Slab On Grade
Slab-on-grade construction places concrete directly on prepared soil, requiring careful moisture control to prevent vapor transmission, structural damage, and flooring failures. Proper assembly design addresses capillary rise, vapor diffusion, thermal bridging, and soil gas intrusion.
Assembly Components
Soil Preparation
Compaction Requirements
- Minimum 95% modified Proctor density for fill soils
- Remove organic topsoil and unsuitable bearing material
- Uniform compaction prevents differential settlement
- Proper drainage away from slab edges (minimum 6 inches in 10 feet)
Capillary Break Layer
- Minimum 4 inches clean gravel or crushed stone
- 1/2 to 3/4 inch aggregate size prevents capillary action
- Open graded material allows drainage and vapor diffusion
- Continuous layer under entire slab area
- Connects to perimeter drainage where required
Vapor Retarder Placement
Vapor Retarder Selection
Material Specifications
| Material | Permeance | Thickness | Application |
|---|---|---|---|
| Polyethylene (Class A) | ≤0.01 perm | 10-15 mil | Standard applications |
| Cross-laminated polyethylene | ≤0.01 perm | 10-15 mil | Enhanced puncture resistance |
| Reinforced vapor retarder | ≤0.003 perm | 10-15 mil | High moisture environments |
| Below-slab vapor barrier systems | ≤0.01 perm | 15+ mil | Critical moisture control |
Installation Position
- Place directly under concrete slab for maximum effectiveness
- Locate above capillary break layer (gravel)
- Position below sub-slab insulation where insulation is used
- Continuous coverage with sealed joints prevents moisture bypass
Installation Details
Joint Sealing
- Overlap all joints minimum 6 inches
- Tape or mastic seal at overlaps
- Seal penetrations for plumbing, electrical conduit
- Seal perimeter connections to foundation walls
- Use compatible tape rated for concrete alkalinity
Protection During Construction
- Minimize foot traffic on installed vapor retarder
- Repair punctures or tears immediately
- Protect from UV exposure before concrete placement
- Inspect completeness before reinforcement installation
Perimeter Termination
- Extend vapor retarder up foundation wall minimum 3 inches
- Seal to wall with compatible mastic or tape
- Coordinate with wall vapor control layer where present
- Maintain continuity across construction joints
Sub-Slab Insulation
Insulation Placement Strategies
Horizontal Insulation
- Full slab insulation for heated spaces
- R-10 to R-20 minimum based on climate zone
- Higher R-values for radiant heated slabs (R-15 to R-30)
- Closed-cell rigid foam board materials required
Edge Insulation
- Vertical edge insulation minimum R-5 to R-10
- Extends 24 to 48 inches below grade depending on climate
- Horizontal wing insulation where frost depth exceeds practical vertical depth
- Protects slab perimeter from thermal bridging
Material Selection
Approved Insulation Types
- Extruded polystyrene (XPS): R-5 per inch, high compressive strength
- Expanded polystyrene (EPS): R-4 per inch, lower cost, requires higher density
- Polyisocyanurate: R-6 per inch, requires moisture protection
- Cellular glass: R-3 per inch, inorganic, high compressive strength
Performance Requirements
- Minimum 25 psi compressive strength for residential slabs
- 40-60 psi for commercial or heavy loads
- Moisture resistant properties essential
- Stable thermal performance when wet
Vapor Retarder-Insulation Sequence
Standard Assembly (Bottom to Top)
- Compacted subgrade
- Capillary break layer (4 inches minimum gravel)
- Vapor retarder (10-15 mil polyethylene or equivalent)
- Rigid insulation (taped joints)
- Concrete slab with reinforcement
Rationale for Sequence
- Vapor retarder below insulation prevents moisture accumulation in foam
- Insulation protects vapor retarder from construction damage
- Slab thermal mass positioned above insulation for heating efficiency
Moisture Testing
Pre-Pour Testing
Subgrade Moisture Assessment
- Verify capillary break layer drainage performance
- Test for groundwater or perched water conditions
- Ensure proper site drainage before slab placement
- Document soil moisture levels if flooring warranty required
In-Slab Moisture Testing
ASTM F2170: Relative Humidity Testing
- Insert RH probes at 40% of slab depth
- Equilibrate minimum 72 hours before reading
- Maximum 75-80% RH for most resilient flooring systems
- Test spacing: one per 1,000 square feet minimum
ASTM F1869: Calcium Chloride Testing
- Measures moisture vapor emission rate (MVER)
- Maximum 3-5 pounds/1,000 sq ft/24 hours typical limit
- Less accurate than RH method for modern flooring systems
- Useful for older slabs without RH probe access
Testing Timing
- Minimum 28 days after concrete placement
- Test after HVAC operational and building enclosed
- Ambient conditions at anticipated service levels (65-75°F, 30-50% RH)
- Re-test if conditions change significantly
Acceptance Criteria by Flooring Type
| Flooring Material | Max RH (%) | Max MVER (lb/1000sf/24hr) |
|---|---|---|
| Vinyl tile/sheet | 75-80 | 3-5 |
| Engineered wood | 75 | 3 |
| Ceramic tile (cementitious thinset) | 85-90 | Not critical |
| Epoxy coating | 75-80 | 3-5 |
| Carpet/pad | 80-85 | 3-5 |
Concrete Curing and Moisture Management
Hydration Process
Initial Curing Period
- First 7 days: maintain moist conditions for proper hydration
- Prevent rapid moisture loss from top surface
- Wet curing, curing compounds, or plastic sheeting methods
- Proper curing develops strength and reduces permeability
Long-Term Drying
- Concrete continues drying for months to years after placement
- Drying rate: approximately 1 inch per month under ideal conditions
- Vapor retarder under slab slows bottom drying significantly
- Top surface drying only for slabs with vapor retarders
Water-Cement Ratio Impact
Low w/c Ratios (0.40-0.45)
- Reduced permeability and higher strength
- Less total moisture requires removal
- Faster drying to acceptable moisture levels
- Better compatibility with moisture-sensitive flooring
High w/c Ratios (0.50-0.60)
- Higher initial moisture content
- Extended drying time required
- Increased slab permeability after curing
- May require vapor barrier over slab for flooring protection
Admixtures and Moisture Control
Internal Sealers
- Crystalline admixtures reduce permeability
- Assist in controlling moisture transmission
- Do not eliminate need for sub-slab vapor retarder
- Enhance long-term durability
Flooring Compatibility
Moisture-Sensitive Flooring Systems
Adhesive Requirements
- Moisture-cure urethanes for high moisture conditions
- Epoxy adhesives for controlled moisture environments
- Verify adhesive MVER tolerance matches slab conditions
- Surface preparation affects adhesive performance
Underlayment Options
- Topical vapor barriers over slab (when sub-slab barrier inadequate)
- Minimum 15 mil polyethylene or equivalent permeance
- Self-leveling underlayments with moisture mitigation properties
- Moisture vapor reduction systems (MVRS) rated for specific conditions
Flooring Installation Timing
Minimum Waiting Periods
- 28 days after placement before moisture testing
- 60-90 days typical for flooring installation on slabs with vapor retarders
- Up to 180 days for slabs without vapor retarders (not recommended)
- Extended periods for thick slabs (>6 inches) or high w/c ratios
Environmental Conditions
- Building enclosed and weathertight
- HVAC operational at service conditions
- Interior temperature 65-85°F
- Relative humidity 30-50% maintained
Radon Mitigation
Radon Gas Sources
Soil Gas Intrusion Pathways
- Cracks in slab and control joints
- Gaps around penetrations (plumbing, electrical)
- Exposed soil in sump pits or crawl spaces
- Porous concrete allows diffusion
Passive Radon Control
Sub-Slab Depressurization System (Passive)
- 3-4 inch perforated PVC piping in gravel layer
- Continuous capillary break layer serves as air flow pathway
- Vertical vent pipe through roof (minimum 3 inches diameter)
- Sealed slab minimizes air leakage pathways
Vapor Retarder Role
- Sealed polyethylene under slab reduces gas entry
- Not sufficient alone for high radon areas
- Must be combined with venting strategy
- Continuity and sealing critical for effectiveness
Active Radon Mitigation
Fan-Assisted Depressurization
- Install radon fan in vent stack (typically in attic or outside)
- Creates negative pressure under slab
- Draws soil gas from sub-slab gravel layer
- Exhausts above roof level away from air intakes
System Components
- Suction points in sub-slab gravel (one per 2,000-4,000 sf)
- Sealed slab surface minimizes short-circuiting
- Manometer monitors system pressure differential
- Electrical connection for continuous fan operation
Radon-Resistant New Construction
EPA Radon Zone 1 Requirements
- Gas-permeable layer (4 inches gravel) under entire slab
- Sealed polyethylene vapor retarder (10 mil minimum)
- Vent pipe from sub-slab to roof (rough-in for future activation)
- Sealed slab penetrations and joints
- Provisions for future fan installation if testing indicates need
Installation Best Practices
Pre-Concrete Checklist
- Verify subgrade compaction and grading
- Install complete capillary break layer
- Place and seal vapor retarder with all penetrations addressed
- Install and seal insulation joints if used
- Inspect for damage before reinforcement placement
- Protect vapor retarder during rebar installation
Quality Assurance
- Document vapor retarder continuity with photos
- Verify insulation thickness and coverage
- Test radon piping for continuity before concrete placement
- Coordinate with concrete contractor on placement procedures
- Plan for moisture testing schedule before flooring installation
Common Deficiencies to Avoid
- Unsealed vapor retarder laps and penetrations
- Missing vapor retarder at slab edges
- Damaged vapor retarder from foot traffic or reinforcement
- Inadequate capillary break layer thickness or coverage
- Insufficient edge insulation depth
- Radon piping obstructions or inadequate sealing