First Step in Cellulose Sealing: Spot the Style

Four styles of house. Top to bottom: the ranch, the colonial, the contemporary, and the finished half-attic.

Figure 1: One of the most notorious heat loss sites in a colonial is the space above the stairwell. When hot air convects up into the attic, it can be replaced by air leaks from below, or by cold attic air descending from above.

Garrison colonials and other homes with cantilevered floors can suffer extreme air intrusion between stories. This house is being manually air-sealed. After removing soffit sheathing, the author is blocking the joist bays and sealing the blocking with expanding foam.

See the Chimney, Seal the Leak Internal chimney structures are home performance failures that can be seen from the street. They are sometimes large enough to function as room dividers, and can be the biggest open chases in the house. But most chimneys have open space around them to comply with fire codes, as no combustibles are allowed within 4 inches of the masonry. It is possible to follow fire codes and still air-seal the chimney chase at the attic floor. The trick is to build a noncombustible sheet metal bridge between the masonry and the ceiling plaster and seal it to each with high-temperature sealant.

In a two-story home like a colonial, recessed lights create a huge aggregate hole into the attic. Covering IC-rated fixtures with airtight boxes seals them dependably. Other recessed fixtures cannot be air-sealed safely.

In contemporaries, interior walls often continue up to the roof, leaving stud bays open at the top. A crew member is air-sealing the tops of these cavities by stuffing plastic bags full of insulation into the bays.

The large, hard-to-access attic of a ranch style home is usually full of surprising, concealed air leaks. Here, an interior wall has leaked so much air through its unsealed top plate that the attic insulation has turned brown from dust. It is best to map the interior of a ranch before attempting to air-seal, in order to know where to look for leaks.

Figure 2. In ranch-style homes, the valances above kitchen cabinets often open directly into the attic. Interior walls are rarely sealed when they enter an attic at such an awkward intersection. The stud bays can be sealed with bags of insulation or with dense-pack cellulose.

Figure 3. The finished half-attic contributes to heat loss in two ways. It is exposed to outdoor air from roof venting, and it usually allows indoor air to leak out through unsealed framing.

There are two ways to air-seal and insulate the side-attics around a finished half-story. On the left, Li Ling Young installs sheathing over the rafters. The space between the sheathing and the roof deck will soon be dense-packed to prevent heat loss and air movement. Right, Fred Lugano solidifies a side-attic, filling the whole space with dense-pack cellulose. While less time-consuming, solidification makes the side-attics useless for storage.

The problems can start below ground: there are usually large holes through the foundation (as in the case of homes with stone foundations or below-grade windows). The basement is often connected to the house by a leaky cellar access door. In this case, the neutral pressure plane drops, placing the top of the house under very high pressure. This pressure means that even small holes into the attic, such as wiring penetrations or the gaps around shrunken partition top plates, blow enough air to heat the attic and ice the roof. This type of air movement causes many two-story homes to develop large black patches on fiberglass insulation in both the basement and the attic. The fiberglass filters out particles as air flows through.

We block the large foundation leaks, but focus primarily on the boundary between the attic and the second story. Every plumbing and wiring penetration and every inch of partition top plate needs to be traced and air sealed with caulk. We've found that leaks around the tops of interior walls are so consistent that we now rarely use pressure from the blower door to guide us. Since the drywall is dimensionally stable, gaps always form when the top plate lumber dries. The gaps may only be 1/16-inch or 1/8-inch wide, but with high stack pressure, this is enough to deliver enormous amounts of heat and moisture to the attic. So every gap gets caulked.

Next Step, Stairways

There is a special problem with the stairwells in two-story houses. When they are balloon framed from the first floor to the attic, the wall bays provide large passages through the conditioned space. When a sloped ceiling is built over the stairs, big problems develop. In this design, a sloped 2 x 4 ledger is nailed to the inside of the stairwell to frame the ceiling over the stairs. This leaves a huge wedge-shaped divot in the attic floor (see Figure 1). However, the interior walls of the stairwell have no top plates, leaving open air passages from the heated space into the attic.

To make matters worse, we find fiberglass batts that have been tented over the void. Air in the divot is then on the attic side of the pressure boundary, but on the house side of the thermal boundary. Once warmed, this air can flow through batt insulation into the attic, to be replaced by cold air.

Cold attic air conducts heat out of the walls. This shows in condensation on (and degradation of) the surrounding wall finishes. When the house is heated, any leaks into the walls (for example, the space around electrical boxes) exhaust enormous amounts of air from the house.

With such extreme leakage to the attic, windows draft, flues backdraft, and roofs ice up, regardless of wind conditions or insulation levels. I've seen three homes where the second floor system was depressurized--the top sashes were sucking in air without help from mechanical depressurization--because the neutral pressure plane was somewhere in the middle of the attic.

So we air seal and insulate the space above the stairs. If access is available, we can build a new attic floor of wood framing, applying normal air sealing methods. But for a quick and utterly reliable air seal, we dense-pack the whole divot. The stairwell is so open that normal pneumatic dense-packing is impossible, so we compress the cellulose with our bodies. At 3 lb/ft3, a typical 3-ft-wide stairwell requires 450 lb of cellulose, which isn't enough to cause structural failure on the ceiling of the stairway below. (Indeed, the main structural stress is having workers in the cavity compressing the insulation.)

Garrison Colonials

The Garrison colonial is known for its cantilevered second-story floor. These same floors show up in many modern contemporaries. This type of construction exposes interior joists between the first and second story to the outdoors (see Moisture Control in Bathrooms, HE Mar/Apr '98, p. 21). Fortunately, they often reside near the neutral pressure plane and are thus not subjected to much infiltration in calm weather. But coupled with balloon framing that is open at the top, these floors are depressurized, and become conduits through which large amounts of inside air can get to the attic. Repair requires air sealing and insulating the ends of the joist bays. There are two methods. We can remove the cantilever's siding and sheathing and manually air seal the joist bays with bags of fiberglass or with rigid materials and caulk. Or, to dense-pack, we can drill holes under the cantilever, over the cantilever, or through adjacent drywall, and pack the ends of the bays with cellulose.

Wood Stoves

In cold climates, where most colonials are found, homeowners sometimes install wood stoves to overcome the heat loss from drafts. At least heat can radiate across the drafts, allowing people to sit still without wearing a coat or blanket. Due to excessive leakage at the top of the house, these same homeowners sometimes have to develop tricks to start a fire. I've seen electric hair dryers, propane torches, and various petroleum accelerants put into wood stoves to light them when they are cold. Once heated, wood heaters pull yet more air out of the house, increasing negative pressures. Even then, some fireplaces backdraft enough that they are unusable unless windows are open. Backdrafting is a sign of strong negative pressures around the wood stove. It shows that air is being lost out the top of the house.

Recessed Cans

Recessed lights are very popular in two-story houses. When installed upstairs, they create a huge aggregate puncture into the attic. I've seen over four dozen used in a single house. Fixtures labeled fire hazard cannot be air sealed safely. If homeowners won't let us replace these fixtures, I do what I can to seal the other gaps in the house, and hope for the best. Building and installing baffles to keep insulation away from these fixtures ends up costing more in labor and energy than just replacing the fixtures with IC-rated ones.

With IC-rated fixtures installed, it is possible to air seal the attic floor. We seal the fixtures by covering them with airtight boxes. We build the boxes by screwing five 12-inch squares of drywall into scrap pieces of steel corner bead. We cut notches to accommodate wiring and framing, and then seal the whole thing to the ceiling with caulk and foam.

Contemporaries often have interior walls that separate living space from attics. Where these open areas butt into the other rooms with lower ceilings, the interior partitions are framed up to the roof. This leaves one side of the wall framing exposed to the attic. Such a wall needs to perform as an exterior wall, blocking the flow of air and heat. However, the partition wall is rarely blocked at the lower ceiling plane, and is often uninsulated. In a complicated design, these critical details are easy to overlook.

These walls destroy thermal comfort by leaking air to the attic or by convecting heat out of the wall. During winter, the lower part of the wall is heated from both sides, heating air within the wall. At the lower ceiling plane, the stud bays are open to the attic, allowing heated air to rise up and out. If the wall is interrupted by electric outlets or other penetrations, house air is drawn through them. If there are no penetrations, dense cold air from the attic falls down the stud bays to be heated. (This can make a thermostat behave erratically, because the wall gets colder as the house is heated.)

To air seal and insulate the confusing spaces at the top of interior walls, we've found that house wrap and dense-pack cellulose are particularly useful. We hang house wrap over the short attic walls and other large areas of strange and leaking framing. To secure it to the framing, we either use wide-crown roofing staples or nails and screws with lath or cardboard reinforcement. Fasteners should be 12 inches on center. We then fill the stud and rafter bays with dense-pack cellulose, delivering insulation and air sealed building joints with a minimum of expense.

Today, exposed beams across cathedral ceilings are a popular theme. The sloped ceilings create mighty drafts that suck huge volumes of air through leaks around the beams. If the air is humidified, either mechanically or by a saturated foundation, destructive condensation often occurs in the roof system. The solution is to pack the cathedral ceiling full of cellulose and turn off the water. Turning off the water can mean sealing the foundation, or at least telling residents not to run mechanical humidifiers all winter. Once air leakage is reduced, humidification should be less necessary.

Typically, the roofs of these houses are framed at a 4-in-12 pitch. Getting out to see and fix bypasses and intrusions at the eaves is difficult. More than with any other type of house, I like to walk the interior and map the walls, lighting, and likely chases before entering the attic. The attic is so tight that it's a waste of physical exertion to hunt for leaks up there.

Each bathroom has a separate plumbing chase leading to unconditioned space. Ceiling fixtures are almost never sealed, providing more routes for air flow to the attic. Every linear foot of interior partition top plate is a potential source of leakage, especially in walls with wall switches and electrical outlets. Shower surrounds present especially big leaks--they are intentionally left open to the attic to let moist walls dry out, but they also let warm air escape all winter long. Chimneys are potent leakage sites (see See the Chimney, Seal the Leak). And kitchens are often open to the attic through valances, exhaust chases, and built-in cabinetry.

Big divots in the attic floor, such as kitchen valances and dropped ceilings (see Figure 2), kill envelope performance in two ways. Wall bays that open into these cavities draft warm air into the attic (just like the colonial staircase described above). This air is replaced either from the house or from the attic. Cold attic air can be drawn down into the wall, or house air can leak into the bays through electrical openings and around trim.

To add architectural interest, ever more ranch houses are featuring cathedral ceilings in living rooms and other public areas. These present the same sort of performance problems found in contemporaries, although in ranch homes, the ceiling anomalies are generally easier to see and fix.

Fixing the Ranch

The goal in the ranch is a continuous air seal at the ceiling plane. Most ranch-style homes have one large, flat ceiling, so the main problem is just locating and accessing the leaks. After walking the interior of the house, looking for likely penetrations, we bring a variety of air sealing materials into the attic, including scraps of drywall, waxed cardboard, and wood blocking, caulk, expanding foam, and sheet metal.

Where adjacent wall bays open into lighting valances, there are two possible repairs. It's possible to block the wall framing at the valance level. Rather than waste time cutting rigid air sealing material, we air seal the bays by stuffing them with bags of fiberglass insulation. Alternatively, the entire valance can be dense-packed with cellulose.

In a house with vented attics, the finished half-attic can be surrounded entirely by outdoor air. This conducts heat in during the summer and out during the winter, to the point that water sometimes freezes when spilled on the floor of these rooms. The extensive exposure to outdoor air also causes massive air leakage. Insulation in the sloped ceilings above the attic often turns black from filtering out particulates in flowing air. I have found homes where sealing the various attic spaces reduces total house leakage (measured by blower door) by over 75%.

Generally, the floor of a side attic is unfinished, consisting of exposed joists and insulation. Some finer old houses have tongue-and-groove subfloors in the side attics, but these pose little obstruction to air movement. Thus, the floor under the finished half-story is connected to the side attic. (Building guides sometimes recommend blocking between the floor joists where the kneewalls sit on the joists. This is a good idea that is practically never implemented.) From the side attic, open rafter bays lead to the top attic. So there is a continuous zone of outdoor air that includes the floor system, the side attic, and the top attic.

The attic living space is effectively outdoors for two reasons. First, roof venting is so effective at circulating air out of the attic that there is a constant flow of outdoor air past the interior space, bringing the attic's temperature ever closer to outdoor conditions. Second, the attics usually leak so much (through the backs of built-in furniture, for example) that the primary pressure barrier becomes the attic ridge vent.

Fixing the Half-Attic

Fixing these houses is extremely difficult with normal air sealing methods. Just getting in there can be a challenge. If residents are using the space for storage, it will be full of belongings, and unused side attics often have no access. The kneewalls tend to be broken up by built-in drawers, bookcases, and shelves, to capture some utility from this otherwise wasted space.

Using traditional air sealing methods, we must treat about 100 ft2 of house to eliminate 100 square inches of effective leakage area. Such a project is not usually cost-effective. But if the side attics are small or unused, it is much simpler to solidify them completely by packing them with cellulose. This eliminates all the hand work in what can be hot, confined, dangerous, and filthy places. The results, unlike conventional air sealing, are extraordinarily reliable and durable, regardless of complications such as built-in furniture.

To solidify a half-attic, we cover the backs of the built-in furniture with house wrap, stapled to the framing. To gain entry, we go through interior walls, through exterior walls, or through the roof. We check the side attic for bad wiring, unlined chimneys, light fixtures labeled fire hazard, disconnected ductwork, roof rot, and dead animals. We then pack the side attics with cellulose at 3.5 lb/ft3, eliminating all attic venting. We treat the top attic conventionally, by air sealing manually and blowing with cellulose. We have found that by blocking off all air movement between the living space and the attic, we can get rid of moisture problems. We guarantee our work, and have yet to have any problems from structural failure or moisture diffusion through the cellulose. Most importantly, we have found that residents are far more comfortable.

In order to preserve side attics, we sheath the rafters with oriented strand board and dense-pack the rafter bays. This is easiest to do from above. If there is no hatch, we gain access to the top attic, either through exterior walls, through the roof, or through gable vents. For uninsulated slopes, we push a 2-inch hose down the rafter bays all the way to the floor. If there is existing batt insulation in the sloped ceiling, we slide a 1-inch hose past the existing insulation. Either way, we blow in cellulose at 3.5 lb/ft3. When we are done, the soffits and rafter bays are packed tight, and any batt insulation in the slopes is crushed flat against the sloped ceiling.

Fred Lugano is a builder and home performance contractor in Vermont.

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