Attic Air-Sealing

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When walking around your house, you’re likely to see only the plaster or drywall. What you don’t see are all the connections where building materials meet (or don’t meet).The biggest leaks are hidden behind the walls, where you can’t see them. The key to air-sealing an attic is to block off the large openings and seal the smaller ones as completely as possible—from one end of the attic to the other.

The best way to start is to walk through the top story of your house, noticing the location of all the partition walls. Also note the location of ceiling-mounted light fixtures, changes in ceiling height, stairwells, hallways, closets, plumbing fixtures, and other features where leaks occur. Then put on your protective gear and head up into the attic to take a closer look.

Start with partition-wall top plates

It’s best to be organized when working your way through an attic, so we usually start at one end and work our way to the other in an orderly fashion. This prevents me from getting mixed up or missing things, particularly when we have to move existing insulation out of the way as we go. And when the attic is cramped (as it often is), it’s better to limit your movements.

In most homes, it makes sense to start by first sealing the top plate along one gable end, then work your way down the center, load-bearing wall. Seal the top plate to the plaster on both sides, and seal any wiring holes with spray foam or caulking .As the center wall meets intersecting partition walls, follow each partition toward the eaves as far as you can reach. Be sure to find all the closet and hallway walls, which can be confusing.

As you pass each room, closet, or hallway, find any light fixtures that may be in the center of the ceiling and seal the electrical boxes (be careful not to squirt any foam inside the box).Treatments for recessed lighting fixtures are covered later in this section. If your attic already has insulation in it, be sure to let the foam or caulking set before putting the insulation back in place.

Along the way, you will probably find some larger leaks, such as duct or plumbing chases, chimneys, and the like. Some of them can be sealed with foam or caulk as you go, but most will require some type of blocking. Make a note of what and where those larger leaks are, so that you can return with the right materials. Also make a note of any electrical junction boxes where wires are spliced. This will be useful later, particularly if you are going to add more insulation. If your house is insulated with fiberglass batts, you may be able to identify some leaks by the black or brown discoloration on the insulation above them, indicating that air has leaked through.

Fig. 32-1: Sealing the gap between the top plates and the drywall or plaster with foam insulation is tedious, but it’s a critical part of any attic air-sealing job. This is a good time to seal wiring holes as well.



Don’t depend on a fiberglass insulation batt lying across an opening to act as an air barrier. Regardless of the type of facing, it will not stop air movement.

Tricks of the Trade

Our first choice for attic air- sealing is a professional foam gun, which we use to apply low-expansion foam with precision control to a wide range of holes and gaps. Hardware-store foam cans are cheaper and smaller, but they are also awkward to use and difficult to control. Once you start a can, you must use it up quickly or throw it away. Caulking is inexpensive for small jobs, but it can’t bridge gaps wider than 3/16 in., and most caulks don’t stick well to dusty or dirty surfaces. Although useful in many places, caulk is not recommended for most sealing jobs in the attic.

Fig. 32-0

Be Safe!

Attics can be nasty places — dusty, uncomfortable, and sometimes hazardous. At a minimum, always wear a tight-fitting, paper dust mask. We prefer a good-quality respirator (one with an HEPA filter); it works better and is more comfortable because of the valves — you’re not breathing right back into your nose. It’s also a good idea to wear coveralls or other protective clothing and gloves.

You should always wear at least a dust mask when working in an attic. Although a HEPA (High Efficiency Particulate Arresting) respirator (left) is more expensive, it is also more effective.

If there is enough clearance between the top plates of the cave walls and the roof sheathing, seal them between the rafter tails. This may be difficult to do. You may want to wait until you have finished the interior partitions and other leaks, then put back any existing insulation you have disturbed before tackling the exterior walls.


Fig. 33-0: attic air leakage details: The greatest attic air leaks typically occur where the ceiling is interrupted: at changes in ceiling height or dropped soffits (A) and at duct and chimney chases (C). Also, check openings around the attic hatch (B); at plumbing vent stacks (D), which vary in size; wiring holes (E); light-fixture boxes (F); and other electrical penetrations, which are small but fairly obvious. Less obvious is the crack that almost always occurs between the top plate and the plaster (G) when the framing lumber shrinks. You may also find walls with no top plate (especially gable-end walls) (H) or gaps of a few inches between the exterior wall’s sheathing and the exterior wall’s top plate (I).



After cutting blocking material to fit and setting it in place, be sure to caulk or foam around all four sides for an airtight, permanent seal.


If your furnace is in a closet with an attic above it, there may be combustion air openings from the closet into the attic. Don’t seal those openings; if the furnace can’t get adequate combustion air, it can generate deadly carbon monoxide gas. Codes require high and low combustion air inlets, and the low one may be fed by a stud bay that opens into the attic, so be sure to leave that unobstructed as well.

More Caution

Bathrooms and kitchens are the rooms where most moisture is produced, and each one usually has one or more penetrations in the ceiling for light fixtures, fans, and plumbing vent stacks. Pay particular attention to air-sealing the attic space above these rooms.


Fig. 34-0: Some contemporary homes have cathedral ceilings flanking both sides of a small attic over the hallway. The studs framing the hallway walls often open into the attic. This one has a plumbing vent, too. Stuff some fiberglass into the opening before sealing it with foam.

Fig. 34-1: One potentially leaky area is the gap between the housing on a bath room ventilation fan and the ceiling drywall or plaster. However, don’t seal this area up yet if you are planning to replace the fan as part of your ventilation strategy.


Through-framing into attics

Now that you are thoroughly familiar with your attic, you should have an idea of where the big holes are located. These holes typically occur wherever framing members, such as wall studs or floor joists, run from the conditioned space to the attic space. These bypasses require a little more thought to seal properly, and they are usually the largest and most important leaks.

The key to creating a good air barrier between the conditioned space and the attic is to make sure you bridge all the gaps and spaces between the plaster and the framing members that you can’t see from the inside. Your job is to find every place where there is a space and then patch it. Follow the plane of the plaster or drywall along the ceiling and any connecting walls. This is sort of like play ing a three-dimensional “connect-the-dots” game where someone has already filled in some of the links but left out a bunch of them. Wherever the ceiling plane is interrupted or changes levels, you need to supply the connecting link.

Keep in mind that insulation batts with paper or foil facing that lie across one of these openings do not constitute an air barrier. Insulation does not stop air! In fact, it’s best to ignore the insulation when assessing and fixing an attic’s air boundary Be aware of some other important issues. First, don’t inadvertently seal combustion air openings. Second, use only noncombustible materials against a chimney Third, recessed lights and some of the situations listed below require special attention.


Think: Safety First!

When working in an attic, make sure you support your weight on the ceiling joists. Otherwise, you may fall right through the ceiling drywall or plaster. It’s a good idea to bring a 4-ft. length of 1 x 12 or a small piece of plywood to help support your weight. Kneepads can also help with the awkward jungle-gym moves.

Fig. 35-0: Airflow in a Cape-Style Kneewall: Floor framing that extends into a side attic, such as the kneewall spaces commonly found in Cape-style houses, and doesn’t contain blocking is an open invitation for cold air to flow between the floor joists.



Although there are a number of variations, here are some common attic bypass situations:

A. Dropped, or soffited, ceilings are common in kitchens above cabinets, in bathrooms over showers or vanities, and sometimes over stairways. (see figs).

Fig. 36-0

Fig. 36-1

B. Plumbing chases (see the bottom photo) can be much larger than the pipes that run through them.

C. Chimney chases often run all the way to the basement of a house (see fig below).

D. Tri-level homes often have wall studs that extend from the lower level into the attic; these stud spaces are bypasses, as are the large openings that may occur near the stairs. (see fig).

Fig. 36-2

E. Older homes and row houses with brick exteriors may have wood furring strips that extend from the attic to the basement. Walls above pocket doors may also have gaps, which open into the channel in which the door slides.



One of the most common through-framing leaks is found in Cape-style houses. As you can see in the drawing above right, the floor joists between the first and second floors usually open into the kneewall attic area, allowing outdoor air to circulate between the floors. Even if there is no access into the kneewall space, it is worth cutting a temporary access through the wall to get in there and block off the joist bays. There are usually many joist bays; you can cut pieces of foam board to fit each one, making them slightly undersized, or cut long strips, notching for the joists. Wedge them into place, and foam or caulk all the gaps. If there is a subfloor in the kneewall area, you will have to pull up some boards to reach that area.

This type of open floor framing occurs in other places. For example, an ell or an addition with a lower roofline, or a finished room over a garage, may be built the same way, so they need the same type of draft-stopping. Also, floor framing between the first and second floors may open into a porch roof or the attic over a single-story addition or garage. Many of these areas are not normally accessible, so you may need to cut an access through the drywall or plaster to get into them; because the leaks are so large, it is worth the trouble. Seal these leaks the same way you would a cape kneewall—foam board or similar blocking material, foamed or caulked in place between the floor joists.

Recessed lights

Recessed “can” fixtures are often big air leaks. Most common styles are full of holes to vent the fixture so it doesn’t get too hot, These lights act like little chimneys when they are on; the heat from the lamp drives warm indoor air into the attic even faster than it would move on its own. If you have a lot of them, the result can be an energy disaster.

The best approach is to replace them with air tight fixtures that are rated for insulation contact, or “IC.” Not all IC-rated fixtures are airtight; new construction energy codes in many states require airtight IC fixtures, so they are not difficult to find. Look for a label that says IC and another one that references the Model Energy Code, MEC, Washington State energy code, or ASTM E 283-91. An airtight retrofit kit is also available for existing recessed cans.

As an alternative to replacing your light fixtures, you can build airtight boxes over them using 1-in-thick duct board. The box must fit between the joists and be large enough to dissipate the heat emitted by the light. For typical 16-in. o.c. framing, for example, make a box that is about 14 in. square and 20-in. high.

Cut out the fiberglass in a “V” behind the corners, being careful not to break through the foil facing, and hold it together with aluminum tape. Make sure the surfaces are clean and free of dust. Clean off any insulation and dust around the recessed light, and cut openings for wires and mounting brackets. Set the box over the light, foam or caulk it in place, then seal the seams with duct mastic.


When working in an attic, watch out for stinging insects (keep a can of insecticide spray nearby in warm weather); rodent droppings (or even the rodents themselves), which sometimes harbor disease; knob-and-tube wiring; and any wires that are spliced without a junction box, frayed, or nibbled by mice, or that otherwise appear unsafe.

According to Code…

To reduce the risk of fire, most chimneys, as well as vent pipes for furnaces and water heaters, are required by code to have a clearance of 2 in. from combustible materials (such as framing). Seal chimney chases with aluminum flashing, light-gauge sheet metal, or duct board (shown in fig.) nailed or stapled to the rough opening. Caulk the joints with high-temperature silicone caulk or furnace cement.

Fig. 37-0: Any draft-stopper installed within 2 in. of the chimney must be made of a material that won’t burn.



Search for hidden attic spaces above additions so that you don’t miss areas that need insulation.

Be Careful…

Recessed lights are very popular, but they can be a major source of air leakage and heat loss, particularly when they are located in a ceiling that has an attic space above. While the light shown here is insulation contact (IC) rated, it’s not air tight, as you can see by the dark fiberglass around it.

Fig. 38-0

Tricks of the Trade…

Polyethylene sheeting has a slippery surface, and it’s difficult to get anything to stick reliably to it. Two products I’ve found that work well are 3M’s builder’s sealing tape and Tyvek sheathing tape. Make sure that the poly is clean, then firmly press the tape in place with a rubbing motion. This tape also sticks to housewraps; plywood sheathing; and clean, dry framing lumber.

Be Safe…

Recessed incandescent lights generate a lot of heat. If you build an air-sealing box around one, it is critical that the material be noncombustible and that the volume of the box is large enough to dissipate heat generated by the fixture.

Fig. 38-1: The floor framing behind this kneewall is open right into the attic. Unless these gaps are blocked, outdoor air will circulate between the first and second floors.

Fig. 38-2: Sealing the floor bays with foam board is the air sealing equivalent of closing up two to four large windows.

Fig. 38-3: Use airtight recessed lights that are certified as meeting current codes. Look for a label that says IC and another one that references the Model Energy Code, MEC, Washington State energy code, or ASTM E 283-91.

Fig. 39-0: Duct board, or rigid fiberglass with foil on one side, is available at HVAC supply houses. The foil facing is the air barrier and is noncombustible. Be careful not to cut through the foil.

Fig. 39-1: Make the box narrow enough to fit between the joists but large enough to dissipate the heat of the light it will cover. Notches at the bottom fit over the light’s mounting brackets.

Fig. 39-2: You can use foam to seal a light box to the ceiling, but be careful not to squirt it in under the box. Duct mastic is not combustible and should be used on the other seams. If the light is not IC rated, you must keep all materials at least 3 in. away from the light’s housing.


Other special situations

Attic air-sealing is complicated by the fact that there are so many types of houses, but there are some fairly common situations. First, don’t miss hidden attic areas. Sometimes, an addition, dormer, or an ell may have a separate attic that you can’t see, because the new roof was built right over the first roof. Cut an access in the roof sheathing once you know where the second attic area is. If you aren’t sure, use a tape measure. The nail pattern may give you a hint as to where the valley rafters were nailed. Use a reciprocating saw and start low. Begin with a small hole, so you can see how far up you can safely cut, and make sure you don’t cut into any rafters.

Suspended ceilings can hide tremendous leak age areas, particularly if they have attic spaces above. In older houses, such ceilings are often installed to hide deteriorating plaster. There may be large holes in the ceiling or high on the walls that are hidden by the dropped ceiling. If you are restoring or renovating the house, the best approach is to remove the suspended ceiling and repair the plaster, or install new drywall over the plaster. Or you can leave the suspended ceiling in place and patch the holes with drywall scraps. Cut pieces that are large enough to cover the holes. Be sure to drive plenty of screws into the lath or ceiling joists, and caulk around the edges to obtain a good seal.

Fig. 40-0: Sometimes, small dormer attics may be hidden behind the roof sheathing; in this case, fortunately, the attic is visible and easily accessed.

Fig. 40-1: The milled tongues and grooves on these boards provide a nice rustic look and blind nailing, but they are notoriously leaky. Always be sure to mount an air barrier on the inside before installing the boards.



Suspended ceilings can hide tremendous leakage areas, particularly if they have attic spaces above them.

In Detail…

Cathedral ceilings are difficult to air-seal and add insulation to. But not all ceilings that appear to be cathedral are. If the slope of the ceiling is shallower than the slope of the roof, the roof was probably built with scissors trusses and is not a true cathedral ceiling. With these roofs, it may be possible— although difficult—to gain access through the attic to reach some of the larger leaks and install more insulation.

Be Aware…

Acoustical tile ceilings present potentially large air leak age problem. These 1-ft-sq. tiles are basically made of compressed paper with tongue-and-groove edges, and the cracks between them leak like crazy. The only reasonable solution is to drywall right over them. If they are on sloped- or cathedral-ceiling areas where there is limited room to add insulation, this is the perfect opportunity to install some rigid-foam insulation prior to the drywall.


Sometimes, a suspended ceiling hides open framing that is insulated with only faced fiberglass batts. This is a huge air leak; the insulation facing does not constitute an air barrier and, of course, the suspended ceiling does not either. There are two ways to deal with this problem. If you plan to do some remodeling, it’s best to remove the suspended ceiling and install new drywall. If this is not an option, you can remove ceiling tiles and staple an air barrier to the underside of the joists. Use 6-mil poly in cold climates or non-perforated housewrap in mixed or hot climates. This can be a tedious task, depending on how many hangers, wires, and other obstructions are in the way, because you must cut and fit the material around each penetration. Be sure to use good-quality tape for the seams. Also, tape the air barrier to the walls or top plates on all sides, and use a sealant (prefer ably foam) for all penetrations.

Another potential air-leakage disaster is a wooden tongue-and-groove ceiling. Whether the ceiling is flat or cathedral, if there is no air barrier behind the planks (which is typical), the hundreds of lineal feet of cracks in even a small room can add up to a big leak. If the aesthetic value of the wood ceiling is important, the only reliable way to add a good air barrier is to carefully remove the planks, create an air barrier, and reinstall the wood, The air barrier can be taped polyethylene (in cold climates) or rigid polystyrene insulation (in any climate).The latter also adds R-value. If the wooden ceiling is not important to you, drywall (an effective air barrier) can be installed over the boards. This is also a good opportunity to add rigid insulation, especially if it’s a cathedral ceiling with limited room for insulation in the rafter cavity.

Before you pull apart a wooden ceiling, check to see whether there is an air barrier behind it; if there is, you may have a much smaller problem on your hands. Poke around a bit at the cracks where the wood fits together; there are almost always some places where the tongue is cracked, the wood is split, or the ends don’t butt tightly. Use a small flashlight to see past the wood. If you don’t see polyethylene, drywall, or some other solid material (kraft paper or foil facing doesn’t count), you probably have no air barrier.

Occasionally, air leaks occur in places that are virtually impossible to reach. If your house has a lot of cathedral-ceiling areas, you may have many plumbing vents and wiring holes that lead from partition-wall cavities up into the roof. These pro vide a path for warm, humid air to escape in winter and for hot attic air to leak into interior walls in summer. Unless you are planning to gut those partitions in a remodel, your best two options involve dense-packed cellulose insulation.

The first option is to dense-pack from indoors the partition walls near the roof, which is messy but very effective. The second option is to dense-pack the roof cavity from outdoors; this adds R-value to the roof, but it may be difficult to do if the roof cavities are already full of batt insulation.

Other hard-to-reach areas include gambrel and mansard roofs. Structurally, they are very similar to the cape kneewalls, but the kneewall area is far too narrow to access. Small eyebrow roofs, or overhangs in the middle of two-story walls, can also be a problem. It may be possible to access the area from the exterior by removing the soffit; if the space is small, you may also be able to fill it with dense-packed cellulose.

Fig. 41-0: The high, narrow kneewall areas in gambrel and mansard roofs can be a big source of air leakage, and they are often difficult to seal.

Fig. 41-1: Small roofs like this one can hide big air leaks; they often have no wall sheathing behind them.


Attached garages can be a source of deadly carbon monoxide and other toxic fumes, as well as cold air, so it is very important to seal the boundary between the house and the garage. Make sure that all joist spaces over garage walls are blocked and sealed. If drywall is in the way, try dense-pack cellulose in the garage ceiling.

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