Do Internal Blinds Affect Overheating (Doc O) or Fuel & Power Consumption (Doc L)?

25th Nov 2025

It’s widely accepted that keeping curtains open on sunny days and closing them at dusk or in dull weather can save on heating costs over the winter months. Conversely, we all know that keeping the blinds down during heatwaves helps keep us sane during hot summer days, and might even soften our craving for energy-hungry cooling equipment.

So why are the Building Regulations Approved Documents L (conservation of fuel and power) silent on these well-reported benefits of blinds and curtains?

Approved Document O (Overheating) does state “internal blinds and curtains provide some reduction in solar gains” but doesn’t provide for using these as part of the shading strategy for Building Regulations. Bear in mind, however, that the ADO also comes with a warning: “Following this guidance does not guarantee the comfort of building occupants.”

And don’t we know it.

The critical thing to bear in mind is that the Building Regulations are limited in scope to the bare "asset rating"—the intrinsic energy performance of the building's structure and fixed services—rather than the net "operational rating" which is, of course, a key consideration of the building owner/landlord/client. 

This is where optional certification schemes bring added value to developers; research demonstrates that health and environmental-certified spaces (such as WELL and BREEAM) attract up to 7.7% uplift in rent values. 

We know from experience that providing residents and occupiers with the option to control their own environment with adjustable blinds reduces energy use and complaints, with clear benefits to landlords in renewal rates and operating costs. This is true for both renovations and newbuilds carried out in accordance with the latest regulations. Decor Systems was recently called in to resolve a serious overheating issue in a newly built 35-story residential unit that had been constructed to post-2022 Building standards, including Part O and L.

The landlord was considering active cooling measures, which would have required significant investment and remedial work. We designed a solution by installing blinds made from a carefully selected fabric that improved the G-value of the existing windows on the south-facing elevations, at a fraction of the disruption and cost of any HVAC option.

So, despite not being factored into formal building regulations, interior blinds still have a significant impact on occupant comfort and well-being – a key commercial and ethical consideration for BTR and landlords.

Below, we outline some of the real advantages of internal blinds for occupier comfort, energy savings, and thermal wellbeing. 

✅ 1. Improved Occupant Thermal Comfort

  • Reduction in radiant heat during summer

Even though internal blinds do not stop solar radiation before it enters the building, they:

  • Block direct solar radiation from hitting the skin, especially so with metallised fabrics.
  • Create a barrier between occupants and the window glare
  • Reduce radiant asymmetry (feeling hot on the side of the body and cold on the other)

This improves thermal comfort perception, a major factor in how occupants rate “overheating” and subsequently, their use of powered cooling equipment.

  • More uniform temperature distribution 

If sunlight falls on only part of a room, that area warms up and increases the air temperature, at least locally. Internal blinds:

  • Reduce direct solar patches, which create hot and cold spots
  • Help maintain more stable internal temperatures by absorbing solar energy during peaks, then slowly releasing it.
  • Make room conditions feel more predictable, controllable and comfortable.

 

✅ 2. Potential Impact on HVAC Loads & Energy Use

Internal blinds can influence operational energy use by reducing demand for active cooling and heating.

Internal blinds:

  • Reduce direct solar heat absorbed by interior surfaces, slowing secondary heat re-radiation
  • Lower “peak discomfort periods” in heatwaves or winter extremes
  • Reduce reliance on fans, local air-conditioning units, and portable heaters.
  • Delay the time when cooling or heating becomes necessary
  • By keeping blinds down during hot summer days, solar heat-gain is trapped between the glazing and blinds, for release during the cooler night by cross-ventilation or opening the window.

This doesn’t eliminate overall heat gain into the building’s fabric over time, but it mitigates its practical impact. Reducing the perceived temperature in occupied zones can lead to energy savings when HVAC or active ventilation is in place, even if the overall air temperature changes minimally.

✅ 3. Benefits for Winter Comfort & Heating Energy

Internal blinds can be effective as a winter comfort measure, as they.

Reduce radiant heat loss

Occupants feel colder near windows because:

  • Glazing has a lower internal surface temperature
  • The human body loses radiant heat to the cold surface

Internal blinds improve comfort by:

  • Creating a radiant barrier between the cold glass and the room
  • Raising the perceived temperature near the window

This is a well-documented thermal effect, that was demonstrated by University of Salford’s Energy House Labs 

Reduce heat loss (thermal resistance improvement)

Blinds add an insulating layer between indoor air and glazing. Benefits:

  • Lower convective heat loss
  • Reduced draughts
  • Slight improvement in window U-value (not enough for regulatory credit, but enough to improve comfort and potentially lower heating demand)

High-performance blinds and curtains, especially honeycomb/thermal fabrics, offer:

  • Up to 20–30% reduction in heat loss through windows (depending on design and airtightness)
    This mainly applies to blinds that create a sealed or semi-sealed air pocket.

✅ 4. Psychological & Environmental Comfort Benefits

Internal blinds and curtains contribute to Perceptions of warmth:

  • The feeling of a “warmer” room
  • Reduced cold-edge perception around windows
  • Improved night-time comfort, especially where blinds come down behind, rather than in front of, radiators.

Even when measured air temperatures do not significantly change, occupants reliably report “warmer-feeling” spaces with the blinds or curtains drawn.

✅ 5. Behavioural Benefits: Occupants Use What’s Easy

Internal blinds are familiar and simple for anyone to operate, unlike some HVAC systems.

Because occupants actually use them reliably:

  • They provide consistent comfort improvement, leading to less energy wastage
  • They support adaptive comfort in both summer and winter, and throughout the day or night.

This “user-friendliness” is one reason they remain so popular in offices and residences, even when not recognised in Building Control Approved Documents.

BREEAM and WELL credits

At Decor Systems we work closely with architects and developers to ensure they can simultaneously achieve their design aspirations and targeted WELL and BREEAM credit potential by correctly specifying their interior shading. 


If you'd like more information on the topic of internal blinds for solar and heat control, or wish to discuss your particular project requirements please reach out to our technical design team, on +44(0)1302 761122 or online