Low-E glass, DGUs, and triple glazing: a technical guide.

The glazing specification of a luxury residential tower is among the most under-examined line items in the buyer's brochure, and among the most consequential for the daily experience of living in the apartment. The glass controls how much solar heat enters the room, how much noise from the main road makes it into the bedroom, how much winter cold radiates off the window surface at 3 am, and how much condensation forms on the pane when the humidity outside is 90 per cent. This brief explains the three most important glazing technologies — low-E coatings, double-glazed units (DGUs), and triple glazing — and describes the specification appropriate for an Indian luxury residence in a climate like Greater Noida West.

The three performance metrics.

Every glazing assembly is described by three numbers: U-value, solar heat gain coefficient (SHGC) and visible light transmittance (VLT).

• U-value — the rate of heat transfer through the window per square metre per degree Kelvin temperature differential. Lower is better. Single glazing has a U-value of approximately 5.7 W/m²K; high-performance DGUs drop this to 1.6–2.0; triple glazing reaches 0.8–1.2.
• SHGC — the fraction of solar radiation (visible + infrared) that passes through the glazing. Lower is better in hot climates. Clear glass has SHGC around 0.86; low-E coated glass drops this to 0.28–0.45 depending on coating type.
• VLT — the fraction of visible light that passes through the glazing. Higher is better for daylight, within reason. Clear glass has VLT around 0.90; tinted glass may be as low as 0.10.

For Indian luxury residences in a hot-humid climate like Delhi NCR, the correct specification is a DGU with a high-performance soft-coat low-E layer, targeting U ≤ 1.8, SHGC ≤ 0.32 and VLT ≥ 0.55. This combination reduces cooling load dramatically without making the apartment visually gloomy.

How low-E coatings actually work.

Low-emissivity (low-E) glass is ordinary float glass that has had a microscopically thin metallic oxide layer deposited on one of its surfaces. The coating is transparent to visible light but highly reflective to infrared radiation. In summer, the coating reflects incoming solar infrared back outward, so much less heat enters the room. In winter, the same coating reflects indoor infrared back into the room, so less radiant heat is lost through the window.

There are two types of low-E coating — hard-coat (pyrolytic) and soft-coat (sputtered). Hard-coat is applied during glass manufacture at high temperature and is durable but has slightly inferior thermal performance. Soft-coat is applied post-manufacture in a vacuum chamber, offers superior thermal and solar control, but is delicate and must be protected inside a sealed DGU. For luxury residential application in India, soft-coat low-E inside a DGU is the correct specification.

The DGU — why two panes beat one.

A double-glazed unit (DGU) is two panes of glass separated by a sealed air or argon-filled cavity. The cavity is sealed by a spacer bar filled with desiccant to keep the internal atmosphere dry. The thermal performance of the DGU is dominated by the low conductivity of the gas in the cavity — air has roughly one-thirtieth the thermal conductivity of glass — so the DGU U-value is roughly one-half to one-third that of single glazing. Argon gas, being heavier than air, reduces convective heat transfer within the cavity and further improves U-value by 10 to 15 per cent.

The optimal cavity width is between 12 and 16 millimetres. Narrower cavities allow too much conductive transfer; wider cavities let convective air loops form that actually degrade thermal performance. A typical Indian luxury DGU specification is 6 mm outer + 12 mm argon + 6 mm inner, with the low-E coating on the cavity-facing surface of the outer pane (for hot climates) or the inner pane (for cold climates). For Delhi NCR, the coating is on the inside of the outer pane — position 2 in façade conventional notation.

Triple glazing — when is it actually needed?

Triple glazing adds a third pane and a second cavity. It is thicker, heavier and more expensive than a DGU, but it delivers materially better thermal and acoustic performance. For an Indian luxury residence in Delhi NCR, triple glazing is not routinely necessary for thermal reasons — a well-specified DGU already reaches U 1.6 to 1.8, which is adequate for the climate. Triple glazing is warranted in two specific situations.

First, triple glazing is used where the façade faces a persistent high-decibel noise source — a highway, a flight path, or a main arterial road. The sound transmission class (STC) of triple glazing is typically 40 or higher, compared to 35 for a standard low-E DGU. The 5 dB improvement represents a 40 per cent reduction in perceived sound. For apartments facing an expressway, the investment is worth it.

Second, triple glazing is used in extreme cold climates where the interior surface temperature of a DGU drops low enough that condensation forms during winter nights. In NCR, this is rare but possible in January for apartments with poor ventilation. Triple glazing eliminates the risk entirely.

For Fab Luxe, the specification is a laminated low-E DGU throughout, with triple glazing as an optional upgrade on specific apartments that face the main approach road. The standard DGU delivers U 1.7, SHGC 0.30, VLT 0.60 and STC 38 — adequate for every climatic and acoustic demand of the site, except where the main road upgrade is selected.

The sealant, the spacer and the frame.

A DGU's performance depends not just on the glass but on the edge seal and the frame. The edge seal prevents the argon gas from leaking out over time — a poor seal can lose 10 to 15 per cent of the argon per year, degrading U-value measurably within a decade. Modern dual-seal DGUs (polyisobutylene primary + silicone secondary) maintain argon retention above 90 per cent at 20 years.

The spacer bar has historically been aluminum, which is highly conductive and creates a thermal bridge at the perimeter of the DGU. "Warm edge" spacers made of stainless steel or polymer composite reduce this thermal bridge substantially and also reduce condensation at the edge of the pane. For Fab Luxe, warm-edge spacers are specified on every DGU.

The frame carrying the DGU contributes approximately 20 to 30 per cent of the total window heat loss. Aluminum frames must have thermal breaks — insulated plastic segments between the inner and outer aluminum profiles — to avoid dominating the total U-value. Fab Luxe windows use thermally broken aluminum frames rated at U ≤ 2.0, which ensures the overall window U-value stays close to the glass-only number.

The broader system — why this matters for AQI and energy.

The glazing specification is one input to the larger building envelope, which determines both the indoor AQI system's effectiveness and the cooling energy consumption over the life of the building. A well-sealed, low-SHGC, low-U glazing assembly cuts the peak cooling load by approximately 25 to 35 per cent, enables the AQI-managed ventilation system to maintain positive pressure more economically, and dramatically reduces the perceived radiant discomfort near the window. The buyer's monthly electricity bill is, in large part, paid at the glazing specification stage.

For the complete façade specification, see the project specs page. Related briefs: HRV and ERV ventilation, and the building plan approval glossary.