Insulation calculation with IsoDim and DS 452

Determine whether the element to be insulated is a pipe, a duct or a flat surface. Pipes are used for anything containing a liquid or solid mass. Ducts are used for air, flue gas or gas. Pipes and ducts with a diameter above Ø400 mm are calculated as a flat surface, as the standard's formulae are not accurate enough for larger sizes. Rectangular ducts are converted to an equivalent circular diameter by dividing the total perimeter by pi (3.14).

If the duct is rectangular, the equivalent circular diameter is calculated as the duct's total perimeter divided by 3.14. Example: a 200×200 mm duct has a perimeter of 800 mm, which corresponds to a diameter of approx. Ø255 mm. This value is entered in IsoDim as a round duct.

Specify exactly where the installation is located: inside (within the building insulation, corresponding to a heated room of min. 5 °C), outside but under a roof (outdoor temperature without wind effect), or above the roof/outdoors (outdoor temperature with wind). The location determines the ambient temperatures and has a direct effect on the insulation class. As a rule of thumb, 20 °C inside and an average annual temperature of 8 °C outside are used for energy calculations.

The insulation classes are set out in DS 452, table 6.4, and are not negotiable. The class depends on the type of installation (e.g. heating system, domestic water, ventilation), not on the operating time. For industrial and process installations, the class is calculated from the operating time, and here the surface temperature — not the energy consumption — is typically the governing requirement.

Uninsulated supports that penetrate the insulation result in a calculation deduction in insulating performance: 15 % for indoor locations and 25 % for outdoor locations. An insulated support must have the same or better insulating performance as the surrounding insulation — a thin 30 mm support is not sufficient if the pipe section is 100 mm.

Specify whether the pipe is installed horizontally or vertically. Vertical pipes typically have a slightly higher insulation requirement because of natural convection — the natural air flow that the hot pipe itself generates along its surface. The difference is usually under 1 mm, but in borderline cases it can mean going up one insulation thickness.

The surface to be specified is the outermost finishing surface — for example a PVC jacket or an aluminium foil. Bright aluminium reflects ambient radiation and raises the surface temperature, which saves insulation thickness on hot pipes, but is unfavourable in condensation calculations. For calculations, 'alu-zinc' (matt aluminium) is recommended, as the surface oxidises over time and attracts dust. The choice of surface applies for the entire service life of the installation, which can be up to 30 years.

For ventilation ducts, the air velocity must be stated in m/s. The lower the air velocity, the greater the surface resistance (surface thermal resistance) between the air and the duct wall — and the more insulation is technically available. Ducts will therefore always require a smaller insulation thickness than equivalent pipes. The flow can be entered in m/s, m³/h or l/s directly in IsoDim. Note that flat surfaces (ducts over Ø400 mm) do not allow flow to be entered.

For energy and class requirements, average temperatures for a whole year are used: typically 20 °C indoors and 8 °C outdoors. For condensation and surface temperatures, the worst case is always calculated — the coldest or hottest conceivable moment. It is wrong to use, for example, -12 °C as the ambient temperature in energy calculations, as this merely gives an artificially improved lambda value and thus a lower — and incorrect — insulation thickness.

Select the relevant insulation product from IsoDim's product list. All products have a maximum service temperature, and the programme gives a warning if the medium temperature exceeds it. Technical insulation has a lambda curve showing the insulating performance at different temperatures — building insulation has only one lambda value (at 10 °C) and is not approved for technical insulation and must not be used for it.

IsoDim calculates continuously and shows the required U-value from DS 452 as well as the corresponding minimum insulation thickness. Minimum requirements are always rounded up to the next available product thickness — never rounded down. If the calculated requirement exceeds the largest available pipe section, a double layer can be used: first calculate the U-value requirement, and then calculate which combination of two product layers meets it.

External condensation occurs on cold pipes (e.g. refrigerant pipes) where the surface temperature falls below the dew point. Enter the medium temperature, ambient temperature and relative humidity. In standard HVAC installations the condensation problem is most often solved with 20 mm insulation. In special conditions such as swimming pools or greenhouses (high relative humidity up to 85 %), 30 mm or more may be necessary. Internal condensation is calculated in the same way.

The 'Flowing media' module calculates the temperature drop for a medium moving through a pipe system divided into sections — useful for e.g. process plants, where the medium must not fall below a critical temperature. The 'Temperature in tanks' module calculates how quickly a tank's contents cool over time under given insulation and weather conditions, and is relevant for sizing the standby time during shutdowns.