Below Grade Insulation

Below-grade insulation protects foundation walls from heat loss and frost penetration while managing moisture exposure from soil contact. Insulation placement, material selection, and thermal bridging control directly affect energy performance and durability.

Insulation Material Properties

Extruded Polystyrene (XPS)

Physical Characteristics:

• Closed-cell foam structure
• Density: 1.3-2.2 lb/ft³
• R-value: 5.0 per inch (aged)
• Water absorption: 0.1-0.3% by volume (ASTM C272)
• Compressive strength: 15-60 psi depending on density
• Dimensional stability: minimal shrinkage below 165°F

Performance Attributes:

• Excellent resistance to moisture intrusion
• Maintains R-value when damp
• High compressive strength for load-bearing applications
• UV degradation if exposed to sunlight
• Thermal drift reduces R-value over time (5.0 vs. 5.6 initial)

Below-Grade Applications:

• Exterior foundation insulation
• Under-slab insulation
• Frost-protected shallow foundations
• Perimeter insulation for heated slabs

Expanded Polystyrene (EPS)

Physical Characteristics:

• Closed-cell bead structure
• Density: 0.7-2.0 lb/ft³
• R-value: 3.6-4.2 per inch (varies with density)
• Water absorption: 2-4% by volume (ASTM C272)
• Compressive strength: 10-60 psi depending on density
• Dimensional stability: excellent across temperature range

Performance Attributes:

• Lower cost than XPS
• No thermal drift (stable R-value over time)
• Higher water vapor permeability than XPS
• Lower compressive strength at equivalent density
• Requires protection from fuel spills and solvents

Below-Grade Applications:

• Exterior foundation insulation where drainage is adequate
• Protected membrane roof assemblies
• Geofoam fill applications
• Frost protection in well-drained soils

Polyisocyanurate (Polyiso)

Physical Characteristics:

• Closed-cell foam with foil facers
• Density: 2.0-2.5 lb/ft³
• R-value: 6.0-6.5 per inch at 75°F (temperature-dependent)
• Water absorption: 0.03-0.05% by volume with intact facers
• Compressive strength: 16-25 psi

Performance Limitations Below Grade:

• R-value degrades significantly below 50°F
• Not recommended for direct soil contact
• Foil facers degrade in alkaline soil conditions
• Limited application below grade due to temperature performance

Mineral Wool (Rock Wool/Stone Wool)

Physical Characteristics:

• Fibrous inorganic material
• Density: 4-8 lb/ft³
• R-value: 4.0-4.3 per inch
• Water absorption: wicks water but drains readily
• Compressive strength: 6-10 psi

Performance Below Grade:

• Excellent fire resistance
• Vapor permeable (dries in both directions)
• Requires drainage system to prevent water retention
• Higher density products suitable for interior applications
• Does not support mold growth

Material Comparison Table

Insulation Placement Strategies

Exterior Foundation Insulation

Installation Configuration:

• Insulation applied to exterior face of foundation wall
• Extends from top of footing to grade or above
• Protected by drainage board or coating at exposed portions
• Continuous thermal barrier minimizes thermal bridging

Performance Advantages:

• Foundation wall remains at near-interior temperature
• Thermal mass of concrete inside insulation envelope
• Eliminates thermal bridging through foundation wall
• Protects waterproofing membrane from thermal stress
• Reduces freeze-thaw cycling in concrete

Installation Requirements:

• Insulation must resist soil pressure and backfill loads
• Protection board required above grade
• Drainage plane between insulation and soil
• Termite inspection gap in high-risk regions
• Mechanical attachment or adhesive to wall surface

Thermal Bridging Control:

• Continuous insulation from footing to rim joist
• Insulation overlap at corners (minimum 12 inches)
• Sill sealer at top of foundation wall
• Rim joist insulation coordination

Interior Foundation Insulation

Installation Configuration:

• Insulation applied to interior face of foundation wall
• Typically framed wall assembly with cavity insulation
• Requires interior finish (fire rating compliance)
• Vapor control strategy critical for moisture management

Performance Considerations:

• Foundation wall exposed to freeze-thaw cycles
• Concrete thermal mass outside insulation envelope
• Potential for condensation on concrete surface
• Reduced below-grade storage temperature

Moisture Management:

• Air gap between insulation and concrete (recommended)
• Vapor retarder placement depends on climate
• Drainage at base of wall to interior drain tile
• Capillary break at bottom plate

Assembly Options:

Option 1: Rigid Foam Direct-Applied

• XPS or EPS adhered directly to concrete
• No air gap (sealed perimeter)
• Strapping for finish attachment
• Minimum R-10 for condensation control

Option 2: Framed Wall with Cavity Insulation

• 2x4 framing 1-2 inches from concrete
• Fiberglass or mineral wool in cavity
• Rigid foam between framing and concrete
• Polyethylene vapor retarder (cold climates)

Option 3: Spray Foam Application

• Closed-cell spray foam directly on concrete
• Vapor barrier and insulation in single layer
• R-10 minimum for condensation control
• Highest cost per R-value

R-Value Requirements by Climate

International Energy Conservation Code (IECC) Requirements

Performance-Based Design

Heat Loss Calculation: Foundation heat loss depends on:

• Insulation R-value
• Depth below grade
• Soil thermal conductivity (0.8-1.5 BTU·in/hr·ft²·°F)
• Groundwater level and movement
• Distance from heated space perimeter

Simplified Heat Loss Equation: Q = U × A × ΔT

Where:

• Q = heat loss (BTU/hr)
• U = overall U-factor (BTU/hr·ft²·°F)
• A = foundation wall area (ft²)
• ΔT = indoor-outdoor temperature difference (°F)

Below-Grade U-Factor Calculation: U_bg = 1 / (R_concrete + R_insulation + R_soil)

Soil resistance increases with depth (approximate):

• 0-1 ft: R-0.5
• 1-2 ft: R-1.0
• 2-4 ft: R-2.0

• 4 ft: R-3.0

Frost Protection Strategies

Frost-Protected Shallow Foundations (FPSF)

Design Principle: Horizontal insulation placed beneath and adjacent to foundation prevents frost penetration by redirecting geothermal heat flow.

Configuration Requirements:

• Heated building (>64°F maintained)
• Horizontal wing insulation extending outward from foundation
• Vertical insulation at foundation wall
• Protection of insulation from physical damage and UV

Insulation Dimensions by Frost Depth:

Installation Details:

• Insulation extends from 6" below finished grade
• Minimum 2° slope away from foundation
• Drainage system below horizontal insulation
• Protection board above horizontal insulation

Deep Foundation Frost Protection

Full-Depth Exterior Insulation:

• Insulation from footing to grade
• Minimum R-value based on climate zone
• Protects against frost heave and adfreezing
• Reduces heat loss from conditioned basement

Partial-Depth Insulation:

• Upper 4 feet of foundation most critical
• R-value doubled for reduced depth application
• Suitable for crawlspaces and unheated basements
• Lower material cost with adequate performance

Thermal Bridging Analysis

Common Thermal Bridge Locations

Foundation-to-Wall Transition:

• Rim joist area (major heat loss location)
• Sill plate thermal bridge
• Foundation wall top course
• Floor framing penetrations

Thermal Bridge Mitigation:

• Continuous insulation across rim joist
• Rigid foam sill sealer (R-3 to R-5)
• Spray foam at rim joist cavity
• Exterior insulation overlap onto above-grade wall

Quantified Heat Loss: Uninsulated rim joist: U-0.30 to U-0.50 Standard fiberglass cavity: U-0.08 Continuous exterior insulation: U-0.05

Corner and Penetration Bridging

Foundation Corners:

• Heat loss increased 20-40% at corners
• Insulation overlap required (12-24 inches)
• Interior corner insulation extends onto adjacent wall
• Exterior corner requires edge protection

Service Penetrations:

• Seal all penetrations with non-expanding foam
• Minimum 2" clearance between insulation and combustion vents
• Electrical conduit sealed at entry point
• Pipe penetrations wrapped with pipe insulation

Installation Best Practices

Surface Preparation:

• Clean concrete surface (remove form release agents)
• Repair major voids and honeycombing
• Level irregular surfaces for foam board contact
• Waterproofing or dampproofing before insulation

Attachment Methods:

• Construction adhesive (beads or ribbons)
• Mechanical fasteners with load-distribution plates
• Shotcrete over insulation (permanent protection)
• Drainage board adhesive for compatible products

Protection Requirements:

• UV-resistant coating above grade
• Impact protection in vulnerable areas
• Insect barrier in termite-prone regions
• Filter fabric between insulation and drainage aggregate

Quality Control:

• Continuous insulation (no gaps at joints)
• Tight fit at corners and penetrations
• Proper drainage path maintained
• Protection board installed before backfill