WFR is the ratio of window area to that of the floor area. ENS requires the building to comply with a minimum WFR. Ensuring minimum WFRop helps in ventilation, improvement in thermal comfort, and reduction in cooling energy
The openable window-to-floor area ratio is calculated as shown
Where,
A openable = Area of the opening
A carpet = Area of the space (net)
The following threshold needs to be met for WFR compliance:
| Climatic Zone | Composite | Hot-Dry | Warm-Humid | Temperate | Cold |
|---|---|---|---|---|---|
| Minimum WFR | 12.50% | 10.00% | 16.66% | 12.50% | 08.33% |
Source: Adapted from Bureau of Indian Standards (BIS). 2016. National Building Code of India 2016. New Delhi: BIS.
Identify your climatic zone as per the map below:
Openable area: The openable area in WFR includes all openable areas that open directly to external/ outside air.
What can be considered as Openable area: Window, ventilator, an open balcony, ‘verandah’, corridor or shaft; and the openable area of the doors opening directly into an open balcony.
Exclusions in Openable area:
Carpet area: The carpet area of dwelling units is the net usable floor area of a dwelling unit.
What can be considered as Carpet area:
Exclusions in Carpet area:
Note: The results displayed here are based on the inputs provided by the user. This implies compliance/ non-compliance to this particular parameter only.
VLT of non-opaque building envelope components (transparent /translucent panels in windows, doors, ventilators, etc.), indicates the potential of using daylight. Ensuring minimum VLT helps in improving daylighting, thereby reducing the energy required for artificial lighting. The challenge with VLT is that less VLT can lead to less daylight and more VLT can lead to glare. Optimization is thus crucial.
The glass used in non-opaque building envelope components (transparent/translucent panels in windows, doors, etc.) shall comply with the requirements given in the following table. The VLT requirement is applicable as per the window-to-wall ratio (WWR) of the building. Hence, it is essential to calculate WWR first.
The following threshold needs to be met for VLT compliance:
| WWR | 0-0.30 | 0.31-0.40 | 0.41-0.50 | 0.51-0.60 | 0.61-0.70 |
|---|---|---|---|---|---|
| Minimum VLT | 0.27 | 0.20 | 0.16 | 0.13 | 0.11 |
Source: Bureau of Indian Standards (BIS). 2016. National Building Code of India 2016. New Delhi: BIS.
The Window-to-Wall-area-Ratio (WWR) should be calculated first to understand the minimum VLT requirements. Check out the ‘Guide to calculate VLT’ to understand calculation of WWR followed by VLT
The Window-to-Wall-area-Ratio (WWR) should be calculated first to understand the minimum VLT requirements.
WWR is the ratio of the area of non-opaque building envelope components of dwelling units to the envelope area (excluding roof) of dwelling units.
Based on this WWR, the minimum VLT as required for compliance (as shown in Compliance threshold ) should be met
Selecting glass with the required VLT
Great! Now you can correlate this with the table under ‘Compliance threshold’ to understand the minimum VLT to be achieved!
Note: The results displayed here are based on the inputs provided by the user. This implies compliance/ non-compliance to this particular parameter only.
Thermal transmittance of a material assembly is its ability to conduct heat through the assembly. Limiting the U value helps in reducing heat gains or losses through the material assembly, thereby improving the thermal comfort and reducing the energy required for cooling or heating.
The ENS categorizes the requirements of U-value under the following:
Limiting the Uroof helps in reducing heat gains or losses from the roof, thereby improving the thermal comfort and reducing the energy required for cooling or heating.
Limiting the Uenvelope,cold helps in reducing heat losses from the building envelope, thereby improving the thermal comfort and reducing the energy required for heating.
The following threshold needs to be met for U-value compliance:
Thermal transmittance of roof shall comply with the maximum Uroof value of 1.2 W/m2.K.
The thermal transmittance of the building envelope (except roof) for cold climate shall comply with the maximum of 1.8 W/m2.K.
Uenvelope,cold takes into account the following:
The following are links to reliable external tools:
U-value calculator
|
Material Name (Outer layer to Inner layer) |
d (thickness) in metres (m) |
K (Conductivity) in (W/mK) |
R= d/k (m2K/W) |
Action | |
|---|---|---|---|---|---|
| Outermost layer | |||||
| Material-2 | |||||
| Material-3 | |||||
| Innermost Layer | |||||
Note: The results displayed here are based on the inputs provided by the user. This implies compliance/ non-compliance to this particular parameter only.
Residential envelope transmittance value (RETV) for building envelope (except roof) for four climate zones, namely, Composite Climate, Hot-Dry Climate, Warm-Humid Climate, and Temperate Climate
RETV is the net heat gain rate through the building envelope (excluding roof) of the dwelling units divided by the area of the building envelope (excluding roof) of the dwelling units. Its unit is W/m2
The following threshold needs to be met for RETV compliance:
The RETV for the building envelope (except roof) for four climate zones, namely, composite, hot-dry, warm- humid and temperate shall comply with the maximum RETV of 15 W/m2.
RETV formula takes into account the following:
The RETV calculation of the building envelope (except roof)
shall be carried out as shown below:
| Aenvelope | : | envelope area (excluding roof) of dwelling units (m2). It is the gross external wall area (includes the area of the wall and the opening such as windows and doors). |
| Aopaquei | : | area of different opaque building envelope components (m2) |
| Uopaquei | : | thermal transmittance values of different opaque building envelope components (W/m2.K) |
| Anon-opaquei | : | area of different non-opaque building components (m2) |
| Unon-opaquei | : | thermal transmittance values of different non-opaque building envelope components (W/m2.K) |
| SHGCeqi | : | equivalent solar heat gain coefficient values of different non-opaque building envelope components |
| 𝜔i | : | orientation factor of respevtive opaque and non-opaque building envelope components; it is measure of the amount of direct and diffused solar radiation that is received on the vertical surface in a specific orientation. |
Links to relevant guidance on RETV calculation:
The following steps can be followed to submit your building for compliance:
| Sl.No. | Compliance Parameter & supporting documents |
|---|---|
| 1 | Architectural drawings with plans, sections & elevations clearly indicating the following |
| (i) Position and sizing of opening | |
| (ii) Building materials used in various envelope components | |
| (iii) External and internal walls | |
| 2 | Bill of Quantities (BOQ) with all materials purchased |
| 3 | Construction photographs |
| 4 | Specification sheets with mentioning parameters for compliance |
| 5 | Narrative elaborating compliance to ENS Part-1 with calculations (wherever relevent) |
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Last Updated : 08/28/2022