Air Gap Resistance Calculator

The Air Gap Resistance Calculator calculates the thermal resistance of air cavities in building assemblies from gap width, orientation, and ventilation.

Air Gap Resistance Calculator Explained

Air gap resistance is the thermal resistance contributed by an enclosed or semi‑enclosed air space between building layers. Thermal resistance, symbol R, measures how well an assembly resists heat flow. Higher R means better insulation. The air in a cavity transfers heat by three paths: conduction through the air, convection within the gap, and radiation between the facing surfaces.

Because all three paths can act together, air spaces are not simply “L divided by k” like solid materials. Orientation (vertical, horizontal up, horizontal down), surface emissivity (how much a surface emits radiation), and gap thickness strongly influence the result. Building standards often use tabulated R-values for common air gaps, while engineering methods compute R from heat transfer coefficients.

The calculator follows both approaches. You can either use physics-based inputs for custom cavities, or select table-based defaults from standards for typical construction details. This allows fast iteration during design and better estimates for energy models, quantity takeoffs, and materials choices.

How the Air Gap Resistance Method Works

The method combines convection and radiation into a single overall heat transfer coefficient, then inverts it to get resistance. Conduction through quiescent air is part of the convection term via correlations. For simple assemblies, you can also add the air-gap R to the R of solid layers to get a total.

• Define the cavity: thickness, height/length, orientation, and whether it is sealed or slightly ventilated.
• Estimate convection using a Nusselt number correlation, which converts the Rayleigh number into a convection coefficient.
• Estimate radiation using surface emissivities and the mean absolute temperature of the cavity.
• Combine convection and radiation to get an overall U-value for the gap, then compute R = 1/U.
• Optionally, use standardized tabular R-values when your geometry matches code defaults.

This approach ties the physics to practical design choices. You can test how changing thickness, adding a reflective foil, or flipping the cavity orientation shifts R, and choose materials that meet targets without extra wastage.

Air Gap Resistance Formulas & Derivations

Below are the key relationships used by the calculator. They balance detail with simplicity so you can evaluate common cavities without heavy modeling.

• Overall relation: R_gap = 1 / U_gap, where U_gap is the combined heat transfer coefficient across the air space in W/m^2·K.
• Combination: U_gap = h_c + h_r, where h_c is the convection coefficient and h_r is the linearized radiation coefficient.
• Convection: h_c = k_air × Nu / d, where k_air is air thermal conductivity, d is gap thickness, and Nu is the Nusselt number.
• Rayleigh number: Ra = g β ΔT d³ / (ν α), with g gravity, β ≈ 1/T_m for ideal gases, ΔT temperature difference, ν kinematic viscosity, α thermal diffusivity, and T_m mean absolute temperature.
• Nusselt correlations (common forms):
  • Vertical cavity: for 10⁴ ≤ Ra ≤ 10⁷, Nu ≈ 0.22 × Ra^0.28.

The radiation term drops sharply when one face has a low emissivity (for example, a foil with ε ≈ 0.05). That is why reflective cavities can yield much higher R, especially when convection is modest (thin vertical gaps or horizontal gaps with heat flow downward).

Inputs and Assumptions for Air Gap Resistance

The calculator needs a small set of inputs to model the cavity. Where possible it offers default values from standards to keep data entry simple and consistent.

• Gap thickness d (mm or m): clear distance between opposing faces.
• Orientation: vertical, horizontal with heat flow upward, or horizontal with heat flow downward.
• Mean cavity temperature T_m (°C or K) and temperature difference ΔT (K).
• Surface emissivities ε₁ and ε₂: representative of paint, foil, board, or membrane finishes.
• Ventilation status: sealed/unventilated (default) or slightly ventilated (reduces R).
• Standard table override: select a code-defined R when geometry matches ISO/ASHRAE cases.

Reasonable ranges: small gaps under 5 mm are dominated by radiation and limited convection; very wide cavities over 200 mm can have strong convection, lowering R. Slight ventilation can erase most of the radiation benefit of low‑e surfaces. Check units carefully and keep material data realistic for your climate and altitude.

Using the Air Gap Resistance Calculator: A Walkthrough

Here’s a concise overview before we dive into the key points:

1. Select your preferred units for length, temperature, and thermal values.
2. Enter gap thickness and choose the cavity orientation.
3. Input mean temperature and temperature difference, or accept defaults.
4. Set surface emissivity for each face, or choose a preset (painted, foil, board).
5. Choose “Physics mode” for custom geometry, or “Table mode” for standard cases.
6. Review the computed U and R for the gap, then add adjacent solid layers if needed.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

Brick veneer wall cavity: A 20 mm vertical, unventilated gap behind cladding uses a reflective foil (ε₁ = 0.05) facing a painted sheathing (ε₂ = 0.9). With T_m ≈ 293 K and ΔT = 10 K, the calculator gives h_c ≈ 2.0 W/m^2·K, h_r ≈ 0.15 W/m²·K, U ≈ 2.15 W/m²·K, R_gap ≈ 0.47 m²·K/W. Adding solid layers and surface resistances yields R_total for the wall. What this means: The low‑e foil turns a thin cavity into a meaningful insulation layer without extra thickness.

Attic plenum above a ceiling: A 100 mm horizontal air space with heat flow upward (room below warmer) and painted faces (ε₁ = ε₂ = 0.9). With T_m ≈ 293 K and ΔT = 15 K, results are h_c ≈ 5.0 W/m²·K, h_r ≈ 4.9 W/m²·K, U ≈ 9.9 W/m²·K, R_gap ≈ 0.10 m²·K/W. A radiant barrier would cut h_r and raise R into the ~0.19 m²·K/W range. What this means: Wide, upward-heated cavities convect strongly; consider low‑e surfaces or insulation to improve performance.

Assumptions, Caveats & Edge Cases

The calculator uses correlations that assume steady conditions and uniform surfaces. It is most accurate for unventilated, plane‑parallel cavities with reasonably smooth faces.

• Slight ventilation reduces R by adding advective heat transfer that the model does not fully capture.
• Moisture and humidity can alter air properties and emissivity; wet, dusty, or oxidized foils are less effective.
• Very thin gaps (< 5 mm) behave almost like radiative layers; convection is minimal.
• Very wide gaps (> 200 mm) can develop complex circulation not covered by simple Nu correlations.
• Edge framing, perforations, and thermal bridges bypass the cavity; include them in whole‑assembly calculations.

Use table mode when your detail matches a standard. Use physics mode for special geometries, unusual materials, or to test sensitivity. When in doubt, be conservative to manage risk in energy modeling and cost estimates.

Units Reference

Accurate units prevent errors when comparing materials and assemblies. This reference shows the symbols and units used by the calculator so you can check inputs and read outputs correctly.

Use consistent units throughout a calculation. For example, if you enter d in millimetres, the tool converts to metres internally to keep U and R in standard SI units.

Common Issues & Fixes

Most errors come from mismatched units, unrealistic emissivity choices, or assuming a sealed cavity when vents exist. The steps below help resolve typical problems.

• If R seems too high, verify emissivity; painted or dusty foil is not ε = 0.05. Try ε = 0.2–0.3 instead.
• If R seems too low, confirm orientation and thickness; horizontal upward heat flow drives convection.
• Recheck ΔT and T_m; extreme values change air properties and the radiation term.
• Use table mode to cross‑check physics mode for the same geometry.

When a cavity is ventilated or irregular, do not rely on air‑gap R alone. Model the assembly with ventilation losses, or choose a solid insulation layer to meet the target.

FAQ about Air Gap Resistance Calculator

Does a bigger air gap always give higher resistance?

No. Past a certain thickness, natural convection strengthens and can lower R. Thin to moderate gaps often perform better than very wide gaps unless you add a low‑emissivity surface.

How much does a radiant barrier help?

A low‑emissivity surface can cut the radiation term by 80–95% in many cases, raising R noticeably, especially in thin vertical gaps or horizontal gaps with heat flow downward.

Can I use this for ventilated rain‑screen cavities?

Use caution. Ventilation adds airflow that the model treats only approximately. For rain‑screens, treat the cavity as ventilated and use conservative R-values or supporting airflow models.

Which emissivity should I pick for common materials?

Typical painted or paper surfaces are ε ≈ 0.85–0.95. New bright aluminum foil is ε ≈ 0.03–0.05; aged or dusty foil can be ε ≈ 0.2–0.3. Check manufacturer data when available.

Key Terms in Air Gap Resistance

Thermal Resistance (R)

A measure of how well a layer resists heat flow, in m²·K/W. Higher R means less heat loss and better insulation performance.

U-Value

The overall heat transfer coefficient, in W/m²·K. It is the inverse of R and represents how easily heat passes through a layer or assembly.

Emissivity

A surface property (0 to 1) describing how efficiently a material emits thermal radiation. Low values greatly reduce radiative heat transfer across an air gap.

Rayleigh Number (Ra)

A dimensionless number that indicates the strength of buoyancy‑driven convection in a fluid layer, based on temperature difference, gap size, and fluid properties.

Nusselt Number (Nu)

A dimensionless measure comparing convective to conductive heat transfer across a fluid layer. It converts to a convection coefficient for the air gap.

Thermal Conductivity (k)

A material property in W/m·K indicating how well heat conducts through a solid or fluid. Air has low k; metals have high k.

Surface Resistance (R_si, R_se)

Standardized resistances at the inner and outer surfaces of an assembly, capturing boundary‑layer effects not included in layer R-values.

Reflective Insulation

A material with low emissivity, often foil‑faced, designed to reduce radiative heat transfer across adjacent air spaces in building assemblies.

References

Here’s a concise overview before we dive into the key points:

• ISO 6946: Building components and building elements — Thermal resistance and thermal transmittance
• ASHRAE Handbook — Fundamentals (Heat Transfer and Building Envelopes)
• Oak Ridge National Laboratory: Radiant Barrier Fact Sheet
• Engineering Toolbox: Emissivity Coefficients of Common Materials
• Thermopedia: Natural Convection in Enclosures
• Wikipedia: Nusselt number

These points provide quick orientation—use them alongside the full explanations in this page.