Adding an insulating panel directly onto the interior surface of a window – GreenBuildingAdvisor

I’m in climate zone 5. I have large old double-glazed windows (R-2?) in rooms that are rarely used. Can I safely apply an insulating panel directly onto the interior surface of the glass? Otherwise, those windows are constant energy drains.

Rigid fiberglass has an R-value of about 4.5/inch. Two inches of this would increase the R-value of the glazed portion of the window to R-11. Three inches would make the assembly as good as a standard stud wall. A good jump for not much money.

Owens-Corning 703 comes with a white “poly-encapsulated paper” facing on one side. If I used two layers of 703, one layer could be installed with the facing against the window to minimize heat gain and thus prevent possible damage to the insulated glass unit’s seals. The other layer could have the white facing pointed towards the room.

If the facing is in direct contact with the glass, there should not be any condensation.

Thoughts? Concerns?