Both routes improve a building's energy performance. One costs a fraction of the other and disturbs nobody; the other fixes problems the first cannot touch. Here are the six checks we make on every window type to choose between them, and why most buildings end up with both.
In short: There are two ways to upgrade the glazing of a commercial building without rebuilding the facade: replace only the insulated glass units in the existing frames, or replace the whole window or storefront unit. Glass-only costs a fraction of full replacement, needs no interior work and no permit for the opening, and is the right answer when the frames are thermally broken aluminum in sound condition with a glazing pocket deep enough for a modern unit. Full replacement is the right answer when the frames have no thermal break, leak at the perimeter, have failed hardware or are too shallow for a low-E argon unit. We decide type by type from the envelope survey, and most buildings end up with a mix.
| Glass-only replacement | Full unit replacement | |
|---|---|---|
| What is removed | The insulated glass unit (and the glazing beads or pressure plates) | The whole window or storefront section, frame included |
| What is new | A new low-E, argon-filled insulated unit, often with a warm-edge spacer, in the old frame | New thermally broken frame, new glass, new hardware, new weatherstrip, new perimeter seal |
| Energy effect | Center-of-glass U-factor from about 0.48 (old clear double) or 1.0 (single) down to 0.25–0.30; frame unchanged | Whole-unit U-factor typically 0.28–0.35; air leakage reset to new; frame conduction removed |
| Interior work | None; done from outside or from the sash | Interior trim, stool and sometimes finishes disturbed and made good |
| Occupied building | Minutes per unit, little disruption | Hours per unit, tenant coordination, dust protection |
| Permits | Usually none for like-for-like glass | DOB filing in most cases; Landmarks if the building is designated |
| Relative cost | Roughly a quarter to a third of full replacement per opening | Full cost |
| Life | 20–25 years for the new unit; frame continues to age | New unit life, 30+ years for aluminum |
The energy numbers above are typical ranges for the glass make-ups involved; the exact figures for a building come from the manufacturer's data for the units chosen, which we provide with the proposal. The energy grade article explains where single glass, old double glazing and unbroken frames each sit.
A building rarely has one window type. A typical Brooklyn mixed-use building from the 1990s might have thermally broken aluminum windows upstairs that only need new glass, a single-glazed storefront in an unbroken frame at the ground floor that needs full replacement, and an entrance that needs a new door for air leakage and accessibility reasons that have nothing to do with glass. We present the schedule with a measure per type, and the owner can see what each line costs and what it does.
On complete facade packages such as 2535 Frederick Douglass Boulevard in Harlem the answer was full replacement of everything, because the building was being rebuilt. On an occupied residential tower with sound frames, the answer has been glass-only for the windows and full replacement for the balcony doors, whose hardware was gone. There is no default.
Where those matter, we say so and price full replacement. Where they do not, glass-only is the fastest and cheapest energy measure a building can make to its envelope, and the one owners most often do not know exists.
Both routes are priced from the same survey schedule. For glass-only we confirm the pocket depth and the bead or pressure-plate system on a sample unit, choose the insulated unit make-up with the owner (low-E type, argon, spacer, laminated where required), and price per unit by size with a crew day rate. For full replacement we produce shop drawings for each type, price the units from the manufacturer or our own fabrication, and add installation, perimeter sealing, interior make-good and the filing. Both prices come with the published performance data for the chosen make-up so the energy consultant can use them. See insulated glass units and window replacement for the products involved, and the property energy performance page for how the routes fit the compliance picture.
Replacing the insulated glass unit inside an existing window or storefront frame with a new low-E, argon-filled unit, leaving the frame in place.
Typically a quarter to a third of the cost per opening, because there is no frame, no interior work and usually no filing. The exact ratio depends on the unit sizes and the frame system.
When the frame has no thermal break, leaks at the perimeter, is too shallow for a modern insulated unit, has obsolete hardware, or is corroded or racked. In those cases the frame is the problem and the unit should be replaced.
Open a sash and look at the frame edge for a plastic or polyamide strip separating the inner and outer aluminum; or feel the interior frame face on a cold day. The survey confirms it from the profile section.
Like-for-like glass replacement in an existing frame generally does not; full window replacement usually does, and landmarked buildings need Landmarks approval for any visible change. Confirm with your expediter or architect.
Yes. Storefront systems hold the glass with pressure plates or stops and are often the easiest units to reglaze, provided the frame is thermally broken and the pocket takes a 1" unit.
Most do. The survey schedule gives a measure per window type; glass-only for the sound frames, full replacement for the rest.
Property energy performance How we survey a building envelope Energy grade D? Windows and storefront Insulated glass units Window replacement Local Law 97 and your windows
About this page. Written by Alex GC Inc. d/b/a Alex Glass Construction and reviewed before publication by Aleksandr Chernovolenko, Managing Partner, with our design and estimating lead and a field foreman. Our foremen hold the NYC DOB Site Safety Training supervisor card and our crews the 40-hour worker card; we are certified to fabricate, install and service Alutech and Everest aluminum systems. Facts are drawn from our own projects, manufacturer test data and the applicable codes; where we are giving an opinion, we say so. Last reviewed November 2026. Credentials
Send the address and your survey or energy report if you have one. We will tell you which openings are candidates for glass-only and which need full replacement, and price both.
