Masonry or Concrete Foundation Walls

Cast-in-place concrete or block-type walls are the most commonly used in Canada. Insulating this type of foundation is best done on the outside if possible. The large amount of thermal mass in a masonry or concrete foundation is included in the interior volume of the house if it is insulated on the outside. As well, the foundation is less susceptible to frost damage and leaking. Rigid insulation or glass fibre sheets can be used. The above grade portion must be protected with stucco, treated plywood or similar rigid exterior finishes. Since masonry or concrete walls are quite porous, a polyethylene air/vapour barrier is added on the interior to eliminate any potential condensation problems with the completed interior walls. The exterior foundation insulation details shows the application of rigid insulation.

Most masonry foundations however, have been, and will continue to be insulated on the inside. A most important step is placing a moisture barrier of polyethylene on the inside of the wall from the exterior grade level to the floor. The wall interior is then insulated and sealed similar to frame wall construction. It is also possible to use rigid insulation, attach an air/vapour barrier, then finish the wall directly over it. The interior finish is difficult to attach through the rigid insulation — which must be 4 to 6 inches thick (100mm to 150mm) in order to have an R-20 value (RSI 3.5). If plastic rigid insulation is used, drywall which is mechanically fastened to the foundation wall must cover it. A 2 x 4 inch (38 x 89 mm) stud wall frame work spaced 1.5 inches (36mm) out from the foundation wall, as illustrated, can be insulated with R-20 (RSI 3.5) batt-type insulation products.

This provides the easiest and most economical route if a foundation wall must be insulated on the inside. The interior foundation insulation detail shows how this is best done to provide a well-sealed and insulated wall.

There are currently a number of products available from manufacturers which offer rigid interior foundation insulation systems. These systems each have their own methods for attaching both the insulation and the interior finish and offer an effective alternative to wood framing and batt insulation.

Pressure Treated Wood Foundation Walls

Wood foundations can easily be made very energy efficient. Often called a ‘PWF’ foundation they can be constructed in almost any type of weather. A wood foundation must, however, be designed by a qualified engineer and constructed by competent builders who understand the importance of proper base preparation, handling techniques for pressure treated materials, the use of correct fasteners, drainage installation, backfilling techniques and sealing requirements. Because the foundation walls are an extension of typical wood frame construction, installing batt insulation and applying the air/vapour barrier is quite straight forward.

PWF Foundation Floors

The floor in a pressure treated wood foundation can be a concrete floor slab. Rigid Insulation should be placed under the foundation floor to a minimum insulation level of R-10 (RSI 1.7). A moisture barrier of at least 6 mil polyethylene (overlap seams) is required. A 3 to 4 inch (75 to 100 mm) layer of sand placed on top of the rigid insulation and the air/vapour barrier protects both during the floor pour and aids in proper concrete curing. Extra insulation to protect footings may be required for shallow footing depths as is often the case in bilevel or crawlspace foundations.

The floor in a pressure treated wood foundation can also be constructed of pressure treated wood. Pressure treated wood foundation floors are constructed using standard floor framing techniques on a gravel drainage bed. The installation of an effective moisture barrier on top of the gravel drainage layer is very important (minimum 6 mil polyethylene sheeting with overlapped and sealed seams).The floor joist cavities can then be filled with standard batt, blown or loose fill insulations. An air/vapour barrier is then installed on top of the floor joists. The attachment of the floor and wall polyethylene sheets is another important step in creating continuity of the air/vapour barrier. Standard floor sheathing and finishes can then be applied.

Polystyrene Foundation Walls

There are two basic techniques used to construct foundation walls using rigid polystyrene insulation. Some systems offer either polystyrene blocks or panels which use concrete and steel reinforcing placed into the cavities for structural support.

Other systems offer solid polystyrene panels using metal or wood studs for structural requirements. In either case, an interior polyethylene air/vapour barrier is applied and covered with a fireproof layer of drywall or plaster which must be mechanically fastened to the structural part of the wall. The outside must also be covered with a rigid material or parging to protect the polystyrene from mechanical damage and degradation from sunlight and soils.

Crawl Space Construction

Many homes have been built with partial depth foundations which are often called crawl spaces. Because the crawl space area under a home usually contains some mechanical services, insulating the foundation walls and floor is recommended to keep the temperature above freezing. A crawl space floor should be treated the same as an exterior wall, insulated and sealed from the house interior space. The crawl space walls can be insulated from the interior or exterior using standard foundation insulation methods. Perimeter insulation is then added as well to ensure that the crawl space retains more heat and is able to resist frost penetration. A moisture barrier, placed over the ground surface, is necessary to keep the space dry.

As well, summer ventilation should be provided by having outside air vents into the crawl space which can be opened in spring and closed in the fall. Any accumulated moisture can then be vented out during the summer months.

Slab-On-Grade Construction

With this type of foundation the concrete slab is the combined foundation and finished floor surface. Rigid polystyrene insulation is used below the slab to lower floor heat loss. Perimeter insulation is also applied to control heat loss from the edge of the floor slab. An insulated skirt of rigid insulation extending down and away from the foundation wall around the entire perimeter will eliminate any potential frost problems, improve drainage and help further reduce heat loss.

The polyethylene air/vapour barrier can be applied on top of the insulation, directly below the slab. A 3 to 4 inch (75 to 100 mm) layer of sand on top of the rigid insulation and the air/vapour barrier will protect both during the floor pour and aid in proper concrete curing. In order to provide continuity with the wall air/vapour barrier, the floor polyethylene layer must be placed so it can conveniently join to the wall layer at some point during construction.

To better anchor a slab-on-grade foundation, it can be attached to concrete piles. Large diameter holes of 8 to 12 inches (200mm to 300mm) are drilled 10 to 12 feet deep (3m to 4m) at intervals around the edge of the foundation. Reinforcing bars tie the thickened slab edge to the piles. In soils where drainage and frost is a problem, additional piles in the centre of the foundation may be required to prevent movement.

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