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Material Selection for Basements

Last issue, the National Research Council Institute for Research in Construction (NRC-IRC) summarized the fundamental site and construction principles for keeping basements dry. This article highlights key considerations in the selection of basement materials essential for building space that is durable and comfortable.
Basements need to meet several performance requirements: structure, drainage, dampproofing or waterproofing, insulation, air/vapour barrier, soil gas barrier, and exterior and interior finishing as required. To this end, material selection is crucial.

Insulating Materials
Insulation materials vary in their physical characteristics, thermal effectiveness and cost, and must be suited for the environmental conditions to which they are exposed. Basement walls can be insulated on either the interior or exterior, but not all insulation materials are suitable for both locations (see table). 
Loose fill insulation, including glass fibre, mineral wool or cellulose products, can be used only on the interior of basement walls to insulate framing cavities. 
Several types of boardstock insulation are used. Low-density moulded/expanded polystyrene (EPS) is referred to as Type I, and higher-density material as Type II or III. All are suitable for interior and exterior basement applications.
Extruded/expanded polystyrene (XPS) is a closed-cell foam material available as Type II, III and IV insulation, based on its density. It has a higher thermal resistance per unit thickness than EPS.  It is also suitable for both interior and exterior basement applications.
Batts and semi-rigid boardstock glass and mineral fibre insulations are available for interior applications. Semi-rigid boardstock can be used below grade on the exterior where it serves as both insulation and foundation wall drainage.
Several types of spray insulations are used for basements. Cellulose and mineral fibre insulations can only be used on the interior. (An adhesive is added to allow the spray to adhere to vertical surfaces.) Wet-spray materials offer thorough-cavity coverage and increase envelope air-tightness slightly. Some require the installation of a mesh over the face of the wall to contain the insulation before installing the gypsum board.
Spray polyurethane foam (SPF) is mixed on site using special equipment and needs to be installed by certified personnel. SPF can be used on both the interior and exterior below-grade areas of the building envelope, but must be covered with a fire-protective covering, such as 12.7-mm (1/2-in) gypsum board, when used on the interior.
Insulating concrete forms (IFCs) make use of either EPS or XPS boardstock insulation to create formwork for cast-in-place concrete and remains in place permanently. ICF systems are permitted in Canada under new provisions of Part 9 of the National Building Code (NBC) 2005. More complex ICF configurations may require Canadian Construction Materials Centre (CCMC) evaluations or may be based on Part 4 engineering specifications.

Drainage Materials
Drainage and dampproofing are used where the foundation drainage system is effectively drained by gravity and the groundwater does not rise above the footings. In areas with periodically ineffective foundation drainage, waterproofing is required. 
Footing drainage is required to remove standing water. Granular drainage layers are used when active foundation drainage (sump and sump pump) is required. When gravity drainage to a storm sewer, ditch or dry well is possible, corrugated plastic drainage pipes are used. 
Foundation wall drainage materials are used to direct water to the footing drainage. Dimpled plastic sheets placed against the foundation wall exterior provide a drainage path. High-density mineral fibre insulation can serve as both external thermal insulation and as foundation wall drainage. Research has shown that both boardstock and spray type plastic insulations successfully manage excess water in the soil next to the basement wall.

Materials for Interior Moisture and Air Control
Materials installed on the basement interior to control moisture and air movement may be selected to deal with one function or to deal with both simultaneously. (Interior moisture build-up is rarely an issue for concrete basements with exterior insulation because interior surfaces are kept warmer and tend to dry toward the inside.)
Interior dampproofing is required when a separate interior finish is applied directly to a concrete or unit masonry wall, or to a concrete slab that is in contact with the soil. It is also required where wood framing is applied in direct contact with below-grade concrete or unit masonry.
Wall dampproofing must extend from the basement floor up to ground level. Permissible dampproofing materials include polyethylene film 0.05 mm or thicker; type S roll roofing, or any membrane or coating with similar properties that meet the requirements for low-permeability materials in Section 9.25 of the NBCC. 
Dampproofing for floors-on-ground is installed beneath the floor, except when a separate floor is provided over the concrete slab. Dampproofing membranes installed below the slab must consist of either 0.15-mm (or thicker) polyethylene film, or type S roll roofing lapped not less than 100 mm (4 in). Where dampproofing is applied above the floor slab, it must consist of two mopped-on coats of bitumen, polyethylene film (0.05 mm thick), or any other material providing equivalent performance.
Vapour diffusion control is essential for all insulated assemblies inboard of the foundation wall and floor slab. It can be effectively provided by polyethylene sheets or by vapour diffusion paint applied to gypsum board wall finish.

Air Leakage Control
Materials for air leakage control must satisfy requirements for resistance to air pressures and must be continuous at all junctions and penetrations to prevent the exfiltration of air and the infiltration of soil gas.
Concrete floors and walls can serve as the air barrier and provision must be made to transfer the basement air barrier to the air barrier system used above the basement. Polyethylene can serve as the air barrier material and must be lapped and sealed at all intersections. It should only be used in basements where concrete foundations have had adequate time to release excess water. Penetrations and joints are typically sealed with gaskets and/or sealants. Evidence now supports the use of spray polyurethane foam as an air barrier.

Soil Gas Control
Basement floors-on-ground must be constructed to deter the entry of radon or other soil gases. In most cases, this can be accomplished by placing 0.15-mm polyethylene under the floor, and sealing the perimeter and penetrations with a flexible sealant. 

This article was drawn from the NRC-IRC publication Options in the Selection of Materials for Basement Construction, Construction Technology Update No. 70.

Mike Swinton is a principal research officer in the Building Envelope and Structure program at NRC-IRC. He will address basement construction at the NRC-IRC cross-Canada seminar series for 2008/09, Single and Multi-Family Houses: Improving Performance Through a Systems Approach. www.bsi.gc.ca, e-mail: mike.swinton@nrc-cnrc.gc.ca.

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