[Technical Series] Insulating Doors & Windows 101 (Part 3): Real Data + How to Pick the Right Glass

Last time, we broke down the heat-transfer paths and the main formulas behind insulating glass. Now we're going straight into the numbers: real test data, real calculations, and the selection rules you actually need.

What Engineering Calculations Tell Us About Insulating-Glass Performance

Calculation basis: the manufacturer measured the samples with a spectrophotometer and used spectral integration formulas to calculate the spectral constants for each pane.

Sample 1: clear glass (CSG 6mm clear glass)

Sample 2: Low-E glass (CSG LB69-1_6 double-silver coated glass)

Sample 3: solar control coated glass (CSG CNY129_6, added for a clearer comparison)

Calculated with LBNL Window 7.2 under the boundary conditions in JGJ/T 151:

• Tvis=0.603，g（SHGC）=0.363，SC=0.417，LSG=1.66，K（U）=1.66 W/㎡·K

Low-E glass results:

• Tvis=0.603，g（SHGC）=0.363，SC=0.417，LSG=1.66，K（U）=1.66 W/㎡·K

• Tv=0.294，g（SHGC）=0.35，SC=0.403，K(U)=2.51 W/㎡·K，LSG=0.84

After switching to solar control coated glass:

• Tv=0.294，g（SHGC）=0.35，SC=0.403，K(U)=2.51 W/㎡·K，LSG=0.84

What the comparison shows:

With similar SC values, the insulating-glass system using solar control coated glass has only about half the LSG of the Low-E system. Why? Low-E lets in much more visible light while reflecting more infrared. Solar control glass gets to a similar SC mostly by cutting visible light.

Basically:

Same SC, very different result. Low-E has much higher visible-light transmittance（0.603 vs. 0.294）and a lower K(U). More daylight gets in, more infrared gets kicked back out. Overall, Low-E wins.

Single-Variable Trend Check

Change one variable, freeze everything else - that's the cleanest way to see how much each parameter actually moves thermal performance.

① Change coated-glass emissivity（0.02-0.25）

When emissivity goes up, K(U) goes up too:

• At ε=0.02, K(U)=2.11

• At ε=0.15, K(U)=2.39

• At ε=0.25, K(U)=2.53

Note: these chart values are assumed parameters used only to isolate the effect of emissivity on K(U). In a real project, emissivity would not change by itself while every other glass parameter stays fixed.

② Change the gas-gap thickness

As the gap gets thicker, air convection drops and K(U) falls. Once the gap reaches 12mm＜d＜16mm, the cavity flow moves from laminar toward transitional flow. Go thicker than that and K(U) barely improves. In practice, 12mm is the sweet spot and the common engineering choice.

③ Change the fill gas:

Lower gas thermal conductivity = lower K(U).

• Dry air: K(U) = 1.66

• Pure argon: K(U) = 1.36

• Pure krypton: K(U) = 1.26

Bottom line: coated-glass emissivity, gas-gap thickness, and gas composition all work together to set the system's thermal performance - its K（U） value.

Basically:

Three clean takeaways: 1. lower coating emissivity = lower K(U); 2. 12mm is the best-value gas gap, because going thicker changes the flow regime and stops giving meaningful K(U) gains; 3. a lower-conductivity inert gas can push K(U) down even further.

Comparing Three Low-E Glass Options by Spectral Performance

All three glass options are from CSG.

Spectral data:

The system uses 6Low-E+12A（air）+6C and is installed at 90° to the vertical plane. Here are the calculated optical results for the three coated-glass options:

All three insulating-glass systems sit in the medium visible-light-transmittance range. Solar blocking ranks: triple-silver ＞ double-silver ＞ single-silver. Triple-silver has the lowest surface emissivity and the strongest reflection in the near-infrared range（780nm-2500nm）, with double-silver next. At the same time, it lets more visible light through, which gives it the best LSG.

Summer Relative Heat Gain（RHG）at a Glance:

RHG gives a quick read on overall summer heat-control performance. Once U value and SC are set, the option with higher visible-light transmittance gives you the better combo of daylight + insulation.

Basically:

All three are medium-transmittance glass, but triple-silver has the lowest emissivity and strongest near-infrared reflection. That gives it the best LSG and the lowest RHG. More daylight, less solar heat, lower cooling demand.

Quick Selection Guide for Insulating Glass

1. Go for lower-emissivity coated glass

This makes a big difference to the whole system. It cuts mid- and far-infrared heat transfer, helping lower K(U), and it also reflects near-infrared solar energy, reducing total solar transmission.

2. Gas gap + gas type:

From the data above, 12mm is the preferred gap. Argon gives the best cost/performance, while xenon gives the best performance. The final thermal metric is K(U).

3. Balance total solar transmittance with visible-light transmittance（Tv）:

Solar energy is mainly visible light + near-infrared. Visible light makes up 44% of the solar-spectrum energy. As a rule of thumb, south-facing windows in southern regions can use Low-E glass with 45-60% Tv. Shaded sides can use high-transmittance Low-E to keep daylight up. Where there is no direct sun, incoming energy is mainly far-infrared, so solar transmittance is not the right insulation metric. And yes: solar control coated glass on a shaded façade does basically nothing for thermal insulation.

4. Near-Infrared Blocking:

Near-infrared is 53% of solar-radiation energy. For summer heat control in southern regions, you want as little of it indoors as possible. Zero is unrealistic, so lower-emissivity glass is the better way to boost near-infrared blocking.

5. Core Metric: LSG

Total solar transmittance sets how much solar heat the insulating-glass system gains. Lower g is better for heat control, but push g too low and you can kill visible light too. So first meet the daylight requirement, then go for the lowest practical g.

Use LSG to optimize the pick: if Tv is fixed - for example 55% - then LSG=Tv/g=0.55/g. Same numerator, so the higher the LSG, the less solar energy gets indoors.

Basically:

Quick selection checklist:

• Pick low-emissivity coated glass to lower both K(U) and solar transmission.

• Use a 12mm gas gap. Argon is the best value; xenon gives the best performance.

• Sun-facing side: Low-E with Tv 45-60%. Shaded side: high-transmittance Low-E. Don't use solar control coated glass on a shaded side - it won't give you meaningful insulation.

• For thermal optimization, check LSG. With Tv fixed, higher LSG = less solar energy indoors.

*Reference standards: JGJ/T151, GB/T2680, ISO15099, etc.*