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Attic mould

Attic Insulation & Ventilation

As recently as 1985, the British Columbia Building code had very modest requirements for insulation standards for houses.

Section 9.26.2 required that: “Buildings of residential occupancy shall be provided with sufficient thermal insulation to prevent moisture condensation on the interior surfaces of wall, ceilings and floors during the winter and to ensure comfortable conditions for the occupants.”

There was no minimum level of insulation standard, the most common insulation material was fibreglass and the average thickness of the attic insulation in the Lower Mainland of BC was 6 inches or R-20 (RSI 3.5).

Requirements for attic ventilation were equally loose in section 9.19, requiring a minimum of “1 square foot of unobstructed vent area for every 300 square feet of insulated ceiling area.” This requirement has remained unchanged since the 1960s.

Insulation and ventilation standards have been upgraded in every subsequent edition of the BC Building code including new requirements in 1998 that required:

“Not less that 63 mm (2.5 inches) clearance shall be provided between the top of the insulation and the underside of the roof sheathing.”

This was in response to condensation problems that have been noted with the increased insulation levels mandated in the 1992 edition of the Code.

The current 2006 edition of the BC Building Code has an extensive list of minimum standards for insulation and ventilation in attics:

Venting Requirements “At the junction of sloped roofs and exterior walls, where preformed baffles are used to contain the insulation, the baffles shall:

  1. provide an unobstructed space between the insulation and the underside of the roof sheathing that is: (i) not less than 25mm (1 inch) and (ii) of sufficient cross area to meet the attic or roof space venting requirements” (still 1/300 of the insulated ceiling area).
  2. extend vertically not less than 50mm (2 inches) above the top of the insulation. “Ceiling insulation shall be installed in a manner that will not restrict the free flow of air through roof vents or any portions of the attic or roof space.”

Insulation Requirements Amount of insulation required in attic spaces is RSI 7.0, R-40. (equivalent depth of loose fibreglass insulation is about 14 inches) Installation
“where soffit venting is used, measures shall be taken:”

  1. “to prevent loose fill insulation from blocking the soffit vents and to maintain an open path for circulation of air from the vents into the attic or roof space”
  2. “to minimize the air flow into the insulation near the soffit vents to maintain the thermal performance” (see

It is generally accepted that with increasing levels of attic insulation, there is a corresponding requirement to increase the free movement of air in the attic.

The problem is that the attic ventilation in the large majority of houses constructed in the Metro Vancouver area after 1998 does not comply with the minimum building code requirements.

The consequence is that we observe the early stages of mould problems (such as is visible in the photograph above) in houses as little as one year old.

Ultimately, MHI recognizes that the builders are responsible to meet the building code standards, but it would be a help if the municipal building inspectors did some obviously needed quality control.

updated November 1, 2010