10.4 Flashing, Sealants, and Air Barriers
Key Takeaways
- IBC 1503.2 and 1404.4 require flashing at roof/wall intersections, penetrations, openings, and wall bottoms.
- Step, base, counter, valley, drip-edge, and through-wall flashing each serve specific joints; drip edge is required at eaves and rakes.
- Masonry weep holes sit directly on through-wall flashing at <=33 in. on center.
- Elastomeric sealants (ASTM C920) use a 2:1 width-to-depth ratio with a backer rod to avoid three-sided adhesion.
- Air barriers limit air leakage (IECC blower-door ACH50 limits) and are distinct from vapor retarders.
Flashing Purpose & Code Basis
Flashing is sheet material installed to direct water away from vulnerable joints and penetrations. IBC 1503.2 requires flashing at wall and roof intersections, changes in roof slope or direction, and around roof openings. IBC 1404.4 requires exterior-wall flashing to divert water to the exterior at the bottoms of walls, above openings, and at roof/wall junctions.
Types of Flashing
| Flashing | Location / Purpose |
|---|---|
| Step flashing | Sidewalls of sloped roof against vertical wall; one piece per shingle course |
| Base/apron flashing | Front of a chimney or wall-to-roof headwall |
| Counter (cap) flashing | Set into a masonry reglet, laps over base flashing |
| Valley flashing | Open valley where two roof planes meet |
| Drip edge | Eaves and rakes; IBC requires drip edge at eaves and gables |
| Through-wall flashing | Masonry cavity walls, with weep holes below |
Drip edge is now explicitly required by IBC 1507.2.9.3 for asphalt shingle roofs.
Masonry Through-Wall Flashing & Weeps
In cavity/veneer masonry, through-wall flashing collects water that penetrates the outer wythe and directs it out via weep holes. Weep holes are spaced at a maximum of 33 in. on center (TMS 402 / masonry standard) and located immediately above the flashing at the wall base, shelf angles, and over openings.
Exam trap: Weeps must sit directly on top of the flashing - if mortar droppings block them or they sit above the flashing line, drainage fails.
Sealants & Joint Design
Sealants accommodate movement. Per ASTM C920 (elastomeric joint sealants), key chemistries:
- Silicone: excellent UV/weather resistance, high movement (+/-50%), glass/metal.
- Polyurethane: good adhesion, paintable, abrasion-resistant, +/-25%.
- Polysulfide: chemical/fuel resistance.
- Acrylic latex: interior, paintable, low movement.
Joint design rule: for working joints, the width-to-depth ratio should be 2:1 (e.g., 1/2 in. wide, 1/4 in. deep). A backer rod (closed-cell foam) controls depth and prevents three-sided adhesion, which would tear the sealant.
Sealant Estimating
Linear coverage of a cartridge depends on bead cross-section. A 10.1 oz cartridge yields about 305 ml (18.6 in3).
Worked example: A 3/8 in. x 3/8 in. joint = 0.375 x 0.375 = 0.1406 in2 cross-section. Per inch of joint = 0.1406 in3. Linear inches per cartridge = 18.6 / 0.1406 = 132 in = 11 ft. For 220 lin ft of joint: 220 / 11 = 20 cartridges (round up).
Air Barriers
An air barrier is a continuous system limiting uncontrolled air leakage through the building envelope; it improves energy performance and prevents moisture-laden air infiltration. The IECC C402.5 / R402.4 requires a continuous air barrier with sealed penetrations, and prescribes a building air-leakage test (blower door) limit of 3-5 air changes per hour at 50 Pa (ACH50) depending on climate zone.
Key distinction: A material can be both an air barrier and a vapor retarder (e.g., sheet poly), but many air barriers (housewrap) are deliberately vapor-permeable to let walls dry. Do not confuse air leakage with vapor diffusion - they are separate transport mechanisms.
A working sealant joint is 1/2 in. wide. Per the standard width-to-depth rule for elastomeric sealants, what is the target sealant depth, and what controls it?
Why might a builder choose a vapor-permeable housewrap as the wall air barrier rather than sheet polyethylene?
Flashing Principles and Locations
Flashing is the metal/membrane that bridges joints and directs water out of an assembly. Apply the shingling (lapping) principle: upper pieces lap over lower so water runs out, never into, a lap. Critical locations: wall-to-roof (step/counter flashing), windows/doors (head, jamb, sill pan), parapets/copings, through-wall at shelf angles, chimneys, and penetrations. A kickout flashing at the roof-wall termination diverts water away from the wall — a notoriously omitted detail that rots wall framing.
Sealants and Joint Design
Sealants accommodate movement; match the type to the joint. Silicone (excellent UV/weather, low adhesion to porous surfaces unless primed), polyurethane (good adhesion, paintable, abrasion-resistant), acrylic latex (interior, paintable, low movement). Design the joint to a proper width-to-depth ratio (~2:1) with a backer rod so the sealant bonds to two sides only (no three-sided adhesion) and can stretch. Over-deep or three-sided sealant tears under movement.
Air Barriers and the Building Envelope
An air barrier stops air leakage, which carries far more moisture and energy loss than vapor diffusion. It must be continuous across walls, roof, foundation, and all transitions/penetrations — sealed at windows, top/bottom plates, and electrical boxes. The IECC requires blower-door testing to a leakage limit (e.g., ~3–5 ACH50). Distinguish: air barrier = stops air, vapor retarder = slows diffusion, water barrier (WRB) = drains bulk water behind cladding.
Common Exam Traps
- Trap: Lapping flashing the wrong way (water runs into the lap).
- Trap: Three-sided sealant adhesion — it should bond two sides over a backer rod.
- Trap: Treating air barrier and vapor retarder as the same thing.
- Trap: Omitting the kickout flashing at roof-wall terminations.
Why is a backer rod installed in an expansion joint before applying sealant?
The Drainage-Plane / Rainscreen Concept
Modern wall design assumes some water gets behind the cladding, so a continuous water-resistive barrier (WRB) drains it back out — building paper or a housewrap lapped shingle-fashion over the flashing below. A rainscreen adds an air gap behind the cladding for drainage and drying. The integration sequence at a window is the classic test point: the sill pan flashing goes in first, the WRB laps over the head flashing and under at the sill, so every lap directs water down and out — reversing one lap funnels water into the wall.