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Thermal performance metrics help architects determine the energy efficiency of buildings - where less energy is required to cool a building due to lesser solar gain through its roofs and envelopes.
As architectural designers, we need to be able to perform the relevant calculations to support our declared ETTV, RTTV and U-values.
In this post, I will share how these calculations are done, including me providing you with key definitions and design considerations.
Post Contents:
Overview of Thermal Performance Metrics
ETTV & RTTV
U-Value, k-values
Shading Coefficient & Correction Factor
Shading Devices
Pro Tips
Resources
Overview of Thermal Performance Metrics
The best way to understand the process of thermal performance calculations is to look at the various scales of the the construction.
Imagine the flow of heat through the roof and envelope from external to internal, there is a certain resistance that will limit the eventual quantity of heat per unit area under steady state conditions entering the building.
Heat transfer can occur in three main processes:
- Conduction
- Heat transferred by molecular collision via physical contact. This is the most common type of heat transfer
- Convection
- Heat transferred by flow through a material by the body movement of particles, only in fluids and gases, not solids.
- Radiation
- Heat transfer through electromagnetic waves
The different layers of the roof and/or wall have varying thermal-physical properties and the combined effect will contribute to the U-value, the thermal transfer value.
With the U-value calculated, we can determine the ETTV and RTTV - more on that below.
What is ETTV & RTTV?
The following are definitions of the two thermal performance metrics:
Envelope Thermal Transfer Value (ETTV)
- Heat conduction through opaque walls
- Heat conduction through glass windows
- Solar radiation through glass windows
Roof Thermal Transfer Value (RTTV)
- Heat conduction through opaque walls
- Heat conduction through glass windows
- Solar radiation through glass windows
The above diagram shows the various parameters required for thermal performance calculations:
- Weighted average U-values:
- Opaque wall type(s)
- Fenestration type(s)
- Window-to-Wall Area Ratio (WWR) or Skylight-to-Roof Area Ratio (SRR)
- Shading Coefficients (SC) & Correction Factor (CF) - discussed below.
Once you have these parameters, you will be able determine the ETTV and RTTV.
More on the formulae below.
What is U-value?
As mentioned above, we must determine the weighted average U-values first.
The overall process to calculate U-value is to obtain the values for the following parameters:
- Thickness
- K-value (discussed in the next section)
K-Values (Thermal Conductivity)
The k-value is a constant value metric of thermal conductivity assigned to the specified material type.
These stipulated values can vary from country to country - so my advice will be to consult your senior architect to get this information.
An example for Singapore’s set of k-values can be found in Table 3 in the Building & Construction Authority Code of Practice, click the link under the ‘Resources’ section below.
Shading Coefficient & Correction Factor
These two values are determined by the selection of the shading system and orientation of the facade.
Each scenario results in a different assigned value.
Again, these values differ from country to country.
An example for Singapore’s set of shading coefficient and correction values can be found in Table 6 onwards in the Building & Construction Authority Code of Practice, click the link under the ‘Resources’ section below.
Shading Systems
The following diagram shows the three main types of shading systems - canopies, fins and eggcrates.
I encourage you to take a look at facades of the buildings around you.
Often times, you will see the employment of one of the three types, or a combination of the different types of shading devices.
In this post, I will cover canopies and fins only - as eggcrates are in essence, a combination of horizontal and vertical shading devices.
1) Horizontal Canopies
Firstly, we have the horizontal canopy placed at the top of the fenestration.
Parameters to establish for your ETTV calculations include:
- Length of shading device
- Height of fenestration
- Solar Angle
- Incline Angle of Shading Device
- Height of Shading Device from top & bottom
To determine the effectiveness of shading - get the ratio of the height of shaded and exposed area of the fenestration. This needs to calculated for the 4 major months of the years, each with a different solar angle indicated.
The solar angle data differ between country to country, so please obtain them from your building authority.
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2) Vertical Fins
Next, we have vertical fins placed at various intervals along the fenestration.
Parameters to establish for your ETTV calculations include:
- Length of shading device
- Height of fenestration
- Solar Angle
- Angle of Shading Device from vertical
To determine the effectiveness of shading - get the ratio of the height of shaded and exposed area of the fenestration. This needs to calculated for the 4 major months of the years, each with a different solar angle indicated.
The solar angle data differ between country to country, so please obtain them from your building authority.
How to Calculate ETTV, RTTV and U-value
Now that you have learnt all of the definitions and parameters to these thermal performance calculations, you might be wondering how to start.
Well, there are some very kind souls who have provided free online calculators that simplifies all of the work for us.
Click the links below to access the U-value and ETTV Calculators.
U-value Calculators
ETTV Calculators
Pro Tips
1) Consider your design strategy to obtain the optimal thermal performance
Use a combination of design strategies comprising of the right building orientation, materials with a lower U-value and shading systems to optimise the U-value and in turn, ETTV/RTTV values.
2) Use tools available for easy calculations
Search for online calculator tools (some are listed in the blog post above), where you can save time to determine ETTV values for your submissions. Just prepare the necessary input values.
3) Don’t forget other building performance & design criteria
As thermal performance depends on the facade (eg. window to wall area/placement of windows) , factors such as natural ventilation, cost and fire safety shall not be neglected.
Resources
Building & Construction Authority Singapore
U-value Calculators
ETTV Calculators
