Air Dome U-Value: Real Measurements and Heating Costs

For sports clubs, municipalities and designers comparing air dome heating costs, insulation claims are useful only when the measurement boundary is clear. This article explains the effective U-value, presents five field results and shows how the same coefficient enters heating-load and seasonal energy calculations. For fuel and equipment choices, see our separate guide to air dome heating systems.

Air dome running costs: why heating matters

The purchase price of an air dome is a one-off number and it is easy to compare. What the operator lives with for the next fifteen years is the running cost, and that is the part most quotations describe least clearly.

For a heated sports hall the running cost is dominated by two energy lines:

• Electricity — the pressurisation fan, which runs continuously, plus lighting, controls and any air-handling auxiliaries.
• Heating — the fuel or energy needed to hold the indoor temperature against the outdoor temperature during the heating season.

Of these two, heating can be the largest energy item in a climate with a real winter; its share depends on the heating system, tariffs, setpoint and operating schedule. Everything else (water, cleaning, staff, insurance, servicing, repairs, financing, taxes) depends on the operating model as well as the hall’s design and equipment.

This article deals with the largest and most build-dependent item: heating. A key technical input is the effective U-value; area, weather, ventilation, schedule and equipment efficiency also matter.

What is the U-value of an air dome?

The U-value (thermal transmittance, sometimes called the heat transfer coefficient) is the standard building-physics number used to describe how well any construction resists heat flow — a wall, a roof, a window or, in our case, an inflated membrane envelope. It is expressed in W/m²K: watts of heat lost per square metre of envelope, per kelvin of temperature difference between inside and outside.

Lower is better. A U-value of 1.0 W/m²K means that one square metre loses 1 watt for every degree of temperature difference; 2.0 W/m²K means it loses twice as much.

The reason a buyer cares is that the whole heating calculation follows directly from it:

where A is the envelope area, U the U-value and ΔT the indoor–outdoor temperature difference. At a constant temperature difference, multiply that power by the operating hours to obtain useful heat. Divide by heating system efficiency to obtain purchased energy, then multiply by the energy tariff. For changing weather, calculate and sum the individual time intervals. Every euro of heating cost in this article traces back to that one line.

Which is exactly why the U-value quoted in a supplier’s offer deserves scrutiny — and why a number taken from a fabric datasheet is not the number that will heat your hall.

Air dome insulation: membrane U-value vs whole-hall performance

A double-skin air dome consists of two thin membranes with an inflated air layer between them. The finished envelope is not uniform: the cavity is deepest in the middle of each inflated panel and closes at the welds. The hall also has a continuous anchoring line, doors, airlocks, transparent sections and an air-handling unit. Because the structure is held up by overpressure, some air continuously leaves through real construction details and must be replaced.

A laboratory value can still be useful, but only if its measurement boundary is stated. Was a complete double-skin assembly tested, or only one membrane? Did the specimen include a weld? Was it taken from the deepest part of the inflated panel? A local result cannot describe all welds, perimeter details and leakage of an operating hall.

That is why this article uses a separate term: effective U-value, U_eff. It describes the observed heat loss of the complete double-membrane hall under its normal pressurised operating condition, divided by the complete envelope area counted once and by the indoor–outdoor temperature difference. Counting the envelope area once is only the normalisation convention; it does not mean that one membrane was measured separately. It is not presented as an accredited material test.

Where on the inflated panel was the U-value measured?

This question matters because a double-skin panel has no uniform thickness. In the centre of the inflated “pillow”, the air cavity is deepest and the local heat transfer is lowest. Moving toward a weld, the two membranes converge, the insulating air layer becomes thinner and the local U-value rises. At the weld, the cavity effectively disappears.

A test taken only at the deepest part can therefore produce an excellent and technically real local value, while saying little about the average performance of the complete envelope. Measuring at a weld would produce the opposite extreme. Neither point alone represents the entire panel.

For comparison and heating design, the meaningful assembly value is an area-weighted average over a complete repeating panel section — from one weld to the next — tested in a geometry and at a cavity pressure representative of the finished hall. The test report should state:

• the exact position and dimensions of the specimen;
• the cavity profile and pressure during the test;
• whether weld bands and edge details were included;
• whether the published result is a local point value or an area-weighted average.

How does U-value affect air dome heating costs?

For the same hall, indoor temperature and weather, heat loss rises in direct proportion to U-value:

Consider a typical double-skin hall with 1,846 m² of envelope and a 30 K temperature difference — for example, +15 °C inside and −15 °C outside.

A difference that looks small on a datasheet becomes large over an entire envelope. Moving from U = 3.0 to U = 2.0 cuts this part of the heating demand by 55 kW in the example above. Over 1,000 equivalent heating hours, that is 55 MWh of delivered heat. At 90% heating system efficiency and an energy price of €0.08/kWh, the difference is about €4,900. The price and weather assumptions are illustrative, but the proportional relationship is not.

This is not a prediction from a gas invoice. A real annual bill also depends on the set temperature, opening hours, weather, doors, occupancy, heating system efficiency and energy tariff. The U-value gives a fair way to compare the thermal performance of two halls before those operating choices are added.

How we estimate effective U-value from a cooling test

The method is deliberately simple. We deliberately carried out the measurements after dark, when no one was playing sports in the hall, so that solar gains would not affect the result. We switched off the heating and monitored the hall’s cooling. We do not start the calculation while hot metal, a combustion chamber or ductwork is still heating the supply air. The usable interval starts only after the measured supply-air temperature has fallen to approximately the outdoor-air temperature. The pressure fan continues to operate normally, because the aim is to measure the hall as it actually stands.

We then read the initial, approximately straight part of the indoor-temperature curve. The heat removed from the indoor air is:

Here, V is the hall volume, A is the complete hall envelope area counted once (not the combined surface area of both membrane layers), ρ is air density (approximately 1.2 kg/m³), and c_p is the specific heat capacity of air (approximately 1,005 J/kgK). If the cooling slope is entered in K/h, it must be divided by 3,600 to obtain watts. Over a short interval we use the mean indoor–outdoor temperature difference.

Worked example: calculating effective U-value

Assume a hall has V = 8,451 m³ and A = 1,846 m². During one selected initial interval, indoor air cools at 12.0 K/h while the mean indoor–outdoor difference is 9.2 K.

This one interval is an illustration of the arithmetic, not the published value for the hall. The hall result is obtained from several suitable heating shutdown events and initial segments, rejecting periods with the heating switched back on, occupancy, open doors, unstable outside temperature or an unclear heating state.

Effective U-values from five operating air domes

The same method and normalisation were used for every site. We include only halls with a usable indoor, outdoor and supply-air record for this test.

Here τ (tau) is the initial cooling time constant: τ = ρ × c_p × V / (A × U_eff), converted to hours. It describes the initial rate of temperature decay, not how long the hall takes to reach outdoor temperature. Displayed values are rounded.

All five results fall between 1.72 and 2.01 W/m²K. That consistency shows what complete halls actually did with fans running, working pressure maintained and normal construction details included.

What does the effective U-value include?

U_eff deliberately combines every heat-loss path that acts on the indoor air during the test: the double-skin envelope, welds, perimeter anchoring, normal leakage at operating pressure, transparent details and the operating air-handling path. We do not subtract the parts that are inconvenient, because the purpose is to compare complete halls and size heating for the complete hall.

Conversely, we do not add assumed heat capacities for membranes, floor slabs or cavity air. The stated method uses the known heat capacity of the indoor air only. Reading the initial cooling segment is intended to reduce the influence of the floor compared with modelling the entire night; it does not demonstrate that this influence is zero.

What this cooling test can and cannot tell us

Interpretation: using the heat capacity of air alone estimates net cooling of the indoor air. The measurements were carried out after dark, so solar gains were not a factor during the evaluated intervals. Heat exchange with the floor and contents, temperature stratification, sensor placement and changing airflow can bias the inferred coefficient. Similar results across five halls do not by themselves quantify that uncertainty. Treat 2.0 W/m²K as a preliminary coefficient for comparable conditions, not a substitute for a project heat-loss calculation.

• It can compare completed halls under their normal pressurised condition using one transparent calculation.
• It can provide a practical preliminary coefficient for heating-system sizing: about 2.0 W/m²K for these double-skin halls.
• It cannot replace an accredited hot-box test when the contractual question concerns a material or a defined membrane assembly.
• It should not use a long cooling tail as if the hall contained only air. The selected time interval and raw temperature curve should always be published with the result.

How much does it cost to heat an air dome? Two seasonal examples

In the two scenarios below, calculated heating-energy costs are €4,017–€10,008 for a 36 × 36 m hall and €5,239–€13,052 for a 54 × 36 m hall. These are illustrative steady-state results using the stated Geneva temperature profile, +15 °C for eight hours a day from October to April, U_eff = 2.0 W/m²K, 90% heating system efficiency and €0.08/kWh. They exclude warm-up energy, night heating and all non-heating costs; they are not guaranteed annual bills.

The following worked example uses the same month-by-month, two-bound structure as our project running-cost worksheets. The milder scenario uses each month’s average high outdoor temperature; the colder scenario uses its average low. “Lower” and “upper” below label these two model scenarios, not statistical confidence limits or guaranteed limits on a real bill. Every input and intermediate monthly result is shown so that the totals can be checked rather than accepted as a black-box estimate.

The worksheet supports separate daytime and night-time setpoints and hours. This example uses 15 °C for 8 daytime hours, no active night heating, and all days from October through April. The outdoor-temperature profile is the Geneva profile used in the supplied running-cost worksheet. Costs use U_eff = 2.0 W/m²K, 90% heating system efficiency and an illustrative energy price of €0.08/kWh.

Detailed monthly calculation — 36 × 36 m hall

Detailed monthly calculation — 54 × 36 m hall

Monthly figures are rounded for display; season totals are calculated from the unrounded values. The range is an illustrative steady-state heating-energy calculation, not a quotation or guarantee. It excludes the extra heat needed to rewarm a hall after an unheated night and any frost-protection or snow-duty heating outside the stated eight hours. Its value is that every input is visible: hall geometry, climate, schedule, setpoint, U_eff, heating system efficiency and tariff can all be replaced with your own.

How much heating output is needed to warm up an air dome?

Holding load and warm-up output are different. The examples below require 120 kW and 156.5 kW of useful heat to hold +15 °C with −10 °C outdoors. Reaching that temperature from cold requires surplus output. All kW values in this section refer to useful output delivered to the hall, not fuel-input ratings; heat-pump capacity must be checked at the stated outdoor condition.

The 120 kW and 157 kW above are steady-state numbers: they replace the heat lost through the envelope once the hall is already at +15 °C. In this idealised model at the stated design outdoor temperature, a heating system delivering exactly that output can warm the hall but cannot reach the exact target temperature in a finite time. As the indoor temperature rises, the losses rise with it, the surplus available for warming the air falls towards zero, and the setpoint is approached only asymptotically — mathematically only after an infinite time; practical control tolerances and changing conditions must be considered separately. Warm-up capacity is therefore a deliberate surplus above the holding load, and it is what decides whether a cold hall is ready in half an hour or in half a day.

Our design software solves the standard lumped heat balance for the hall air, C · dT/dt = P − k · (T − Tout), which gives the working formulas:

where ΔT₀ is the starting and ΔT₁ the target temperature difference to outdoor air. The condition P/k > ΔT1 is exactly the statement that the useful heating output must exceed the holding load, otherwise the target is unreachable at any time. Applied to the same two halls, warming a cold hall from 0 °C to +15 °C with −10 °C outside:

These times are an optimistic lower limit, because the balance above carries only the heat capacity of the air. The floor and sports surface, the membranes, the anchoring line and the equipment all have to be warmed as well, doors are opened, and infiltration increases with the temperature difference. A real hall therefore needs a further margin on top of the figures in the table; a preliminary option is useful heating output of roughly 1.5 to 2 times the holding load, but the required margin and modulation range must be checked for the project rather than treated as a universal sizing rule. The heating system must also be able to deliver that output into the hall: the air-handling unit needs enough airflow at an acceptable supply-air temperature, which is a second sizing check alongside the useful heating output.

How to compare air-dome insulation and heating quotations

A low U-value is useful only when you know what was measured. Three very different numbers may all appear under the same label:

Ask the supplier to state the measurement boundary, the area used in the denominator, whether normal leakage and construction details are included, the indoor and outdoor design temperatures, and the required heating output at those temperatures. If the value comes from a cooling test, ask to see the indoor, outdoor and supply-air curves.

Air dome U-value and heating costs: frequently asked questions

Are air dome heating costs the same as total operating costs?

No. Heating is one part of the operating budget. Continuous pressurisation, lighting, maintenance, staffing and other site expenses must be budgeted separately. The seasonal examples here also exclude warm-up and night heating. For the wider investment context, see our air dome cost guide.

What effective U-value should I use for a double-skin air dome?

For preliminary design, measurements from five operating halls support approximately 2.0 W/m²K for the complete double-membrane hall, normalised to the envelope area counted once. A project calculation should still use its actual geometry, design temperatures, openings and required operating pressure.

How much does a lower U-value save?

For the same hall, weather and indoor temperature, the measured heat loss changes in direct proportion to U-value. Reducing U_eff from 2.5 to 2.0 W/m²K reduces this part of the heating demand by 20%. The reduction in the total energy bill will be smaller if fans, lighting or other uses form a significant share of the bill.

Why do you wait for supply air to reach outdoor temperature?

Immediately after the heating is switched off, a hot combustion chamber or duct can continue adding heat. Starting only after the supply air has cooled to approximately outdoor temperature removes that obvious residual input from the selected interval.

Does the test measure the fabric alone?

No. It measures the effective behaviour of the completed pressurised hall. That is intentional.

A laboratory test on a specimen requires a different measurement boundary — and for a double-skin air dome that boundary is the problem. The inflated cavity is not the same anywhere on the hall. It is deepest in the middle of each pillow, thins out towards the welds and disappears at the weld itself; it also changes with panel geometry, curvature, operating pressure and position on the dome. A specimen cut from one place is a correct measurement of that place and tells you very little about how the envelope as a whole will behave.

A local laboratory point value cannot simply be multiplied by the full envelope area. A representative, area-weighted assembly value can be a valid transmission-loss input, but doors, perimeter details, ventilation and leakage must still be accounted for separately. That is the practical reason we publish an effective whole-hall value instead: it is measured on the finished, pressurised, operating structure — welds, perimeter, doors, airlocks and leakage included — which is the only condition your heating bill actually sees.

This article reports an engineering field method, not an accredited material certification. The raw interval, geometry, sensor channels and calculation assumptions should accompany any published result.