[ Basement Guidelines - intro | Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Bibliography | Related publications | PDF version ]

Michael C. Swinton, NRC-IRC
Dr. Ted Kesik, University of Toronto

October 2005

A survey of new home warranty programs across Canada showed that the combined action of water and soils on basements was responsible for most major basement failures in new homes in 1994 and 1995. Frost action on basement walls was cited as a contributing factor in 40% of the failures; swelling clays (resulting from strong fluctuations of wetting and drying in clay soils) were responsible for another 36%; and frost action on the footings, a high water table and the presence of water-borne soluble salts contributed another 9%, for a combined total of 85% of all failure cases surveyed. This survey led the National Research Council (Institute for Research in Construction) to undertake a basement research project, which was guided by a large industry steering committee broadly representative of the issues. After completion of the research, this comprehensive publication on basement guidelines was developed. The guidelines will facilitate the design and construction of cost-effective basement systems that will achieve satisfactory performance in a cold climate like Canada's. They are intended for technical decision-makers in the home building industry along with material manufacturers, home warranty agencies, building officials, as well as educators. The guidelines are divided into six parts: 1) performance requirements; 2) envelope system selection; 3) materials and equipment; 4) critical design details; 5) quality assurance; 6) cost-benefit analysis.

This publication is dedicated to achieving satisfactory basement performance in cold climates

Preface

These guidelines were developed by the National Research Council of Canada (NRC) under the guidance and review of a Steering Committee formed by representatives of industry associations and government agencies. This committee was formed to oversee the development of the Guidelines and to ensure that they reflect the best collective knowledge of Canadian industry and related public and private agencies . The following were members of the Steering Committee:

As well, participation in the survey by the following New Home Warranty Agencies:

Funding from the following agencies and associations is gratefully acknowledged:

• Canada Mortgage and Housing Corporation
• Polystyrene and Polyurethane Divisions of the Canadian Plastics Industry Association
• Canadian Urethane Foam Contractors Association
• Canadian Association of Man-made Vitreous Fibre Manufacturers
• Owens Corning Canada Inc.
• Roxul Inc.
• Cellulose Insulation Manufacturers of Canada
• Canadian Home Builders Association
• Canadian Portland Cement Association
• Ready Mixed Concrete Association of Canada
• Network of Centres of Excellence on High Performance Concrete
• Canadian Wood Council
• Natural Resources Canada
• National Research Council

Authors who have contributed to the writing of the guidelines include Dr. Ted Kesik, University of Toronto and Mike Swinton, IRC/NRC, with extensive inputs and review by the Steering Committee. Our thanks to Marianne Manning for incorporating Steering Committee comments.

A special acknowledgement to Dr. John Timusk, whose expertise in the physics of basement performance and contributions to the field of knowledge, both inspired and informed the authorship of this publication. It would not have been possible without his insights and collegial support.

Audience

These Guidelines were written primarily for the technical decision-makers in the home building industry. They have been written to appeal to a broad audience, including people involved in the following sectors of the industry:

• home builders and contractors
• materials manufacturers
• codes and standards developers
• warranty agencies
• materials evaluators
• regulators
• building officials
• educators and trainers

Organization

The Guidelines have been organized into six parts:

Part 1 - Performance Requirements for Basements

Part 2 - Basement Envelope System Selection

Part 3 - Selection of Materials and Equipment for the Basement System

Part 4 - Critical Design Details

Part 5 - Quality Assurance

Part 6 - Basement System Cost/Benefit Analysis

The role of the basement envelope is reviewed in Part 1. Our general expectations of what basements are for and how they are expected to perform in what circumstances are laid out – these are the performance objectives of the basement envelope. From these follow the technical performance requirements – the structural requirements, the environmental separation functions, and the qualitative properties of the envelope system: buildability, durability, etc.

Part 2 reviews the main construction approaches – the basement envelope systems that can be selected to address the performance requirements. The combination of environmental conditions (inside and out) and occupant expectations, combined with the selected envelope system determines the performance requirements of the materials to be used within the construction system.

Part 3 identifies the roles of the materials within the envelope system and indicates what performance characteristics have to be met by those materials for their given roles.

Part 4 addresses some key detailing issues, and the special requirements put on the design detail and the materials used; e.g., the wall-soil interface, window well detailing, etc.

Part 5 reviews various quality control tools available to the Canadian construction industry. Quality control is an essential element of achieving envelope system performance targets. Its role is to ensure that performance objectives laid out at the design stage are satisfied throughout the production chain: material manufacturing or site forming, assembly and finishing.

Part 6 introduces the concept of cost/benefit analysis as a planning tool for achieving a balance between long-term basement envelope system performance and first cost, for a range of scenarios and locations.

Introduction

The basement can be a challenging environment in which to build livable space. By its very nature, it is the lowest location in the house, and often the coolest, the most humid and the darkest. It is surrounded by earth that can be dry, moist, wet or frozen, and sometimes all of these at the same time. As a result, the envelope components are subjected to greater structural, water and moisture loads than the above-grade portions. And although it is generally agreed that from a thermal standpoint, the above-grade components are subjected to more extreme loads, the duration of the below-grade heating season can be longer and is out of phase with the rest of the house and the outdoors.

As well, the context in which the basement system is expected to operate varies from site to site. It is affected by local climate, site grading conditions, a seemingly infinite variety of soil conditions – some of which represent a challenging environment for materials and the structure. On the inside, the basement interior is often allowed to run cold and damp, and the envelope can be isolated from the interior air by storage boxes, cold storage spaces, etc.

In these conditions, the basement envelope has some difficult and often contradictory functions to perform, and these are generally not well understood.

Most envelope systems used in Canadian house construction have evolved into present-day practice through a sequence of improvements based on trial and error. When a particular system and its materials become recognized (often marked by a reduction in the overall cost of construction, including cost of 'errors' or call-backs) the approach becomes mainstream practice. It may eventually be incorporated into the National Building Code or officially recognized by evaluation agencies, if it meets the intent of code requirements. With this 'evolutionary system,' we know the functions of the building envelope are addressed if there are no problems – but we don't know which functions and why. This leads to problems when systematic errors start to occur.

On occasion, the cost of 'errors' becomes excessive for builders, homeowners, manufacturers and warranty programs alike. Recently, symptoms have been emerging for insulated wall systems for applications below grade. The symptoms appear to be varied and sporadic, with some being localized regionally and others being more widespread. In both older and new homes, problems created by moisture seepage through the basements walls are reported. Excessive moisture in walls may, in addition to structural problems, cause mold and mildew, which are currently associated with potential health risks. In some locations, difficult soil conditions have been the cause of major problems that have entailed costly repair.

The guidelines recognizes that there are any number of design solutions that can be applied to address particular conditions and circumstances, but that some may be more appropriate than others to achieve the intended performance at least overall cost to the consumer. In many cases, the applicable code, which is a minimum standard, can represent a good solution. Even in these circumstances, it is not always clear what role, or how many roles, each material or system of materials is expected to play within the envelope – conventional approaches to basement design just seem to work on average and fail in certain circumstances. Not knowing why some basement systems work and others fail makes innovation of materials and systems difficult to introduce to the industry.

Innovative materials and existing materials used in innovative ways need a set of rules against which performance requirements are to be assessed.

In more difficult environmental conditions (both interior and exterior), it is not clear whether commonly used basement materials can be expected to take on more than their primary function, and may even require special formulation even to address their primary function. Some circumstances may require that a material, which normally plays many roles in the envelope system, give up some of those other roles to more specialized materials designed for that purpose. When this happens, additional construction costs may occur. Does the improved performance; i.e. reduction in numbers of failures and associated repair or replacement costs, warrant the additional expenditure? In what circumstances? The means of making such assessments are the central focus of this guide.

Main Messages Contained in this Document

• Performance expectations for environmental separation have become more demanding as basements are used as living space.
• The National Building Code of Canada and applicable provincial counterparts, being documents of minimum requirements, can't anticipate all of the variations in conditions for which a basement is expected to perform. Nor can the NBC force everyone to build to those conditions. As yet, regionally/site sensitive requirements have not been proposed. This leaves considerable decision-making and responsibility to the designer/builder.
• The tools available for this task are: builders' experience, builders' guides, material standards and application standards, evaluation guides, engineering manuals, engineering/architectural services from consultants and municipal offices. These guidelines are intended to facilitate the process of accessing these tools – a pathfinder – to promote performing basement envelopes.
• Basement envelopes featuring multiple materials must generally be specified to make sure that all of the functional requirements expected of the envelope are covered by at least one of those materials or system of materials, and that all of the materials are working together to achieve satisfactory performance – the 'basement as a system.'
• There are many different approaches to building a basement envelope and more are emerging every year. All should be capable of functioning as intended, if performance requirements are satisfied for the existing environmental conditions.
• An understanding of the intended role that is to be played by the materials and systems is needed by the people designing and constructing the envelope and the people responsible for inspecting the assembly process, to ensure that assembly techniques don't defeat the intended properties or function of the materials and systems.
• There is a balance to be achieved between first cost, cost of repair (including warranty work), and cost of maintenance and operation. That balance changes with conditions. Approaches to achieving a good balance are proposed.

Intent

The ultimate objective of these guidelines is the specification of basement envelope and material systems that perform better and last longer, within the broad range of Canadian climates, soil conditions and indoor environments.

The guidelines are intended to be a communication tool between construction materials manufacturers and designers, specifiers and builders involved in the construction of residential basements. It is a given that materials manufacturers know the business of making their products and builders know the business of building houses. The key intermediate step between the activities of materials manufacturing and house construction is the specification of the appropriate material for the appropriate task within the envelope system.

The following questions can be asked:

• what turns a material or a product that is available for many applications, e.g. plastic film, into a specialized building product destined to play a key role in the envelope systems of houses; e.g. the polyethylene sheet playing the dual role of the vapour barrier and an integral part of the air barrier system?
• how is the right material chosen and installed to form an integral part of an envelope system?

Designers and builders generally know the answers to these questions but specific answers for specific materials and envelope systems may be less clear cut. For example, the decision to select a particular product may be influenced by a combination of:

• long-term conventions generally used by the home building community
• individual builder experience and preference
• product performance characteristics, cost, product availability and consumer appeal
• product literature
• product track record
• applicable standards
• current and past codes requirements
• warranty issues
• materials evaluations criteria for assessing conformity

The link between material performance properties and their selection for envelope systems is therefore not necessarily a simple matter of cause and effect. As well, with evolving materials and methods of construction, the rationale for specifying a particular material and using it in a particular way may become lost or obsolete.

These guidelines record the technical rationale for specifying particular basement envelope materials and systems based on the best information available today. This is to promote a greater level of coherence in the process of specifying appropriate envelope systems and materials for basements and foundations. This will be achieved by reviewing the performance requirements of the envelope and its related systems, reviewing the performance capabilities of available constructions systems, and finally, sorting out the host of regulatory requirements that must be met by the building materials and the systems in which they are incorporated.

Once the technical link is clearly made between material performance characteristics and intended function within the basement envelope system, then the host of other factors affecting the process of material selection can enter into play.

Overview of the Problems

The National Building Code of Canada, being universally applicable, has to strike a balance between first cost and probability of basement failures and expensive repairs. Forcing everyone to pay a premium because someone else is going to develop a problem down the road, leading to possible health and safety issues, may not be good economics. On the other hand, having a consistently large number of homeowners (or warranty claims) pay for repair bills that are many times the original cost of prevention is not good economics either. A sound economic balance is needed.

Unfortunately, in many cases we are not good at predicting where the problem areas are: 'minimum basement configurations' get built in less than ideal situations – these show up as failure statistics. Sometimes the situation is too challenging for the 'minimum basement configuration.'

Sometimes site assembly practices defeat the intended functions, resulting in a poorer envelope than is expected with a 'minimum basement configuration.'

These guidelines try to fill the gap between minimum-code basements and what is actually needed or desirable for a given set of client expectations, site considerations, materials availability, and cost.

Approach to Basement Systems and Material Selection

The selection of basement systems and materials through design involves the following process:

• Understand the building physics.
• Identify the environmental constraints.
• Select the appropriate basement system.
• Review all functions expected of the system and identify the roles that materials have to play within that system.
• Select the appropriate materials to satisfy the needs of the system.
• Review both the envelope system and the materials for durability criteria.
• Review the material Evaluation Reports and applicable Codes & Standards to ensure that compliance is achieved.
• Review the initial material and labour costs, cost of operation, maintenance and repair, and weight as appropriate for builder, client and societal needs.
• Refine the design as needed.
• Set up and follow a quality assurance program to ensure that the product meets the specs and the expectations.

These Guidelines are formatted to accommodate this design process.