Cracks between partition top plates and partition drywall.

A surprising volume of indoor air can escape through these cracks. Usually, the conditioned air enters the partition stud bays through electrical outlets and cracks at the bottom of the wall.

To seal these cracks, peel back the insulation above each partition top plate. The cracks can be sealed with caulk. If the top plates have lots of wiring penetrations, it’s often easiest to seal the entire top plate with spray foam from a two-component spray foam kit.

Access hatches and pull-down stairs

There are two problems with attic access hatches and pull-down attic stairs: they usually aren’t properly insulated, and they are rarely weatherstripped.

It’s easier to deal with an attic hatch than pull-down stairs. As long as the existing hatch is sturdy, the usual solution is to fasten or glue multiple layers of rigid foam on the attic side of the hatch.Then install weatherstripping on the lip where the hatch rests, as well as at least two latches that pull the hatch tightly against the weatherstripping.

To stop air leaks at pull-down attic stairs, you’ll need to install an elaborate cap.