Another insulation situation

[gearman]

I am insulating the ceiling of an old building. This building was originally flat roofed. Later a 4/12 roof was built on top of the flat roof. The flat roof was covered with felt or membrane, tar and pea gravel. The pitched roof has tar paper and shingles.

The question is...When insulating between the ceiling joists (2x8s)of the old flat roof, should I leave a space for air to circulate above the insulation batts?

The upper, pitched roof has gable end and ridge vents. The bottom, flat roof has a few soffit vents, but no path for air to travel into the attic (air space between the old flat roof and new pitched roof). Im inclined to fill the ceiling joist cavities, and close off the soffit vents, but not sure.

Thoughts and suggestions

 

[Falcon67]

Referring back in memory to an older Holmes episode - yes, you need air space and circulation above that insulation. Any moisture traveling up though the insulation will be trapped. I imagine that a tightly sealed vapor barrier on the ceiling would reduce that - that probably depends on the climate.

 

[StartInsulating]

I disagree. You should definitely air seal the old roof first (foam around any penetrations) then fill the cavities. Dense packed cellulose is the best value. Fiberglass only if the old ceiling joists are perfectly spaced 16 or 24" on center.