Solar Reflectance Index in the built environment
The Solar Reflectance Index (SRI) is a measure of the solar reflectance and emissivity of materials that can be used as an indicator of how hot they are likely to become when solar radiation is incident on their surface. The lower the SRI, the hotter a material is likely to become in the sunshine.
This is important as solar radiation can cause buildings to overheat if their external fabric has a high solar absorptance and is unable to 'lose' (emit) absorbed heat by re-radiating it to the outside. In addition, the high solar absorbent nature of many human-made surfaces means that urban areas can have a higher average temperature than surrounding green spaces. This is referred to as the urban heat island effect.
The Solar Reflectance Index can be used in 'cool roof' calculations to demonstrate compliance with LEED (Leadership in Energy and Environmental Design), a voluntary environmental certification system developed by the U.S. Green Building Council.
SRI is a scale from 0 to 100 on which materials that absorb and retain solar radiation (and so become hotter in sunshine) have a lower number, whilst highly reflective materials (which remain cooler in sunshine) have a higher number.
The US department of energy defines the Solar Reflectance Index (SRI.) as '…a measure of the roof's ability to reject solar heat, as shown by a small temperature rise. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. For example, the standard black has a temperature rise of 90 deg. F (50 deg. C) in full sun, and the standard white has a temperature rise of 14.6 deg. F (8.1 deg. C). Once the maximum temperature rise of a given material has been computed, the SRI can be computed by interpolating between the values for white and black.'
NB This definition means that it is possible to have materials with an SRI of less than 0 or more than 100.
SRI can be calculated based on the pitch of a roof, its weight, its solar reflectance, thermal emittance (emissivity) and its ageing characteristics. Ref Cool Roof Rating Council Solar reflectance index calculation worksheet.
The selection of materials with a high SRI can reduce indirect solar gain in buildings and reduce the urban heat island effect. For example, metal roofs finished with a high SRI coating will have a lower temperature under sunny conditions compared to a similar, uncoated metal roof. However, effective solar design is more complicated than simply selecting high SRI materials, as in cooler locations solar gain may actually be desirable.
SRI can be considered a better indicator of a materials response to solar radiation than 'albedo' (from the Latin for 'whiteness') as albedo is a simple ratio of reflected to incident radiation and does not take account of emissivity.
Source: Cambridge Science Park
[edit] Related articles on Designing Buildings Wiki
- Albedo.
- Amber warnings raise building overheating concern.
- Cool roofs.
- Emissivity.
- Green roofs.
- G-value.
- Light reflectance value.
- Low-e glass.
- Part L.
- Passive building design.
- Preventing overheating.
- Shading coefficient.
- Solar gain.
- Solar heat gain coefficient.
- Solar transmittance (gtot).
- Thermal comfort.
- Thermal mass
- Thermal optical properties.
- Types of cool roofs.
- Urban heat island effect.
[edit] External references
- Autodesk, Direct solar gain.
- The US department of energy, Cool roofs.
- Cool Roof Rating Council Solar reflectance index calculation worksheet.
Featured articles and news
Amendment to the GB Energy Bill welcomed by ECA
Move prevents nationally-owned energy company from investing in solar panels produced by modern slavery.
Gregor Harvie argues that AI is state-sanctioned theft of IP.
Heat pumps, vehicle chargers and heating appliances must be sold with smart functionality.
Experimental AI housing target help for councils
Experimental AI could help councils meet housing targets by digitising records.
New-style degrees set for reformed ARB accreditation
Following the ARB Tomorrow's Architects competency outcomes for Architects.
BSRIA Occupant Wellbeing survey BOW
Occupant satisfaction and wellbeing tool inc. physical environment, indoor facilities, functionality and accessibility.
Preserving, waterproofing and decorating buildings.
Many resources for visitors aswell as new features for members.
Using technology to empower communities
The Community data platform; capturing the DNA of a place and fostering participation, for better design.
Heat pump and wind turbine sound calculations for PDRs
MCS publish updated sound calculation standards for permitted development installations.
Homes England creates largest housing-led site in the North
Successful, 34 hectare land acquisition with the residential allocation now completed.
Scottish apprenticeship training proposals
General support although better accountability and transparency is sought.
The history of building regulations
A story of belated action in response to crisis.
Moisture, fire safety and emerging trends in living walls
How wet is your wall?
Current policy explained and newly published consultation by the UK and Welsh Governments.
British architecture 1919–39. Book review.
Conservation of listed prefabs in Moseley.
Energy industry calls for urgent reform.