usually manufactures recommend a opening 1/2″ wider and taller than the window to enable shimming. If shimming is not done than the window can bow out in the middle of the frame and allow air to leak in or the sashes to not operate correctly.
THANKS,
MATTHEW BURR
BUYER – WINDOWS AND DOORS
VILLAGE HOME CENTER / dba COOPER BUILDING MATERIALS
4650 HIGHWAY 7 NORTH
HOT SPRINGS VILLAGE, AR 71909
EMAIL: MBURR@CBMCCI.COM