Thermal insulation is a layer or component that slows the flow of heat between two areas at different temperatures by obstructing conduction, convection and thermal radiation. How well it does this is described by thermal conductivity λ: insulation materials sit below 0.1 W/(m·K), mineral wool at 0.035 to 0.045, and an aerogel blanket at 0.020 W/(m·K) at 0 °C according to the manufacturer data sheet. Plastic films for electrical insulation, at 0.12 to 0.2 W/(m·K), let six to ten times more heat through. In industrial plants, 15 to 21 percent of the fuel is lost as heat, according to the Austrian guideline on technical insulation. This article shows how to calculate the heat flow through a layer, which three calculation errors become expensive in practice, and which thin thermal layers sit inside devices, cut and die-cut by GOBA from its range of insulation materials.
Lagging, insulating, separating: what each term means
In German technical usage, heat is damped (gedämmt) and current is insulated (isoliert). English uses insulation for both, and data sheets add thermal lagging, heat insulation and thermal protection for the same thing. That is fine as long as it is clear which job the layer has to do.
For purchasing, the difference is tangible. You select thermal insulation by λ and service temperature, and electrical insulation by dielectric strength and thermal class. Materials that block voltage at high temperature are covered in the article on high-temperature insulation, and both worlds side by side are compared in the article on insulating material. This page is about heat flow at two scales: the pipe with 20 to 100 mm of insulation, and the device, where a thermal layer can rarely be thicker than a few millimetres.
How thermal insulation works: three heat paths, three countermeasures
Heat always flows from hot to cold, along three paths at once. An insulation material works because it traps still air in pores or between fibres and blocks radiation with its solid skeleton. That is how the technical insulation guideline describes it, with reference to VDI 2055. Each heat path has its own countermeasure and its own property value:
| Heat path | What happens | What slows it | Property value |
|---|---|---|---|
| Conduction | energy passes from molecule to molecule through a solid or still gas | little solid, plenty of trapped air (still air: 0.026 W/(m·K)) | thermal conductivity λ in W/(m·K) |
| Convection | moving air or liquid carries the heat along | fine pores and fibres that hold the air in place, tight joints | heat transfer coefficient h in W/(m²·K) |
| Thermal radiation | electromagnetic waves, even through a vacuum | bright, reflective surfaces | emissivity ε between 0 and 1 |
Radiation is the path most often underestimated. Under the Stefan-Boltzmann law it grows with the fourth power of absolute temperature, which makes it the largest item on hot surfaces. According to the guideline, bright aluminium foil has an emissivity of 0.05, dusty galvanised steel 0.44. The foil therefore radiates almost nine times less. That is why heat protection panels between a cooker and a fridge are aluminium-laminated, such as the 3 mm Isoplan panel rated for 750 °C, and why heat shields on exhaust systems are made of bright aluminium.
How much heat passes through thermal insulation
For a flat layer the relationship is simple: heat flux density q in W/m² equals λ times the temperature difference ΔT divided by the thickness d. The reciprocal is easier to work with, the thermal resistance R = d / λ in m²·K/W. Several layers add up their resistances. The U-value used in building physics also includes the transfer to the adjacent air. The method in five steps:
- Define the temperature on the hot and the cold side and take the mean temperature.
- Read λ at that mean temperature from the data sheet, not the room-temperature value.
- Calculate R = d / λ for each layer and add the resistances.
- Determine the heat flux density as q = ΔT / R.
- Add allowances for joints, fixings and thermal bridges.
A worked example with a 10 mm aerogel blanket: the hot side is at 400 °C, the cold side at 20 °C. At a mean temperature of about 210 °C, the Pyrogel XTE data sheet gives λ of 0.028 W/(m·K). That gives R = 0.010 / 0.028 = 0.36 m²·K/W and a heat flow of 380 K / 0.36 = about 1,060 W/m². A 0.1 mm polyester film with λ = 0.2 W/(m·K) reaches R = 0.0005 m²·K/W, roughly 700 times less. An electrical insulating film hardly slows heat at all. In a motor this is exactly what you want, because the loss heat has to get out of the winding. With values for Kapton, Nomex and PET, the article on thermal conductivity shows how much the film thickness governs heat transmission.
Three calculation errors that make thermal insulation expensive
The formula is quick to write down. The errors hide in the input values, and all three push the same way: the layer lets through more heat than planned.
Lambda rises with temperature
Data sheets like to lead with λ at 0 °C or 10 °C. On the hot side that value does not apply. The aerogel blanket Pyrogel XTE, measured to ASTM C177, shows the trend clearly:
| Mean temperature | λ in mW/(m·K) |
|---|---|
| 0 °C | 20 |
| 100 °C | 23 |
| 200 °C | 28 |
| 300 °C | 35 |
| 400 °C | 46 |
| 500 °C | 64 |
| 600 °C | 89 |
Between 0 °C and 600 °C, λ rises to four and a half times its starting value. Using the 0 °C value in the example above gives 760 instead of 1,060 W/m² and underestimates the heat flow by almost 30 percent. Ceramic fibre behaves in a similar way: 0.09 W/(m·K) at 400 °C, 0.48 at 1,400 °C.
The data sheet value is not the operating value
The λ on a data sheet is a nominal value measured on a clean sample. Once installed, open joints, compressed areas and convection inside the insulation add to it. The technical insulation guideline works through a vessel with two-layer insulation according to VDI 2055-1: the allowances raise λ by more than 17 percent, and with supports and fixings as thermal bridges by almost 28 percent. For uninsulated fittings it gives a rule of thumb: a flange loses as much heat as half a metre of bare pipe of the same nominal size, a valve as much as a full metre.
On thin lines, insulation increases heat loss
Around a pipe or a wire, every added layer increases the surface that releases heat to the air. Below the critical radius r = λ / h this effect wins, and more insulation increases heat loss. A worked example: a plastic with λ = 0.2 W/(m·K) and an assumed heat transfer to still air of 10 W/(m²·K) give a critical radius of 20 mm. For cable insulation this is good news, because the plastic sheath helps the conductor shed its Joule heat. If a thin hot line really needs to hold its heat, it needs a material with very low λ. With aerogel (λ = 0.02) the critical radius drops to 2 mm.
Thermal insulation of plant and pipework: standards and minimum thickness
In industrial plants, operating temperatures range from minus 160 °C to above 600 °C. There, insulation decides energy costs and worker safety. According to a study by the Forschungsstelle für Energiewirtschaft with the European Industrial Insulation Foundation, up to 10 percent of plant components are uninsulated or damaged, and insulating those parts alone cuts heat losses by 23 percent. In Germany, these rules apply:
- VDI 2055 for calculating heat loss and economic insulation thickness
- DIN 4140 for the execution of insulation work on industrial installations
- AGI worksheets such as Q 101 for insulation work on power plant components
- VDI 4610 for the energy efficiency of existing plants, including a thermal bridge catalogue
For heating and hot water pipes in buildings, Annex 8 of the German Building Energy Act sets minimum thicknesses, based on an insulation material with λ = 0.035 W/(m·K):
| Inner diameter of the pipe | Minimum insulation thickness |
|---|---|
| up to 22 mm | 20 mm |
| above 22 to 35 mm | 30 mm |
| above 35 to 100 mm | equal to inner diameter |
| above 100 mm | 100 mm |
Materials with a higher λ need correspondingly more thickness. For touch protection, the guideline targets surface temperatures of about 50 to 60 °C. Where insulation cannot achieve that, perforated guard sheets to DIN 4140 go in front. Pipe sections, wired mats and insulation jackets come from insulation manufacturers and specialist insulation contractors. GOBA does not supply them, and anyone insulating a plant is best served by a VDI 2055 calculation and a specialist contractor.
Thermal insulation inside devices: millimetres instead of centimetres
Inside a device there is no room for 100 mm of mineral wool. The heating element sits a few millimetres from the control board, the battery cell a few millimetres from the next one. The thermal layer has to be thin, fit a contour and often block voltage at the same time. Typical locations for which we supply cut and die-cut parts:
- Heat shields and separating layers between heating element and electronics in dishwashers, ovens and coffee machines.
- Mica carrier plates for heating coils that withstand heat and keep voltage away from the housing.
- Cell-to-cell barriers in battery modules made of aerogel composite and mica, described in the article on thermal insulation in battery systems.
- Heat protection textiles for the engine compartment, made to size.
- Insulating strips of PE or EPDM foam for thermal decoupling in ventilation and air conditioning units.
Five material groups are candidates for these layers. They differ by more than a factor of 30 in thermal conductivity and in whether they may block voltage:
| Material | Thermal conductivity | Temperature limit | Blocks voltage | Source |
|---|---|---|---|---|
| Aerogel fibre blanket (Pyrogel XTE) | 0.020 W/(m·K) at 0 °C, 0.028 at 200 °C | 650 °C | not rated for it | Aspen data sheet |
| Foams (EPS, PU) | 0.025 to 0.040 W/(m·K) | EPS up to about 80 °C, PUR up to about 130 °C | not rated for it | GOBA glossary, insulating material |
| Aramid paper (Nomex) | about 0.14 W/(m·K) | recognised by UL as a 220 °C insulating material | yes, 17 to 33 kV/mm depending on thickness | DuPont Technical Data Sheet |
| Mica plate (micanite, K-Therm AS M) | 0.26 to 0.75 W/(m·K) at 100 °C | continuous 500 to 700 °C, short-term up to 800 °C | yes, up to 25 kV/mm | AGK material data |
| Ceramic fibre and ceramic paper | 0.09 W/(m·K) at 400 °C | classification temperature from 1,260 °C to EN 1094 | no published breakdown values | EN 1094, GOBA glossary, insulating material |
The table reveals a conflict that often gets lost in the specification. Mica conducts heat ten to thirty times better than aerogel. It is a heat shield against the temperature peak and an electrical insulator, but a poor thermal insulator. Aerogel insulates best of all five, yet it is not tested as an electrical insulating material. Where both are needed, the answer is a composite: an aerogel core for the heat flow, a mica layer against the peak and the voltage. We therefore recommend designing the thermal and the electrical task separately and only then choosing the material. Above 1,000 °C only ceramic fibre, calcium silicate and phlogopite mica remain, and the article on high-temperature insulation sorts materials by thermal class.
How GOBA produces thermal layers
Aerogel blankets, mica plates and foams arrive from the manufacturer as rolls or sheets. We slit rolls from 5 to 1,600 mm wide with a width tolerance of plus minus 0.1 mm and die-cut parts from 0.023 to 3.0 mm material thickness up to 1,000 x 2,000 mm, using steel rule or solid steel tools. Complex geometries are formed under heat. Each material needs its own handling. Aerogel sheds dust and chips at the edge, so we run the knife shaft more slowly. Rigid mica plates can be die-cut, but the tool needs a hold-down, otherwise the mica splits.
An objection we often hear: is cutting worth it if the blanket arrives as a roll anyway? Inside a device, yes. The layer needs cut-outs for screws, sensors and cables, an edge that does not shed dust, and a size that goes into assembly without rework. That saves more on the line than the cutting costs. Examples of appliance parts are on the industry page household appliances, and for battery modules on the page on battery film slitting. Tools, tolerances and delivery forms for die-cut parts are described under stamping and forming.
Selecting thermal insulation: five questions before you enquire
With these five answers we can design a thermal layer for your device and produce a sample:
- What temperature acts at the hottest point, continuously and short-term?
- How much installation space is available? The thickness determines how low λ has to be.
- Does the layer also have to block voltage, and which thermal class does the application require?
- Which mechanical load applies: contact pressure, vibration, swelling of a cell?
- Which fire rating does the appliance standard require, and may the material shed dust?
The second question decides most often. If the space is limited to 2 mm, a foam layer is not enough, and the path leads to aerogel or a composite. Send us your drawing, temperatures and installation space, and you will receive a technical quotation with a material proposal.
