Building envelopes are the barriers between indoor and outdoor environments – they include walls, roofs, windows, and doors. Their role is much like clothing: we adapt what we wear to suit the weather, light shades on hot summer days, or waterproof jackets during rain. Similarly, we interact with building envelopes instinctively- opening windows for a breeze, pulling down blinds to block glare, or extending awnings to keep out the rain.
That’s why the design of the envelope isn’t just an architectural detail, it’s the starting point of a good building design. Why does envelope design matter now more than ever? Because we are living in a time when buildings must respond not just to comfort, but to climate, resilience, and rising energy demands.
Take Delhi, for example. Last May, the city recorded an unprecedented 52°C, a temperature disturbed daily lives of many. It was a stark reminder of how deeply our built spaces affect our comfort. Delhi is just one example among many regions increasingly exposed to such heat stresses.
India currently witnesses an annual demand of over 14 million air conditioning units, with the residential sector alone experiencing a nine-fold increase in just the past year. Air conditioning now accounts for nearly a quarter of the nation’s electricity demand which is driven by rising temperatures and a growing consumer preference for comfort and well-being.
In such a climate, the building envelope helps shield us from extremes while reducing the load on cooling systems. As temperatures continue to rise, designing for thermal comfort should be a norm. To achieve this, we need measurable ways to evaluate how much heat our buildings let in. One such metric, particularly relevant for residential construction, is the Residential Envelope Transmittance Value, or RETV.
Globally, RETV has seen strong policy uptake and has been adapted into building regulations and energy codes in several countries with region-specific customizations. The Bureau of Energy Efficiency (BEE) has formally recognized the importance of thermal envelope performance by integrating RETV evaluation into India’s residential energy code, the Eco Niwas Samhita. Introduced in 2018, this code sets measurable benchmarks for heat gain through external walls, windows, and shading in residential buildings.
What is RETV?
The RETV, measured in Watt per square meter, quantifies the net heat gain through a building’s envelope (excluding the roof). It assesses the thermal performance of a building envelope by considering heat gain through external walls, windows or glazing, and the shading elements applied to these non-opaque surfaces. A lower RETV signifies a more thermally efficient envelope, leading to cooler interiors and reduced reliance on air conditioning.
While RETV quantifies the envelope’s efficiency, the parameters assessed are also the key determinants of cooling energy needs of a building. A report by India Energy Transformation Platform demonstrated that reducing RETV with better envelope materials and design strategies from 18.5 to 8 can reduce discomfort hours indoors by 56%.
To evaluate savings on cooling costs, let’s take the example of an average home in Delhi with a 100 square meter of air-conditioned space. An 18.5 RETV is considered as a business-as-usual case with walls made of burnt clay bricks, single glazed window with aluminium frame, a window-to-wall ratio of 15% with minimum fixed shading. An example high performing envelope with 8 RETV comprises a walling assembly made with clay brick or concrete block cavity wall with insulation, double glazed window with uPVC frame, and optimized fixed shading. Reducing RETV by 10.8 units can save around 3062 units of electricity, amounting to Rs 15,000 a year to a resident. Investing about 11% delta capex on a high performing envelope can unlock close to Rs 3,75,000 throughout the building’s lifetime (25 years). It’s not just an investment in energy savings, but in long-term comfort and climate resilience.
Benefits of RETV beyond savings
Enhances Passive Design Evaluation
RETV provides a clear, quantifiable way to assess how effectively passive elements like shading, wall insulation, and glazing reduce heat gain. This helps designers evaluate and refine envelope strategies early in the design process so that cost to changes are low and engineer principles of comfort into their designs.
Meets Regulatory Compliance
The Eco Niwas Samhita 2018 (Energy Conservation Building Code for Residential Buildings) recommends an RETV of 15 W/m² as the minimum performance criteria, and even as low as 12 W/m² for some climate zones.
Green Building Rating Credits
For certifications (GRIHA, IGBC), achieving lower RETV is a pathway to excellence. It allows projects to go beyond the minimum requirement and be rewarded for superior design. This presents a strong opportunity for projects to gain a market advantage and showcase genuine innovation in material use and design quality.
Designing for Resilience
Achieving RETV beyond compliance involves optimizing envelope performance to limit heat gain under extreme climatic conditions, rather than just meeting minimum code thresholds. Such an approach enhances thermal autonomy, reduces cooling loads, and contributes to building resilience.
Staying Ahead of Regulatory Shifts
With BEE’s recent proposal to standardize air-conditioner set points between 20 to 28°C, building envelopes must be optimized to perform under these narrower comfort bands. Designing envelopes with lower RETV ensures the longevity and improved performance of mechanical cooling systems, while also enabling more comfortable indoor environments.
Incentives for Strong RETV Performance
The Energy Efficient Housing Program (EEHP) by State Bank of India, supported by KfW, offers concessional loans and technical assistance to projects achieving at least 25% energy savings and RETV targets below 12 W/m² (as low as 10). The program also provides an investment grant of around €10 million to incentivize developers who achieve 40% or greater energy savings.
While evaluating RETV clearly helps improve a building’s thermal performance, it’s not widely used yet. This is partly because it can seem complex or costly at first, and it is mostly used in projects planned for green building certifications. As a result, many regular buildings miss out on its benefits, slowing progress toward designing better envelopes.
To move forward, integrating performance metrics like RETV at the early design stage is the key. In India’s fast-growing residential sector, RETV serves as a standardized method to quantify envelope heat gain, that enables design teams to optimize wall, window, and shading configurations for thermal efficiency regardless of project scale or certification intent. Its widespread application can bridge the gap between policy goals and on-ground construction practice, making energy efficiency measurable, actionable, and scalable.
References:
- Maithel, S., Chandiwala, S., Bhanware, P., Rawal, R., Kumar, S., Gupta, V., & Jain, M. (2020). Developing cost-effective and low-carbon options to meet India’s space cooling demand in urban residential buildings through 2050. India Energy Transformation Platform. link
- Bhanware, P., Chandiwala, S., Maithel, S., Rawal, R., & Kumar, S. (2020). Methodology for RETV formula development for ENS 2018. Bureau of Energy Efficiency/Eco-Niwas Samhita. link
- Wienerberger India & Greentech Knowledge Solutions Pvt Ltd. (n.d.). Calculation of RETV for residential projects: A comparative study for different walling materials across various climate zones. Wienerberger India. link
- SBI, KfW sign $277 million agreement for energy-efficient housing programme in India. Business Standard, Dec. 16, 2019. link
