Walls, windows and air: insulation, thermal bridging, airtightness and heat recovery ventilation in BC homes

How the code measures insulation

The BC Building Code describes insulation in two numbers. RSI is the metric measure of how well an assembly resists heat flow, in square metre kelvins per watt; a higher RSI holds heat better. The U-value, used for windows and doors, is the rate at which heat passes through, in watts per square metre per kelvin; a lower U-value is better.

The code asks for effective values, meaning the whole wall as built. Sentence 9.36.2.4.(1) says the calculation must account for the heat loss through closely spaced framing such as studs and joists, and through lintels, sills and plates. The code treats each stud as a thermal bridge: a path where heat escapes faster than through the insulation beside it.

Under the Step Code, an energy advisor can trade one part of the shell against another as long as the whole building meets its heating demand target. The code's prescriptive tables still show what a baseline home looks like, and the energy model compares the design with a reference house built to the base rules. The table below lists the values in Tables 9.36.2.6.-C and 9.36.2.7.-D, which Sentence 9.36.1.3.(6) applies to houses and to buildings that hold only homes.

These tables apply where a heat recovery ventilator or a heat pump is installed, under Sentence 9.36.2.6.(1). For buildings without either, Tables 9.36.2.6.-A and -B list walls at RSI 2.78 in places under 3,000 degree-days.

Thermal bridges and where insulation must stay continuous

A shell loses the most heat where insulation stops. Article 9.36.2.5 sets rules for those places. Where an interior wall, foundation wall, firewall, party wall or structural element passes through an outside wall or insulated roof, Sentence (2) says it must be insulated in one of three ways: on both sides for a distance of 4 times its uninsulated thickness, within the plane of the insulation to at least 60 percent of the required value, or within itself to the full value.

This matters for multiplexes. The party wall between two side-by-side homes meets the outside wall at every floor. That junction has to be detailed so the insulation runs past it. Sentence 9.36.2.5.(9) adds that joints between walls and other parts of the shell must be insulated to at least the lower of the two values that meet there.

Large structural parts that pass through the shell, such as balcony slabs, beams and columns, can be left out of the wall calculation under Sentence 9.36.2.4.(3), but only if insulation is fitted tight around them and their total area is no more than 2 percent of the gross wall area. A design with many balconies or exposed beams has to count them.

Pipes, ducts and wires inside walls are covered too. Sentence 9.36.2.5.(7) allows a reduced value at plumbing and electrical boxes and pipes, at least 60 percent of the required value, if the insulation behind them on the cold side is continuous. Sentence 9.36.2.5.(8) requires at least RSI 2.78 where ducts, pipes or cables run inside an insulated floor or ceiling.

The air barrier

An air barrier is the layer of materials that stops air from moving through the shell. It can be a sheet membrane, taped sheathing, sealed drywall or a mix, as long as the pieces join into one continuous layer. Article 9.36.2.9 requires a continuous air barrier system and says it must stay continuous across construction, control and expansion joints, across junctions between materials and assemblies, and around penetrations through every assembly.

That list explains where houses fail their air tests. A wall can be airtight across its middle and leak at the rim joist, where floor framing meets the wall, or around every pipe, vent and wire that passes through. Sentence 9.36.2.9.(3) also requires windows, doors and skylights to meet the air leakage limits in the North American fenestration standard and its Canadian supplement.

The step sets how good the result must be. Step 3 allows 2.5 air changes per hour at 50 pascals, Step 4 allows 1.5 and Step 5 allows 1.0, under Table 9.36.7.4. Going from Step 3 to Step 5 cuts the allowed leakage by 60 percent. Name one person on site as responsible for the air barrier, because every trade that drills through the shell can break it.

Fresh air: ventilation and heat recovery

A tight house needs a planned supply of fresh air. Article 9.32.3.1 requires every home with electrical power to have a mechanical ventilation system that meets CAN/CSA-F326-M or Subsection 9.32.3. The system must include a principal ventilation fan system that supplies air and has an exhaust fan, plus kitchen and bathroom exhaust fans, under Article 9.32.3.3.

Article 9.32.3.4 lists ways to supply that air. One is a ducted forced-air heating system with an outdoor air inlet and a fan that runs all the time. Another adds a heat recovery ventilator to that system. A third uses a heat recovery ventilator with its own ducts to each bedroom and each floor without a bedroom. A heat recovery ventilator, often shortened to HRV, passes outgoing stale air and incoming fresh air past each other so the fresh air picks up heat without the two streams mixing.

Sentence 9.32.3.4.(6) allows passive vents for supply air only in limited cases: a one-storey home under 168 square metres, with no ducted forced-air heating, in a mild climate, and not in a building that complies through the Step Code. Since new houses comply through the Step Code, this exception does not reach them, apart from a secondary suite.

Where an HRV is installed, Sentence 9.36.3.9.(3) sets its efficiency. In places where the January design temperature is -10°C or warmer, it must recover at least 60 percent of sensible heat at an outdoor test temperature of 0°C. Table C-2 lists the Metro Vancouver locations between -6°C (Ladner) and -9°C (Haney), so the 60 percent rule applies across the region.