Basement Slab Vapor Barrier Placement Guide

The Vapor Barrier Placement Debate

One of the most debated details in basement construction is the proper placement of polyethylene vapor barriers in relation to sub-slab foam insulation. Should the poly go above or below the foam? The answer affects moisture control, insulation effectiveness, and radiant floor heating installation.

Understanding the Confusion

As Ryan noted when reviewing GBA’s “Fixing a Wet Basement” article, seeing polyethylene placed above rigid foam can seem counterintuitive. After all, if EPS/XPS foam is already waterproof, why not put the vapor barrier below it to protect the foam? And what about stapling radiant heating tubes?

The Building Science Behind Placement

Key Principle from Martin Holladay:

“Unlike an air barrier, a vapor barrier can have holes in it without significantly reducing its effectiveness as a vapor barrier. If a vapor barrier consists of 96% membrane and 4% holes, it is still 96% as effective as a membrane with no holes.”

This principle is crucial for understanding why minor penetrations from staples don’t compromise the system.

Standard Assembly (Top to Bottom)

The Proven Stack:

1. Concrete slab (4-6” typical)
2. Polyethylene vapor barrier (6-10 mil)
3. Rigid foam insulation (R-10 to R-20+)
4. Gravel drainage layer (4-6” clean stone)
5. Earth

Why Poly Above Foam Works

Moisture Control:

• Prevents concrete moisture from entering living space
• Creates continuous barrier at slab level
• Easier to seal at edges and penetrations
• Protects foam from concrete alkalinity

Installation Benefits:

• Provides smooth surface for concrete placement
• Protects foam during concrete pour
• Easier to repair tears before pour
• Better edge termination details

Performance:

• Foam stays dry below poly
• No moisture accumulation in foam
• Vapor drive is primarily upward
• System proven over decades

Radiant Floor Heating Considerations

Traditional Radiant Installation

When installing radiant tubes directly on poly:

• Staples do create small holes
• Holes are minimal (< 1% of area)
• 99% effectiveness still excellent
• Concrete encases and seals penetrations

Modern Solutions: Bekotec System

As Ryan mentioned, Schluter Bekotec offers an elegant solution:

Assembly with Bekotec:

1. Concrete slab (thinner possible)
2. Bekotec panels with tube channels
3. Polyethylene vapor barrier
4. R-20 rigid foam (EPS/XPS)
5. Drainage stone
6. Earth

Benefits:

• No staples penetrating vapor barrier
• Additional R-2 from Bekotec EPS
• Faster heating response
• Lower concrete volume
• Structured tube layout

Alternative Assemblies

Poly Below Foam (Less Common)

When used:

• Extremely wet conditions
• Temporary construction moisture
• Specific engineer requirements

Drawbacks:

• Harder to seal edges
• Foam can trap moisture above
• More difficult installation
• Less proven track record

Double Vapor Barrier (Overkill)

Not recommended:

• Can trap moisture between layers
• Unnecessary expense
• Complicates installation
• No performance benefit

Technical Details for Success

Vapor Barrier Specifications

Material Requirements:

• Minimum 6 mil polyethylene
• 10 mil preferred under slabs
• Virgin material (not recycled)
• ASTM E1745 compliance
• Low permeance (< 0.1 perms)

Installation Best Practices:

• Overlap seams 6-12 inches
• Seal with compatible tape
• Turn up at walls 6 inches
• Protect during construction
• Repair all tears/punctures

Foam Insulation Choices

EPS (Expanded Polystyrene):

• Lower cost option
• R-4 per inch
• Handles moisture well
• Compressive strength varies
• Good for most applications

XPS (Extruded Polystyrene):

• Higher R-value (R-5 per inch)
• Better compressive strength
• More moisture resistant
• Higher environmental impact
• Premium option

Polyiso (Not Recommended):

• Loses R-value when wet
• Not suitable below grade
• Avoid for slabs

Stone Base Requirements

Critical for Performance:

• 3/4” clean washed stone
• No fines (self-draining)
• 4-6 inch minimum depth
• Connects to perimeter drains
• Provides capillary break

Common Questions Answered

”Won’t foam get wet below the poly?”

No. The foam sits on free-draining gravel that doesn’t hold water. Any moisture drains away. The vapor drive is upward, not downward.

”What about punctures from radiant staples?”

Minor punctures don’t significantly impact performance. The concrete fills and seals these small holes. Use proper staples and techniques.

”Can I skip the poly since foam is waterproof?”

No. While foam resists liquid water, it’s not a true vapor barrier. You need polyethylene to control vapor diffusion effectively.

”What if my floor heat needs different layout?”

Systems like Bekotec eliminate stapling concerns. Otherwise, use aluminum plates or other attachment methods that minimize penetrations.

Radiant Heating Integration Tips

With Standard Poly-Above-Foam:

1. Use plastic staples (not metal)
2. Minimize penetration depth
3. Consider welded wire mesh instead
4. Use zip ties to mesh
5. Apply sealant to larger holes

With Bekotec or Similar:

1. No vapor barrier penetrations
2. Follows standard assembly
3. Additional insulation benefit
4. Faster response times
5. Professional appearance

Quality Control Checklist

Before Concrete Pour:

• Vapor barrier continuous
• All seams sealed
• Edges turned up walls
• No standing water
• Foam joints tight
• Stone base level
• Perimeter drains connected
• Photos documented

Critical Details:

• Plumbing penetrations sealed
• Column bases detailed
• Wall-floor junction sealed
• No foam compression
• Proper concrete thickness

The Bottom Line

The standard assembly with polyethylene above foam insulation is proven, effective, and recommended by building science experts. While it may seem counterintuitive, the physics support this arrangement. For radiant heating, minor penetrations from proper installation don’t compromise the system’s effectiveness.

Advanced Assembly (With Bekotec)

For those wanting the ultimate basement slab:

1. Finished floor (optional)
2. Concrete slab (3-4”)
3. Bekotec panels (R-2)
4. Polyethylene vapor barrier (10 mil)
5. Rigid foam (R-20)
6. Drainage stone (6”)
7. Undisturbed soil

Total assembly: R-22+, superior moisture control, integrated radiant heating

References and Resources

• Polyethylene Under Concrete Slabs - GBA
• Fixing a Wet Basement - GBA
• Sub-Slab Insulation Guide
• Basement Waterproofing Systems

Professional Installation

Proper basement slab construction requires attention to detail and understanding of moisture dynamics. Our certified teams ensure your assembly follows building science best practices.

Call (437) 545-0067 for expert basement slab installation.

DrySpace Waterproofing - Building Science-Based Solutions Since 2008