Interior vs. Exterior Waterproofing Systems

Interior vs. Exterior Waterproofing Systems

Approach and Technique 


– Protects against water intrusion altogether. The process begins by excavating the soil around a building down to the foundation’s footings, typically 8 to 10 feet deep. To prevent cave-ins, the excavation must be “stepped back” progressively further from the foundation, potentially expanding the excavated area beyond initial estimates. 

Exterior waterproofing is labor-intensive and involves significant disturbance to the surrounding landscape. It may require temporary removal of structures like decks, patios, or even gardens. Despite its disruptive nature, this process is a one-time event with long-lasting benefits. 

Once excavation is complete, the exposed foundation walls are treated with a combination of waterproof coatings—such as liquid tar or polymer-based products—and a self-adhesive rubber membrane. This creates a continuous barrier that prevents groundwater from penetrating the foundation. Additionally, drainage systems are often installed at the base of the foundation to channel water away. These systems are ideally directed to naturally sloping terrain where the water can exit, or “daylight.” In cases where no such natural drainage option exists, the water is directed to a sump pump system installed inside the building. This system then safely discharges the water away from the property. 


– Most employed when water has already penetrated the building. This method typically includes the installation of a perimeter drainage system beneath the basement floor to intercept water before it can build pressure and leak out through paths of least resistance, like cracks and the space where the foundation wall meets the basement floor. 

This labor-intensive work can be completed on the inside of the basement without affecting the exterior of the home. Installation might include breaking concrete to install drains and requires careful handling to ensure that the building’s interior finishes are protected. 

The collected water drains towards a sump pump system, which safely discharges it from the building. Furthermore, sealants may be applied to walls and floors to address and prevent minor seepage, providing an extra layer of moisture protection inside the building. 

Assessment & Diagnosis 

Diagnosing interior and exterior waterproofing systems requires distinct approaches due to the differences in their exposure and function.

Interior systems are typically reactive, addressing moisture that has already penetrated the building. The diagnosis involves identifying signs like damp basement walls, pooling water, and musty odors indicative of mold or mildew. 

Tools such as moisture meters are essential for measuring internal moisture levels, and inspections may require invasive methods to examine the functionality of sump pumps, sealants, and internal drainage systems directly. 

Exterior waterproofing diagnosis focuses on the building’s outer defenses against moisture, checking aspects such as soil grading, gutter functionality, and the condition of existing drainage systems. Thermal imaging can be particularly useful for spotting hidden moisture seepage. 

Regular checks are necessary to ensure that perimeter drainage systems are unobstructed and effectively channeling water away from the foundation. Due to the complexity of these systems, professional evaluations are often advisable, combining visual assessments with advanced diagnostic tools to pinpoint both the symptoms and underlying causes of water intrusion. 

Installation Processes for Interior and Exterior Waterproofing 

Exterior Waterproofing Installation 

  1. Preparation and Planning:

– Site Evaluation: Assess the area to identify any obstacles such as gardens, patios, or utilities. 

– ObtainPermits: Check local regulations and secure any required permits before beginning work. 

  1. Excavation:

– Set Up Safety Measures: Install shoring to prevent collapses. 

– Begin Excavation: Use excavation equipment to dig down to the foundation footings, ensuring the dig is stepped back from the foundation to avoid undermining the structure. 

  1. Foundation Preparation:

– Clean the Foundation Walls: Remove any dirt, debris, and old waterproofing materials. 

– Repair Cracks: Use hydraulic cement or epoxy injections to repair any cracks or holes in the foundation. 

  1. Waterproofing Application:

– Apply Primary Waterproof Coating: Applying a liquid tar or a polymer-based product to create the first layer of protection.

– Rubber membrane: After the initial coating has cured, apply a self adhesive rubber membrane to cover the seam between the footing, all the way up above grade to prevent water leaking behind it after backfill. 

– Install Drainage board: This helps protect the waterproofing layer and facilitates water flow to the drainage system. 

  1. Drainage System Installation:

– Ensure Proper Slope: Confirm that the drainage system is sloped properly to carry water away from the foundation. 

– Install Perimeter Drains: Lay landscape fabric and a base layer of gravel at the base of the foundation. Place perforated pipes and plumb to gravity draining exit (hillsides are ideal) or to interior sump system. Cover with remaining gravel 

  1. Backfill and Landscaping:

– Backfill: Carefully replace the excavated soil, using compaction equipment to prevent settling. 

– Restore Landscaping & Hardscaping: Replant any disturbed vegetation and repair or replace hardscaping elements like paths or patios. 

Interior Waterproofing Installation 

  1. Preparation:

– Remove Interior Obstructions: Clear furniture, appliances, and other items from the work area. 

– Safety Preparation: Lay runners to protect floors in high traffic areas, set ventilation to mitigate dust as much as possible. 

– Prepare the Floor and Walls: Break up perimeter concrete where necessary and remove finished materials like sheet rock, studs and insulation. 

  1. Installation of Drainage Systems:

– Install Drain Tile: Dig trench down to the bottom of the footer and lay a base layer of gravel. Place sump basket. Place drain tile around the interior perimeter of the trench. 

– Connect to Sump Basin: Plumb the drain tile to the sump pump basin. 

– Install Sump Pump: Install sump pump and 1-½” PVC discharge pipe. Plumb to exterior and ensure water flows downhill and away from the property. 

  1. Sealant Application:

– Apply Sealants: Use optional waterproof masonry sealants or vapor barriers on walls and floor joints to cover and seal any minor cracks or openings. 

  1. Finishing Touches: 

– Re-Concrete Floors: Pour new concrete over the floor systems, ensuring a smooth, level finish. 

– Final cleaning: Remove all tools and debris from the site, ensuring a fast start for refinishing professionals that may follow the waterproofing team. 


Exterior Waterproofing: Pros 

  1. Effective Moisture Barrier:

– The combination of waterproof coatings and self-adhesive rubber membranes provides a highly effective barrier that prevents groundwater from penetrating the foundation walls. This significantly reduces the risk of water damage and mold growth within the building structure. 

  1. Long-term Solution:

– Exterior waterproofing addresses water issues at their source outside the building, making it a more permanent solution compared to interior methods. By preventing water from entering the foundation in the first place, it offers lasting protection and peace of mind. 

  1. Added Property Value:

– The installation of comprehensive exterior waterproofing can increase the property value and marketability, as it assures potential buyers that the building is well-protected against water-related issues. 

Exterior Waterproofing: Cons 

  1. High Initial Cost:

– The process involves extensive excavation and the application of expensive materials, which can make exterior waterproofing significantly costlier than interior options. This includes not only the materials but also labor and potentially landscaping repairs after the work is completed. 

  1. Disruptive Installation:

– Excavation around a building can be disruptive to the existing landscaping and may require temporary relocation of features like pathways, gardens, or even utilities. The need to “step back” the excavation can also increase the disturbed area, affecting more of the property. 

  1. Dependence on Terrain:

– The effectiveness of the drainage systems often depends on the property’s ability to naturally divert water (daylight). In flat terrains where natural slopes are absent, additional systems like sump pumps are necessary, which can increase maintenance needs and operational costs. 

Interior Waterproofing: Pros 

  1. Less Invasive Installation:

– Unlike exterior waterproofing, interior methods do not require extensive excavation around the property. This makes the installation process quicker, less disruptive to daily activities, and does not affect the exterior landscaping or structures. 

  1. Cost-Effective:

– Interior waterproofing is generally less expensive than its exterior counterpart. The costs are lower due to the reduced amount of labor and the absence of heavy machinery needed for excavation. Additionally, the materials used for interior systems, like sealants and interior drain tiles, are more affordable. 

  1. Responsive to Immediate Needs:

– Interior waterproofing is designed to manage water that has already entered the building, making it an effective solution for immediate water issues. It helps to relieve hydrostatic pressure and direct water away from the foundation, preventing further penetration and damage. 

Interior Waterproofing: Cons 

  1. Reactive Rather Than Preventative:

– Interior waterproofing addresses water problems after they have entered the building. Although it manages the symptoms of water ingress effectively, it does not stop water from entering the foundation as exterior waterproofing does. 

  1. Potential for Ongoing Maintenance:

– Systems like sump pumps require regular maintenance to ensure they operate effectively. Failure of these systems, due to power outages or mechanical failure, can lead to immediate and severe water issues. 

  1. May Not Address All Moisture Issues:

– While effective at handling water that enters through the floor or lower walls, interior waterproofing might not fully address moisture entering through higher levels of the foundation or through condensation within the walls, potentially leading to ongoing humidity and mold issues inside the building. 

Preventative Measures

You can help avoid water damage before it even begins a couple of ways. Proper landscaping can direct water away naturally, ensuring that the ground slopes away from the foundation to prevent water pooling near the structure. 

Additionally, gutters should be kept clear of debris to ensure efficient water flow away from the building. Regular cleaning and inspection prevent overflows and back-ups that can lead to water spilling over the sides and ending up near the foundation, causing potential water issues. 

Designing and installing a surface drainage system, or French Drain, can effectively channel water away from the property, provided the landscaping will allow for it. These systems are designed to intercept water before it seep into the ground and infiltrate the foundation, routing it safely into storm sewers, dry-wells or to exit a hillside. 

Cost Factors 


– Generally more expensive due to the extensive labor, specialized machinery, materials like rubber membranes and liquid coatings, timeframe, scope of demolition and post-work repairs to landscaping and hardscaping. The overall expense can further escalate based on the accessibility of the foundation and any obstacles that complicate the excavation, such as tight property lines or existing landscaping. 

Once the waterproofing is complete, the cost of restoring the landscaping and hardscaping must also be considered. This includes replacing disturbed grass, flowers, and shrubs, as well as repairing or reconstructing pathways and patios affected by the work. These restoration efforts are necessary to return the property to its original condition or better, adding to the comprehensive cost of the project. 


– Notably less expensive upfront, making it a financially accessible option for many homeowners. The cost-effectiveness of this method stems largely from the lack of extensive excavation and the simpler, less costly materials used, such as sealants and internal drainage systems. These factors contribute to making interior waterproofing a popular choice for those working within a tighter budget or timeframe, or for homeowners who prefer a phased approach to managing moisture problems. 

Interior waterproofing can be implemented quickly, often without the disruptions associated with major construction projects. While homeowners may need to refinish areas affected by water damage, such as drywall and flooring, costs are generally less costly than the substantial exterior repairs. 

Common Obstructions for Exterior Waterproofing:

  1. Utilities: Pipes and electrical lines running under or around the foundation can hinder excavation and installation processes, requiring careful planning and sometimes relocation of these utilities. 
  2. Property Lines: Tight property lines limit the space available for excavation equipment and may require more precise and thus time-consuming techniques to navigate the restricted space. 
  3. Patios and Decks: These structures often need to be partially or fully dismantled to allow access to the foundation walls, adding time and cost to the project. 
  4. Gardens and Landscaping: Waterproofing can disrupt landscaping, requiring significant restoration work post-project. Plants, shrubs, and even trees may need to be temporarily removed and replanted. 
  5. Built Features: Features like built-in barbecue areas, permanent outdoor furniture, or water features near the foundation can pose significant challenges and may need to be moved or worked around carefully. 

Common Obstructions for Interior Waterproofing: 

  1. Built-in Furniture: Permanent or heavy furniture like bookcases, bars and shelving that is difficult to move can restrict access to floors and walls where interior drainage systems or sealants need to be installed. 
  2. Flooring: Existing flooring must often be removed or altered to install subfloor drainage systems, which can be a significant undertaking especially with materials like hardwood or asbestos (!) tile. 
  3. Fireplaces: Particularly if located along basement walls, fireplaces can complicate the installation of internal waterproofing systems due to their immovability and the materials involved. 
  4. Internal Utilities: Interior plumbing, wiring, and HVAC systems may interfere with the installation of waterproofing systems, especially when these systems run through basement floors or walls. 
  5. Storage Areas: Basements often serve as storage areas, and clearing the space to work effectively can be a logistical challenge, requiring temporary relocation of stored items. 

Maintenance Requirements 

Maintaining both exterior and interior waterproofing systems is important for ensuring their effectiveness and longevity, but each requires a different approach to maintenance.


Generally designed to be low maintenance. However, periodic inspections are recommended to ensure long-term effectiveness. It is advisable to inspect the drainage systems annually to ensure they are not clogged with debris, which could impede their ability to channel water away from the foundation. 

Additionally, the waterproof barrier should be checked regularly for any signs of damage or wear that could compromise its integrity. These inspections help identify and rectify minor issues before they escalate into major problems, thus maintaining the protective capabilities of the exterior waterproofing system. 


Tends to be more maintenance-intensive. It requires regular checks of the sump pump system to ensure it is functioning correctly, especially before and during the rainy season. The interior drain tile system, which captures and directs water to the sump pump, does not typically clog. 

Still, a prudent decision would be to install a cleanout access along the system to allow for possible cleanings. 

Furthermore, internal sealants applied to basement walls and floors should be inspected regularly for effectiveness and might need reapplication to seal any new or worsening cracks that develop over time. Proactive maintenance can prevent a disaster before it occurs. 



– Particularly well-suited for both new constructions and existing homes. On new homes, it can be integrated into the building process seamlessly, significantly reducing cost and damage to landscaping and hardscaping (it hasn’t been created yet!). 

However, for existing homes, especially older ones, the costs and complexities increase. Challenges and complications may arise in the form of aging structures, existing damage or foundation degradation may require additional repair work prior to waterproofing. Despite these challenges, exterior waterproofing remains a robust solution for long-term moisture management by preventing water from penetrating the foundation walls. 


– On the other hand interior systems offer a versatile, adaptable solution that is suitable for almost any building type. This method is particularly beneficial in situations where external excavation is impractical or impossible, such as in urban environments with limited space around the property or buildings where exterior access is restricted by close property lines or hardscaping.

Interior waterproofing is also a preferred choice for older buildings where extensive exterior excavation might risk structural integrity. By installing systems like sump pumps and internal weeping tiles, interior waterproofing effectively manages water that has entered the building, making it a practical choice for a wide range of properties. 

Weather and Seasonal Constraints 


– Ideally performed in dry, warm conditions to allow for optimal application of waterproofing materials and a more manageable excavation process. The effectiveness of materials like membranes and sealants is highly dependent on temperature; colder conditions can reduce their efficacy and make them more difficult to apply properly. 

Winter installations pose a myriad of challenges, such as frozen ground (aka “excavation hell”) difficult or even impossible. Consequently, planning exterior waterproofing during warmer months is not only more efficient but also necessary to ensure the quality and durability of the waterproof barrier. 


– Can be installed regardless of the season. This method does not require weather-dependent conditions since all the work is conducted inside the building. As such, interior waterproofing projects can proceed during any time of the year, including during winter when exterior waterproofing would be problematic. 

This option is attractive for urgent waterproofing needs or in climates where exterior work is limited to specific seasons. The ability to install interior drain tile and sump pump systems without concern for external weather boasts superior flexibility.