Thermal comfort

The best reason to insulate is to save energy; but another, more immediate reason is to feel comfortably warm.  In the building trade this is called thermal comfort, and I used to teach it to first year construction management students.  Thermal comfort itself is simple enough: feeling warm but not too warm. What I used to teach them are the factors controlling thermal comfort so they could apply them to building offices and other public buildings; but the principles apply just as well to a flat and provide useful pointers to sustainable living.

There are six factors affecting thermal comfort:

  1. Clothing : light clothing keeps you cool, heavy clothing keeps you warm
  2. Physical activity : doing things keeps you warmer than sitting still
  3. Air movement : a flow of air encourages sweating  and this keeps you cool
  4. Humidity : in low humidity you sweat more easily which keeps you cool
  5. Air temperature : the reading of a thermometer exposed to the air
  6. Radiant temperature : the reading of a thermometer inside a black ball

The first three factors are common knowledge, but can be effective. The last three are more complicated.

Humidity is the amount of water contained in the air as a percentage of the maximum possible content. For health reasons we need to maintain a humidity between 40% and 60%. Too low and we may experience throat and nasal discomfort; too high and moulds and dust mites proliferate, which can cause breathing and allergy problems. In a flat, ventilation is limited, as is access to outside space: thus, humidity is often high – particularly if washing is dried in the flat and pots are boiled without an extractor.  If humidity is too low, indoor plants are a good way of increasing it. (Plants also reduce air-borne toxins such as formaldehyde liberated by some timber products).  Humidity meters are cheap to buy, and usually show air temperature as well.  For comfort, thermal and otherwise, we should try to keep humidity within the 40-60% range.

Air temperature for comfort depends on what we are doing.  Most people sleep best with a covering over them, so to maintain thermal comfort bedrooms should be cooler than living rooms.  Sitting quietly in a chair probably needs a higher temperature for comfort, and a related factor is temperature gradient – how the air temperature changes with height in the room.  Many heating devices (including all night storage devices) are convectors that heat air directly. The hot air then rises towards the ceiling until it cools and then falls to the floor: so the feet are colder than the head which, in turn, is colder than the ceiling (if you get up on a ladder). Air temperature is connected with radiant temperature in a complex way, which we discuss below.

Radiant temperature is a way of measuring the heat that passes by radiation rather than conduction.  On a sunny day we can be comfortably warm even when the air is cold because sunlight shines through the air without heating it and gets absorbed by our bodies directly. This is called radiant heat and the sun – for building purposes – is just a large, free radiator.  We can provide other radiators inside – usually flat, water-filled panels connected to a heating system. Other powerful radiators are fires – either open or  better still in a closed woodstove (ten times more efficient).  All of them work by shining heat through the air directly at whatever is in front of them, and so they can affect a thermometer inside a closed black bulb. Radiation heats the bulb, this heats the air inside and that operates the thermometer.

What is not normally appreciated is that the walls of the room itself can act as radiators.  They give off less heat than fires, but they are large and cover the whole room from floor to ceiling.  This is why a room which has been unheated for days takes a while to feel comfortable again.  If the walls can be warmed by convected air and kept warm by insulation they can deliver a level of thermal comfort difficult to quantify without a radiant thermometer. Another thing they can do is store heat, like a storage heater.  Try to place the insulation outside, so the walls inside can use more material for heat storage.