PVB vs SGP interlayers for impact
The interlayer is the member that makes laminated glass impact-rated: it holds the fragments together after the strike, and the question is how much load it carries once the glass is broken. PVB is the standard interlayer, elastic and effective on ordinary impact lites; SGP is several times stiffer, carries the load longer after breakage, and is the specification for the large lite and the position that has to stay in place after an event. The choice is set by the lite size and the residual-strength case, not by habit.
What the interlayer does after the break
A laminated lite is two or more panes bonded by the interlayer. On impact the glass cracks, and the interlayer holds the pieces so the opening stays closed — no hole through which a hand can reach. How well it holds, and for how long, depends on the interlayer. PVB is elastic: it stretches and retains the fragments, which is enough for the ordinary impact lite. SGP is far stiffer: after breakage it carries a load like a structural member for longer, which matters where the lite has to stay in place, not just stay closed.
Where PVB is enough
On the standard impact lite — the door, the window in the debris-zone position — PVB does the job and is the cost-effective default. It meets the projectile criteria, it holds the fragments, and it is what most impact units carry. The specification names it and moves on; the heavier interlayer is not automatically better on a small lite.
Where SGP earns its place
As the lite gets larger and heavier, and as the post-breakage requirement gets stricter, SGP pulls ahead. The stiff interlayer keeps a broken lite in the frame and carrying load where a PVB lite would sag out of the opening, which is why the large fixed pane, the tall door and the position with an occupancy requirement behind it move to SGP. It costs more and the lite is heavier again, so it is specified where the residual-strength case demands it rather than across the board.
What to put on the schedule
Name the interlayer, not just the thickness: a laminated lite reads as PVB or SGP on the data sheet, and the two behave differently after breakage. State the lite size and the residual-strength case — whether the opening must stay weathertight and load-bearing after the event, or just closed. Where the case is the stricter one, the schedule specifies SGP and the hardware follows the added lite mass.
Questions
It carries more load after breakage, but it costs more and adds mass. On a small standard lite PVB meets the criteria, so SGP is specified where the size or the residual-strength case calls for it.