Foundation Assemblies

Foundation assemblies represent the most challenging moisture control application in building envelope design due to continuous contact with ground moisture, hydrostatic pressure, and capillary suction forces. Below-grade conditions impose liquid water exposure that exceeds the moisture loads encountered by above-grade assemblies by several orders of magnitude.

Moisture Transport Mechanisms Below Grade

Foundation assemblies must resist three distinct moisture transport mechanisms:

Liquid Water Intrusion

• Hydrostatic pressure from groundwater table elevation
• Lateral water migration through saturated soil
• Surface water infiltration from precipitation events
• Pressure differential can reach 5-10 psi at 10 ft depth

Capillary Transport

• Wicking action through porous concrete and masonry
• Vertical rise can exceed 8-12 ft in concrete depending on pore structure
• Continuous moisture supply from soil contact
• Rate governed by pore size distribution and surface tension

Vapor Diffusion

• Concentration gradient from soil (100% RH) to interior
• Secondary mechanism compared to liquid transport
• Significant only when liquid water pathways are controlled
• Permeance of assembly components determines vapor flow rate

Waterproofing vs Dampproofing

The distinction between waterproofing and dampproofing systems is critical for foundation moisture control.

Dampproofing Systems

Dampproofing provides resistance to moisture transmission by capillary action but does not resist hydrostatic pressure.

Application Requirements

• Sites with water table at least 6 in below basement slab
• Free-draining soils with functional foundation drainage
• No history of periodic groundwater elevation
• Maximum moisture load from dampened soil contact

Materials and Performance

• Spray-applied or trowel-applied asphalt emulsion
• Thickness: 1/16 in minimum
• Permeance: typically 0.5-1.0 perm when dry
• Does not bridge cracks or accommodate movement
• Service life: 10-20 years in favorable conditions

Limitations

• No resistance to liquid water under pressure
• Degrades when submerged
• Ineffective against lateral groundwater flow
• Cannot repair after backfilling

Waterproofing Systems

Waterproofing provides a continuous impermeable barrier that resists liquid water under hydrostatic pressure.

Application Requirements

• Sites with seasonal or permanent high water table
• Poor-draining soils (clay, silt)
• Below-grade occupied spaces requiring dry conditions
• Locations where foundation drainage may become compromised

Sheet Membrane Systems

Fluid-Applied Membranes

• Spray or roller application
• Polyurethane, polymer-modified asphalt, or acrylic base
• Seamless coverage around penetrations
• Thickness: 40-60 mil dry film
• Requires surface preparation and priming
• Quality control depends on applicator skill

Bentonite Clay Systems

• Hydrates when wet to form impermeable gel
• Thickness expands 10-15× when saturated
• Self-healing properties for minor cracks
• Requires 20-30 psi confinement from backfill
• Vulnerable to contaminated groundwater (high salinity)

Foundation Drainage Systems

Effective drainage reduces hydrostatic pressure and prevents water accumulation at foundation perimeter.

Perimeter Drain Configuration

Footing Drain Placement

• Located at footing bearing elevation
• Minimum 4 in diameter perforated pipe
• Invert at or below bottom of slab elevation
• Slope: 1/8 in/ft minimum, 1/4 in/ft preferred
• Filter fabric envelope prevents soil migration

Drainage Layer Requirements

• Clean graded gravel: 3/4-1.5 in stone
• Thickness: 12 in minimum at wall
• Hydraulic conductivity: >100 ft/day
• Extends from footing to grade or drainage board

Discharge Methods

Drainage Board Systems

Prefabricated drainage boards replace gravel drainage layers at foundation walls.

Dimpled Sheet Products

• HDPE sheets with 1/2-3/4 in dimple height
• Flow capacity: 5-15 gal/min/ft at 1/4 in/ft slope
• Combines drainage and protection functions
• Filter fabric integrated on exterior face
• Reduces backfill settlement and hydrostatic load

Performance Characteristics

• Compressive strength: 10,000-40,000 psf
• Long-term flow capacity maintained under backfill pressure
• Provides air gap for foundation wall drying
• Protects waterproofing membrane during backfilling

Vapor Retarder Placement

Vapor retarder location in foundation assemblies depends on insulation strategy and climate conditions.

Exterior Insulation Assemblies

Vapor Retarder on Interior

• Waterproofing membrane functions as exterior vapor retarder
• Interior polyethylene not required with exterior insulation
• Concrete remains warm, reducing condensation potential
• Preferred configuration for consistent performance

Assembly from Exterior to Interior

1. Drainage board or gravel
2. Waterproofing membrane (Class I vapor retarder)
3. Concrete foundation wall
4. Rigid insulation (XPS or polyiso)
5. Gypsum board finish

Interior Insulation Assemblies

Vapor Retarder Considerations

• Concrete wall exposed to cold soil temperature
• Interior insulation creates condensing surface at wall
• Vapor retarder placement is critical
• Risk of interstitial condensation increases in cold climates

Assembly Options

Critical Details

• Interior vapor retarder must be continuous and sealed
• Penetrations for electrical, plumbing require sealing
• Sill plate area is vulnerable intersection point
• Air leakage dominates over vapor diffusion in most failures

Insulation Placement Strategies

Foundation insulation location impacts thermal performance, moisture control, and constructability.

Exterior Insulation

Performance Advantages

• Foundation wall remains warm and dry
• Thermal mass inside conditioned space envelope
• No interior space consumed by insulation
• Protects waterproofing membrane from temperature extremes
• Eliminates thermal bridging through concrete

Material Selection

Installation Requirements

• Drainage board or protection course over insulation
• Mechanical attachment or adhesive above grade
• Backfill protection to prevent damage
• Thickness: 2-4 in typical (R-10 to R-20)
• Extend to frost depth minimum

Interior Insulation

Application Scenarios

• Retrofit of existing foundations
• Limitations on exterior excavation
• Waterproofing membrane already installed
• Cost constraints (no excavation required)

Moisture Management Challenges

• Cold concrete wall at dew point temperature
• Condensation risk on wall surface behind insulation
• Air leakage through rim joist/sill plate connection
• Reduced drying capacity of foundation wall

Recommended Assemblies

Spray Foam Application

• Closed-cell foam: 2-3 in minimum (R-12 to R-18)
• Seals air leakage pathways simultaneously
• Vapor impermeable (Class II at 2 in, Class I at 3 in)
• Highest performance but highest cost

Rigid Foam Board

• XPS: 2-3 in thickness, sealed seams with foam/tape
• Provides both thermal resistance and vapor control
• Must detail air sealing separately
• Gypsum board finish requires furring strips

Batt Insulation (Not Recommended)

• High condensation risk without vapor retarder
• Difficult to achieve air sealing
• Vulnerable to moisture accumulation
• Only acceptable in dry climates (CZ 1-3)

Slab-on-Grade Assemblies

Foundation slabs require integrated moisture and thermal control.

Vapor Retarder Requirements

Sub-slab Vapor Retarder

• Minimum 6 mil polyethylene (IRC code minimum)
• 10-15 mil cross-laminated PE preferred for durability
• Overlaps: 6 in minimum, sealed with tape or mastic
• Extended to interior face of perimeter insulation
• Protects from ground moisture and soil gas intrusion

Placement Sequence

1. Compacted granular fill (crushed stone, 4 in minimum)
2. Vapor retarder (all seams sealed)
3. Concrete slab (4-6 in thickness)
4. Floor finish (tile, wood, carpet)

Granular Fill Function

• Capillary break between soil and vapor retarder
• Provides firm base for concrete placement
• Allows vapor retarder drainage if punctured
• Facilitates radon dispersal to perimeter venting

Slab Insulation Strategies

Horizontal Sub-Slab Insulation

• XPS rigid foam below entire slab area
• Thickness: 1-3 in (R-5 to R-15) depending on climate zone
• Prevents heat loss to ground
• Reduces slab edge thermal bridging
• Required for radiant floor heating systems

Vertical Perimeter Insulation

• XPS at slab edge extending to frost depth
• Minimum R-10 for CZ 4-8
• Width: 2-4 ft horizontally from slab edge
• Reduces edge heat loss (dominant path in slab assemblies)
• Can replace full sub-slab insulation in mild climates

Crawlspace Foundation Assemblies

Crawlspace design has shifted from vented to unvented/conditioned configurations based on moisture research.

Unvented Conditioned Crawlspace

Configuration Requirements

• Ground surface covered with continuous vapor retarder
• Foundation walls insulated (interior or exterior)
• Crawlspace included in conditioned building volume
• Mechanical ventilation or passive connection to occupied space
• Code compliant per IRC Section R408.3

Ground Cover Specification

• Minimum 6 mil polyethylene vapor retarder
• Overlaps sealed with tape or mastic (Class I vapor retarder)
• Extended 6 in up foundation walls and sealed
• Covered with 3-4 in sand or concrete for protection (optional)
• Eliminates soil moisture evaporation (primary moisture source)

Moisture Performance Advantages

• Eliminates humid outdoor air entry during cooling season
• Prevents condensation on cool crawlspace surfaces
• Reduces latent load on HVAC system
• Lower wood moisture content in floor framing
• Superior to vented crawlspace in all but arid climates

Vented Crawlspace (Traditional)

When Applicable

• Dry climates with low humidity (CZ 2-3 arid regions)
• Existing construction where conversion is impractical
• Flood-prone areas requiring venting for water removal

Ventilation Requirements

• 1 ft² vent area per 150 ft² floor area (with ground cover)
• 1 ft² vent area per 1,500 ft² floor area (without ground cover)
• Vents located on multiple walls for cross-ventilation
• Minimum two vents, operable or fixed

Performance Limitations

• Introduces humid air during summer in humid climates
• Causes condensation on cool surfaces and ductwork
• Increases cooling loads and moisture problems
• No longer recommended by building science research
• Floor insulation required (crawlspace outside thermal envelope)

Foundation Penetration Details

Penetrations through foundation waterproofing require careful detailing.

Pipe and Conduit Penetrations

• Sleeve pipe through wall with oversized opening
• Pack annular space with hydraulic cement or non-shrink grout
• Install waterstop or link seal compression fitting
• Extend waterproofing membrane over penetration with boot or mastic
• Test seal before backfilling when possible

Mechanical Anchors and Ties

• Waterproofing membrane must seal around anchor points
• Self-adhered membrane patches over mechanical fasteners
• Bentonite plugs for form ties in clay systems
• Liquid membrane striping over fastener heads

Hydrostatic Pressure Considerations

• Penetrations are highest risk failure points
• Hydrostatic pressure forces water through any opening
• Back-side waterproofing cannot seal penetrations
• Interior sealants experience positive pressure (failure mode)
• Exterior sealing is primary defense

Interior Drainage and Sump Systems

When exterior drainage fails or is compromised, interior perimeter drainage provides backup moisture control.

Interior Perimeter Drain Installation

Configuration

• Saw-cut slab 12-18 in from foundation wall
• Remove concrete and excavate to footing level
• Install 4 in perforated pipe in gravel bed
• Slope to sump pit location (1/4 in/ft minimum)
• Replace concrete or leave accessible trench with grate

Connection to Foundation Wall

• Drill weep holes through footing at 4-6 ft spacing
• Allows water at exterior to drain to interior system
• Relieves hydrostatic pressure against waterproofing
• Requires functional exterior drainage or waterproofing

Sump Pump Sizing and Selection

Pump Capacity Calculation

• Based on maximum groundwater inflow rate
• Perimeter drain inflow: estimate 50-100 gal/hr per 100 linear ft
• Safety factor: 2× anticipated flow
• Typical residential: 1/3-1/2 HP, 40-50 gal/min capacity

System Components

Discharge Location

• Minimum 10 ft from foundation perimeter
• Daylight to grade or underground piping to storm sewer
• Freeze protection required in cold climates
• Backflow prevention if connected to municipal system

Thermal Bridging Considerations

Foundation assemblies create significant thermal bridges that impact whole-building energy performance.

Foundation Wall to Floor Slab

• Continuous concrete path from exterior to interior
• Heat loss coefficient: 0.5-1.0 Btu/hr·ft·F per linear foot
• Mitigated by thermal break at slab edge
• Exterior insulation eliminates bridge entirely

Rim Joist/Band Joist Connection

• Wood framing bears on foundation wall or sill plate
• Air leakage pathway between sill and foundation
• Interior insulation must seal this junction completely
• Spray foam application provides optimal air sealing

Foundation Penetrations

• Steel columns, anchor bolts, mechanical sleeves
• Point thermal bridges with high conductivity
• Insulation continuity difficult to maintain
• Energy impact usually minor compared to wall area

The design of foundation assemblies requires integration of waterproofing, drainage, vapor control, insulation, and thermal bridging mitigation. Below-grade moisture loads exceed above-grade conditions by orders of magnitude, requiring robust liquid water management as the primary design strategy. Vapor diffusion control is secondary but necessary to prevent interstitial condensation in cold climates. The shift toward unvented conditioned crawlspaces and exterior foundation insulation reflects building science research demonstrating superior moisture and energy performance compared to traditional vented and interior-insulated approaches.

Sections