Where the Air Barrier Actually Is in a House

Why this matters

Ask where the air barrier is in an existing house and most people point at the insulation. The insulation is usually where the drawings put the boundary, and the air goes somewhere else: up an open chase beside the chimney, through the floor joists under a kneewall, over the top of a dropped soffit, behind a tub on an outside wall. Those openings are invisible from the living space and often from the attic too, and they are where the frost, the ice dams, the cold rooms and the dirty insulation come from. A trade that knows where the barrier breaks can explain a complaint in five minutes, and can avoid making a new break with its own drill.

What an air barrier has to be

An air barrier is the layer that stops air moving between inside and outside. It can be drywall, a sheet of plywood, a membrane, sealed rigid foam, or poured concrete. What makes it a barrier is not the material, it is continuity: every piece joined to the next, every edge sealed to the framing, every penetration closed. A barrier with holes in it is a sieve with a fine mesh. For the concept and how it relates to the water, vapor and thermal layers, see Building Envelope Fundamentals.

Insulation is not an air barrier. Fiberglass batts and loose-fill cellulose and fiberglass slow heat but let air pass through them. Air moving through or around insulation carries heat with it and makes the insulation far less effective than its R-value says. In most houses the insulation and the air barrier are supposed to be in contact, the insulation on the outside face of a sealed layer. Where the two separate, air finds the gap.

The common places the barrier breaks

The drawn boundary in a typical house runs along the attic floor, down the exterior walls and across the basement walls or the floor over a crawlspace. These are the places it most often fails, and they match the common bypass locations in ENERGY STAR's thermal bypass and air sealing guidance:

Kneewalls. In a story-and-a-half or a bonus room over a garage, the sloped ceiling and short kneewall are insulated and drywalled, and the attic space behind the kneewall is outside. The floor joists run under the kneewall, out into that attic, with nothing blocking the end of each bay. Cold attic air flows straight into the floor cavity between stories. The drawn barrier follows the kneewall; the real one has a hole at the bottom of every joist bay.

Dropped soffits and dropped ceilings. A soffit framed over kitchen cabinets, a tray ceiling, or a lowered hallway ceiling for ducts is often framed and drywalled before or without the main ceiling being run above it. Its top opens into the attic. From the attic it is hidden under the insulation; from below it looks like a solid box.

Chases. The framed shaft around a chimney, a flue, a plumbing stack bundle or a duct riser runs from the basement to the attic with nothing sealing either end. Air leaks in from the basement and out into the attic, the whole height of the house. Chimney chases need a noncombustible seal at the clearances the fuel-gas or chimney code requires, which is a job for the trade that knows those clearances.

Tubs, showers and stairs on exterior walls. The wall behind a tub was often insulated but never drywalled, because the tub went in first. The tub is not an air barrier, so the barrier for that section of wall is missing. A staircase against an exterior wall has the same problem behind its stringer.

The attic hatch and pull-down stairs. A loose panel with no weatherstrip and no latch, or a folding stair with gaps all around the frame.

The rim joist. The band of framing sitting on the foundation, between the floor and the basement or crawlspace wall. The sill plate, the rim and the subfloor edge meet here with gaps at every joint, and in a basement it is often covered only with a batt stuffed into the bay. See related: How to Air Seal a Basement Rim Joist.

Top plates and wiring holes. Every hole drilled through the top plate of an interior wall for wire or pipe is a path from the wall cavity into the attic. Interior walls are open to the house through outlet boxes and gaps at the floor, so the wall cavity becomes a chimney.

  x = where the drawn barrier usually breaks

        attic (outside the boundary)
  ==x=======x==========x=========x===  attic floor
  | hatch   soffit top  chase top  top |
  |         open        open      plate|
  |  [soffit]           |c|      holes |
  |                     |h|            |
  |  upper floor        |a|            |
  x tub on              |s|            |
  | exterior wall       |e|            |
  =x====================|=|=========x=  floor framing
   rim joist            |c|    rim joist
  |  basement           bottom open    |
  +------------------------------------+
  story-and-a-half: add an x at every open
  joist bay under each kneewall

The drawing is the drawn boundary as a continuous double line, and the places it usually is not. The attic floor carries four breaks (the hatch, the open top of the soffit, the open top of the chase, and drilled top plates), the chase carries a second at its open bottom in the basement, the tub section of the exterior wall was never drywalled, and both rim joists leak where sill, rim and subfloor meet. The kneewall break is the same idea in a story-and-a-half, and the fix there is blocking each joist bay at the kneewall line with sealed rigid material, or moving the boundary out to the roof deck.

Worked example: what the visible holes are worth

A story-and-a-half house. The homeowner caulked around the plumbing vent where it passes into the attic and weatherstripped the attic hatch, and the upstairs is still cold and the floor is cold underfoot. Compare the size of the openings, to see where the air actually goes. These are opening sizes, not leakage rates, but airflow goes with opening size, so the comparison holds.

Attic hatch. A 22 by 30 inch hatch has a perimeter of 104 inches. With an average 1/8 inch gap all around, the opening is 104 times 0.125, or 13 square inches.

Plumbing vent. A 2-inch pipe, about 2.375 inches outside diameter, through a 3-inch hole. The ring between them is the area of a 3-inch circle, about 7.07 square inches, minus the pipe's 4.43, leaving about 2.6 square inches.

Flue chase. A framed chase 16 by 20 inches, open to the attic at the top: 320 square inches.

Kneewall joist bays. Two kneewalls, six open joist bays each side, so 12 bays. Each bay is 14.5 inches wide between joists and 9.25 inches deep for 2x10 framing: 14.5 times 9.25 is about 134 square inches. Twelve of them is about 1,610 square inches.

Total and share. 13 plus 2.6 plus 320 plus 1,610 is about 1,946 square inches. The two holes the homeowner sealed, 13 plus 2.6, or about 15.6 square inches, are 15.6 out of 1,946, under one percent of the opening area. The kneewall bays alone are about 83 percent, 1,610 out of 1,946.

What that tells the trade. The cold floor is cold attic air in the floor cavity, and no amount of caulk at the visible penetrations would have touched it. The useful work was blocking 12 joist bays and capping the chase. If the house has a dropped soffit over the kitchen with its top open to the attic, that opening can be as large as the kneewall bays and adds to the same list.

How to find it in a house you did not build

Follow the insulation, then ask where the air stops. In the attic, look at every place the insulation surface drops, changes level or has a hole: soffits, chases, kneewalls, the hatch, recessed lights. At each one ask what solid, sealed material sits below the insulation. If the answer is "nothing" or "a batt," the barrier is broken there.

Read the dirt. Fiberglass insulation that air has been moving through filters the air and turns gray or black where the flow is strongest. Dark insulation at a top plate, a light box or a chase edge is a leak map.

Read the frost and the snow. In cold weather, frost on the attic sheathing directly above a spot, or a bare patch of roof in fresh snow, points down at the leak under it. Ice dams that start above an interior location, not at the eave, often do too. See related: Stack Effect and the Neutral Pressure Plane, for why the top of the house pushes air out.

Use a pressure difference. With the house depressurized by a blower door, or by every exhaust fan in the house running, a smoke pencil or a hand at a suspect gap shows air coming in. See How to Find Air Leaks While the House Is Depressurized.

Check behind what was installed first. Tubs, stairs, cabinets and fireplaces on exterior walls all hide a wall section that may never have been drywalled. An infrared camera on a cold day, with the house depressurized, shows them as cold streaks.

The trade that makes the hole

Most new breaks in the barrier are made by trades doing something else: a plumber running a new vent up through the top plate, an electrician drilling for a new circuit, an HVAC tech cutting in a return or a bath fan, a cable installer feeding a line into the attic. Each hole is small. A remodel adds dozens. The rule is simple: if you cut through the air barrier, you close it, with a material suited to the location and the clearance rules, before you leave. See How to Close a Penetration You Made Through the Air Barrier, and How to Air Seal Penetrations Into the Attic for the attic side.

Where the answer changes

The attic or crawlspace is inside. If the roof deck is foamed or the crawlspace is encapsulated, the barrier has moved to the roof or the foundation, and the attic floor or the subfloor is no longer the boundary. Sealing the attic floor in an unvented attic does little; sealing a gap in the foam at the roof deck matters a lot. See related: The Attic and Crawlspace, Inside or Outside the House.

Combustion appliances remain. Closing large holes tightens the house. That is good for comfort and moisture and it makes every exhaust fan pull harder on any natural-draft appliance left. Whoever does the sealing flags that and routes it to a qualified combustion-safety tech for a test. See related: Which Trade Jobs Change House Pressure and Moisture.

References

• See related: Building Envelope Fundamentals; Building Science and Envelope Reference
• See related: How to Air Seal Penetrations Into the Attic; How to Air Seal a Basement Rim Joist
• See related: How to Close a Penetration You Made Through the Air Barrier
• ENERGY STAR, thermal bypass and air sealing guidance for existing homes (common bypass locations)