Introduction: A 2 mm coating layer turns a building roof from a “70°C frying pan” into a “35°C pergola”
At noon in summer—you touch the roof and it’s “scalding hot,” temperature >60-70°C—this heat continuously transfers indoors—the air conditioner runs at full load—electricity bills soar. The core value of thermal insulation coating is through the synergy of three mechanisms: solar reflection (reflecting >85% of solar radiation back into the atmosphere), infrared thermal radiation (actively emitting the small amount of absorbed heat as 8-14μm far-infrared rays), and thermal barrier (hollow glass microspheres/aerogel prolonging the heat conduction path)—forming a “thermal barrier” on the building surface, reducing summer roof surface temperature from >65°C to 20-40%. The latest 2024 research data (SiO2 aerogel + hollow polymer microsphere composite system) has pushed the thermal conductivity to 0.0277W/(m·K)—lower than still air (0.026). “A 2mm layer of coating = 5cm of traditional insulation board—no occupation of indoor space—no change to building appearance—application only requires—brushing—rolling—spraying”—this is a revolutionary solution for energy-saving retrofits of existing buildings.
Thermal Insulation Coating is a class of water-based environmentally friendly specialty coatings that achieves triple synergy through high-reflectance pigments (rutile TiO2/ZnO—solar reflectance ≥0.85), high-emissivity fillers (silicon carbide SiC/cordierite—hemispherical emissivity ≥0.85), and low thermal conductivity fillers (hollow glass microspheres/aerogel—thermal conductivity 20-40%—and also providing decorative and protective functions. According to the GB/T 25261-2018 “Reflective Thermal Insulation Coatings for Buildings” standard—it is divided into three types: thermal insulation middle coat (thermal conductivity ≤0.15W/m·K), reflective thermal insulation flat finish paint (solar reflectance ≥0.85/hemispherical emissivity ≥0.85—high lightness L*>95), and reflective thermal insulation textured finish paint.
I. Synergistic Effects of the Three Major Thermal Insulation Mechanisms and Key Fillers
| Insulation Mechanism | Physical Principle | Key Fillers | Core Indicators | Contribution Ratio | Applicable Scenarios |
|---|---|---|---|---|---|
| Solar Reflectance | High-refractive-index pigments reflect 300-2500nm solar radiation (including visible and near-infrared) back into the atmosphere—reducing coating heat absorption “white = cool—” | Rutile TiO2 (refractive index R=2.72 / near-infrared reflectance >85%) / ZnO (R=2.2) / infrared reflective pigments (chrome green / iron red) | Solar Reflectance (SR) ≥0.85 / Near-Infrared Reflectance (NIR) ≥0.80 | 50-60% (dominant—summer—daytime) | Roofs / exterior walls “the stronger the sunlight—the better—the effect—” |
| Infrared Thermal Radiation | High-emissivity materials actively radiate the small amount of heat absorbed by the coating to outer space as 8-14μm far-infrared rays (atmospheric window) “passive—heat dissipation” even at night it is “cooling” | Silicon carbide SiC (emissivity >0.9) / cordierite / transition metal oxides (Fe2O3 / Cr2O3) | Hemispherical Emissivity ≥0.85 | 10-15% | Regions with large day-night temperature differences “nighttime—heat dissipation—faster—” |
| Thermal Barrier (Conduction Barrier) | Low-thermal-conductivity fillers extend the heat conduction path (maze effect)—reducing the overall thermal conductivity of the coating “heat—is trapped—on the coating—surface—cannot—reach—the substrate—” | Hollow Glass Microspheres (HGM—thermal conductivity 0.04-0.08) / SiO2 aerogel (thermal conductivity 0.015-0.02) / hollow polymer microspheres (HPM) | Thermal Conductivity (λ) ≤0.05W/(m·K) (GB/T 25261 requires ≤0.15) | 25-35% | Building exterior walls / industrial pipelines “year-round—continuous—heat—blocking—” |
II. Core Standard System and Testing Methods for Thermal Insulation Coatings
| Standard No. | Standard Name | Core Indicators | Limit Requirements | Test Methods | Scope of Application |
|---|---|---|---|---|---|
| GB/T 25261-2018 | Reflective Thermal Insulation Coatings for Buildings (National Standard) | Solar reflectance / hemispherical emissivity / thermal conductivity / thermal insulation temperature difference | High lightness (L*>95): SR≥0.85 / HE≥0.85; Mid-coat: λ≤0.15 | Spectroscopy (300-2500nm) / emissometer method / thermal conductivity tester / thermal insulation temperature difference test device | Building exterior walls / roofs “China — Building — Energy Saving — Mandatory — Standard” |
| JG/T 235-2014 | Architectural Reflective Thermal Insulation Coatings (Industry Standard) | Solar reflectance / near-infrared reflectance / hemispherical emissivity / lightness L* | Introduces lightness grading — colored coatings can also be tested — near-infrared reflectance ≥0.80 | UV-Vis-NIR spectrophotometer / D&S AE1 emissometer | Building exterior walls “Introduces — Lightness — Concept — Colored — Coatings — No Longer” cannot — be tested “” |
| JGJ/T 359-2015 | Technical Specification for Application of Architectural Reflective Thermal Insulation Coatings | Equivalent thermal resistance / solar radiation absorption coefficient / construction process | Equivalent thermal resistance calculated from SR and HE — layered construction — >168h curing | Equivalent thermal resistance calculation method / on-site random inspection | New / existing building energy retrofit “Design — Construction — Acceptance — Full Process” |
| ASTM C1371 | Standard Test Method for Determination of Emittance of Materials Using Portable Emissometers (USA) | Hemispherical emissivity — temperature difference method | >0.85 is excellent — D&S AE1 type emissometer | Thermopile heating detection — linear relationship between temperature difference and emissivity | Internationally recognized “Portable — On-site — Also — Measurable” |
FAQ
Q1: What is the difference between thermal insulation coating and insulation board—why “2mm coating = 5cm insulation board”?Insulation board (EPS/XPS—thermal conductivity ≈0.03-0.04W/m·K—thickness >5-10cm) relies on “thickness” to block heat conduction—”passive blocking”. Thermal insulation coating (composite type—solar reflection + thermal radiation + thermal barrier) relies on “triple mechanism” (1) Reflection—>85% solar radiation—”fundamentally prevents heat from entering the coating” (2) Radiation—actively dissipates the small amount of absorbed heat (3) Barrier—remaining heat is blocked by HGM/aerogel maze—coating “rejects heat from the source” from entering; insulation board only “blocks” the conduction after entry. Different mechanisms—coating is more efficient—2mm under direct sunlight conditions is equivalent to the cooling effect of 5cm insulation board—but in winter with no sunlight, the coating has no insulation effect—needs to be used with an insulation system.
Q2: Why can hollow glass microspheres (HGM) block heat conduction? How to quantify the “maze effect”?HGM are hollow glass spheres with a diameter of 10-150μm and a wall thickness of 1-2μm—the internal sealed cavity contains rarefied gas (thermal conductivity ≈0.006W/m·K—far lower than that of the resin base ≈0.2-0.5). When heat flow encounters HGM in the coating, it must “detour” and conduct along the outer wall of the microspheres—the actual heat conduction path length is 5-10 times the coating thickness—this is the “maze effect”. At the same time, the rarefied gas inside the microspheres hardly conducts heat—the “thermal bridge” is “interrupted” by the “hollow” structure. The particle size of HGM requires a “three-level gradation”—large beads (100-150μm) provide the barrier skeleton—medium beads (50-80μm) fill the gaps—small beads (1-20μm) fill the submicron voids—maximizing packing density—minimizing heat conduction pathways. Adding 10-20wt%—can reduce the coating thermal conductivity from >0.5 to 90%.
Q3: SiO2 aerogel—why is its thermal conductivity lower than that of air (0.026), the “lowest among solids” material?Aerogel’s nanoporous structure (pore size <50nm—smaller than the mean free path of air molecules ≈70nm)—air molecules are "imprisoned" in the nanopores and cannot perform convective heat transfer—"air—loses—convective—ability—only—extremely—low—gaseous—thermal—conduction—remains". Aerogel's solid SiO2 skeleton occupies <10% of volume—"solid—thermal—conduction—path—extremely—long—and—narrow". Combined effect: aerogel's thermal conductivity (0.015-0.02W/m·K) is even lower than that of still air (0.026)—it is one of the solid materials with the lowest thermal conductivity in nature. The latest 2024 study on PVA/SiO2 composite aerogel + hollow polymer microsphere (HPM) coating—thermal conductivity as low as 0.0277W/m·K—2mm coating thickness provides thermal insulation temperature difference up to 40°C—outdoor experimental house temperature change rate reduced by about 40%. Adding only 1.5-3% aerogel to thermal insulation coatings can reduce the coating's thermal conductivity by 30-40%—"with minimal addition—huge insulation improvement is gained".
Q4: Reflective heat insulation—why is the near-infrared reflectance of rutile TiO2 >85%—while that of anatase is only >70%?Rutile TiO2 has a denser crystal structure—lattice vibrations scatter near-infrared photons (700-2500nm) more efficiently—and surface Al2O3/SiO2 coating treatment further reduces UV absorption, “turning TiO2—from—light-absorbing—to—light-reflecting”. Anatase TiO2 has a slightly wider band gap (3.2eV) than rutile (3.0eV)—a larger absorption cross-section for near-infrared photons—some near-infrared photons are absorbed rather than reflected—reflectance is 15-20 percentage points lower. Outdoor heat-insulating coatings must use rutile TiO2: “hiding power = reflection—heat insulation = rutile”. The high-lightness (L*>95) reflective insulating flat topcoat required by GB/T 25261-2018—solar reflectance ≥0.85—must use a combination of rutile TiO2 + infrared reflective pigments.
Q5: Construction of thermal insulation coating — why the “four-layer system” is indispensable — what does each layer do?The construction system of thermal insulation coating is usually four layers: ① Substrate treatment — clean the base surface — leveling — achieve bond strength “base surface — not — firm — = — coating — hollow — drum”; ② Putty layer — layered construction — each layer fully dried — leveling + crack resistance “smooth — surface — = — reflection — efficiency — maximum”; ③ Mid-coat paint (key thermal insulation layer) — contains HGM/aerogel — thickness 1.0-2.0mm — layered application — 24 hours interval between coats “thermal — conductivity — = — determined — by — the — thickness — and — filler — of — the — mid-coat” this is — the — core — of — the — thermal — insulation — performance — of — the — entire — system”; ④ Topcoat layer (reflection + decoration) — high brightness — uniform — spraying — control viscosity and spray gun pressure — no bare base — no sagging — recommend matching finishing layer (improve mildew resistance / anti-soiling / anti-UV aging) — cure at least 168 hours (7 days) after painting “7 days — = — coating — fully — cured — = — performance — stable”. “four-layer — system — = — substrate — (anchoring) — + — mid-coat — (blocking — heat) — + — topcoat — (reflection) — + — finishing — (protection) — missing — any — layer — thermal — insulation — performance — discounted — > — 50 — %”.
Q6: Energy-saving renovation of existing buildings—why is thermal insulation coating more suitable than insulation boards? The energy-saving renovation of existing buildings (old communities/historical buildings) faces: exterior walls cannot be damaged (cannot attach insulation boards—alters appearance—requires approval), indoor space cannot be reduced (internal insulation occupies >5-10cm per wall of indoor area), construction cannot affect residents’ normal lives. Thermal insulation coating “directly—applied—on—exterior wall—/—roof—surface” does not change—building—appearance—(same-color—coating)—does—not—occupy—indoor—space” construction—simple—(brush—/—roll—/—spray)” can—cover—hundreds—of—square—meters—per—day” does—not—affect—residents” “it—is—precisely—these—”cannot”—”that—make—thermal—insulation—coating—the—”only—feasible” solution—for—energy-saving—renovation—of—existing—buildings”. JGJ/T 359-2015 provides an equivalent thermal resistance calculation method—at—the—design—stage—the—energy-saving—effect—can—be—estimated.
Q7: How to test the “thermal insulation temperature difference” of heat-insulating coatings—why is it the core indicator to distinguish “real” from “fake” heat-insulating coatings?Thermal insulation temperature difference test—using artificial simulated light source (AM 1.5 solar spectrum)—measuring the difference between the temperature of the reference black board and the metal surface temperature on the side of the test panel facing away from the heat source. A ordinary white coating—without HGM/aerogel—its solar reflectance can also be >0.80—but it “has no—barrier—function” heat—still—passes—through—the coating—conducting—to the substrate” thermal insulation temperature difference—is very small—(—15—-—40°C” and “fake—insulation—(ordinary—white—coating)—temperature difference—<—5—°—C"". Industry experts call for—incorporating thermal insulation temperature difference (or insulation index) into—GB—/—T—2—5—2—6—1—as a—mandatory—inspection—item" no—thermal insulation—temperature difference—data—=—cannot—be called—heat-insulating—coating".
Q8: Aerogel thermal insulation coating—why is the price high—but the whole-life-cycle cost may be lower?Aerogel price >300 RMB/kg—addition level 1.5-3%—makes the material cost of thermal insulation coating >2-3 times higher than ordinary coating “First—time—application—material—cost—is—higher”. But—whole—life—cycle—(>—10—years): the thermal insulation temperature difference of aerogel—coating—(>—30—-—40°C)—is—>—50%—higher—than—HGM—coating—(>—15—-—25°C) “Every—year—in—summer—air—conditioning—saves—>—3—0—-—5—0—%—electricity—bill” “10—years—electricity—bill—savings—far—exceed—the—initial—coating—price—difference” “High—end—buildings—=—aerogel” “Ordinary—buildings—=—H—G—M” “Selection—=—a—calculation—of—initial—cost—vs—long—term—return”.
Q9: Thermal insulation coatings on industrial pipelines and equipment—how to apply—and how do they differ from architectural coatings?Industrial pipelines (steam pipes/storage tanks/reaction kettles)—temperature >100-300°C (higher than buildings’ “normal temperature”)”Industrial—thermal insulation—=—prevent—heat—from—pipelines—loss—to—environment” (heat preservation)—while—buildings—=—prevent—solar—heat—from—entering—indoors—(thermal insulation)”opposite direction”. Industrial—thermal insulation—coatings—usually—adopt “barrier-type” formulations (HGM/aerogel)—reflection—and—radiation—mechanisms—inside—internal—pipelines (no sunlight)—not applicable”require—thermal conductivity—lower (200°C (silicone resin binder). JG/T 517 (hollow glass microsphere thermal insulation materials)—and—HG/T 4341 (heat-reflective thermal insulation coatings for metal surfaces)—are—relevant—standards—for—industrial—thermal insulation—coatings.
Q10: Heat-insulating coating — Winter — useful —? Will it — instead — make — the house — colder —?The reflection mechanism of heat-insulating coating — only — reflects — solar — near-infrared — radiation — (wavelength — 0.7 — -2.5 — μm) — for — indoor — heating — dissipation — of — long-wave — infrared — (> — 10 — μm) — almost — does not — reflect — will not — reflect — indoor — heat — back — but — in winter — no — sunlight — = — reflection — useless — heat-insulating — coating — only — the — blocking — mechanism — in — winter — plays — a — heat — preservation — role — (reduces — wall — thermal — conduction) — heat-insulating — coating — = — summer — energy — saving — > — winter — neutral — winter — also — will not — have — negative — impact — but — cannot — replace — insulation — system —.
Summary
Thermal insulation coatings—through the synergy of three mechanisms: solar reflectance (TiO₂—>85% reflection) + infrared thermal radiation (SiC—active heat dissipation) + thermal barrier (HGM/aerogel—maze-like extension of heat conduction path—thermal conductivity <0.03 W/m·K)—achieve "2 mm coating = 5 cm insulation board—no space occupation—no change to appearance—easy application". The three standards GB/T 25261-2018 (solar reflectance ≥0.85/hemispherical emissivity ≥0.85/thermal conductivity ≤0.15), JG/T 235-2014 (brightness grading—colored coatings detectable) and JGJ/T 359-2015 (design, construction and acceptance—equivalent thermal resistance method) constitute the complete standard system for China's architectural thermal insulation coatings. Energy-saving retrofitting of existing buildings, with the unique advantages of "no damage to exterior walls/no occupation of indoor space", is the largest incremental market for thermal insulation coatings. Kexin New Materials provides customers with coating selection, formula optimization and energy-saving effect evaluation, "dressing every building in a summer energy-saving cool coat".