Double-Sided Foundation Waterproofing: When Forms Can’t Be Removed

The Row House Dilemma

Jamie’s situation represents a common challenge in urban construction, particularly relevant for Toronto’s dense row house neighborhoods. With 45 linear feet of foundation wall where exterior forms couldn’t be removed due to tight space constraints, the question of double-sided waterproofing becomes critical.

Understanding the Specific Challenge

Jamie’s Situation:

• Rear basement extension in row house
• 45 linear feet of new foundation wall
• Elastomeric liquid membrane applied where accessible
• Wood forms trapped against exterior (bottom sections)
• Concern about water infiltration through exposed concrete
• Contractor suggesting interior waterproofing too

The Core Question: “If I waterproof or seal both sides of a concrete wall, will this create any issues?”

Martin Holladay’s response provides clarity: No, it won’t create issues.

Debunking the Concrete “Breathing” Myth

The Science Behind Concrete and Moisture

Common Misconception: Many believe concrete walls need to “breathe” or dry to the interior. This is false.

Martin Holladay’s Key Insight: “It’s a myth that concrete walls need to ‘breathe’ or to dry to the interior… Damp concrete is strong.”

The Reality:

• Concrete gains strength in moist conditions
• Fully cured concrete is essentially waterproof
• Moisture in concrete isn’t harmful to the concrete itself
• The concern is moisture migration to interior finishes

Why Double-Sided Waterproofing is Safe

No Trapped Moisture Issues:

• Concrete doesn’t deteriorate from internal moisture
• No organic materials to rot
• No freeze-thaw cycles within the wall
• Concrete actually prefers to stay moist

Critical Questions Before Interior Waterproofing

Martin Holladay’s Essential Checklist

Before worrying about interior waterproofing, verify these critical details:

1. Capillary Break Installation

• Between footing and foundation wall
• Prevents moisture wicking up from footing
• Essential for long-term performance
• Often overlooked detail

2. Footing Drain System

• Properly installed perimeter drain
• Connected to daylight or sump
• Adequate gravel envelope
• Filter fabric protection

Why These Matter More: If these fundamental details are correct, the trapped forms may not cause problems even without interior waterproofing.

Understanding the Trapped Form Scenario

What Happens with Forms Left in Place

Immediate Effects:

• Wood forms will eventually deteriorate
• Create voids against the wall
• May allow water channels
• Reduce waterproofing coverage

Long-Term Considerations:

• Forms decompose over 5-10 years
• Leave irregular surface
• May create water pathways
• Increase hydrostatic pressure points

Why Interior Waterproofing Makes Sense Here

In Jamie’s Case:

• Known exterior waterproofing gaps
• Unable to achieve full coverage
• Belt-and-suspenders approach justified
• Relatively low cost for peace of mind

Interior Waterproofing Options

Products Martin Holladay References

1. Thoroseal

• Cementitious waterproofing
• Becomes integral with concrete
• Good for negative side pressure
• Requires proper surface prep

2. UGL Drylok

• Latex-based coating
• DIY friendly application
• Multiple coat system
• Limited hydrostatic resistance

3. Xypex

• Crystalline technology
• Penetrates into concrete
• Self-healing properties
• Premium option

Application Considerations

Surface Preparation Critical:

• Remove form release agents
• Clean all laitance
• Open pores for penetration
• Achieve proper profile

Application Timing:

• Allow concrete to cure (28+ days)
• Check moisture content
• Apply when conditions are right
• Follow manufacturer guidelines

The Physics of Water Movement

Understanding Capillary Action

Jamie’s Concern: “Ground water at the exposed parts of the concrete, through capillary action will eventually bring in moisture issues”

How Capillary Action Works:

• Water moves through porous materials
• Against gravity in small spaces
• Concrete has capillary pores
• Movement continues until equilibrium

Mitigation Strategies:

• Break capillary paths
• Reduce pore connectivity
• Create vapor barriers
• Manage water sources

Hydrostatic Pressure Reality

Martin Holladay’s Warning: “If the hydrostatic pressure against the outside of your foundation wall is high enough, these products won’t be able to keep out the water”

Understanding Limits:

• Interior coatings have pressure limits
• Typically 2-10 PSI resistance
• High water tables exceed this
• Drainage remains critical

Best Practices for Double-Sided Waterproofing

When It Makes Sense

Recommended Scenarios:

1. Known exterior deficiencies (Jamie’s case)
2. High-risk water exposure
3. Critical spaces below grade
4. No ability to repair exterior
5. Historical building constraints

Implementation Strategy

Exterior First (Where Possible):

• Maximum coverage achievable
• Focus on critical areas
• Document gaps for interior treatment
• Use compatible products

Interior Application:

• Target known weak points
• Full coverage for insurance
• Choose appropriate product
• Consider vapor permeability

Common Mistakes to Avoid

Application Errors

1. Incompatible Products

• Check chemical compatibility
• Avoid reactions between systems
• Consider expansion coefficients
• Test small areas first

2. Insufficient Coverage

• Pin holes defeat system
• Multiple coats essential
• Check mil thickness
• Overlap applications

3. Poor Timing

• Too soon after pour
• Wrong weather conditions
• Inadequate cure time
• Rushed application

System Design Flaws

1. Ignoring Drainage

• Waterproofing isn’t drainage
• Water management essential
• Reduce hydrostatic pressure
• Provide escape routes

2. Forgetting Details

• Cold joints need attention
• Penetrations are weak points
• Tie holes require treatment
• Corners need reinforcement

The Complete Water Management System

Beyond Waterproofing

Essential Components:

1. Surface Water Control

   • Proper grading
   • Gutter systems
   • Downspout extensions
   • Impermeable surfaces

2. Subsurface Drainage

   • Footing drains
   • Drainage aggregate
   • Filter protection
   • Positive outflow

3. Foundation Protection

   • Waterproofing/dampproofing
   • Capillary breaks
   • Vapor barriers
   • Protective boards

4. Interior Management

   • Sump systems
   • Dehumidification
   • Vapor barriers
   • Climate control

Cost-Benefit Analysis

Interior Waterproofing Addition

Typical Costs:

• DIY: $500-1,500 materials
• Professional: $2,000-4,000
• Time: 2-4 days
• Disruption: Minimal

Benefits:

• Peace of mind
• Added protection
• Relatively low cost
• Can DIY if desired

Compared to Future Repairs:

• Water damage: $5,000-20,000
• Mold remediation: $3,000-10,000
• Lost use of space: Invaluable
• Health concerns: Priceless

Special Considerations for Row Houses

Toronto-Specific Challenges

Tight Space Issues:

• Zero lot line construction
• No exterior access
• Shared party walls
• Limited equipment access

Solutions:

• Interior waterproofing systems
• Injection technologies
• Crystalline treatments
• Preventive approaches

Party Wall Considerations

Water Migration Between Units:

• Shared foundation walls
• Interconnected drainage
• Neighbor’s water problems
• Legal considerations

Protective Strategies:

• Comprehensive interior treatment
• Monitor party wall conditions
• Document existing conditions
• Coordinate with neighbors

Professional Recommendations

For Jamie’s Specific Situation

Recommended Approach:

1. Verify capillary break installation
2. Confirm footing drain functionality
3. Apply interior waterproofing system
4. Choose crystalline technology if possible
5. Monitor performance first year

General Best Practices

For Similar Situations:

1. Don’t panic about trapped forms
2. Focus on complete water management
3. Use interior waterproofing as backup
4. Document everything thoroughly
5. Plan for long-term monitoring

Monitoring and Maintenance

First Year Critical

What to Watch:

• Efflorescence patterns
• Moisture meter readings
• Visible dampness
• Musty odors
• Crack development

Documentation:

• Photo record
• Moisture logs
• Weather correlation
• Seasonal changes
• Performance trends

Long-Term Management

Annual Inspection:

• Check for new cracks
• Test sump operation
• Verify drainage flow
• Inspect waterproofing
• Update documentation

Conclusion

Jamie’s concern about double-sided waterproofing is understandable but unfounded. As Martin Holladay clearly states, there’s no risk in waterproofing both sides of a concrete foundation wall. The myth of concrete needing to “breathe” has been thoroughly debunked.

In situations where exterior waterproofing is compromised—whether due to trapped forms, tight spaces, or other construction constraints—interior waterproofing provides valuable additional protection. The key is understanding that waterproofing is just one component of a comprehensive water management strategy.

For row houses and tight urban sites particularly common in Toronto, this belt-and-suspenders approach makes practical sense. The relatively small investment in interior waterproofing compared to potential water damage makes it a wise choice when exterior coverage is incomplete.

Key Takeaways

1. Double-sided waterproofing is safe - No adverse effects on concrete
2. Concrete doesn’t need to breathe - Moisture in concrete isn’t harmful
3. Drainage matters more - Waterproofing without drainage often fails
4. Capillary breaks are critical - Often overlooked but essential
5. Document everything - Especially important with known deficiencies

Resources for Further Research

• Interior Waterproofing Systems
• Crystalline Waterproofing Technology
• Foundation Drainage Best Practices
• Row House Foundation Solutions

For expert consultation on complex waterproofing scenarios in Toronto, call (437) 545-0067