Window and Glass Installation, Measured to Your Openingin DC, MD & VA

These are two different jobs and the right one depends on what has actually failed. If the frame is sound, the sash runs, and only the glass is broken or the sealed unit has fogged, the glass is what gets replaced — the unit is made to the template and set back into the existing sash, which is faster and leaves your trim untouched. If the frame is rotten, the sash no longer meets the frame, or the window is a single-glazed original you want brought up to current performance, then the window itself is replaced. We will tell you which one your opening is, including the cases where the smaller job is the right one.

Width is taken at the head, the middle and the sill; height at each jamb and up the centre. The smallest of each governs, because a unit made to the largest simply will not go in. Both diagonals are measured to check the opening is square: if they differ, the opening is racked and the new frame is shimmed square rather than following the old one. For glass going back into an existing sash we measure the daylight opening and add the bite on each side — the amount the bead or stop actually holds. Anything arched, angled or with a hole in it gets a physical template. Tempered glass cannot be trimmed on site, so the template is the job.

An insulated glass unit is two or three lites separated by a spacer bar and sealed at the edge. The spacer is filled with desiccant to absorb whatever moisture is trapped when the unit is built. A primary polyisobutylene seal between the spacer and each pane is the vapour barrier; a secondary structural seal fills the channel behind it and holds the sandwich together. The cavity is usually filled with argon, which is denser and less conductive than air. Warm-edge spacers replace the traditional aluminium bar with a low-conductivity section, which raises the temperature at the sightline and is the main reason a modern unit does not run with condensation along its bottom edge in January.

The four glass faces in a double-glazed unit are numbered from the outside in: surface 1 faces the weather, surfaces 2 and 3 face the sealed cavity, and surface 4 faces the room. A low-emissivity coating is a microscopically thin metallic layer that passes short-wave daylight and reflects long-wave heat. In the mixed-humid climate the DMV sits in, that coating goes on surface 2, where it turns summer solar heat back outside before the cavity picks it up and still reflects room heat back inside in winter. Sealed inside the cavity it cannot be scratched, cleaned off or degraded. The performance difference is not subtle: in the numbers we publish, going from an uncoated double unit to a Low-E argon unit takes the centre-of-glass U-factor from 0.48 down to 0.27 and cuts solar heat gain by half.

Vinyl is welded uPVC with hollow chambers doing the insulating; it performs well, needs no painting, and is what most DMV replacement work uses. Aluminium is strong with slim sightlines and dominates commercial openings, but it must have a polyamide thermal break or the frame conducts straight through and sweats in winter. Clad wood gives a real wood interior with a maintenance-free exterior skin, and is usually what a historic review board will accept in Georgetown, Old Town Alexandria or Capitol Hill. Fibreglass is pultruded glass fibre that expands and contracts at close to the same rate as glass, so the sealed unit is worked far less through a DMV year of 95 degree Julys and 20 degree Januarys.

An insert, sometimes called a pocket replacement, sets a new frame inside the existing one. The exterior trim and siding are never disturbed, the job is quick, and the interior casing usually survives. The trade is that you lose roughly an inch of glass in each direction and you inherit whatever the old frame and sill are doing. A full-frame replacement removes everything back to the rough opening. It costs a day rather than an afternoon and it disturbs the exterior, but it is the only way to install a sill pan, lap the flashing correctly, insulate the shim gap and deal with rot you cannot see from inside. Neither is the default; the condition of the existing frame decides.

Almost every leak blamed on a window is an installation detail rather than a product fault. The sill pan goes in before the window and turns up at the back and the ends. Flashing laps shingle-fashion: sill first, jambs over the sill, head over the jambs, so gravity carries water out rather than in. Shims go at the load points and fasteners go through the frame into structure, never through a shim. The shim gap is filled with low-expansion foam only — the high-expansion kind will bow a jamb far enough to stop a sash from running. The exterior joint is backed with rod and sealed with a product suited to the substrate. Sealant is the last line of defence, not the first.