What insulation actually does
Insulation does not create warmth. It slows the movement of heat from a warm place to a cooler one. In winter, that means heat leaves the house more slowly. In summer it means heat from outside reaches the inside more slowly. Either way, the building changes temperature more gradually, so heating or cooling has less to do.
Heat always flows from warmer to cooler, and it flows faster when the temperature difference is larger. Insulating a wall or roof does not stop this flow, but it reduces the rate.
Why trapped air does most of the work
Still air is a poor conductor of heat. Most insulating materials are, at heart, ways of holding a lot of air in tiny pockets so it cannot move around and carry heat by convection. Mineral wool, glass wool, cellulose, wood fibre, foam boards and sheep’s wool all work on this principle in different ways. Some foam boards use trapped gas rather than air, which can let them be thinner for the same effect. Reflective foils work by reducing radiant heat transfer across an air gap, and their performance depends on that gap being present and facing the right way, so claims for thin products are worth treating with caution.
Insulation performance is often expressed as a thermal resistance (R-value) or, for a whole element such as a wall, a U-value. A higher R-value, or a lower U-value, means less heat flows through. Different countries use different conventions, so compare like with like. Thickness matters, but so does the material, so a thicker layer of one type is not necessarily better than a thinner layer of another.
Where heat escapes in a typical house
The distribution of heat loss differs for every building, and general claims about percentages are rarely reliable. It depends on the age, construction, size, shape, how many walls are shared with neighbours and how much glazing there is. As a general picture, heat tends to escape through:
- The roof or loft. Warm air rises, and a poorly insulated roof can be a big route for heat loss, so loft insulation is often one of the first measures considered. It also reduces summer heat gain through the roof.
- The walls. In many homes, walls make up the largest surface area. Cavity walls, common in more recent construction, have a gap that may be filled with insulation. Solid walls, common in older buildings, have no cavity and can be insulated on the inside or the outside, which is more involved.
- The floor. Suspended timber floors over a void can lose heat to the air below, while solid ground floors usually lose less per square metre.
- Windows and doors. Glazing has far higher heat loss per square metre than an insulated wall, so even small areas count. Options are covered in the article on window insulation.
- Air leakage. Warm air escaping through gaps carries heat with it. This is separate from conduction through materials and is addressed by draught proofing.
- Thermal bridges. These are places where a conductive material such as concrete, steel or timber bridges the insulation, creating a cold spot. They are common around window reveals, lintels and where floors meet walls.
A survey, a thermal imaging report or even a careful walk-round on a cold day, noticing cold walls and ceilings, can show which areas are weakest in your own home.
Common types of insulation
- Loft insulation: laid between and over joists, usually in rolls or loose fill. Keep it away from recessed lights unless they are rated for covering, and do not block eaves ventilation.
- Cavity wall insulation: a material injected into the cavity of a suitable wall. It needs the right wall, dry conditions and a competent installer, as faults and unsuitable sites can cause damp.
- Solid wall insulation: internal lining or external cladding. It is more expensive and disruptive, may need planning permission or listed-building consent, and requires attention to moisture.
- Floor insulation: between joists in suspended floors, or rigid boards over solid floors, often combined with other work.
- Pipe and tank insulation: simple covers on hot water pipes and cylinders that cut heat loss from the system itself.
The trade-offs of materials such as sheep’s wool, wood fibre and cellulose, compared with foam products, are covered in the article on natural insulation. Some materials have a lower embodied carbon than others, which matters if you are weighing manufacturing impact against years of savings.
Moisture and ventilation
Adding insulation changes how a building handles moisture. Walls that once dried out easily may dry more slowly. Colder surfaces behind insulation can attract condensation if warm, moist air reaches them. In older buildings built with breathable materials, using non-breathable insulation can trap moisture in the wall. These risks are manageable with the right design, but they are a good reason to get advice from someone experienced with your type of building, particularly for solid walls.
Insulation also reduces natural air leakage in some cases, so ventilation needs a second look afterwards. The article on condensation control explains the effect in more detail.
Safety and hazards
If you are working on an older property, materials such as asbestos may be present in ceilings, lagging or old boards. Disturbing them can release harmful fibres. Do not drill, sand or remove suspect material, and get it tested by a qualified professional first. Some insulation products can irritate skin and lungs during installation, so follow protective-equipment advice. More on this is in the article on renovation hazards.
Is it worth it?
Insulation usually has no running cost once installed and lasts for many years, so over a long period it often costs less than the heat it prevents from being wasted. But payback depends on energy prices, the starting condition of the house and the cost of installation, and it is better described as likely than guaranteed. Cheaper, simpler measures such as loft insulation often come first, and the order of upgrades is discussed in the article on retrofit order. Local grants or support schemes sometimes exist, but they vary and change, so check current local information.
Key points
- Insulation slows heat flow; it does not stop it.
- Trapped air is what makes most materials work.
- Heat tends to escape through the roof, walls, floors, windows and air gaps, in proportions that depend on the house.
- Moisture, ventilation and building type should shape what you choose.
This is general information, and a qualified surveyor or installer can advise on a specific building.