Building Envelope Design Guide - Foundation Walls

by Mark Postma, PE
Carl Walker, Inc.

Last updated: 03-14-2006

Introduction

The foundation wall of a building may be a cast-in-place concrete retaining or basement wall or a structural wall complete with load-bearing pilasters. Materials used may be concrete or reinforced masonry. The foundation wall system may include an earth retention system of soldier piles and wood lagging or shotcreted rock requiring consideration of waterproofing applied to the earth retention system. For most portions of the foundation wall, water removal and control is of prime importance. In the upper areas of the foundation wall thermal loading considerations must be addressed.

Description

This section provides specific description of materials and systems common in foundation walls and below grade building enclosure systems in general. Descriptions and guidelines are provided in the following sections:

• Drainage Materials
• Filter Fabrics
• Damproofing
• Waterproofing Membranes
• Protection Board
• Insulation Materials
• Waterstops
• Drainage Pipe

Drainage Materials

Drainage Materials for below grade enclosures include:

• Aggregate Drainage Layers
• Prefabricated Synthetic Drainage Layers

Aggregate Drainage Layers—Aggregate drainage layers include graded pea-gravel aggregate or coarse sands. Graded pea gravel refers to naturally rounded stone between 3/16 inch and 3/8 inch in diameter. Coarse sands varying from No. 30 to No. 8 sieve are suitable. Gap-grading the sand provides uniform grain size, which accelerates drainage flow rates.

Prefabricated Synthetic Drainage Layers—These products are comprised of a combination of plastic composite drainage cores with adhered geotextile fabrics. The plastic composite drainage cores are available in all type of configurations of drainage cores and are available in polypropylene, polystyrene and polyethylene. The geotextile prevents adjoining soil from clogging up the drainage system and are available in various forms including nonwoven for clay type soils and woven or small opening geotextiles for sandy or high-silt type soils.

Design considerations include selection of appropriate design to achieve flow rate required. Typical widths of ¼ to ½ inch provide drainage flow rates 3 to 5 times the capacity of commonly used natural backfill materials. These systems are advantageous in their light-weight design and cost effectiveness. Although marketed to be used with excavated soils as backfill instead of granular drainage layer, it is recommended that a full system approach be used in key applications that include a synthetic drainage layer and granular drainage layers.

Filter Fabrics

Geotextile filter fabrics are also used for separation of differing soil types in below grade enclosure applications. This separation of differing soil types maintains flow rates of soils used as drainage layers and minimizes settlement from finer materials filling in more coarse materials. Geotextile fabrics include polypropylene, polyester or nylon fabrics in either woven or non-woven varieties resulting in varying flow rates.

Damproofing

Damproofing materials are primarily spray or roller applied bitumen based coatings applied up to 10 mils (0.25 mm) in thickness. These materials can be solvent based or water emulsions. Damproofing is always applied to the positive side, or wet side, of the structural element.

Waterproofing Membranes

Waterproofing membrane systems include both negative and positive side waterproofing. Positive side waterproofing systems are applied to the face of the element that is directly exposed to moisture, the exterior face. Negative side waterproofing systems are applied to the surface of the element opposite the surface exposed to moisture. Positive systems are available in numerous materials and forms. Negative systems are limited to cementitious systems.

Waterproofing membranes can be categorized into 4 types:

1. Cementitious Systems—These systems contain Portland cement with and sand combined with an active waterproofing agent. These systems include metallic, crystalline, chemical additive and acrylic modified systems. These systems can be applied as negative or positive side waterproofing.
2. Fluid Applied Systems—These systems include urethanes, rubbers, plastics and modified asphalts. Fluid membranes are applied as a liquid and cure to form one monolithic seamless sheet. Fluid systems can be applied to vertical and horizontal applications. For foundation wall applications typical fluid applied systems are 60 mils in thickness.
3. Sheet-Membrane Systems—Sheet membranes used in below grade applications are similar to the materials used in roofing applications and include thermoplastics, vulcanized rubbers and rubberized asphalts. The thickness of these systems varies from 20 to 120 mils.
4. Bentonite Clays—Natural clay systems, known as bentonite act as waterproofing by swelling when exposed to moisture thus becoming impervious to water. This swelling can be 10 to 15 percent of the thickness of the base material. Clay panels and sheets are popular for use in blind-side waterproofing applications such as on retaining earth systems and elevator and sump pits.

Protection Board

Protection Boards are used to shield waterproofing membranes from construction damage and ultraviolet radiation. The most commonly used protection board is an asphalt board material that may have a polyethylene film on one side if it is desired to prevent bonding. For some membrane applications, such as hot applied bitumen systems the protection board is embedded into the wet membrane and forms an integral part of the waterproofing membrane. Asphaltic protection boards are available in 1/16, 1/8 and ¼ inch in thickness. Other materials such as insulation boards or prefabricated synthetic drainage layers are sometimes used as protection for membranes.

Insulation Materials

Insulation materials used in below grade enclosure applications are primarily limited to rigid extruded polystyrene board among insulation materials due to the need for high compressive strengths and moisture resistance. For vertical wall applications, grooved insulation boards with applied geotextile fabrics can be used as a protection board for the waterproofing membrane and serve the function of a synthetic drainage layer.

Waterstops

Waterstops should be utilized at construction joints in below grade walls, footings and other elements where a waterproof system is required. These systems prevent the passage of water across these cold-joints. Waterstops are manufactured products available in a wide range of configurations and sizes. Common materials include polyvinyl chloride (PVC), neoprene, and thermoplastic rubber.

Drainage Pipe

Drainage pipes, typically 4" or 6" in diameter, used in below grade systems are primarily made of corrugated PVC or polyethylene and in some cases porous concrete. PVC and polyethylene pipes are available in smooth or corrugated configurations and are slotted on the bottom half of their cross-section to allow water infiltration.

Fundamentals

Figure 2 is an overall schematic that characterizes the four functions i.e. Structural Support, Environmental Control, Finish, and Distribution as they relate to the below grade enclosure elements of foundation walls.

The four function categories, i.e. Structural Support, Environmental Control, Finish and Distribution, are expanded in general terms for foundation walls.

Structural Support Functions—The foundation wall system of the below grade building enclosure must be designed and constructed to support both vertical and lateral loadings.

Vertical loads exist from the dead, live and lateral loads from the structure and the wall itself. The foundation wall may be an integral part of the load bearing design of the building carrying column and floor loads from above, either as distributed loads on the wall or point loads on pilasters integral to the wall system. These walls may also be used in the lateral resisting system for the building.

Lateral loads on foundation walls exist from the soil, surcharge and hydrostatic pressure loads. Soil loadings vary with soil type and whether soil is treated as active or passive. Hydrostatic pressure loads may exist in cases of high water tables or flood events. Typical hydrostatic and soil pressures generally range from 30 to 62.4 psf per foot of depth. Surcharge loadings may include live loads from pedestrian walkways or from vehicular roadways. In many cases around commercial office buildings, areas designed as pedestrian areas must also include consideration for emergency vehicular loadings.

In many cases the foundation wall is required to resist all of these loads directly with the wall designed as a cantilever retaining wall with a large base footing or as a basement wall spanning vertically between the foundation element and supported floors. Other cases may include an earth retention system, such as piling and timber lagging, facilitating construction and designed to resist the lateral loads leaving the foundation wall to resist primarily vertical loads.

Special loadings such as blast loads are a design consideration in parking areas under and next to buildings. While the first control of these abnormal load events are through security entry control systems and restricted access, structural design considerations may also be required in the design of the foundation wall system.

Environmental Control Functions—The exterior environment that the foundation wall is subjected to includes environmental control loadings such as thermal, moisture, tree roots, insects, and soil gas. The interior environment that the foundation wall is subjected to includes environmental control loadings such as thermal and moisture. The performance of the foundation wall system depends on its ability to control, regulate and/or moderate these environmental control loadings on each side of the foundation wall to desired levels.

Likely, the most predominant environmental control loading for foundation wall systems is moisture. Moisture control is dealt with in a multiple screen/barrier type of design approach. For surface moisture loadings such as rain, snow and sprinklers the first line of control is the upper screen at the exterior surface. This upper screen may be comprised of relatively permeable landscape areas to impermeable pavers, concrete or asphalt surfaces that will shed the majority of surface moisture. The effectiveness of this initial screen in shedding moisture may influence the design of the other components of the system.

Moisture that penetrates through the upper screen needs to be directed to the exit drain located at the base of the foundation wall. This is accomplished through a drainage system to the exterior of the wall that is typically a free draining granular material. Backfilling with native, poor draining soil is not recommended as this will maintain an active water load on the foundation wall and limit its ability to control moisture ingress to the interior. As moisture moves from the upper screen through the drainage system on the exterior towards the exit drain, moisture will inevitably make its way toward the surface of the foundation wall itself. Depending on the quantity of water that makes its way through the upper screen, a drainage system at the surface of the foundation wall may be required to direct this water expeditiously toward the base of the foundation wall and the exit drain.

In many foundation wall situations with low water table elevations, the combination of the upper screen, the exterior drainage system, the near surface drainage system and the exit drain will control the majority of the water. The key question that remains is whether to provide damproofing or waterproofing to the surface of the foundation wall or not at all. Damproofing resists vapor migration in the absence of hydrostatic pressure. Waterproofing resists both vapor migration and hydrostatic pressure. Generally, damproofing can only be eliminated in sites with exceedingly dry soil; however, most building codes require damproofing as a minimum amount of moisture protection. In these cases the remaining part of the system is a damproofing applied directly to the exterior surface of the foundation wall. Building codes also generally require waterproofing if the ground-water level cannot be maintained at least 6 in. below the bottom of the slab-on-ground. This can be accomplished with pumping systems. In areas with greater moisture loads from hydrostatic pressure from high water tables or sensitive interior environments, a waterproofing membrane should be applied to the surface of the foundation wall in lieu of damproofing. Waterproofing membranes are predominantly applied to the positive (exterior) face of the foundation wall, however, there are negative side waterproofing systems that can be applied to the interior of the foundation wall.

Even when it is necessary to apply a waterproofing membrane, it is recommended to also utilize a system approach including components of exterior drainage system, near surface drainage system and exit drain. The removal of the moisture in the most complete and expeditious manner will decrease the probability for intrusion of the system.

Thermal considerations are of limited concern with depth along the foundation wall as there is a constant, thermal design condition on the exterior. As most foundation wall systems have substantial mass, e.g. concrete, insulation may only be of importance to moderate interior temperatures in the upper portions of the foundation wall where temperature conditions will fluctuate. However, the use and location of the insulation is more important on the control of moisture in terms of preventing condensation on interior wall faces for the entire height of the foundation wall. Condensation is possible in below grade conditions in warmer more humid summer conditions as below grade spaces tend to be cooler in the summer because of the insulating effect of the backfill soil. This cooling effect combined with general poor air circulation in underground spaces can result in condensation on interior wall surfaces. The higher soil temperatures on the exterior also create the need to provide at least a damproofing on the exterior of the foundation wall to resist the strong interior vapor drive. In fact, in some situations, conditioned below grade spaces are subjected to a constant inward vapor drive as in the summer the interior space is air conditioned and in the winter the interior space is heated resulting in a lower vapor pressure than the exterior condition as the soil stays relatively constant in terms of vapor pressure.

Finish Functions—There are two areas of finishes that are of importance in relation to foundation walls. The first area is the finish to the interior space. This finish is dependent on the interior use whether it be a controlled office environment or a non-controlled parking environment. Typical finish systems may include paints, stucco, or framed walls with drywall. In many applications the interior finish is simply the interior surface of the material used for the foundation wall, i.e. concrete or concrete masonry units.

The second area is the finish to the exterior near grade level. Proper treatment of this area is critical in terms not only of aesthetics but also durability. Damproofing/Waterproofing in all situations should be carried up above the upper screen and integrally tied into the building faç flashing and waterproofing. Many waterproofing membranes need to be shielded from ultraviolet radiation to prevent deterioration and as such some type of exterior finish is required. The most basic finish is parging or an external insulation finish system applied at the base of the wall. In many cases the exterior faç element, whether it be brick, stone, etc. is carried down to just below grade level to properly transition and protect this sensitive area.

Distribution Functions—Foundation walls may contain distribution systems such as electrical and electronic runs. At times these systems are run internally in the interior surface finish system or in ceiling space. Distribution systems within the foundation walls themselves must be treated with careful consideration, as they can also be conduits transporting air and moisture within the structure.

Applications

Upper Screen Design Considerations for Surface Runoff

Many areas around a building's perimeter at grade are subjected to high amounts of surface runoff from the high use of fenestration and impermeable wall faç materials such as thin stone and EIFS. The first and most effective defense against this water is to slope the upper screen surface away from the building a minimum of 5% near the building edge. Proper design to connect downspouts into perimeter drain systems directly instead of flowing onto the area directly adjacent to the foundation wall is prudent in design.

Important design considerations include sloping the surface away from the structure. Providing a suitable drainage system from the upper screen through the granular backfill and synthetic drainage layer down to the exit drain.

Exit Drain Design Considerations

Drainage pipe at the perimeter of the foundation wall should be surrounded by a free draining granular material that is wrapped in filter fabric to prevent fines from filling in the porous spaces of the granular material. Drainage pipe should have a slope of at least 0.5%, but preferably 1.0%.

Damproofing/Waterproofing Membrane Selection

The designer must consider the overall water management system relative to site conditions and loadings to determine if damproofing or waterproofing is required. If in doubt, it is clearly prudent to err on the conservative side and provide a waterproof system.

For waterproof systems the first consideration is whether to use positive or negative side water proofing. Although negative side waterproofing is advantageous from the standpoint of repairability most foundation wall applications utilize positive side waterproofing.

For positive side waterproofing the next design decision is to use fluid applied or sheet products. Sheet products are advantageous in terms of consistency in product material properties and thickness but have significant drawbacks due to the numerous laps that are required. Laps should be installed so that the upper sheet is lapped over the lower sheet so that water is shed naturally across the lap.

With fluid membrane systems, proper application in terms of coverage and thickness is important to performance. Fluid systems are applied as positive side waterproofing and need to be protected by a protection layer. Most fluid applied systems will degrade with exposure to ultraviolet radiation. The key advantage to fluid systems is their self-flashing abilities as the material is applied in liquid form.

Membrane Protection

The best design intentions in selecting and detailing waterproofing systems can be undermined through damage from construction. The installation of protection boards or insulation layers as quickly as possible after membrane installation is critical. Prefabricated synthetic drainage layers are sometimes used in lieu of protection board for protecting waterproofing membranes. Caution is advised with use over softer liquid applied materials as the drainage layer may dig into and breach the membrane. With these softer waterproofing membranes protection board is recommended under the synthetic drainage layer.

Building Façade Termination—Of critical importance in any building is the proper detailing and integration of the vertical building faç system and the below grade building system. The integration of the two systems requires careful consideration to insure that all moisture, air and thermal criteria for each system are satisfied at the transition interface. There is a combination of environmental design loadings at this interface such as surface water, runoff, and cavity wall drainage.

Façade terminations often produce the accumulation of moisture at or near the grade line of the building with the surrounding area. A special flashing behind face stones of buildings or special flashing and treatment of the exterior slab edge where it is adjacent to ground features is required.

Special treatment is also needed at all door entries. Common practice for wall terminations or door entries is to provide slope away from the building as previously indicated. Limiting the direct contact of moisture with the isolation or flashing detail at the envelope seal is a very effective practice.

Penetrations

Condition appraisals and trouble shooting of below-grade structures reveals common sources of leakage that arise from penetrations. These penetrations are any openings in the wall or structural system that once waterproofed provide an avenue of breech for moisture entry into the building. Sewer pipe penetrations, water line entry penetrations, drain basins in the floor slab or sleeves for electrical, gas or communication are all common penetrations, typically with their own design or detailed features. These features, however, leave much to be desired with respect to sealing and waterproofing. Penetrations can also become quite exotic such as steam penetrations or other features that require special treatment. Because of the unique nature of penetrations and Special Features, no single rule or criteria can govern or apply to the effective treatment thereof. However, classification of common penetration types and features helps to ensure effective treatment and proper function.

Isolation, insulation and waterproofing of certain piping that undergoes large temperature changes is often under estimated. Where expansion and contraction of the services or pipes entering the building occurs, a sleeve through the wall that is discontinuous with the penetration piping is required. Sealing these generally requires application of preformed neoprene or elastomeric boots that seal to the housing and to the exterior pipe. Other surfaces such as gas pipes, signal or electrical should generally be done with due consideration for the nature of the sleeve through the exterior wall and the depth penetration below grade.

Common knowledge suggests that the seals serve as a back-up function and that avoiding moisture build-up is the principle objective for obtaining a leak free building at penetrations.

Wall Expansion Joints

Wall expansion joints should be designed to accommodate the anticipated movement. Historical evidence indicates that wall movements below grade are generally nominal and the effective work of the seal is limited. For treatment of leaks providing amplified external drainage media similar to that required on the exterior wall is highly effective. Special emphasis is placed on evacuating the water at the wall base to avoid water build-up in the back fill or drainage system.

Wall/Floor Construction Joints

Construction joints have been effectively treated in most applications by a manufacturer's recommended details. The construction joints typically exhibit only limited movement and as such are generally not as susceptible to leakage as expansion joints. A multiple layer detailing of the membrane, proper isolation and allowance for joint detailing is generally effective for construction joints. A cementitious or mason's mortar cant covered with an elastomeric flashing extending to the edge of the footer and several inches above the cant has proven historically effective.

Details

The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Autodesk® DWF Viewer. Download Adobe Reader.

Foundation Wall—Typical System (Detail 1.2.1)  DWG | DWF | PDF

Foundation Wall System—Foundation Lagging (Detail 1.2.2)  DWG | DWF | PDF

Foundation Wall—Liquid Membrane System (Detail 1.2.3)  ;DWG | DWF | PDF

Foundation Wall—Sheet Membrane System (Detail 1.2.4)  DWG | DWF | PDF

Foundation Wall—Pipe Penetration Detail (Detail 1.2.5)  DWG | DWF | PDF

Foundation Wall—Construction Joint (Detail 1.2.6)  DWG | DWF | PDF

Foundation Wall—Wall Expansion Joint (Detail 1.2.7)  DWG | DWF | PDF

Foundation Wall—Façade Transition (Detail 1.2.8)  DWG | DWF | PDF

Emerging Issues

For emerging issues refer to General Overview section.

Relevant Codes and Standards

For codes/standards refer to General Overview section.

Additional Resources

WBDG

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MASTERSPEC®

For resources including texts, guides, and web pages refer to General Overview section.