I am looking for suggestions to add R-value to the roof insulation in the single room occupancy apartment building shown in the attached photos. Prior to my site visit, I assumed the roof was flat or low slope. It turns out there is an unused attic with 3′ poorly insulated knee walls and sloped rafters above with very poorly done fiberglass batts. The attic floor has 7-8″ of blown cellulose between joists over air sealed top plates (assumed a maximum possible R-value of 28). With no plans for future attic storage but a need for occasional access to roof hatch, I would like to add 2″ of polyisocyanurate (taped seams) over the entire sheathed floor plus a plywood path to roof hatch laid directly on the polyiso. I will create a polyiso box over the attic access below. This is in keeping with the spirit of the Stretch Code in this Massachusetts “Green Community”, which requires R-38 in the ceiling.

This technique will hopefully keep the attic floor sheathing warm, and reduce the chance of condensation from moisture-laden air from units below driven upward by the stack effect.

I am concerned that irregularities in the existing floor and mild settling of existing cellulose in joist bays will prevent the two insulation types from being “aligned and continuous”. If there is air movement between, it seems a waste.

Any suggestions on how to address this with an extremely limited budget?

Thank you!