Capillary Breaks

Capillary breaks are material layers or assemblies that interrupt the continuous pore structure in building assemblies to prevent moisture migration through capillary action. These breaks are critical components in foundation design to prevent ground moisture from entering the building envelope.

Capillary Rise Mechanism

Physical Principles

Capillary action occurs when:

• Liquid molecules exhibit surface tension and adhesion to solid surfaces
• Pore diameter in porous materials creates meniscus formation
• Height of rise inversely proportional to pore diameter: h = 2γcosθ / (ρgr)
  • h = height of capillary rise
  • γ = surface tension of liquid
  • θ = contact angle
  • ρ = liquid density
  • g = gravitational acceleration
  • r = pore radius

Moisture Transport Characteristics

Capillary rise height in common materials:

• Fine sand (0.1 mm pores): 1-3 ft
• Silt: 3-10 ft
• Clay: 10-30 ft or higher
• Concrete (micropores): 3-10 ft
• Brick masonry: 5-15 ft

Transport occurs when:

• Continuous pore network exists from ground to material
• Pore size permits meniscus formation
• Sufficient moisture available at base
• No intervening non-porous layer breaks the path

Capillary Break Materials

Granular Materials

Coarse aggregate specifications:

• Minimum particle size: 1/2 inch
• Maximum particle size: 1-1/2 inch
• Clean, washed aggregate (no fines)
• Particle distribution: uniformly graded
• Layer thickness: minimum 4 inches
• Effective pore size: prevents capillary rise
• ASTM C33 compliance for drainage aggregate

Performance characteristics:

• Pore diameter exceeds capillary rise threshold
• Maximum capillary rise: 1-2 inches
• Provides drainage path when properly graded
• Requires separation from finer soils

Sheet Membrane Materials

Polyethylene vapor retarder:

• Material: 6-10 mil polyethylene sheeting
• Standard: ASTM E1745 Class A (0.01 perms)
• Permeance: 0.1 perms or less
• Primary function: vapor control, secondary capillary break
• Overlap: minimum 6 inches at joints
• Continuous under entire slab area

Bituminous membranes:

• Self-adhered modified bitumen
• Torch-applied systems
• Cold-applied liquid membranes
• Thickness: 40-60 mils typical
• Provides both capillary break and waterproofing

Closed-cell foam materials:

• Extruded polystyrene (XPS)
• Polyisocyanurate with foil facing
• Dual function: thermal insulation and capillary break
• Minimum R-value per energy code
• Compressive strength: 25 psi minimum for under-slab

Placement Requirements

Under-Slab Applications

Base preparation sequence:

1. Undisturbed or engineered fill subgrade
2. Compacted base course (6-12 inches)
3. Capillary break layer (4 inches minimum coarse aggregate)
4. Polyethylene vapor retarder (when required)
5. Concrete slab

Critical installation details:

• Granular layer extends 12 inches beyond slab perimeter
• No migration of fines into granular layer
• Geotextile separator fabric between subgrade and aggregate
• Vapor retarder placed on capillary break, not directly on subgrade
• All penetrations sealed with mastic or tape

Footing to Foundation Wall Interface

Horizontal break at footing:

• Membrane placed on top of footing before wall construction
• Material: bituminous dampproofing or sheet membrane
• Coverage: full width of footing plus 2 inches overhang
• Prevents moisture rise from footing into wall
• Critical in high water table conditions

Installation sequence:

1. Strip footing poured and cured
2. Surface prepared (clean, dry)
3. Membrane applied over entire footing top
4. Foundation wall constructed on membrane
5. Vertical dampproofing connects to horizontal membrane

Wall-Slab Connections

Monolithic Slab-Grade Beam

Capillary break placement:

• Horizontal membrane under thickened edge
• Continuous from exterior to interior
• Vertical membrane extends from below footing to grade
• Connection detail seals horizontal to vertical transition

Slab-on-Grade with Stem Wall

Multi-layer protection:

• Membrane at top of footing (below wall)
• Sill plate gasket (wood construction)
• Under-slab vapor retarder
• Perimeter insulation as capillary break
• Drainage system at wall-slab joint

Detail requirements:

• Interior slab isolated from foundation wall
• Compressible filler at perimeter
• Vapor retarder terminates at wall face
• No continuous path from exterior soil through assembly

Basement Floor to Wall Connection

Critical interfaces:

• Exterior waterproofing on wall continues to below-slab level
• Under-slab drainage system connects to perimeter drain
• Interior vapor retarder lapped with wall membrane
• Cove detail at floor-wall junction prevents water entry
• Hydrostatic pressure considerations require full waterproofing

Dampproofing vs Waterproofing

Dampproofing Systems

Function and application:

• Resists moisture vapor and capillary water
• Not designed for hydrostatic pressure
• Applied to above-grade portions of foundation walls
• Materials: bituminous coatings, cementitious parge coats
• Thickness: 1/32 inch minimum for bituminous

Typical dampproofing materials:

• Asphalt emulsion: ASTM D1227
• Coal tar pitch: ASTM D43
• Polymer-modified asphalt
• Cementitious coatings
• Single-coat application typical

Waterproofing Systems

Function and application:

• Resists hydrostatic pressure and bulk water intrusion
• Required when groundwater above floor level possible
• Required when interior conditions sensitive to moisture
• Multiple material layers
• Thickness: 60 mils minimum for membranes

Waterproofing membrane types:

System selection criteria:

• Depth of water table below finished floor
• Soil drainage characteristics
• Building use and moisture sensitivity
• Installation season and conditions
• Required service life

Capillary Break Specifications

Material Requirements

Granular capillary break:

• Material: ASTM C33 No. 57 stone or equivalent
• Particle size: 1/2 to 1-1/2 inch
• Fines content: maximum 2% passing No. 200 sieve
• Layer thickness: 4 inches minimum, 6 inches preferred
• Compaction: not required (self-draining)

Vapor retarder under slab:

• Material: Polyethylene per ASTM E1745 Class A
• Thickness: 10 mil minimum for Class A
• Permeance: 0.3 perms maximum
• Laps: 6 inches minimum, sealed with compatible tape
• Puncture resistance: 440 gf minimum

Membrane at footing:

• Material: Self-adhered modified bitumen or fluid-applied
• Thickness: 40 mils minimum
• Elongation: 300% minimum
• Tensile strength: 30 lbf/inch minimum
• Temperature rating: -20°F to 180°F

Installation Standards

Substrate preparation:

• Surface clean, dry, and free of protrusions
• Concrete surfaces: no laitance or curing compounds incompatible with adhesive
• Temperature: membrane installation per manufacturer (typically 25°F minimum)
• Concrete cure time: 7 days minimum before membrane application

Quality control:

• Verify material certifications before installation
• Inspect substrate condition before membrane placement
• Check for continuous coverage with no voids
• Test laps and seams for proper adhesion
• Protect installed membranes from damage during construction

Performance testing:

• Flood testing for waterproofing systems (24-hour minimum)
• Electronic leak detection for critical applications
• Moisture vapor emission testing for slab-on-grade
• Visual inspection of all penetrations and terminations

Critical Details

Penetration Treatments

Pipe penetrations:

• Boots or sleeves seal membrane to pipe
• Mastic or sealant at all interfaces
• Granular capillary break removed locally, replaced with concrete or grout
• Annular space sealed to prevent bypass

Wall-to-footing transitions:

• Horizontal membrane laps with vertical membrane (6 inches minimum)
• Compatible materials used throughout
• Continuous seal prevents water entry
• Reinforcing fabric at outside corners

Termination Details

Grade terminations:

• Membrane extends minimum 6 inches above finished grade
• Mechanical termination bar at top edge
• Sealant bead at termination bar
• Backfill protection board over membrane
• No exposed membrane to UV degradation

Slab edge terminations:

• Vapor retarder terminates at slab perimeter
• Sealed to vertical membrane or sealed with mastic
• Slab perimeter insulation may act as additional capillary break
• Compressible joint filler at construction joints