If you want to improve the energy performance of an older house, one of the first steps is to plug your attic air leaks. Although many GBA articles address aspects of attic air sealing, no single article provides an overview of the topic. This article is an attempt to provide that missing overview.
I’ll try to explain how you can seal air leaks in a conventional vented, unconditioned attic. If your house has cathedral ceilings — that is, insulated sloped roof assemblies — the air sealing tips in this article don’t apply to your house.
There are four basic steps to sealing attic air leaks:
- Inspecting your attic;
- Patching the big holes;
- Sealing the cracks and small holes; and
- 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. (For more information on these two pieces of equipment, see Blower Door Basics and Pinpointing Leaks With a Fog Machine.)
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.
Warning: if your attic floor is insulated with vermiculite, a type of insulation that may contain asbestos, don’t touch the insulation. Don’t attempt any air sealing work in an attic insulated with vermiculite. Removal of vermiculite insulation should only be performed by a certified asbestos abatement contractor.
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. For example, it isn’t too unusual to find gross insulation or ventilation system defects in the attic, including joist bays with no insulation and bathroom fans that direct exhaust air into the attic. Needless to say, these defects will need to be remedied. The first order of business, however, is to seal the air leaks.
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. Perhaps the easiest material to use, however, is foil-faced polyisocyanurate, since it’s easy to cut and easy to tape.
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. Here’s a link to a GBA video that demonstrates this type of air sealing: How to air seal soffits and chases in an attic.
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; of course, you’ll need to cut each piece with a half-moon hole that corresponds to the chimney diameter.
Manufacturers of metal chimneys and most building codes require a 2-inch air space between the chimney and any framing lumber. Respect this air space; avoid the temptation to fill the air space with insulation.
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. Recessed can lights are bad news; 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. For more information on this topic, see Recessed Can Lights.
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. Don’t make the mistake of filling the box with canned spray foam — that would be a code violation and would create a fire hazard.
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. Unless the drywall contractor installed a bead of adhesive along the top plate — most don’t — air will continually escape along this crack.
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. Here’s a link to a GBA video that demonstrates how to seal these cracks: How to Air-Seal Ceiling-to-Wall Connections in Attics.
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. (Make sure that any adhesive you use is compatible with rigid foam). 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. For more information, see How to Insulate and Air Seal an Attic Hatch.
To stop air leaks at pull-down attic stairs, you’ll need to install an elaborate cap. For more information on this topic, see these two articles:
So far, we’ve mostly been talking about ceiling leaks. But air leakage routes get more complicated if your attic includes kneewalls.
A typical Cape-style home has several different attics. There are usually two cramped attics behind the second-floor kneewalls; there is also a low attic above the second-floor ceiling. Kneewalls are also common in some new homes built in the 1980s and 1990s, especially large homes with chopped-up roofs, dormers, and multiple ceiling planes.
There are at least two problems with most kneewalls:
- Kneewalls often lack an attic-side air barrier, even though they are insulated with air-permeable insulation (usually fiberglass batts) that require such a barrier.
- When builders install kneewalls, they often forget to install solid blocking between the floor joists under the kneewall bottom plate, as well as solid blocking between the rafters above the kneewall top plate. If the kneewall is insulated, such blocking is essential; moreover, each piece of blocking needs to be carefully air sealed. Without the blocking, cold outdoor air can migrate horizontally between the floor joists, robbing heat from the house.
If you discover that your kneewalls are uninsulated, don’t despair. It’s possible that your builder decided to insulate the sloped roof assembly above the attic instead of the kneewall. If that’s the case, you’re in luck. As long as the work was done conscientiously, the best way to insulate attics behind kneewalls is at the sloping roof.
Ventilation baffles, insulation dams, and duct leaks
Once you have completed your air sealing work, there are a few more details that you may need to address before you add more insulation to your attic: ensuring that the ventilation channels that connect your soffit vents to the attic aren’t blocked by insulation; making sure that all of your insulation dams are high enough; and checking any attic ducts for problems.
If your attic has soffit vents and ridge vents, you’ll need a ventilation gap under the roof sheathing at the perimeter of your attic — a clear channel that allows air to flow from your soffit vents to your attic. In some attics, this area is blocked by insulation. The solution is to install ventilation baffles — either commercially available baffles or site-built baffles. (For more information on this topic, see Site-Built Ventilation Baffles for Roofs.)
If you need to install these baffles, you’ll also need to install insulation dams from the top plates of your exterior walls to the underside of the ventilation baffles. Here’s a link to a GBA video that shows how to do this work: How to Ventilate Rafter Bays When Adding Insulation.
If you plan to add more insulation to your attic floor, you’ll probably need to extend the insulation dam around your attic hatch. This dam needs to be at least as high as your anticipated insulation depth. You can use either 2-by lumber or plywood to build this insulation dam.
Remember that installing insulation on top of live knob-and-tube wiring is a code violation and a possible fire hazard. If you have knob-and-tube wiring in your attic, you can only insulate on top of the knob-and-tube if you are sure that it has been permanently disconnected. When in doubt, call an electrician to have your wiring evaluated before proceeding.
Finally, if there are any ducts in your attic, you’ll want to inspect them. Make sure that there are no disconnected ducts, that all duct seams are sealed with mastic or high-quality HVAC tape, and that the ducts are adequately insulated.
It’s usually a bad idea to locate ducts in an unconditioned attic — but just because it’s a bad idea, doesn’t mean that attic ducts don’t exist. If you’ve got them, you’ll have to do your best to deal with them. For more information on attic ducts, see these three articles: Sealing Ducts; How to Install Flex Duct Properly; and Keeping Ducts Indoors.
Martin Holladay’s previous blog: “All About Wood Stoves.”