Century Home Basement Retrofit - Comprehensive Waterproofing & Insulation

Understanding Your Ambitious Plan

Your comprehensive approach to retrofitting a 100-year-old double brick house basement shows excellent planning. Let’s analyze each component to ensure they work together effectively while addressing some critical code and practical issues.

Your Proposed System Overview

Interior Work:

• French drain with T-pipe for radon
• Sealed sump pump system
• 2” EPS on walls
• 2x4 stud wall with Rockwool
• Air sealing around joists

Exterior Work:

• Skirt insulation
• Underground waterproof “roof”

This multi-faceted approach addresses water, radon, insulation, and freeze-thaw protection - but needs some refinement.

Critical Code Issue: Sump Pump Discharge

The Septic Tank Question

Short Answer: NO - You cannot discharge a sump pump into your septic system.

As plumb_bob correctly noted: “When you pump storm or other drainage water to a septic system… it is considered cross connected and not allowed.”

Why It’s Prohibited:

• Overwhelms septic capacity
• Dilutes bacterial action
• Causes system failure
• Violates plumbing codes
• Can back up sewage into home

Legal Discharge Options:

1. Municipal storm sewer (if available)
2. Grade discharge (20+ feet from foundation)
3. Dry well (if soil percolates)
4. Rain garden (with overflow provision)

Interior French Drain Analysis

Your Proposed System

Strengths:

• Addresses wall-floor joint leakage
• Can integrate radon mitigation
• Less disruptive than exterior work
• Year-round installation possible

Considerations:

• Manages water, doesn’t prevent entry
• Requires breaking concrete floor
• Must maintain proper slope to sump
• Concrete cap over drain unusual

Radon Integration Challenges

Your T-Pipe Concept: While creative, combining drainage with radon mitigation requires careful design:

Potential Issues:

• Water in drain can block air flow
• Radon fan not designed for moisture
• May reduce drainage effectiveness
• Could create odor problems

Better Approach:

• Separate radon system under slab
• French drain for water only
• Both discharge independently
• Maintain system integrity

Wall Insulation Strategy Review

Your 2” EPS + Stud Wall Plan

Good Elements:

• EPS won’t absorb moisture
• Rockwool provides additional R-value
• Maintains air space

Critical Details:

1. Moisture Management

   • No vapor barrier on concrete
   • EPS acts as vapor retarder
   • Allow wall to dry inward

2. Bottom Plate Detail

   • Sits on French drain? Problematic
   • Use treated lumber
   • Consider raising plate above drain
   • Gasket under plate essential

3. Air Sealing

   • Embedded joists are challenging
   • Caulk is good start
   • Spray foam in joist pockets excellent
   • Consider rim joist insulation

Century Home Specific Challenges

Double Brick Walls

• Highly porous
• No cavity for drainage
• Moisture moves through readily
• Interior insulation traps moisture

Concrete Foundation (100 years old)

• Likely unreinforced
• May have deteriorated
• Porous concrete common
• Check structural integrity

Embedded Joists

• Major air leakage path
• Potential rot risk
• Difficult to air seal
• Fire-stopping requirements

Exterior Skirt Insulation Analysis

Your Underground “Roof” Concept

Benefits:

• Reduces freeze-thaw cycles
• Directs water away
• Protects foundation
• Increases soil temperature

Implementation Details:

1. Excavate 2-4 feet out
2. Install rigid foam (slope away)
3. Cover with membrane (EPDM/similar)
4. Protect with soil/gravel
5. Ensure positive drainage

Why Not Full Exterior Waterproofing? As plumb_bob asked: “If you are going to dig up the outside of the foundation why not install perimeter drainage?”

Good question - partial excavation for skirt vs full depth for drainage.

Integrated System Recommendations

Modified Interior Approach

French Drain System:

1. Perimeter drain at footing level
2. Minimum 1% slope to sump
3. Filter fabric and clean stone
4. NO concrete cap - use removable covers
5. Separate from radon system

Radon Mitigation:

1. Sub-slab depressurization separate
2. Sealed penetrations through slab
3. Dedicated piping to exterior
4. Proper fan sizing for system

Sump Pump Installation:

1. Sealed lid (as planned)
2. Check valve (as planned)
3. Discharge to exterior/storm sewer
4. Battery backup recommended
5. Water alarm essential

Modified Wall System

Better Assembly (inside to out):

1. Drywall
2. 2x4 studs with Rockwool
3. 1” air gap (critical)
4. 2” EPS foam board
5. Existing concrete wall
6. No vapor barrier

Bottom Detail:

1. Pressure-treated bottom plate
2. Sill gasket under plate
3. Plate sits ON concrete floor
4. Not directly on French drain
5. Leave gap at bottom of drywall

Air Sealing Strategy

Embedded Joists:

1. Remove debris from pockets
2. Apply borate treatment
3. Spray foam to seal
4. Maintain 1” gap from brick
5. Fire-stop as required

Rim Joist Area:

1. Cut-and-cobble rigid foam
2. Seal with spray foam
3. Add mineral wool if needed
4. Ensure vapor permeability

Phasing Your Project

Phase 1: Water Management

1. Install interior French drain
2. Set up sump pump system
3. Test and refine drainage
4. Monitor moisture levels

Phase 2: Exterior Work

1. Install skirt insulation
2. Create underground “roof”
3. Improve surface grading
4. Add downspout extensions

Phase 3: Interior Insulation

1. Confirm dry conditions
2. Install EPS on walls
3. Build stud walls
4. Complete air sealing
5. Install finishes

Phase 4: Radon (if needed)

1. Test radon levels first
2. Design appropriate system
3. Install sub-slab system
4. Test post-mitigation

Cost Estimates (Toronto Area)

Component	DIY Cost	Professional	Priority
French Drain	$2,000-3,000	$6,000-10,000	High
Sump System	$500-1,000	$2,000-3,000	High
Wall Insulation	$3,000-4,000	$8,000-12,000	Medium
Skirt System	$1,500-2,500	$4,000-6,000	Medium
Radon System	$500-1,000	$2,000-3,000	As needed

Total: $7,500-11,500 DIY / $22,000-34,000 Professional

Common Mistakes to Avoid

1. Discharging to septic - Never do this
2. Vapor barriers wrong side - Causes rot
3. Insulation touching concrete - Moisture trap
4. Ignoring structural issues - Check first
5. Poor drainage slope - System fails

Success Factors

What Makes This Work:

1. Comprehensive approach
2. Addressing root causes
3. Proper material selection
4. Phased implementation
5. Code compliance

Critical Monitoring:

1. Moisture meters in walls
2. Sump pump operation
3. Indoor humidity levels
4. Radon levels (if applicable)
5. Foundation movement

Final Recommendations

Your plan is ambitious and well-thought-out with these modifications:

Must Change:

• Sump pump discharge location
• Separate radon from drainage
• Wall bottom detail

Should Consider:

• Full exterior drainage if excavating
• Professional structural assessment
• Permit requirements
• Neighbor considerations

Nice to Have:

• Battery backup sump
• Smart home monitoring
• Annual maintenance plan

Conclusion

Retrofitting a century home basement requires balancing modern performance with respect for the original structure. Your comprehensive approach addresses multiple issues simultaneously, which is efficient. With the modifications suggested - particularly the sump discharge and system separation - you’ll create a dry, comfortable, healthy basement that preserves your home for another century.

Resources

• Interior French Drain Systems
• Century Home Foundations
• Basement Insulation Guide
• Toronto Building Codes

Need professional help with your century home basement? Call (437) 545-0067 for expert retrofit solutions.