The continuous, unbroken skin or membrane around the entire building is called the “airtight layer” (ATL) or the “pressure boundary.” The airtight layer is where air sealing should happen. If there is a hole or a gap—for example, a missing gasket under a bottom plate—in the airtight layer, air will pass through that gap and drift around the wall cavities and may then emerge through several outlets in the living room. The outlets are secondary leaks. The gap under or at the outside face of the bottom plate is the primary leak. While sealing the outlets may be a good idea, it will do nothing to stop the primary leak, whereas sealing one primary can shut off five secondaries. If you can successfully air seal the full run and surface of the airtight layer, your air sealing is done. No other air sealing will be necessary.
This airtight layer simply establishes the environment (wild side) on one side and the controlled indoor environment on the other. Tape, liquid elastomers, and fabric are the chief air sealing materials applied to the structural skin. The airtight layer can also be established somewhere between the structural skin and the interior finish wall (typically drywall), and it can have insulation on either side.
Six common ATL configurations. Above are six common configurations for an airtight layer (ATL) and a basement, crawlspace, or slab (slab configurations can be the same as A, B, or C). Any combination of these can be used. The blue line (ATL) must be continuous and unbroken around the entire building enclosure. Insulation can be applied on either or both sides of the ATL. In colder climates, insulation, usually in the form of continuous sheet material, is added to the exterior of the ATL.
Selecting the Best Surface for the ATL
To avoid problems, mostly with dry rot and mold, you need to study and understand the materials of the assembly and the climate in which you are building along with the possible uses of the building.
“The significant disadvantage of exterior air barriers is their inability to control the entry of air-transported moisture into cavities from the interior. Indoor moisture traveling out—chasing dryer exterior air.
“One big drawback with interior air barriers is the potential for moisture and mold on the backside of drywall in humid climates. In an air-conditioned house, the drywall will be cool in summer. Without an exterior air barrier, humid, outdoor air can get into a wall cavity, find that cool drywall, and get the drywall wet enough to damage it or grow mold.” —Allison Bailes of Energy Vanguard
Typically, the ATL on the exterior of the building works well in many climates and building uses. But in many renovation situations, such as a gut rehab of an old building when exterior renovation is not part of the scope of work, it makes sense to establish the ATL on the interior. And it’s especially challenging in older homes that predate the widespread use of plywood and OSB panels for sheathing and decking. Nevertheless, it is possible to improve airtightness.
Making the Exterior Surface the ATL
There are several options for creating the ATL on the exterior of a building.
Plywood and OSB as the exterior ATL. Plywood entered the housing market in the 1930s. Its popularity grew slowly, but as plywood production went up, prices went down, and builders realized plywood was faster, stronger, and more economical. Oriented strand board (OSB), introduced in the 1970s, came in as an alternative to plywood. Using plywood- or OSB-wrapped buildings was the most significant leap in air sealing since the chinking of log cabins. And unfortunately, except for Passive House, that, for the most part, is where air sealing has stopped advancing.
Since most wood-frame buildings built after the 1960s are wrapped with plywood or OSB, those materials are the obvious choices for the primary airtight layer. Plywood and OSB provide a sturdy and thick continuous skin. Moreover, the surface is relatively easy to “read” for punctures. Any breaks or holes in the plywood/OSB should be sealed. There have been reports of OSB sheets leaking even though they appear undamaged, but this is an exception to the rule. Still, suppose you are going for a very airtight assembly. In that case, you may consider using a good-quality five-ply plywood or applying an elastomeric coating or a self-adhesive fabric on the surface of the OSB.
Using tapes on plywood/OSB. There are numerous durable (long-lasting) tapes on the market that have super adhesion and flexibility. I call them “Super Tapes.” They will adhere to almost any kind of material, including plastic, glass, or concrete.
Super tapes. Both blue tapes shown above are by Pro Clima. The green and white tapes are by Siga. The tape at bottom left is by Partel.
Lumber sheathing as the exterior ATL. Before the 1960s, solid wood planks with up to 1/4-inch gaps between the boards were commonly used to wrap a wood-frame building. By itself, lumber sheathing could not be the ATL. It relied on tar paper, wood siding, or stucco to be the primary exterior ATL. Except for stucco, most of these coverings were not airtight.
Lumber sheathing. Pictured above is a single layer of 30 lb. tar paper torn off a diagonal lumber sheathing wall. Note the demo’d stucco at right. Any air that gets behind the stucco will quickly leak into and out of the wall assembly.
Stucco Plaster as the Exterior ATL
There are millions of stucco (plaster) wood-frame buildings and houses in the United States. They vary in how they are put together and present a variety of air sealing problems; therefore, they require a variety of solutions. Early stucco ran below the framed walls and was bonded to the foundation walls. If done properly, bonded stucco creates a relatively airtight plane along the stucco bottom. Tightly fitting stucco around door and window openings and other punctures in the stucco could yield a relatively airtight assembly.
Early three-coat stucco coatings (cement, sand, and lime) have been used on stock houses and buildings since the early 1900s. They were mixed in the field, on site. Three-coat stucco is more vapor open and can dry out faster (which is good) than new formulas and mixes.
Bonded stucco. Seen above is a 1949 house with three-coat, bonded stucco applied over lumber sheeting.
In the mid-1970s, one-coat, premixed stucco came out in a bag. Over the years, different additives—fly ash, acrylic, fibers—were added to the sand, lime, and cement. One-coat stucco now dominates the stucco world.
The introduction of a drainage plane (a gap) behind the stucco and an added weep screed (drainage bottom) in the early 1980s allowed any possible water trapped behind the stucco to escape. The introduction of weep screeds introduced a new and massive primary air leak into our buildings. Since most wood-frame buildings have no gasket under the bottom plate of the framed wall, a weep screed within inches of the plates will allow air to flow in and out under the bottom plate. The weep screed is where air sealing efforts should be concentrated.
Air sealing bonded stucco—no weep screed. The house in the pictures below was built in 1947. Because the stucco is sealed (bonded) to the foundation or slab (no weep screed), and the stucco is sealed tight against the door and window trim, it will be more airtight than stucco assemblies with weep screeds. Caulking or paint will keep the joints airtight. If stucco is below grade, dig down to where the stucco terminates on a concrete footing or slab. Make sure the bond is good. Break away loose bonds and repair. Not only can air get behind the stucco, but moisture (from soil) can also creep up the gap. Subterranean termites and bugs can also enter this gap.
A. In the wall assembly above, stucco is applied over chicken wire. The wire is nailed over tar paper, which is nailed onto lumber sheathing. Stucco runs down past the framing and is bonded over the stem-wall footing. (The stem wall is part of a conditioned basement room.) The white arrow shows the surface of stucco bonded to the foundation.
B. Shown above, stucco is sealed tight against wood “stucco mold” and wood trim at all openings. Any failed caulking (caulk line indicated by black arrows) could allow water to reach the tar paper. Any weak spots in the tar paper would allow water to come in contact with the wood and cause rot.
C. This cutaway section (above) shows hairline cracks (black arrows) in the stucco, leaving only the paint to make it watertight and airtight. Cracks such as these, over time, could transport water. Note the rotted wood (white arrow).
If possible, it may be a good idea to break away the stucco and finish the edge above grade.
Air-sealing bonded stucco on slab footing. The example below is from a 1972 commercial building. The stucco is bonded to the concrete above grade.
A. A mirror on the ground shows the weak bonding of the stucco to the concrete slab. Leaks will occur along this edge when the bonding layer breaks out.
B. Remove loose mortar or stucco, scrub with a wire brush, then caulk joint. Use high-adhesion, highly elastic caulk.
In a circa-1970 wood-frame office building (below), there was no way to tell what the wall assembly was. But a mirror was laid on the ground under the stucco to view up into the screed reveals a sizable gap between weep screed and the wall.
A folded dollar bill slips behind the screed, illustrating the ability of air to go behind the screed and enter the wall cavities.
If the gap is wide enough, you could fill it with a backer rod or gun foam after cleaning it. This will do nothing to stop air from entering the screed holes, but you may get lucky and close off some of the air flow that gets under the bottom plate.
After cleaning (with a wire brush) all the surfaces, seal with your best, highest quality elastomeric caulk. While this can shut off myriad direct leaks, air can still enter the weep holes, wash the surface of the tar paper, and find ways into the wall cavity.
Two possible weep screed configurations.
Sheathing flush with face of concrete. Caulk between the weep screed and concrete (above, left). Be sure caulk does not clog up the weep screed holes.
Sheathing hanging over and past concrete face. This gap creates a leak that goes directly into the wall cavities. Caulk the gap between the plywood and concrete (above, right right).
Stucco on shear wall hanging over concrete slab or footing. Sometimes the shear wall is flush with the concrete. Sometimes it hangs over and below the edge. With no gasket under and at the edge of the bottom plate, there will be a massive opening that can create a hidden leak directly into the wall assembly.
Air pathways. The image below illustrates (using butter knives) three ways air can enter the wall cavity.
A. Slipping under the gap between the bottom plate and the slab. This common gap, with no gasket under the bottom plate, will allow air to quickly enter the living space.
B. Slipping between the edge of the bottom plate and the back of the plywood. This gap allows air to easily enter and go up into the insulated wall cavity.
C. Slipping between the metal weep screed and the tar paper. This gap will allow air to run outside the plywood and find gaps or holes in the WRB.
The fix. This is how you would fix the multiple leaks through the bottom of a stucco wall:
1. Clean the concrete and any dirt accumulated in the gap.
2. Use one of two options below for sealing the air leaks:
Both tape and caulk must leave the weep holes open. This fix will still allow air to enter the weep holes and potentially leak behind the stucco wire and tar paper, but it will shut down the sizable leak pathway under the bottom plate.
Weep screed on the surface of a concrete footing. Below is an example of a weep screed circa 1998. This is a typical one-coat stucco assembly, found in newer buildings (1980 to present).
Above: One-coat stucco was applied over wire (not visible) nailed on top of tar paper. Tar paper was stapled onto OSB or plywood, with a metal drain screed at the bottom. The weep screed is tight against the concrete.
Other than caulking the screed where it touches the concrete, there is no way to air seal this type of assembly unless you demo the entire bottom portion of stucco. Most of the outlets in this wall leaked air.
Note: Tar paper behind stucco usually warps and crinkles. While this trait helps the drainage plane behind the stucco, it also provides a pathway for air (and termites) that get behind tar paper.
Air sealing stucco over framing—no shear wall. The figures below show one option for air sealing a stucco house built in 1978.
A 12-inch-wide run of drywall was removed from the interior of all the exterior walls to expose the massive gaps between the back of the bottom plate and the tar paper.
Thickened elastomer was sprayed on using a texture gun, then brushed to fill gaps up to 1/4 inch wide. While this will stop the big leaks between the tar paper and bottom plate, air can still leak between the stucco and the tar paper and travel up until it finds a puncture in the tar paper or, more notably, at tar-paper seams.
Make sure the tar paper is intact and the elastomer does not make contact with stucco, or it will block the flow of trapped water on the exterior surface of the tar paper.
Note: Removing a smaller, 4 1/2-inch-wide section of drywall from the bottom of the wall would limit access to seal the gaps, but it would make for an easy repair using a 6-inch baseboard.
Air Sealing Stucco—Going Deeper
Removing all the stucco allows you to make the building airtight but is very expensive. Another option is to score and break off the stucco at 6 to 12 inches above the weep screed around the entire building. This may seem extreme and expensive, but if you leave the stucco as it is, you may never achieve any significant air leakage reduction in the building unless you choose to form the airtight layer on the interior. The gaps along the bottom perimeter of the building will be a substantial contributor to the stack effect during heating and cooling demands. The gaps will also be a runway for termites, powder post beetles, insects, moisture/vapor, and smoke, and will accelerate flame spread.
Demo the entire perimeter at the weep screed. While this may seem extreme and expensive, it will stop a significant amount of air from entering the entire perimeter of the building. It may be very cost-effective with large, multistory buildings or offices.
To stop air from traveling behind and up the tar paper, tape the existing tar paper down over the first layer of tape or set the existing paper onto double-sided tape. (See photo, above right. Note: The tape in the image is no longer available. Use any of the “super tapes” shown earlier in this article.)
Caulk and Tape options. The overall goal is to “shingle” all the new materials and seal each layer with tape or caulk as it is being installed.
If the existing tar paper gets destroyed or is crumbly, you can slip new paper under and behind the old. Tape can be used in place of caulk. Only use good-quality elastomeric caulk that you have tested for adhesion. Use only super tape for the tape option.
After demo of the stucco, as shwn above,, you can slide a new strip of tar paper under the existing. If the shear wall extends below the screed, add new tar paper in a bed of caulk at the top.
A good stucco tradesperson can replaster and join the old with the new. (The new wire must overlap with the old wire to avoid hairline cracks.)
If there is a crawlspace or basement, you must seal the subfloor, blocking, and joists from inside the building. This combination of air sealing will ensure you have air sealed all but one potential leak problem—the air running up the outer surface of the tar paper and leaking in or out through the seams of the tar paper covering the plywood. This step will work only if the stucco is backed by plywood or OSB—not lumber sheathing.
New construction.
In this photo, stucco screed, tar paper, and wire were applied over a completed airtight layer, so there was no need for further air sealing. Flashing was added to cover up the visible tape.
Exterior insulation and finish systems (EIFS). In the 1960s, an exterior insulation finish system (EIFS) entered the United States from Europe. A thin “stucco coating” (synthetic material) is laid over EPS sheet foam. While the stucco coating is airtight, there is no way of telling how leaky the assembly is behind it. There is a drainage plane behind the EPS with a drip cap at the bottom, much like a weep screed. Leakage could occur there as it does with all other stuccos at the weep screed and at the junctures of other materials. If the assembly is not airtight behind the sheet foam, there is not much you can do to shut down leaks other than caulk all the open visible junctures of the EIFS.
EIFS in Detail (above):
1. Stone fascia
2. Elastomer stucco coating
3. EPS sheet foam
4. Drip cap
Brick veneer. Many wood-frame buildings are wrapped with brick or stone. The brick is primarily the weather barrier.
Code specifies a gap between brick and a wood-framed wall. (Typically there is some type of sheathing over the framing, even if not detailed as shear panels). This gap serves as a drainage plane for bulk water as well as a drying vent for the framed portion of the wall. The roof and floor assembly are the same as on any other standard wood-frame building, and the house can have a full or partial basement.
Air-sealing protocol follows the procedures outlined elsewhere in the book; however, the brick wrap prevents access to the exterior. Air sealing the brick-facing walls must be done from the inside.
Most brick veneer has a 1- to 2-inch gap between the brick and the framed wall. There is no effective way to seal brick veneer from the exterior.
Wood Siding as an Exterior Airtight Layer
If your goal is less than four or five air changes per hour, you must remove solid wood siding, or you could move the ATL to the interior walls. If there is no tar paper behind the siding, you should remove all the siding and install some kind of weather-resistant barrier. This would be a good opportunity to establish a new airtight layer on the exterior.
Using a blower door, the 90-year-old house shown below tested in at 15 air changes per hour. Upon completion, it achieved just under one air change per hour. All the work was done outside the conditioned space, including the attic, sidewalls, and crawlspace.
Removing original siding.
A. The original shiplap siding was taken off (see lead photo, page 41). There was no building paper or sheathing under the siding.
B. New OSB shear wall was gasket-sealed by setting the OSB into a wet bed of gun foam; alternatively, you can let the foam set up for five to 10 minutes before installing the OSB.
C. The wet foam bed is oozing out of the seams. This same concept of gasket-sealing can be done with caulk or foam gaskets between the shear wall and the framing surface.
D. Old wire and plumbing lines were cleaned up. Main lines were stubbed out below the floor joists to be finished after the OSB was installed.
E. The OSB was sealed using tape and gun foam.
Leaving the original siding on. Suppose you are not removing the exterior siding and have no access to the interior wall. In this case, you can significantly reduce the leakage in a house with a can of foam (for big gaps), some backer rod, and a case of good caulking. You do, however, run the risk of trapping moisture behind the paint, which can “throw” the paint off the wood.
