There are five options for installing an air barrier system in an unfinished attic:

• concentrate on air sealing
• install polyethylene over top of the joists
• install polyethylene between the joists
• spray foam between the joists (hire a contractor)
• spray foam under the roof to create a conditioned attic (hire a contractor)

The first option is the most practical, since installing a polyethylene air and vapour barrier in an existing attic is fraught with obstructions and requires painstaking attention to detail. On the other hand, spray foam offers air sealing and an initial layer of high quality insulation that can be topped up to the desired RSI (R) level.

If the attic retrofit is being completed in conjunction with interior renovations, the easiest approach is to install a new, single air and vapour barrier on the underside of the ceiling joists.