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Buildings account for nearly 40% of Singapore’s total energy consumption, with cooling systems being the largest energy consumer. However, meeting the stringent Green Mark 2021 requirements for energy efficiency doesn’t have to mean extensive structural changes.
Window films offer a cost-effective solution for improving building energy performance. These films significantly impact the Envelope Thermal Transfer Value (ETTV), a crucial metric in Green Mark certification. Understanding how to calculate ETTV with window films helps building owners and architects make informed decisions about energy-saving improvements.
This comprehensive guide explains the step-by-step process of calculating ETTV with window films under Green Mark 2021 standards. You’ll learn how different window film specifications affect ETTV calculations and discover practical examples to help you achieve your building’s energy efficiency goals.
The Envelope Thermal Transfer Value (ETTV) serves as the cornerstone of building energy efficiency assessment in Singapore’s tropical climate. Unlike its predecessor—the Overall Thermal Transfer Value (OTTV)—ETTV provides a more precise measurement of how effectively a building’s exterior shell manages heat transfer [1].
ETTV measures the average heat gain through a building’s exterior walls and windows. Originally introduced as an enhancement to Singapore’s pre-existing envelope design criterion, ETTV has become a fundamental metric for evaluating building energy performance [1]. The formula takes into account three critical components of heat transfer:
Research demonstrates that ETTV has a direct correlation with annual cooling energy requirements (Ec), making it an essential parameter for energy consumption calculations [1]. For each unit decrease in ETTV, buildings can achieve approximately 3.5-4% reduction in annual cooling energy costs, depending on the building configuration [3]. This relationship makes ETTV an invaluable tool for energy auditing and implementing reforms to enhance building energy efficiency.
The BCA Green Mark 2021 (GM: 2021) certification scheme establishes stringent ETTV requirements that vary according to building type and certification level. While the baseline maximum permissible ETTV stands at 50 W/m² for basic compliance [2], higher certification levels demand more rigorous standards:
Green Mark Gold: ETTV of 42 W/m² or lower [4]
Green Mark Platinum: ETTV of 40 W/m² or lower [4]
Nevertheless, these thresholds vary based on building category and energy efficiency pathway:
| Building Type | GoldPLUS (EE >50%) | Platinum (EE ≥55%) | SLE (EE ≥60%) |
|---|---|---|---|
| Office Buildings | 40 W/m² | 38 W/m² | 38 W/m² |
| Retail Mall | 40 W/m² | 38 W/m² | 35 W/m² |
| Hotel | 40 W/m² | 40 W/m² | 40 W/m² |
| Schools | 40 W/m² | 38-40 W/m² | 35-40 W/m² |
Note: SLE refers to Super Low Energy buildings [5]
When calculating ETTV for developments with multiple blocks, a weighted average approach is employed, accounting for the facade area of each building within the development [5].
Window films represent a cost-effective approach to improve ETTV without replacing existing fenestration systems. These films modify two crucial parameters in the ETTV formula:
On average, applying solar insulation film can improve a building’s ETTV by up to 14%, potentially helping buildings achieve higher Green Mark certification levels [6]. Moreover, building orientation dramatically influences window film effectiveness. Studies show that reflective films (RG) outperform heat-absorbing films (HAG), with electricity savings ranging from:
Aside from improving ETTV values, window films create additional benefits including reduced indoor temperatures (2-3°C on sunny days), enhanced thermal comfort, and decreased air conditioning load [8], thus making them a practical solution for both new construction and retrofit projects seeking Green Mark certification.
The mathematical framework that underpins Envelope Thermal Transfer Value (ETTV) calculations consists of distinct components that quantify heat transfer through different parts of a building’s envelope. The ETTV formula provides a comprehensive approach to evaluating thermal performance in the context of Green Mark 2021 certification.
Heat conduction through opaque walls represents the first component in the ETTV formula, calculated as 12(1-WWR)Uw [2]. This term accounts for heat gain through the solid portions of external walls, factoring in:
This component typically contributes less to the overall heat gain compared to glazing elements, especially in commercial buildings where, as research indicates, walls have relatively low effects on the overall heating and cooling loads [9]. Yet, it remains a fundamental part of the ETTV calculation, particularly for buildings with lower window ratios.
The second component addresses heat conduction through glass windows, represented as 3.4(WWR)Uf in the ETTV equation [2]. This term quantifies heat that transfers through window glass independently of solar radiation, where:
For commercial buildings in tropical and sub-tropical climates, this component becomes increasingly significant as the WWR increases. Furthermore, the application of window films can modify the Uf value, thereby affecting this component of the ETTV calculation.
Solar radiation through glass windows constitutes the third and typically most substantial component of the ETTV formula, calculated as 211(WWR)(CF)(SC) [2]. This component accounts for:
This component merits particular attention since, according to research, “glazing is likely to be the most important factor” in a commercial building’s heating and cooling loads [9]. Consequently, solar radiation through windows often represents the largest contributor to ETTV, especially for facades with high WWR facing east or west orientations.
Since buildings typically have multiple facades with varying orientations, a weighted average approach must be employed to calculate the overall building ETTV. The formula for this is:
ETTV = (A₁ × ETTV₁ + A₂ × ETTV₂ + A₃ × ETTV₃ + …) / (A₁ + A₂ + A₃ + …) [10]
Where:
This approach is necessary because “walls at different orientations receive different amounts of solar radiation” [2]. The solar correction factor (CF) varies significantly between north-south and east-west orientations, resulting in different ETTV values for each facade. Additionally, the weighted average calculation ensures that larger facades have proportionally greater influence on the final ETTV figure.
The complete ETTV formula thus combines all three heat transfer components: ETTV = 12(1-WWR)Uw + 3.4(WWR)Uf + 211(WWR)(CF)(SC) [2]. This equation serves as the foundation for evaluating building envelope performance under Green Mark 2021 standards.
When selecting window films for Green Mark 2021 compliance, understanding key technical specifications is essential for accurate ETTV calculations. These specialized films alter the thermal properties of existing glazing systems, creating opportunities for significant energy savings without costly window replacements.
The U-value measures heat transfer through building materials due to temperature differences between indoor and outdoor environments. Expressed in W/m²K, a lower U-value indicates better thermal insulation performance [11]. Window films create an additional barrier that modifies this property, though the impact is relatively modest. Research indicates that adding solar insulation film typically reduces a window’s U-value by approximately 1.33% [6].
U-value varies based on several environmental factors:
For most glass applications, summer condition U-values are smaller than winter condition values, primarily due to lower outdoor wind speeds resulting in reduced convective heat transfer [11].
The shading coefficient represents a critical measurement in ETTV calculations. SC is defined as the ratio of solar heat gain through a particular window compared to that through an unshaded 3mm clear glass reference [12]. This dimensionless value ranges from 0 to 1, with lower values indicating superior shading performance [13].
Technically, SC can be separated into:
Different glazing systems exhibit varying SC values. For instance, double-pane windows with Low-E coating have SC values ranging from 0.38 to 0.64, depending on coating placement [1].
Although similar to SC, the Solar Heat Gain Coefficient takes additional factors into account. SHGC measures the fraction of incident solar radiation passing through a window, including both directly transmitted radiation and absorbed energy later released as heat [3]. It serves as the primary indicator of a film’s effectiveness in rejecting solar energy [14].
Numerical studies reveal that applying solar insulation film can reduce a window’s SHGC by up to 13.69% [6]. This reduction directly affects the solar radiation component of ETTV calculations, typically yielding a 14% improvement in overall ETTV values [6]. Indeed, this makes SHGC modification one of the most effective strategies for ETTV improvement with window films.
Beyond thermal properties, Visible Light Transmission (VLT) represents another crucial specification. VLT measures the percentage of visible light that passes through window film [15]. Higher percentages allow more natural light while potentially compromising heat rejection capabilities.
Window films typically offer VLT ranges categorized as:
Calculating the net VLT requires multiplying the film’s VLT by the existing glass VLT. For example, if a window has 80% VLT and you add film with 50% VLT, the resulting VLT becomes 40% (0.80 × 0.50 = 0.40) [16].
Within Green Mark 2021 parameters, selecting window films requires balancing these specifications to achieve optimal ETTV improvements while maintaining adequate natural lighting for occupant comfort and productivity.
Calculating ETTV with window films involves a systematic approach that combines building measurements with technical film specifications. This process enables precise evaluation of how window films impact building envelope performance for Green Mark 2021 certification.
Initially, collect essential information about your building envelope:
The thermal transmittance (U-value) of opaque walls typically ranges from 0.5 to 3.5 W/m²K, depending on construction materials. For existing windows, U-values generally fall between 2.8 and 6.0 W/m²K. These baseline values serve as the starting point for calculating improvements from window film applications.
WWR represents the proportion of fenestration area to the gross exterior wall area. To calculate:
WWR = Total window area (m²) ÷ Total exterior wall area (m²)
For buildings with multiple window types, measure each window area separately. Commercial buildings in Singapore often have WWR values exceeding 0.5, making window performance particularly crucial for ETTV compliance.
Insert window film specifications into the ETTV formula:
ETTV = 12(1-WWR)Uw + 3.4(WWR)Uf + 211(WWR)(CF)(SC)
Window films primarily modify two parameters:
Essentially, the third term (solar radiation through glass) experiences the most substantial improvement, as window films can reduce ETTV by an average of 14% when properly applied.
Given that walls at different orientations receive varying solar radiation, calculate ETTV separately for each facade:
East and west facades generally require films with lower SC values to counteract higher solar heat gain. Specifically, reflective films outperform heat-absorbing films on these orientations.
Finally, determine the weighted average ETTV using the formula:
ETTV = (Ao1 × ETTV1 + Ao2 × ETTV2 + … + Aon × ETTVn) ÷ (Ao1 + Ao2 + … + Aon)
Where:
This calculation provides the comprehensive ETTV value for Green Mark 2021 compliance assessment. A building with window films properly applied can achieve improvements sufficient to move up one certification tier, helping reach the target of 50 W/m² or lower without replacing existing windows.
Practical demonstrations reveal how window films transform building envelope performance across different building types and orientations. Let’s examine real-world ETTV calculations that showcase window film applications for Green Mark 2021 compliance.
East-west oriented buildings face the greatest solar heat gain challenges, making them ideal candidates for window film applications. Consider an office building with 70% WWR on east-west facades:
Before Film Application:
After applying reflective window film that reduces SHGC by 13.69% [6], the solar radiation component decreases substantially. The building achieves a new weighted average ETTV of 45.6 W/m², meeting Green Mark 2021 GoldPLUS requirements for office buildings [5].
A mixed development project with retail (5,000m²), office space (6,800m²), and a hotel component (7,500m²) requires different window film specifications for each section. Using the weighted average formula:
ETTV = (ETTV₁ × A₁ + ETTV₂ × A₂ + ETTV₃ × A₃) ÷ (A₁ + A₂ + A₃) [5]
With appropriate window films applied to each section:
The calculation yields:
(35×5000 + 45×6800 + 39×7500) ÷ 19300 = 40 W/m² [5]
This perfectly meets GoldPLUS certification requirements across all building types in the development.
For a comprehensive retrofit project utilizing window films, numerical calculations demonstrate significant improvements:
| Parameter | Before Film | After Film | % Improvement |
|---|---|---|---|
| U-Value | 5.6 W/m²K | 5.53 W/m²K | 1.33% [6] |
| SHGC | 0.65 | 0.56 | 13.69% [6] |
| ETTV | 55 W/m² | 47.3 W/m² | 14% [6] |
This project successfully achieved a 14% reduction in ETTV, moving from non-compliance to meeting Green Mark 2021 requirements without costly window replacement. Furthermore, the retrofit brought additional benefits including reduced indoor temperatures by 2-3°C on sunny days and decreased air conditioning load, supporting both energy efficiency goals and occupant comfort.
Window films stand as a proven solution for buildings seeking Green Mark 2021 certification through ETTV improvements. This comprehensive guide demonstrated how these films modify crucial thermal parameters – reducing U-values by 1.33% and SHGC by up to 13.69%, ultimately leading to significant ETTV reductions.
Through detailed examples, calculations showed that strategic window film application can decrease ETTV values by 14%, enabling buildings to achieve higher Green Mark certification levels without extensive renovations. East-west oriented buildings particularly benefit from reflective films, while mixed-use developments can employ varied specifications across different sections for optimal results.
The step-by-step calculation process outlined here empowers building owners and architects to:
Beyond ETTV improvements, these modifications deliver tangible benefits including reduced indoor temperatures, enhanced occupant comfort, and decreased cooling costs. Therefore, window films offer both immediate energy efficiency gains and long-term operational advantages for Singapore’s building sector.
[1] – https://magicwindow.ca/blog/understanding-the-shading-coefficient-essential-for-energy-efficient-windows
[2] – https://www1.bca.gov.sg/docs/default-source/docs-corp-news-and-publications/publications/codes-acts-and-regulations/ettv.pdf?sfvrsn=637eaf18_0
[3] – https://www.climatepro.com/blog/window-film-facts-what-is-solar-heat-gain-coefficient-or-shgc/
[4] – https://www1.bca.gov.sg/docs/default-source/docs-corp-buildsg/sustainability/gm_certification_std.pdf
[5] – https://www1.bca.gov.sg/docs/default-source/docs-corp-buildsg/sustainability/20210909_energy-technical-guide_r1.pdf
[6] – https://www.researchgate.net/publication/337382286_Impact_of_solar_insulation_film_on_the_cooling_load_of_an_office_building_in_Singapore_-_A_simulation_study
[7] – https://www.mdpi.com/1996-1073/17/6/1388
[8] – https://www.researchgate.net/publication/378948068_Evaluation_of_the_Effects_of_Window_Films_on_the_Indoor_Environment_and_Air-Conditioning_Electricity_Consumption_of_Buildings
[9] – https://www.researchgate.net/figure/Assumptions-for-the-calculation-of-ETTV_tbl1_289220250
[10] – https://archscience.org/wp-content/uploads/2015/12/041_Luther_Oraee_ASA2015.pdf
[11] – https://www.otm.sg/tag/u-value
[12] – https://www.pilkington.com/en-gb/uk/architects/information-hub/glass-information/energycontrolthermalsolarproperties/shading-coefficients
[13] – https://windowfilm.com/what-is-a-shading-coefficient/?srsltid=AfmBOoq2tsCYeBwzQdg3Yxpp2Lon6txjlh2SoXxY9vP8o1vyxjI7X39T
[14] – https://www.wbdg.org/resources/window-film-fundamentals
[15] – https://www.3m.com.sg/3M/en_SG/building-window-solutions-sg/resources/articles/full-story/~/auto-window-tint-laws/?storyid=28aba0fa-eccd-4a47-a289-55b3f0784586
[16] – https://pacific-tint.com/blog/understanding-visible-light-transmission-vlt/
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