Total R-value Calculations (AS/NZ 4859.2 + NZS 4214)
Speckel performs thermal resistance calculations for walls, roofs, and floors in accordance with J 1.5 - Total System R-value. These calculations are based on:
AS/NZS 4859.2:2018: Thermal Insulation Materials for Buildings
NZS 4214:2006: Methods of Determining the Total Thermal Resistance of Parts of Buildings
The National Construction Code (NCC) 2019 references two standards: the primary standard, AS/NZS 4859.2:2018, and the secondary standard, NZS 4214:2006. Speckel adheres to both standards as necessary.
The primary standard introduces new calculation requirements that consolidate various existing standards. Specific requirements for roof thermal performance are detailed in J1.2 - Thermal Construction — General (Roof and Floors).
For roof systems without structural thermal bridging, Speckel exclusively uses AS/NZS 4859.2:2018. However, for roof systems with structural thermal bridging, Speckel calculates using both AS/NZS 4859.2:2018 and NZS 4214:2006 to meet the performance requirements for roofs, floors, and walls. Both primary and secondary standards are necessary to satisfy Deemed-to-Satisfy and Performance Solutions criteria.
Speckel Procedure
To achieve a calculated outcome, Speckel follows these steps. Note that this process is iterative and not defined by a single equation:
Temperature Corrections:
Apply temperature corrections to all airspaces and insulation layers in the system.
Assign equidistant temperature values to each layer.
Correct the declared R-Value of insulation for AS/NZS temperatures.
Calculate the airspace R-Value and perform any necessary derating.
Recalculate Temperature with Corrections:
Calculate the Total System R-Value.
Perform the NZS 4214:2006 thermal bridging calculation on any insulation layers with framing as needed.
Determine the contribution of each layer to the Total System R-Value based on temperature values.
Iterative Process:
Repeat the above steps iteratively until the R-Values of all layers stabilize to three significant figures.
Final Calculation:
Calculate the final Total System R-Value.
Perform the full NZS 4214:2006 thermal bridging calculation as necessary.
By following these steps, Speckel ensures accurate and reliable thermal resistance calculations for building components.
Equidistant Temperature Values
Before calculating the Total System R-Value, two key calculations must be performed: the insulation value correction and the cavity R-Value calculation (including possible derating).
Insulation Value Correction:
The mean temperature across the insulation must be determined. This is essential for both the insulation and air cavity calculations.
The mean temperature is calculated based on the percentage that each layer contributes to the Total System R-Value.
An iterative process is necessary because the Total System R-Value depends on these initial calculations, creating an impasse.
Iterative Process:
To resolve this, initial assumptions are made about the mean temperatures to start the calculations.
The mean temperatures across each layer are initially assumed to be an equal portion of the total temperature change across the system.
The process iterates to balance the final Total System R-Value, adjusting mean temperatures and layer contributions until the R-Values stabilize to three significant figures.
By making initial assumptions and using an iterative process, Speckel ensures accurate calculation of the Total System R-Value, accommodating the interdependence of insulation value correction and cavity R-Value calculation.
Temperature Values Recalculated
According to AS/NZS 4859.2:2018 section 5.2(1), we need to determine FT, the calculated conversion coefficient, to adjust the insulation R-Value. This process involves the following steps:
Calculate the Mean Temperature:
Infer the mean temperature across the insulation layer. This is based on the declared mean temperature at which the insulation was tested (23°C).
Determine the Conversion Coefficient (fT):
Use the mean temperature to find the conversion coefficient.
The conversion coefficient is selected from a series of tables specific to the insulation material type (e.g., Mineral Wool, Expanded Polystyrene).
Each material type has additional requirements such as density or thickness, which are used to determine the exact coefficient.
Apply linear interpolation within the tables if necessary to refine the coefficient.
By following these steps, we accurately calculate the insulation R-Value adjustment, ensuring it aligns with the standards set out in AS/NZS 4859.2:2018. This meticulous approach helps achieve precise and reliable thermal resistance values for building components.
Airspace R-Value
According to AS/NZS 4859.2:2018, there are three types of air cavities: Unventilated, Slightly Ventilated, and Well Ventilated. The type is determined by the ventilation area per meter of the wall length.
Well Ventilated airspace (6.3.2)
The air cavity R-Value is 0, and a derate of 100% is given to all layers between the cavity and external air.
Slightly Ventilated airspace
The air cavity is a mixture between Unventilated and Well Ventilated. The calculation for this type (6.3.1) only results in the Total System R-Value; however, since individual layer R-Values are important for the temperature correction calculations, we cannot use this result.
The calculation is simply the linear interpolation of two systems; one with Unventilated and one with Well Ventilated airspace. Using this logic, we then do the same to instead calculate the individual airspace R-Value (between 0 and calculated Unventilated R-Value) and the derating amount (between 0% and 100%).
Unventilated airspaces
Calculate the R-Value using the conduction/convection coefficient (ha) and other required coefficients from Tables 12 and 13.
Table 12
0°
0.12 × d⁻⁰·⁴⁴
1.95
0°
45°
0.83 × d⁻⁰·⁰⁵
1.95
45°
90°
1.25
-
90°
d = thickness of the airspace in the direction of heat flow, in meters. If larger, ha = 0.025/d.
Table 13
Here is the requested table based on the provided data for temperature differences greater than 5 K:
0°
0.09 × (ΔT)⁰·¹⁸⁷ × d⁻⁰·⁴⁴
1.14 × (ΔT)¹/³
0°
45°
1.00
1.14 × (ΔT)¹/³
45°
90°
0.73 × (ΔT)¹/³
-
90°
d = thickness of the airspace in the direction of heat flow, in meters. ΔT = temperature difference in Kelvin.
The radiative coefficient (hr) is calculated using the emissivity of bounding layers and the temperature change across the layer; as per 6.2(2). The emissivity of bounding layers is given an uplift of 1.36 if they are lower than e0.15 as per 6.2(4). No derating is applied with this airspace type.
In 4859.2 Section 6.2, For airspaces ranging from 10 mm to 300 mm in thickness, the thermal resistance, 𝑅 𝑔 R g , is determined using the formula 𝑅𝑔= 1/(ℎ 𝑎 + ℎ 𝑟). Here, ℎ𝑎 represents the conduction/convection coefficient and ℎ𝑟 denotes the radiative coefficient, both measured in W/(m²·K). The formula integrates the effects of conductive/convection and radiative heat transfer to provide a comprehensive measure of the airspace's resistance to heat flow.
NZS 4214:2006 Thermal bridging
Now that we have performed our corrections, we calculate the actual temperature values as per 10.3. The only usable values are the mean temperature and temperature change across each layer. These values are calculated by the percentage the layer contributes to the Total System R-Value. However, due to the inclusion of clause 10.2, which cites NZS 4214:2006 for calculation of thermal bridging (which impacts Total System R-Value), we must first perform a simplified thermal bridge calculation.
As per NZS 4214:2006, we are required to combine any adjacent air cavity and insulation layers into one unified thermally-bridged layer, however, since we need to know individual layer R-Values, we must simplify this. The conjoining of airspace and insulation is omitted for the sake of individual layer calculations, but it will be applied later after results are stabilised through iteration. Only the insulation layer is subject to NZS 4214:2006 for thermal bridging in this initial processing. The Total System R-Value can now be calculated by simply adding all layer R-Values together (including any thermally-bridged insulation layers).
R-Values Stabilisation
The temperature values for the next iteration of corrections can then be calculated. The above process is performed until values have stabilised to three significant values (as per 10.1).
Full NZS 4214:2006 Thermal Bridging Calculation
After determining individual layer R-Values, the Total System R-Value can be calculated. As per NZS 4214:2006, the proper (adjacent airspace and insulation layers are combined) thermal bridge calculation is performed as necessary.
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