Ok if we say it is drywall, would you count each wall as being made of 2 layers of drywall? Or does the structure rating account for the fact that walls usually have one layer of drywall on each side of the wooden frame?
Well according to destroying barriers, the armor/structure rating is for: "1 meter square and about 10 cm thick (SR4A, pg. 166)."
10 cm is just under 4 inches (3.94 rounded) and Drywall is usually used in 1/2" sheets (some areas use 3/8" sheets for interior wall and 5/8" greenboard in high moisture areas). Using the 1/2" standard, that means one structure/armor rating worth of Drywall is nearly 8 sheets of Drywall (4 walls effectively). In all reality, this makes a lot of sense, Drywall is pretty much just a privacy screen, it does not stand up to any kind of force really. For more evidence on this view:
Box of Truth. Even a .22 pistol will go through 3
drywalls (6 sheets of drywall) without any other factors. Anything that serious runners are firing is going to swiss cheese drywall. This is why hosing an area inside a building is bad when you don't know what's on the other side of the walls.
That said, that 10cm wall is going to have other stuff inside/on it as well: Studs, wiring, insulation, switch housings, paint, posters etc. It would be a fair call for the GM to adjust things to make it one armor/structure rating per wall or two, especially in decent neighborhoods.
In areas like the barrens, a drywall may literally just be a single sheet put up, so it would vary from area to area (a lot of buildings use this tactic for maintenance/employee only interior sections as well.
All that said, for many modern buildings, plascrete (which can be made in clear for window use according to fluff), plastiboard, and Kevlar Wallboard are all also common materials with the latter being likely most reserved for secured/risk locations.