Replacement Windows

An insert — also called a pocket or retrofit replacement — removes the old sash and hardware and fits a new complete window inside the existing frame, which stays in the wall. The exterior trim, siding, and interior casing are left undisturbed. It is faster, cleaner, and less expensive, and it is the right choice when the existing frame and the sill beneath it are structurally sound and dry. The cost is glass area: the new frame sits inside the old one, so the visible glass shrinks by an inch or more per side.

A full-frame replacement takes everything out down to the rough opening — frame, sill, and all — and installs a new window with a nailing flange that gets integrated into the wall’s water barrier. It is required, not optional, when the existing frame is rotted or corroded, when the sill or the framing below shows water damage, when the original installation was never flashed correctly, when the opening is being resized, or when the existing frame is out of square enough that an insert would never seal. It is the only version that lets the installer actually flash the opening, which is why it is the honest answer on a house with a history of leaks at the windows.

• Vinyl — the volume choice. It insulates well because the frame is hollow and chambered, it never needs finishing, and it costs the least. It expands and contracts more than the other materials, and dark exterior colors on a large west-facing window are where that shows.
• Fiberglass — dimensionally stable, strong enough to support narrow sightlines and large units, and its expansion rate is close to that of glass, which reduces stress on the seal over decades. It costs more than vinyl and less than wood-clad.
• Aluminum — strong, thin-profiled, and durable, but a conductor. Older aluminum windows without a thermal break are the reason so many Texas houses have sweating window frames in winter. Modern thermally broken aluminum performs far better and remains common on commercial and contemporary residential work.
• Wood-clad — a wood interior for appearance with an aluminum, fiberglass, or vinyl exterior skin for weather. The premium option, and the usual choice where interior millwork matters.

An insulated glass unit is two or three panes separated by a spacer, sealed at the perimeter, with the cavity filled with an inert gas — usually argon, sometimes krypton in thinner cavities. The gas is denser than air, so it slows heat transfer across the gap. Triple glazing adds a third pane and a second cavity. In northern climates the gain is substantial; across Texas and the Southeast the return over a good double-glazed unit is much smaller, and the weight and cost increase is real, so double glazing with a strong coating is usually the better value here.

Low-emissivity coatings are the part that matters most. Low-E is a microscopically thin metallic layer on one of the glass surfaces that reflects long-wave infrared while passing visible light. Which surface it goes on changes what it does — a coating tuned for a heating climate keeps interior heat in, while a coating tuned for a cooling climate rejects solar heat before it enters. For most of CMAC’s service area you want the cooling-climate version.

Two numbers appear on every window label. U-factor measures how readily heat conducts through the whole window assembly, frame included; lower is better, and it is the number that governs performance when there is a temperature difference across the glass — which is to say, in winter, at night, and in heating climates.

Solar Heat Gain Coefficient measures the fraction of solar energy striking the window that ends up as heat inside. It runs from zero to one, and lower is better in a cooling climate. This is the number that matters most across Texas, Oklahoma, Arkansas, the Southeast, and anywhere else with long, hot, sunny cooling seasons. The reason is simply where the energy comes from: in a hot climate the dominant load on your air conditioner is sunlight coming through glass, not conduction through it. A window with an excellent U-factor and a high SHGC will still cook a west-facing room all afternoon. Orientation compounds it — south and especially west glass takes the most direct load, and that is where a low SHGC pays for itself fastest.

Visible transmittance is worth checking alongside SHGC, because it tells you how much daylight you keep. Modern spectrally selective coatings hold visible light reasonably high while pushing SHGC low, which is what lets you cut solar gain without living in a dim room.

Haze or moisture between the panes that you cannot wipe off from either side means the perimeter seal on the insulated glass unit has failed. Once the seal opens, the inert gas fill escapes and humid outside air moves in and out with daily pressure and temperature cycling. The desiccant in the spacer absorbs moisture until it is saturated, and after that the unit fogs on cool mornings and clears in the afternoon, gradually progressing to permanent haze and mineral staining etched into the interior glass surface.

Two things follow. First, the insulating performance you paid for is gone — the cavity is air, not argon. Second, it does not heal, and no cleaning reaches it. Seal failure is accelerated by heat cycling, by standing water in a sill that does not drain, and by south and west exposure, which is why fogged units on a Texas house so often appear on one elevation first.

Sliding patio doors are large horizontal openings low on the wall, which is a demanding condition. The sill sits where wind-driven rain collects, so the pan flashing and the weep path under the track carry most of the burden. Operation problems are usually rollers or track, not the panel, and doors that need shoulder pressure to move are running on flat-spotted rollers or in a track packed with grit. Foggy glass in a slider is the same insulated-unit failure as any window, on a much larger and more visible pane.