Attic spaces in older homes in BC were often designed as vented spaces. If the attic is not vented and has not been insulated at the roof plane, it is possible that the original attic vents were at some point blocked off incidentally during re-roofing (e.g. cedar shingle/shake to asphalt shingle conversion) or other home renovations. Attic ventilation is an important factor in controlling moisture in the attic, reducing heat build-up in summer, and reducing ice damming in snowy climates. Poor attic ventilation, coupled with the leakage of indoor air into the attic, causes most attic moisture and mould problems in B.C.

For attic ventilation to be most effective, outdoor air should enter the attic low at the attic perimeter (i.e. at the soffits) and exit high near the attic ridge (i.e. ridge vents or cap vents installed near the ridge). The soffit vents can be provided in the form of perforated soffit material, discrete vents in the soffit material, or - if no soffit is present - button vents near the edge of the attic. If there is a gable end wall, vents may also be placed near the bottom of these walls above the insulation level. Powered or winddriven attic vents such as "whirly birds" should not be used as a means of attic ventilation as they can lead to depressurization of the attic space and cause additional air leakage. Soffit vents must connect through to the attic and past the insulation, typically by the use of insulation baffles or guards. Typically, these are placed between trusses at every bay or every second bay, depending on the roof configuration and available soffit area for venting.

The area and size of attic vents can be determined by referencing the BC Building Code, which requires attics to be vented with a net free vent area of 1:300 (2012 BCBC).