Overheating is now a regulated risk on UK projects. Approved Document O (Part O) sets out how new residential buildings in England must limit solar gains and remove excess heat, either via a simplified route or dynamic thermal modelling; shading and solar control are recognised components of both approaches. Design teams typically assess risk using CIBSE TM59 for residential and CIBSE TM52 for non‑domestic spaces, in which shading can materially improve outcomes.
Independent measurements show external shading (e.g., brise‑soleil, external venetians, external screens) rejects heat before it enters the glazing system and can achieve much lower system g‑values than internal devices, meaning lower peak temperatures and smaller cooling loads. For schools, BB101 (2018) adopts the adaptive overheating method and often necessitates shading to manage classroom conditions during teaching hours.
Implication for QS/Commercial: If your mitigation strategy relies on shading, make sure it’s in the base build scope and reflected in the model, leaving blinds to tenant fit‑out can invalidate the compliance evidence and cause expensive re‑work.
Homes & residential‑type schemes
TM59 defines overheating using adaptive comfort (linked to prevailing outdoor conditions) and a bedroom night‑time limit: from 22:00–07:00 the operative temperature should not exceed 26 °C for more than 1% of annual hours. When blinds form part of the mitigation, TM59 guidance is clear: they must be included in the base build and the model, along with any impact on natural ventilation.
Non‑domestic buildings
TM52 uses three criteria: Hours of Exceedance, Daily Weighted Exceedance, and an Upper Temperature Limit, based on the adaptive comfort model for free‑running buildings. Exceeding two of the three criteria indicates unacceptable risk.
Schools
BB101 adopts the TM52 approach for 09:00–16:00 teaching hours between May–September, with an emphasis on passive measures (ventilation, thermal mass, shading) to maintain comfort without excessive reliance on mechanical cooling.
External shading sits outside the thermal envelope, intercepting solar radiation before it hits the glass; most absorbed energy is shed to the outdoor air. Long‑term field measurements at Lund University found average system g‑values around 0.3 for external devices, ~0.5 for products placed between panes, and ~0.6 for internal devices, illustrating why externals can halve peak solar gains compared with internals (product specifics vary).
Methodologies from Lawrence Berkeley National Laboratory show how to calculate an adjusted SHGC (or g‑value) for windows with external shading; the physics is simple: reject heat before it enters and peak cooling loads fall. Laboratory comparisons also indicate that venetian blinds (because of slat geometry) can achieve lower SHGC than roll screens under the same glazing, reinforcing the need to select and angle products carefully.
Where internal solutions shine: Internal blinds and contract curtains are critical for glare control, privacy, black‑out (AV/meeting/bedrooms), cost, and programme. They can support compliance when appropriately specified, and in many refurbishments, they are the only practical option. However, for overheating specifically, expect less impact than a well‑designed external system.
To compare options fairly, specify performance using EN 14501:2021 (+A1:2025), which classifies blind, shutter and awning systems for thermal (e.g., total solar factor, g_tot, direct solar transmittance) and visual comfort (e.g., glare control, daylight utilisation, privacy) characteristics.
Specifier tip: Always request system‑level g_tot for the actual glazing + shading build‑up (not fabric alone). The g_tot figure you put into TM59/TM52 is the number that matters for overheating risk.
BREEAM Hea 01 (Visual Comfort)
BREEAM requires a glare control strategy in relevant spaces (DSE offices, classrooms, receptions, etc.). Compliant measures include building‑integrated shading (overhangs/fins), external devices, and appropriately specified internal blinds; the design should not reduce daylight so far that electric lighting energy increases.
NABERS UK (operational performance for offices)
With CIBSE appointed scheme administrator in April 2024, more UK office clients are targeting measured reductions in operational energy. Intelligent solar control (e.g., automated external venetians linked to BMS/KNX and daylight sensors) can reduce cooling energy and enable daylight harvesting, supporting better NABERS UK star ratings post‑occupancy.
Preventing overheating is most effective when solar control is considered early, quantified properly (at system g_tot level), and coordinated across façade, ventilation, and controls. The right mix of external shading for heat rejection and internal blinds/curtains for glare, privacy, and flexibility de‑risks Part O/TM59/TM52/BB101 sign‑off, improves occupant comfort, and can help control both capex and opex. With clear performance targets and robust evidence, project teams can confidently specify systems that work on paper and in operation.
Both describe the total solar energy transmittance of a window system; in European practice and EN 14501 you’ll often see g and g_tot (the latter for the glazing + shading combination). For overheating assessments, always use system g_tot for the actual build‑up.
Yes, but if they’re used as mitigation, blinds must be in the base build and represented in the model; they also must not unduly reduce the ventilation assumed for compliance.
Because externals intercept and shed solar energy outside the envelope, achieving lower system g‑values and lower peak room temperatures than internal devices under the same glazing. Field measurements and calculation methods consistently show this effect.
Specify BS 5867‑2 (Type B for most commercial/public, Type C for healthcare or frequent hot laundering) and keep certificates with O&M documentation.
BREEAM Hea 01 expects a glare control strategy using compliant measures that don’t drive up lighting energy, while NABERS UK rewards operational energy performance, both benefit from intelligent solar control that reduces cooling loads and preserves daylight.
