Planning Ahead

Providing the Required Effective Thermal Resistance

Code requirements for minimum levels of effective thermal resistance in wall and other building assemblies depend on climatic conditions and the type of energy source used for space heating.

It is important to understand the difference between the nominal and effective thermal resistance of insulated building assemblies. The nominal thermal resistance refers to the thermal resistance rating of the insulation which is installed. For example, a 2 x 6 in. (38 x 140 mm) wall cavity containing a glass or mineral fibre insulation batt has a nominal thermal resistance of about R-20 (RSI-3.52). The effective thermal resistance takes into account interior and exterior sheathings and finishes, and most importantly. the effect of the wood studs which conduct heat at a higher rate than insulation. This effect, known as thermal bridging, reduces the nominal thermal resistance of the building assembly and is reflected in the elfective thermal resistance rating. For a wali constructed with wood studs at 16 in. (400 mm) on centres, the effective thermal resistance is R-17.4 (RSI-3.06). In general, the effective thermal resistance is lower than the nominal rating. It is important to select an assembly which will comply with the minimum Code requirement applicable to the house being constructed. The following points should be considered when providing adequate effective thermal resistance values:

-* Effective thermal resistance values of common building assemblies are provided in the Model National Energy Code for Houses. It is available from the National Research Council.

-» Wider spacing of framing members reduces thermal bridging and provides higher effective thermal resistance.

-» Select insulation materials with higher nominal thermal resistance

Use insulating sheathings in place of structural sheathings to increase the effective thermal resistance of wall assemblies.

-* Take into account the effect of thicker building assemblies when design ing details at the foundation wall/main floor and wall intersection, and at the wall and roof intersection.

Additional consideration should also be given to factors affecting the amount of latjour associated with various insulated wall assemblies, particularly in relation to the placement of air and vapour barriers.

adequate bracing to resist lateral loads and keep the wall square. Others, such as rigid glass-fibre, polystyrene or polyurethane board, will not. In this latter case, the wall should be reinforced with a diagonal wood or metal bracing let into the studs.

The complete wall sections are then raised and put in place, temporary braces added and the bottom plates nailed through the subfloor to the floor framing members (Fig. 42). The braces should have their larger or flat dimension upward and should permit adjustment of the vertical position of the wall.

Once the assembled sections are plumbed, they are nailed together at the corners and intersections. A second top plate, with joints offset at least one stud space away from the joints In the plate beneath, is then

Wall framing used with platform construction: (l) top plate end-nailed to each stud with two 3 "A in. (82 mm) nails; (2) top plates nailed together with 3 in. (76 mm) nails 24 in. (600 mm) on centre; (3) stud toenailed with four 2 1/2 in. (63 mm) nails or end-nailed to bottom plate with two 3 'A in. (82 mm) nails; (4) top plates at corners and loadbearing partitions are lapped and nailed together with two 31/4 in. (82 mm) nails or the plates are butted together and tied with a metal plate fastened to the top plates with three 21/2 in. (63 mm) nails on each side of the joint; (5) doubled studs at openings and multiple studs at corners and intersections nailed with 3 in. (76 mm) nails 30 in. (750 mm) on centre; (6) bottom plate nailed to joist or header joist with 3 1/4 in. (82 mm) nails 16 in. (400 mm) on centre.

butt joint with metal butt joint with metal temporary brace stud and jack stud cripple/trimmer stud window opening lintel

Trimmer Studs Jack Studs Lintel

temporary brace stud and jack stud cripple/trimmer stud window opening lintel bottom plate subfloor let-in bracing or metal strapping when no or non-structural sheathing is used

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