Thermal Insulation · Vacuum Insulation Panels · Core Materials

Fumed Silica for Vacuum Insulation Panels & Thermal Insulation

Vacuum insulation panels demand core materials that maintain ultralow thermal conductivity under long-term vacuum conditions and resist structural collapse. Fumed silica — with its sub-micron particle network, surface area exceeding 200 m²/g, and intrinsic opacity to infrared radiation — is the benchmark VIP core material for building envelope, cold chain logistics, and high-efficiency appliances. SEMISIL 200 and SEMISIL 300 deliver engineered porosity, controlled tap density, and compliance-ready particle-size distributions for demanding OEM and construction applications.

Vacuum Insulation Panels VIP Core Material Thermal Conductivity Building Energy Efficiency Cold Chain Appliance Insulation Microporous Silica SEMISIL 200 / 300

Overview VIP Performance Applications Specification FAQ

Overview: Why Fumed Silica Is the VIP Core Material of Choice

Vacuum insulation panels achieve thermal resistance values 5–10× greater than conventional rigid foam by evacuating the interstitial gas that carries the majority of conductive and convective heat transfer. The structural integrity of the evacuated core — and its long-term lambda stability — depends entirely on the core fill material’s ability to support the vacuum barrier film without mechanical creep while simultaneously suppressing the residual radiative and solid conduction pathways.

Fumed silica satisfies all three requirements simultaneously. Its three-dimensional chain-aggregate microstructure — primary particles of 5–40 nm fused into aggregates of 100–300 nm — creates a tortuous, open-pore network with pore diameters well below the mean free path of air at modest vacuum levels (1–10 mbar). This Knudsen suppression mechanism reduces gas-phase conduction by more than 90% even before the vacuum pump-down stage reaches ultimate pressure, providing a critical processing tolerance margin for panel manufacturers.

Knudsen Effect at work: When pore diameter falls below the mean free path of the gas molecules (~70 nm at atmospheric pressure, extending to several micrometers at 10 mbar), gas-phase thermal conductivity drops sharply. Fumed silica’s aggregate network operates in this regime, making VIP performance far less sensitive to residual vacuum level than glass-fiber or open-cell foam cores.

In contrast, expanded polystyrene (EPS) and polyurethane foam (PU) — the dominant alternatives — cannot exploit the Knudsen effect because their cell diameters (50–300 µm for EPS; 100–400 µm for PU) are orders of magnitude too large. EPS achieves λ ≈ 0.033–0.038 W/mK at atmospheric pressure; PU achieves λ ≈ 0.022–0.028 W/mK with blowing agents. Fumed silica VIP cores achieve λ = 0.003–0.008 W/mK under operating vacuum, delivering insulation performance in panels 5–8× thinner for equivalent thermal resistance — a decisive advantage in space-constrained building facades, refrigerators, and insulated shipping containers.

Ultralow Thermal Conductivity

λ = 0.003–0.008 W/mK under vacuum, compared to 0.033–0.038 W/mK for EPS and 0.022–0.028 W/mK for PU foam. Panels as thin as 20 mm deliver R-values equivalent to 150 mm of mineral wool.

Infrared Opacity

Fumed silica nanoparticles absorb and scatter mid-infrared radiation (3–8 µm wavelength), the dominant radiative heat transfer band at near-ambient temperatures. Radiation accounts for up to 30% of total heat transfer in evacuated panels at 25 °C — fumed silica directly suppresses this pathway.

Mechanical Stability Under Vacuum

The sintered aggregate network resists compressive creep under the atmospheric pressure load (~10 kN/m² per 1 bar differential) applied to the VIP barrier film. Low tap density (50–150 g/L) enables panel production without high-force compaction equipment.

Chemical Inertness & Moisture Tolerance

Amorphous SiO₂ is inert to alkaline building materials (concrete, lime plaster) and acidic refrigerant decomposition products. Surface silanol groups can be controlled via post-treatment to manage moisture adsorption — critical for VIP service life projections of 25–50 years.

Vacuum Retention Compatibility

Low outgassing rate and controlled BET surface area allow manufacturers to reach target vacuum levels (0.1–10 mbar) with standard rotary-vane or dry-screw pumping systems, without the cryopump investment required for glass-fiber cores.

Processability for Panel Manufacture

Free-flowing powder morphology allows automated filling of laminate envelopes. Controlled particle size (D50 typically 5–20 µm for aggregates) avoids puncture risk to barrier films and supports high fill-line throughput with standard gravimetric dosing systems.

VIP Core Performance: Thermal Conductivity & Lambda Stability

The total effective thermal conductivity (λ_eff) of a fumed silica VIP core is the sum of three parallel pathways: solid conduction through the particle network (λ_solid), gas-phase conduction through residual air (λ_gas), and radiative transfer (λ_rad). SEMISIL 300 is engineered to minimize all three simultaneously.

Solid Conduction

Despite the high surface area of fumed silica, inter-aggregate contact area is extremely small — fused primary particle necks are narrow, and aggregate-to-aggregate contact is point-like. This tortuous, highly resistive solid pathway gives fumed silica VIP cores λ_solid values of 0.005–0.010 W/mK at atmospheric conditions, compared to 0.020–0.040 W/mK for glass fiber mats. Increasing BET surface area from 200 to 300 m²/g further fragments the solid network, reducing λ_solid by approximately 15–20%.

Gas-Phase Conduction and the Knudsen Effect

At atmospheric pressure, gas-phase conduction through air accounts for ~60–70% of total thermal conductivity in porous media. For fumed silica VIP cores, the characteristic pore diameter (d_p = 50–200 nm) is comparable to the molecular mean free path at pressures well above 1 mbar, meaning Knudsen suppression activates at readily achievable vacuum levels. The critical pressure (p_c), at which gas-phase conduction is halved, is approximately 200–600 mbar for SEMISIL 300 — an order of magnitude higher than for glass-fiber or open-cell foam cores. This means VIP panels using SEMISIL 300 tolerate micro-leaks and remain performant far longer before service lambda exceeds specification thresholds.

Critical pressure advantage: For SEMISIL 300 (300 m²/g, d_p ≈ 50–100 nm), p_c ≈ 200–400 mbar. For a glass-fiber core (d_p ≈ 50 µm), p_c ≈ 0.5–2 mbar. A VIP panel sealed at 0.5 mbar will perform well with fumed silica even if internal pressure rises to 5–10 mbar over 10 years; the same pressure rise would cause immediate lambda exceedance in a glass-fiber panel.

Radiative Transfer Suppression

At 25 °C, radiative heat transfer contributes 20–35% of total λ_eff in evacuated silica cores, depending on panel temperature differential and thickness. Fumed silica absorbs infrared radiation strongly in the 8–12 µm atmospheric window due to Si-O-Si stretching modes, providing inherent opacity without opacifier additives. For applications requiring λ < 0.004 W/mK at 25 °C — such as high-performance building façade panels — SEMISIL 300 can be blended with silicon carbide (SiC) or carbon black opacifiers at 2–8 wt% to extend opacity into the 3–6 µm band, reducing λ_rad by 40–60%.

Lambda Values by Application Condition

Condition	Internal Pressure	Temperature	SEMISIL 200 λ_eff	SEMISIL 300 λ_eff	EPS Reference
New panel, building envelope	0.1–0.5 mbar	23 °C	0.006–0.008 W/mK	0.003–0.005 W/mK	0.033 W/mK
Aged panel (10 yr), building	2–10 mbar	23 °C	0.008–0.012 W/mK	0.005–0.008 W/mK	0.033 W/mK
Refrigerator/freezer compartment	0.5–5 mbar	−20 °C to +5 °C	0.005–0.007 W/mK	0.003–0.005 W/mK	0.033 W/mK
Cold chain container (pharma)	1–5 mbar	2–8 °C	0.005–0.008 W/mK	0.003–0.006 W/mK	0.033 W/mK
High-temp industrial (opacified)	1–10 mbar	200–600 °C	0.015–0.025 W/mK	0.010–0.020 W/mK	Not applicable

Thermal Bridging at Panel Edges

The dominant thermal weakness in VIP assemblies is the metallic barrier film edge, which creates a thermal bridge reducing effective panel R-value by 10–25% depending on panel geometry and installation details. Fumed silica VIP cores themselves do not create edge bridges, but their superior centre-of-panel lambda means the edge correction term represents a larger fraction of total heat loss in thin, square panels. Engineers specifying fumed silica VIP must account for linear thermal transmittance (Ψ-value) of the barrier film edge in compliance calculations per EN ISO 10211 and EN 16012.

Applications by Sector

Fumed silica VIP technology crosses three major market segments, each with distinct performance requirements, panel geometries, regulatory frameworks, and lifetime expectations. SEMISIL 200 and SEMISIL 300 are qualified and in serial production for all three.

Building Envelope & Energy Efficiency

In high-performance building envelopes — passivhaus walls, retrofits of masonry facades, flat roofs, and floor slabs — thickness constraints prevent conventional insulation from achieving target U-values without sacrificing usable floor area. A 20–30 mm fumed silica VIP panel achieves U ≈ 0.10–0.15 W/m²K, equivalent to 150–200 mm of mineral wool. Key application formats include:

Facade Cladding Panels

Pre-assembled panels (600 × 600 mm to 1200 × 600 mm) bonded to ventilated facade systems. SEMISIL 300 enables centre-of-panel λ ≤ 0.005 W/mK, supporting passivhaus compliance with panel thicknesses of 20–40 mm and 25-year design life per EN 16012.

Flat Roof & Floor Slab

Load-bearing VIP elements with compressive strength ≥ 100 kPa under mechanical load. Fumed silica core maintains structural integrity under point loads from foot traffic and ballast layers without lambda degradation — critical for inverted roof assembly.

Prefabricated Wall Elements

Factory-integrated VIP in sandwich panels for modular construction. SEMISIL 300 particle morphology enables automated core filling at production speeds of 20–60 panels/hour with gravimetric dosing — no manual tamping required.

Heritage & Interior Retrofit

Where external insulation is prohibited by planning constraints (listed buildings, conservation areas), internal VIP boards as thin as 15 mm achieve thermal resistance equivalent to 100 mm of PIR, minimising loss of internal room dimensions.

Refrigerators, Freezers & Household Appliances

Appliance manufacturers face competing demands: tighter energy efficiency regulations (EU Regulation 2019/2016, DOE STEP levels), reduced cabinet wall thickness to maximize food storage volume, and 10–20 year product lifetimes. Fumed silica VIP panels in refrigerator side, top, and door panels allow wall thickness reductions of 20–30 mm versus PU foam alternatives while meeting A+++ energy labels. SEMISIL 300 is preferred for appliance-grade VIP due to its lower critical pressure and consequent greater tolerance of micro-seal defects during the panel lamination process at refrigerator production speeds.

Appliance qualification note: Refrigerator OEMs typically require VIP core materials to demonstrate <5% lambda increase after 10-year accelerated ageing (ISO 8318 or manufacturer-specific protocols). SEMISIL 300 consistently meets this threshold across tested vacuum levels of 0.5–5 mbar, with <3% lambda increase in third-party testing.

Cold Chain Logistics & Pharmaceutical Packaging

Temperature-controlled shipping containers for biopharmaceuticals (2–8 °C, −20 °C, −70 °C), fresh food, and specialty chemicals require insulation that is lightweight, dimensionally stable across −80 °C to +50 °C ambient cycles, and reliably performant for single-use or multi-use lifecycles of 3–10 days or 50–200 trip cycles. Fumed silica VIP walls in ISO-compliant shippers reduce thermal mass and tare weight versus expanded polyurethane alternatives, extending thermal autonomy or reducing coolant payload. SEMISIL 200 provides a cost-optimized solution for single-use cold chain formats; SEMISIL 300 is specified for reusable, premium-tier passive shippers.

Industrial & Cryogenic Applications

Beyond ambient applications, opacified fumed silica microporous panels are qualified for high-temperature process pipework (200–600 °C) and cryogenic LNG storage (−162 °C). In LNG tank insulation, the absence of organic binders ensures non-combustibility per EN 13501-1 (Class A2-s1,d0), and the low temperature coefficient of λ ensures consistent performance across the full service range. Custom particle size distributions and opacifier loadings are available on request for these specialty applications.

Product Specification: SEMISIL 200 & SEMISIL 300

Both grades are produced via high-temperature vapor-phase hydrolysis (pyrogenic process), yielding amorphous SiO₂ with precisely controlled BET surface area, aggregate morphology, and particle size distribution. No crystalline silica phases (quartz, cristobalite, tridymite) are present — SEMISIL grades carry a crystalline silica content of <0.1% as certified by XRD analysis per each production lot.

Parameter	Test Method	SEMISIL 200	SEMISIL 300
BET Surface Area	ISO 9277 (N₂)	200 ± 15 m²/g	300 ± 15 m²/g
Average Primary Particle Diameter	TEM image analysis	~12 nm	~7 nm
Aggregate D50 (laser diffraction)	ISO 13320	8–15 µm	5–12 µm
Tap Density	ISO 787-11	100–130 g/L	50–90 g/L
pH (4% aqueous dispersion)	ISO 787-9	3.6–4.5	3.6–4.5
Loss on Drying (105 °C, 2 h)	ISO 787-2	≤1.5%	≤1.5%
Loss on Ignition (1000 °C)	ISO 787-3	≤1.0%	≤1.0%
SiO₂ Content (dry basis)	ICP-OES	≥99.8%	≥99.8%
Fe₂O₃ Content	ICP-OES	≤0.003%	≤0.003%
Crystalline Silica (XRD)	ISO 16258-2	<0.1%	<0.1%
VIP Lambda (centre-of-panel, 0.5 mbar, 23 °C)	EN 12667 / EN 12939	0.005–0.008 W/mK	0.003–0.005 W/mK
Critical Pressure (p_c)	Guarded hot plate	300–500 mbar	200–400 mbar
Recommended VIP Application	—	Cold chain, single-use, cost-sensitive	Building, appliances, premium cold chain

Surface Chemistry & Moisture Management

Standard SEMISIL 200 and SEMISIL 300 carry a high density of surface silanol groups (SiOH, ~2.5–3.5 /nm²), making them inherently hydrophilic. For VIP applications where moisture ingress through barrier films could degrade lambda over service life, a hydrophobic surface treatment variant (SEMISIL 200H / SEMISIL 300H) is available, providing a moisture pickup of <0.5% at 50% RH versus 3–5% for untreated grades. Pre-drying VIP cores prior to panel assembly (typically 120–150 °C, 4–12 h) is mandatory for all grades to reach moisture content <0.5% at point of vacuum sealing.

Packaging & Handling

SEMISIL grades are supplied in sealed polyethylene-lined multiwall kraft bags (10 kg, 15 kg) and bulk bags (500 kg, 1000 kg). Minimum re-order quantities: 1 MT (bags), 5 MT (bulk). Standard lead time 10–15 business days from confirmed purchase order. Certificates of Analysis (CoA) including BET surface area, tap density, particle size D10/D50/D90, SiO₂ purity, and VIP lambda are issued per production lot. REACH-registered (EC No. 231-545-4). SDS available on request.

Handling precaution: Fumed silica is a respirable nuisance dust. When handling in open environments or during panel filling, use P2/FFP2 respiratory protection and follow occupational exposure limits (ACGIH TLV-TWA: 2 mg/m³ inhalable fraction; 0.05 mg/m³ respirable fraction). SEMISIL grades contain no crystalline silica and do not present a silicosis risk — however, dust generation should be minimised using enclosed conveying and gravimetric filling systems.

Frequently Asked Questions

What makes fumed silica superior to glass fiber mats as a VIP core material?

Glass fiber VIP cores rely on conventional conduction-reduction strategies: low bulk density and fiber-to-fiber contact area. Their pore diameters (10–100 µm) are too large for Knudsen suppression at practically achievable vacuum levels, meaning their critical pressure p_c ≈ 0.5–2 mbar — panel performance collapses rapidly if internal pressure rises above 5 mbar. Fumed silica exploits the Knudsen effect at pore diameters of 50–200 nm, with p_c ≈ 200–600 mbar (SEMISIL 300). A fumed silica VIP panel sealed at 0.5 mbar retains ≥80% of its initial lambda advantage over conventional insulation even when internal pressure rises to 5–10 mbar after 10 years of service — a realistic in-wall scenario given barrier film permeation rates. Additionally, fumed silica’s inherent infrared opacity suppresses radiative transfer without additives, whereas glass fiber is transparent to infrared and requires opacifier integration at significant cost. The tradeoff is mass and compressive stiffness: glass fiber VIP panels are lighter (bulk density 150–300 g/L vs 120–200 g/L for fumed silica panels), but fumed silica panels offer better long-term lambda stability in real-world installations.

What is the difference between SEMISIL 200 and SEMISIL 300 for VIP applications, and when should I specify each?

SEMISIL 200 (200 m²/g BET) and SEMISIL 300 (300 m²/g BET) differ primarily in primary particle size (~12 nm vs ~7 nm), tap density (100–130 g/L vs 50–90 g/L), and achievable VIP lambda. SEMISIL 300 produces smaller primary particles, finer aggregates, and a denser surface area network — resulting in a lower critical pressure (200–400 mbar vs 300–500 mbar) and a centre-of-panel lambda 30–40% lower than SEMISIL 200 at equivalent vacuum conditions. Specify SEMISIL 300 for: building envelope panels targeting λ ≤ 0.005 W/mK, premium household appliances with 15-year+ design life, pharmaceutical cold chain shippers requiring multi-use requalification, and any application where panel internal pressure is expected to rise above 2–5 mbar during service life. Specify SEMISIL 200 for: cost-sensitive single-use cold chain boxes, industrial pipe insulation where lambda targets are ≤0.008 W/mK, and applications where production volumes justify higher-density core fill (lower tap density of SEMISIL 300 increases panel fill time by 15–25%). Both grades carry identical purity and regulatory compliance profiles.

How should fumed silica VIP cores be pre-treated before panel assembly to ensure maximum service life?

Moisture is the primary degradation driver for VIP service lambda. Surface silanol groups on fumed silica adsorb atmospheric moisture (3–8 wt% at 50% RH for untreated grades), and water vapor re-released inside a sealed panel raises internal pressure — potentially by 0.5–2 mbar over a 25-year service life. Pre-drying the loose fill or pre-formed core boards immediately before vacuum sealing is therefore mandatory. Recommended protocol: heat core material at 120–150 °C for 4–12 hours in a convection oven, handling exclusively under <10% RH conditions thereafter, with time-to-seal ≤60 minutes after removal from the oven. For hydrophobic grades (SEMISIL 200H / SEMISIL 300H), surface treatment reduces equilibrium moisture pickup to <0.5% at 50% RH, significantly relaxing drying protocol severity (90–120 °C, 2–4 h) and extending allowable time-to-seal to 2–4 hours. In-line moisture monitoring of the filling line environment and core material via Karl Fischer titration is recommended for high-volume panel production to ensure lot-to-lot consistency. SEMISIL technical support can provide detailed drying protocol validation data specific to your panel format and barrier film permeation rate model.

Are SEMISIL grades compliant with REACH, RoHS, and building product regulations in the EU?

Yes. Amorphous fumed silica (EC No. 231-545-4, CAS No. 7631-86-9) is REACH-registered under the current dossier and is not listed as an SVHC (Substance of Very High Concern). Both SEMISIL 200 and SEMISIL 300 are free of crystalline silica (<0.1% per lot XRD certification), which is the substance subject to REACH Annex XVII restriction No. 75 in occupational contexts. Under EU Construction Products Regulation (CPR 305/2011), fumed silica VIP cores are incorporated into products covered by EN 16012 (Thermal insulation for buildings — Reflective insulation products) and EN 13165 / EN 13167 harmonized standards by analogy. RoHS (2011/65/EU) Annex II restricted substances are not present. Full regulatory dossiers, REACH SDS, and Declaration of Performance templates are available from the technical sales team upon request under NDA where applicable.

Can SEMISIL fumed silica be used in non-evacuated microporous insulation boards?

Yes — and this is an important secondary application segment. Microporous silica insulation boards (also called microporous panels or MPS boards) combine fumed silica with opacifiers (SiC, titanium dioxide, or carbon black at 10–30 wt%) and fiber reinforcement, pressed at 2–5 MPa to bulk densities of 200–350 g/L. Without evacuation, Knudsen suppression is not operative at atmospheric pressure; however, the combination of low solid conductivity, opacifier-enhanced infrared opacity, and silica’s low temperature conductivity yields λ ≈ 0.018–0.025 W/mK at 200 °C and λ ≈ 0.030–0.040 W/mK at 25 °C — better than mineral wool at elevated temperatures. These boards are used in industrial furnace linings, backup insulation behind fibrous blanket in high-temperature kilns, and fire-rated building panels requiring A1 non-combustibility (EN 13501-1). SEMISIL 300 is preferred for microporous boards due to its higher surface area, which enhances Knudsen effects at the micropore scale (5–20 nm intra-aggregate pores remain within the Knudsen regime even at atmospheric pressure) and provides superior opacifier particle separation. Contact our technical team for recommended formulation ranges and pressing protocols.