Should I add vapour barrier to the ceiling of an old house before blowing in fibreglass? – GreenBuildingAdvisor

I have a 1920s wood frame house with asphalt shingle roof above an unfinished attic in Victoria BC (similar climate to US Pacific Northwest, so 4C but with less rain). 

I removed the blown in cellulose above the ceiling, which is drywall + lathe and plaster + wood joists, and plan to get a contractor to blow in R50 fiberglass (there’s a rebate for adding significant insulation). 

There was no vapour barrier originally, and no mould that I saw. Multiple contractors told me not to bother installing a vapour barrier above the studs + new wiring, but their reasons seemed vague to me. It seems to me the dew point will be within the insulation in winter, and the building being old doesn’t change that. 

I have some 10 mil poly, and was thinking of laying it over the joists + wiring + LED pot lights, taping the joins, and blowing in over that. What do you think?

Thanks,

Chris