b'Variance in Measurements 1. Windows under 60 U.I.shouldbecutback1/4.Windows between 61 and 94Measure instructions request that the width and the U.I. be cutback 1/4 to 3/8.height be measured in three placesend the ends Windows above 95 U.I. cutand in the middleand the smallest be used inback 1/2.providing the rough opening dimension to a windowmanufacturer to product the properly sized replace- 2. If the cutback is 1/2, thenment window. While it is desirable to have a window backer rod is mandatory and probably exterior capping whenthat fitsespecially with any irregularities in plumb outside in. Additionally new interior stops will be needed toand square -making the glass area smaller by create proper seal if it is installed inside out.shrinking the new window, or restricting the egress3. Windows cutback 1/4 to 3/8, require no backer rod and canof the replaced window are not desirable so the be conventionally installed using existing blindstop or interiorlargest window that fits is the goal. stop. Capping would be optional. The most common replacement is blind-stop or Different procedures are also being recommendedpocket install where the old operating sash, weights depending on the space between the new windowand parting stops are removed to make a space for ordered and sized by the factory, and the rough the new replacement window to fit. Alternatively,openingcreated by the removal of the old window.windows with plastic or aluminum tracks have theold sash and the track assemblies removed leavingthe same opening for the new window. 3/8 TrimStop SurfaceCommon industry traditional cut-back has beenon the width andon the height when a headexpander is used and when the new window is to befit against the exterior stop.Cut-back for an outside-in install, where the interi- 3/4 Sill Dropor stops are left intact but the exterior blindstop iscut or fractured off to allow the new window to beinserted into the opening without a head expander, 3/8 Blindstop has traditionally beento 3/8. SurfaceThese cut back dimensions have been the most common because stop surfaces (interior stop orSealant 1/4 GAPblind stop) are 3/8. Cutting back any more than Batte Insulation1/4 (which leaves 2/8 on each side) risks havingtoo little contact surface for the new window impeding proper sealing and mounting and riskingair and water intrusion.However, many openings are not square and whenthe three measures are taken, while they may even Sealant & Backer Rodbe equal, they are not square and the new windowwould need to be racked out of square to fit withthese minimum tolerances. This has led manyreplacement cut backs to become largerto nominal Sealant 1/8 GAP1/2 cutback on width and 3/4 on height whenusing a head expander, and 1/2 on height when not. Foam InsulationAWDI recommends three circumstances to determinethe appropriate cutback, because as a windowbecomes taller (and wider), the out-of-square condition of the opening becomes a more progressive problem. For example, a 1/16 deviance over 2 feettall will become 1/8 out at 4 feet tall. An un-level sill Sealant(more common) might require shimming the new window off the old sill, and that distance will lessen * PU Foam or otherthe room at the top of the new window. appropriate insulation CappingWindow Fitters Guide to Window Installation252020 AWDI, LLC Do not reproduce without permission'