The interior design guide
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The interior design guide. A definitive knowledge base for interior designers.

From first principles to the finishing touches: shell, surfaces, space planning, furniture, electrics, plumbing and joinery, in the order a project runs.

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The Lexicon.

Every term, in plain words.

From hue and light reflectance to bonding and kick space, every term the guide uses has a short definition, what to know in practice and the pages that use it.

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The interior design guide.

A definitive knowledge base for interior designers.

Eleven chapters take a home from first principles to the moment you show it to the client. Start with Foundations and read in order, or search for a topic, a term or a rule of thumb.

Chapters.

Eleven steps, from principles to presentation.

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Where to go next.

Sections.

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Where to go next.

Temperature,

how warm a room feels, not just reads.

A thermostat reads one number, but a room is a mix of temperatures: warm air, cold glass, a sunlit floor. Learn how heat moves, why the same reading can feel warm in one room and cold in another, and how layout, materials and shading keep a home comfortable through the year.

Temperature basics.

What temperature is and how we measure it.

Temperature says how hot or cold something is, in degrees Celsius (°C) or Fahrenheit (°F). In a room, the air and every surface have their own temperatures. A thermostat measures only the air near its sensor, so it misses what a person actually feels: a cold window, a sunlit floor or a warm ceiling changes comfort while the air stays the same. Read temperatures where people actually sit and stand, away from direct sun and heat sources.

Point at the air, the window, the wall or the sunlit floor to read its temperature. One room, four numbers.

Living rooms

20–22°C

For sitting still in everyday clothes.

Bedrooms

16–19°C

Cooler air helps sleep, under covers.

Humidity

40–60%

Relative humidity that feels fresh and keeps mould away.

How heat moves.

Why rooms warm up and cool down.

Heat always flows from warmer to cooler, in three ways. Conduction carries it through materials, such as a wall or a window. Convection carries it between surfaces and moving air, such as the warm air rising off a radiator and circling the room. Radiation carries it between surfaces with no contact at all, such as a person losing warmth to cold glass across the room. Sunlight, people, lamps and appliances add heat; the walls, windows and fresh air add or remove it depending on the weather. A room’s temperature is the balance of all of these.

Conduction

Through materials. Heat passes through the glass, from the warm room to the cold outside.

Convection

Through moving air. Air warmed by the radiator rises, cools against the glass and sinks again.

Radiation

Across space, with no contact. A person loses warmth to cold glass across the room.

Thermal comfort.

Why the same temperature can feel different.

Comfort depends on more than the air. The surfaces around us exchange heat with our bodies by radiation; moving air takes heat away faster; humidity changes how easily sweat evaporates; and clothing and activity change what feels right. Operative temperature combines the air temperature with the mean radiant temperature of the surroundings, which is why two rooms with the same thermostat reading can feel different. Local conditions matter too: a draught at the ankles or sun falling on one person can make part of a comfortable room uncomfortable.

Beside cold glass

The air is 21 °C, but the person loses heat to the cold window and feels cooler.

Among warm walls

The air is 21 °C too, but warm surfaces give heat back and the person feels warmer.

Designing for temperature.

Using layout, materials and controls to improve comfort.

Control unwanted sun with shading, ideally outside the glass, where it stops heat before it enters. Place seating away from cold glazing, hot surfaces and draughts, and keep radiators, supply outlets and return grilles clear of furniture and curtains. Put thermostats and blind controls where people can reach and understand them. Insulation slows heat flow through the envelope. Thermal mass, such as stone, concrete or brick, stores heat and gives it back later, evening out the swings between day and night. Ventilation cools a room when the outside air is cooler, but adds heat when it is warmer. Design for every season, and give people simple ways to adjust.

Summer

Shade the glass from outside, open windows when the air outside is cooler, and use thermal mass to soak up the day’s heat.

Winter

Let low sun in, close up and seal draughts, and move seating away from cold glass. Never block a radiator with a sofa or long curtains.

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Where to go next.

Previous

Acoustics

Next

Representation

Questions.

What people ask most about indoor comfort.

What is room temperature?

Room temperature usually means about 20–22 °C, the range most people find comfortable sitting still indoors in everyday clothes. Bedrooms feel better a little cooler, around 16–19 °C. What you feel also depends on the surfaces around you, draughts, humidity and clothing.

What should the humidity be in a house?

Aim for 40 to 60 percent relative humidity. Below about 30 percent, air feels dry and static builds up; above 60 percent, condensation, mould and dust mites become likely. In cold winters, 30 to 40 percent avoids condensation on windows.

How do I lower the humidity in a house?

Remove moisture where it is made: use extractor fans while cooking and bathing, dry clothes outside or in a vented dryer, and open windows briefly to air rooms. Keep rooms evenly heated, avoid blocking radiators, and use a dehumidifier in damp rooms or basements.

What should I set my thermostat to in winter?

About 20 °C when you are home and awake suits most people, with 16–18 °C overnight and when you are out. Each degree lower saves noticeable energy. Avoid turning it off completely in very cold weather, to protect pipes and prevent condensation.

What is radiant heat?

Radiant heat warms people and surfaces directly, without heating the air first, the way the sun warms your face on a cold day. Underfloor heating, radiant panels and warm walls work this way. It feels comfortable at a lower air temperature and avoids draughts.

How do I keep a house cool without air conditioning?

Block the sun before it reaches the glass, with external shades, awnings or shutters, and close curtains on sunny windows during the day. Open windows on opposite sides at night to let cool air through, use ceiling fans, and switch off heat sources such as lamps and appliances.

How do I increase the humidity in a house?

Use a humidifier in the rooms you spend most time in, dry laundry indoors on a rack, and add plants. Lowering the heating slightly also helps, because warmer air feels drier. Keep humidity below 60 percent to avoid condensation.

What is a smart thermostat?

A smart thermostat controls your heating or cooling from a phone app and learns your routine. It can lower the temperature when you leave, warm the house before you return, and control rooms separately. Most save energy by heating only when and where it is needed.

How do I keep a baby's room cool?

Keep the room at about 16–20 °C. Shade the window during the day, air the room in the cool evening, and use a fan to move air, never pointed directly at the baby. Dress the baby in light layers and use a room thermometer to check.

What causes high humidity in a house?

Everyday activities release moisture: cooking, showers, drying clothes and breathing. Poor ventilation keeps it inside, and cold surfaces make it condense. Leaks, rising damp and unvented appliances add more. Better extraction, steady heating and fixing water leaks bring it down.

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