Introduction
According to the Indoor Air Quality research briefing published by the UK Parliament in 2023, people spend about 80-90% of their time indoors. Given this statistic and knowing the fact that our environment has a direct impact on our health, it becomes quite crucial that we pay special attention to the design of the internal environments in our buildings.
Some of the key factors requiring improvement in our internal environments include thermal performance of the fabric of the building and an improved response to our changing climate. As our climate continues to change, it is more likely that occupants will suffer temperature related health issues, so it is vital our design considers in detail aspects such as thermal comfort, ventilation, daylighting, solar gain and solar shading from an early stage in the design process.
In this post, we will be taking you through these key aspects to help you design the best indoor spaces for your clients. Read on for all the insights!
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Internal Environment
The internal environment refers to the conditions within a building that affect how people feel and function.
These include:
- Temperature/Thermal Comfort
- Air Quality
- Acoustics
- Lighting
- Spatial Quality
- Materials
- Views
All of which can influence a person’s comfort, health, productivity, experience and more.
Indoor environments can be complex as they are composed of several overlapping layers of these conditions. Let’s take a closer look.
Thermal Comfort
Thermal comfort is linked to a feeling of wellbeing of an occupant. For instance, if there is a high level of wellbeing, a high-performance level can be expected in a work environment.
Thermal comfort can vary from person to person and has many contributing factors. Our bodies are constantly producing heat energy from the food we consume, which is dissipated in the form of convection, radiation and evaporation in order to keep our bodies at a constant temperature. Our heat production can depend on our size, our age, our sex, the activity we are carrying out and the clothes we are wearing.
The level of thermal comfort we experience within a space can be affected by various factors. Some of which are personal to us, while others relate to the conditions around us.
Personal variables include:
- Activity
- Age
- Gender
- Clothing
- Adaptation and acclimatisation
- Room occupancy
- Body condition
- Wellbeing and sickness
Physical variables include:
- Air temperature
- Air movement
- Air quality
- Surface temperature
- Humidity
- Building design
- Visual influences
- Acoustic conditions
You will find information on thermal comfort such as that from the Health and Safety Executive often highlights six out of the aforementioned factors as the main ones influencing thermal comfort.
The first two are physical variables:
Activity
The greater the activity level, the more heat our body will emit. The rate this heat emits will depend on the metabolic rate and surface area of the individual.
As shown in the table below, a person who is immobile while sleeping emits way less heat as opposed to a person engaging in heavy lifting.
[Table adapted from Environmental Science in Building – Randall McMullan]
Clothing
Clothes help us maintain a more constant body temperature by providing a thermal insulation layer. Too much clothing can lead to heat stress while too little can lead to cold stress. This is why we wear varied clothing at different times of the year and our buildings must be adaptable to provide comfortable temperatures for these variables.
Air Temperature
The temperature of the air we are in contact with can have an impact on our thermal comfort. Temperature tends to be the most decisive factor of an occupant’s thermal comfort. This can depend on mood, duration of stay and so on.
Air Movement
Air movement helps increase the heat lost from the body by convection and can give the sensation of draughts. On a warm day this can be very pleasant, but on a cold day a draught can be very uncomfortable. Thermal discomfort tends to be experienced more when there is a draught when the occupant is sitting, so when people are sitting in an office, school or conference type setting it is important to minimise the feeling of air velocity (draught). Natural draughts can occur near windows or in rooms with high ceilings.
Mean Radiant Temperature
Our bodies naturally emit heat and continuously exchange thermal radiation with our surroundings. This exchange is primarily driven by the temperature differences and emissivity levels between our skin (or clothing) and the surrounding surfaces.
Mean Radiant Temperature (MRT) represents the weighted average temperature of all surfaces in an environment. This includes walls, windows, heat-producing equipment, and the influence of the sun and sky. It is a key factor in thermal comfort, as it determines whether our bodies will gain or lose heat to the physical objects around us, regardless of the air temperature.
Humidity
The ideal relative humidity (RH) range is between 40 and 60% for comfortable conditions according to most professional bodies. High humidity with high temperatures can lead to an oppressive feel and decrease the ability of the human body to naturally cool by perspiration. High humidity can also lead to mould growth which can aggravate asthma and allergies.
Extreme thermal discomfort can lead to thermal stress, which occurs during particularly hot summers or even moderate winters. Factors such as draughts, high relative humidity and cold temperatures from poor heating can lead to thermal stress, which causes a number of medical conditions. Thermal stress is more common in the elderly, very young or infirm.
Some key considerations when designing for thermal comfort are:
- Ideal internal temperatures in winter: 18-21˚C
- Ideal internal temperatures in summer: 22-27˚C
- Occupants can experience thermal discomfort when the vertical air temperature difference between the head and foot level exceeds approximately 3˚C. This can be alleviated with underfloor heating.
- Ensure relative humidity (RH) is between the comfortable levels of 40%-60%
- To avoid high levels of CO2 concentrations, ensure there is suitable ventilation to keep concentrations below 1000-1200ppm (parts per million).
- Use Mechanical Extract Ventilation (MEV) or Mechanical Ventilation with Heat Recovery (MVHR) to create good air quality in winter.
- Allow users to operate elements to achieve improved thermal comfort – for example blinds, window openings, adjusting the heating and cooling.
Here are some questions to guide your design thinking as well:
- Do internal spaces respond to the sun path?
- Is there natural purge ventilation for cooling?
- Is there sufficient year-round background ventilation?
- Is daylight maximised by providing a shallow plan?
- Is there good night cooling (especially in thermal mass designs)?
- Is solar shading provided (horizontal on the south and vertical on the east and west)?
- Are all heating, cooling and solar shading systems easy to control and maintain?
[Reference: Environmental Design Pocketbook – Sofie Pelsmakers]
Optimising thermal comfort should be woven into the general architectural design process, with the tips above proving a useful checklist to get you started.
You can also check out our Building Fabric Series on our Architecture and the Environment page for further guidance.
Air Quality
Optimal air quality is another vital consideration during building design. Not only do humans need air to breathe and live, the quality of air in our buildings whilst at home, at school, at work or during recreational activities also has an impact on our feeling of wellbeing. Poor air quality can result in people suffering from a number of ailments including skin irritations, headaches, tiredness and general feelings of being unwell.
Air quality requirements will vary according to the different activities carried out in the space, the number of people and the length of their stay in that space, amongst other factors. It is also worth noting that outside air quality will have an impact on the type of ventilation specified.
Human emissions also contribute to the interior air quality, along with emissions from materials in the room. To maintain a healthy internal environment, any building materials that have a negative impact on air quality like those with high chemical off gassing, should not be specified.
Both mechanical and natural ventilation can be used to provide consistent clean air to a building, and usually a combination of both of these works well.
Ventilation in buildings has a number of objectives including:
- supply of oxygen for human breathing
- removal of carbon dioxide from human breathing
- control of humidity for human comfort
- control of air velocity for human comfort
- removal of odours
- removal of micro-organisms, mites, moulds etc.
- removal of excessive heat
- removal of water vapour to help prevent condensation issues
- removal of particles such as dust and smoke
- removal of volatile organic compounds (VOCs) from cleaning solvents, carpet, furniture and building products
- removal of ozone gas from photocopiers and laser printers
- removal of cooking and heating combustion products
[Reference: Environmental Science in Building – Randall McMullan]
Acoustics
Sound can have an impact on our physical and mental wellbeing. Numerous surveys have indicated that people find their home in some way unsatisfactory because of noise intrusion – this includes people living in new homes built to modern building standards. It is important when designing buildings that we consider the sound quality of our built environment. Unwanted sound is perceived as a nuisance and can cause a number of emotional effects.
The primary noise intrusion is outside noises, particularly traffic both in permanent noise levels (a busy road), and short term, (passing airplane, train etc). Outside noise levels can influence levels of concentration, work performance and interrupt our rest or sleep.
A factor to consider when designing ventilation strategies is the opening of windows in warm temperatures and whether this activity will be impacted by external noise levels. Reports indicate that occupants are more willing to accept higher levels of noise from outside for the sake of natural ventilation through windows on a warm day.
Other noise sources can include industrial noise, construction noise and noise from leisure and entertaining. Inside buildings we can experience noise issues from other occupants, machinery and appliances and so on. It is also worth noting some activities require specialist sound design, for example recording studios.
The following table provides a range of different environments and the sound pressure levels reached in each:
Lighting
Achieving visual comfort in a building is usually a combination of both natural daylight and artificial lighting. Designs should attempt to include a good amount of natural daylight without risking summer overheating.
Daylighting
Daylight enters a building through skylights or windows, but these windows can also transmit heat, sound and air. It is therefore important to consider natural lighting along with all the other environmental variables.
Daylighting depends on:
- the amount of daylight available
- any obstructions or reflectance
- internal reflectance of room finishes
- location shape and size of the window
- depth of the plan of the room
Artificial Lighting
The selection and specification of artificial lighting will be designed in conjunction with other environmental factors such as daylighting and window openings, heating and cooling, extent of the plan, use of the building and so on.
The main functions for artificial lighting are:
Task – lighting to help building occupants to carry out specific activities
Movement – lighting to allow the building occupants to move safely around the building
Display – lighting to illuminate a particular feature of the building or particular element within the building
[Reference: Environmental Science in Building – Randall McMullan]
You can explore these articles for more tips on lighting:
Designing for Daylight – 45 and 25 Degree Test
How to Maximise Light in House Extensions
How to Get Natural Light Into Basements
Lighting Design Concepts
Spatial Quality
The building itself is the container in which the internal space exists and so it quite literally shapes it. It’s not just the floorplan that matters but the vertical volume that affects how a space feels to a user. Large open spaces can feel expansive and airy while more enclosed spaces can offer a sense of security or maybe even a feeling of restriction.
The primary structure like the columns, load-bearing walls, and floor slabs define the permanent boundaries of the space. Keeping this structural grid simple allows for easier reconfiguration later on.
How the space can be adapted to user needs for different activities or life stages can add to its desirability. Including things like movable walls or modular furniture can help a room transition into a live/work zone or change as a family grows.
If you’re looking for a deeper dive on layouts, you might find our guide on Space Planning Basics useful.
Materials
The materials used within a space also add to how people interact with the buildings we design. They can enhance internal spaces visually and create different types of atmospheres, be it through warm textured timber or cool polished stone. Material transitions in a building can be used to intuitively guide users from one space to another.
Varying material textures and finishes adds depth and character. These sensory layers encourage users to explore the space in a tactile and haptic manner.
The right specification of materials matters not only to achieve aesthetic goals but also in terms of promoting health and wellbeing. Building regulations prioritise Low-VOC (Volatile Organic Compound) finishes to ensure that the air remains free from toxins. Beyond air quality, natural materials like clay plasters and lime offer unique advantages, from moisture regulation to carbon sequestration. We explore these in detail throughout our Natural Building Materials series.
Materials can also be strategically used to absorb unwanted sounds and improve internal acoustics. High thermal mass materials can store heat and radiate it slowly, maintaining a steady, comfortable temperature that reduces the need for mechanical heating and cooling.
Views
Having views to the outside overlooking nature or even the cityscape can have positive impacts on people’s wellbeing. Windows don’t just let light in but also create a connection between the inside and the outside. They also provide visual relief as we spend the majority of our time glued to screens.
Framed views can provide people with focal points to look at. Perhaps this could be a singular tree that stands out on your site. These types of views can also be seen as living artwork that gradually shifts as day turns into night.
Different types of openings can create different effects and can be used to bring interest to a space. Panoramic windows offer a sense of transparency and scale. Window nooks and seats create an intimate, tactile connection to the outdoors and can encourage rest. Low-level or vertical slit windows can bring in shifting light patterns and glimpses of the ground plane or garden without exposing the interior.
Prioritising views help us link to a specific context, making the internal environment feel like an extension of its surroundings rather than an isolated box.
You might also find our posts on Biophilia in Architecture – Designing with Nature for Better Living and What is Liminal Space? relevant to this topic.
Books
Here are some recommendations for further reading:
You Might Also Be Interested In…
The following articles from our Building Fabric Series, study some of the design requirements for the many different internal environment factors in more detail.
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Conclusion
To summarise, how we design our internal environment directly impacts our end users and their overall experience of our buildings. The careful consideration and balance of the key conditions we discussed in this post is vital in ensuring that experience is positive. By focusing on how light, materials, and space interact, we can create environments that don’t just function but actively support the health and happiness of the people inside. Ultimately, a building is only as successful as the comfort of the people who use it every day.
We hope this post helps you think about your next project from the inside out.
Don’t forget, you can read all of the posts in our Building Fabric Series here:
Architecture and the Environment
Thank you for reading!
Your Comments
Internal environments can be tricky to get right. What is your best advice? Do share it in the comments below.
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Author
Written by Emma Walshaw, Architectural Technologist and founder of First In Architecture and Detail Library. Emma has written a number of books about construction and architectural detailing.
Informative article with useful references
Thank you Jonathan, much appreciated.