Expert guideHeat always moves from warm to cold. Insulation cannot stop that, it can only slow it down. This guide explains how, what the numbers on a product sheet mean, and why the edges of a job matter as much as the board in the middle. There is a calculator so you can try it on your own walls.
By Miles Sharples and the Energy Giants installation team · Updated October 2026The basics
Your heating puts energy into the room. Everything after that is about how fast it gets out again. It goes three ways, and good insulation work deals with all three.
Heat passing through solid material, molecule to molecule. Brick and stone conduct heat well, which is why an uninsulated solid wall loses so much. Insulation is mostly material that conducts heat badly: lots of tiny pockets of still air or gas held in a solid that barely conducts.
Heat carried by moving air. Warm air rises, cools against cold surfaces and falls, and it escapes through gaps and leaks. This is why airtightness matters as much as the insulation itself, and why a gap behind a board can undo it.
Heat moving as infrared from a warm surface to a cold one, which is why you feel cold sitting next to a cold wall even in a warm room. Low emissivity foil facings, like the ones on many rigid boards, reduce radiant heat transfer across an unventilated air gap next to them.
Product sheets throw three numbers at you. They describe different things, and mixing them up is how people end up comparing apples with walls.
How easily heat passes through a material, regardless of thickness. Lower is better. Kingspan Kooltherm is 0.019. It describes the material, not your wall.
How well a particular thickness resists heat: thickness in metres divided by lambda. Higher is better. 50 mm of Kooltherm is 0.050 ÷ 0.019 = 2.63, which Kingspan declares as 2.60.
| Kooltherm K118 insulation thickness | Declared R-value (m²K/W) |
|---|---|
| 25 mm | 1.30 |
| 40 mm | 2.10 |
| 50 mm | 2.60 |
| 60 mm | 3.15 |
| 80 mm | 4.20 |
Source: Kingspan Kooltherm K118 Declaration of Performance and brochure (Eighth Issue, 03/2026). The 12.5 mm plasterboard adds 0.065 m²K/W.
Because U is one divided by the total resistance, every extra millimetre of insulation saves less than the one before. On a 215 mm solid brick wall that starts at 2.1 W/m²K, the first 25 mm of Kooltherm takes it to about 0.54. That is roughly three quarters of the heat loss gone. The next 55 mm only takes it from 0.54 to 0.21.
That curve is why the Building Regulations set a sensible target rather than “as thick as possible”, and why they let you stop short where the extra thickness would cost too much room or would not pay back. Our Building Regulations guide covers the rules.
Heat takes the easiest path. A thermal bridge is anywhere it can get round or through the insulation: a floor that cuts through the wall lining, a window reveal left bare, a party wall, a timber stud. As homes get better insulated these weak spots matter more. Kingspan says they can account for as much as 30% of total heat loss in a well insulated building.
The cold spot is also where condensation and mould start, because it is the coldest surface in the room. So on site we line every reveal with a thinner board, take insulation round junctions as far as the structure allows, and on framed walls take the floor covering back so the board runs down past the floor. The BBA certificate for Kooltherm K118 says that in England and Wales the risk of surface condensation is adequately limited where the wall’s U-value is no worse than 0.7 W/m²K at any point and the junctions are designed properly. That is a statement about the edges, not the middle of the wall.
Air that leaks in through gaps carries heat out with it, and air moving behind a lining bypasses the insulation altogether. The Scottish Government’s building standards handbook gives a striking example: in a typical 1960s house with poorly fitted windows, around 20% of the heat can be lost through air leakage, rising to over 40% if the walls and roof are insulated but the leaks are not dealt with.
That is why, before a board goes up, existing gaps at floors, ceilings and corners are sealed, and afterwards the perimeter and every socket, pipe and cable through the lining is sealed too.
Damp and mould
Air always holds some water vapour. Cool it far enough and the vapour turns back into water. The temperature where that happens is the dew point. When a surface in your home is colder than the dew point of the room air, water forms on it. Long before that, the air right next to a cold surface gets very humid, and mould can grow where it stays above about 80% humidity for long periods.
So two things decide whether a wall gets mould: how cold its surface gets, and how much moisture is in the room. Insulation deals with the first. Ventilation deals with the second. Try it below.
How it works: inside surface temperature = room temperature − U × Rsi × (room − outside), with Rsi 0.13 m²K/W for an open wall and 0.25 m²K/W for still air behind furniture. Dew point and surface humidity use the Magnus formula. The 1.4 W/m²K wall is an illustrative older wall. A simple steady-state model to show the idea, not a condensation risk assessment.
Surface condensation is the visible kind. Interstitial condensation happens inside the wall, where warm moist air finds its way to a cold layer. Insulating on the inside makes the masonry colder, so the design has to stop room air reaching it. That is the job of a vapour control layer on the warm side, sealed at every joint and penetration. The BBA certificate for Kooltherm K118 requires walls to be designed to BS 5250, with a more detailed dynamic analysis to BS EN 15026 where the simpler calculation shows a risk.
Government guidance on damp and mould estimates that in 2019, damp and mould in English homes were associated with around 5,000 cases of asthma and 8,500 lower respiratory infections. It lists poorly insulated homes among the risk factors, and notes that an energy efficient home is less likely to suffer condensation, provided it is adequately ventilated.
Source: UK Government, “Understanding and addressing the health risks of damp and mould in the home” (updated 1 April 2026).
A warmer, more airtight home still produces moisture: cooking, washing, drying clothes and breathing all add to it. It has to get out. On funded retrofit work, the ventilation is assessed before any insulation goes in, and the designs we work to typically upgrade bathroom and kitchen fans to quiet continuous extract units, add trickle vents to windows, and make sure internal doors have a gap at the bottom so air can move through the house.
Insulation works best as a planned whole rather than a measure at a time. On funded work a retrofit assessor surveys the whole house and a retrofit coordinator plans the sequence, so that loft, walls, floors, windows and ventilation each work with the next. For a homeowner, the same thinking applies: deal with damp and draughts, then the roof, then the walls, then floors, and make sure the ventilation keeps up.
FAQs
The rate heat passes through a whole wall, roof or floor, in watts per square metre for every degree of difference between inside and outside. Lower is better. An uninsulated 215 mm solid brick wall is around 2.1 W/m²K; the Building Regulations target for an internally insulated wall in England and Wales is 0.30.
Lambda describes a material: how easily heat passes through it, whatever the thickness. R-value describes a particular thickness of it: thickness in metres divided by lambda. Kooltherm has a lambda of 0.019 W/mK, and 60 mm of it has an R-value of 3.15 m²K/W.
Thicker always loses a little less heat, but each extra millimetre saves less than the one before. On a solid brick wall the first 25 mm of high performance insulation removes roughly three quarters of the heat loss. Past the regulations target, extra thickness mostly costs room space and money.
Done properly, it reduces condensation by making wall surfaces warmer. Done badly, it can cause problems: insulation over a wall that is already letting in rain, gaps that let room air reach cold masonry, or a home left without enough ventilation. That is why damp is fixed first, edges and penetrations are sealed, and ventilation is checked.
Usually some improvement, yes. A warmer, more airtight home still needs moisture to get out. On funded work this is assessed before installation, and typically means continuous extract fans in bathrooms and kitchens and trickle vents in windows.
Anywhere heat can get round or through the insulation more easily, such as a floor cutting through a wall lining or an uninsulated window reveal. They are often where mould first appears, because they are the coldest surfaces in the room.
Go deeper
A free survey looks at damp, ventilation and the walls themselves, and gives you a straight answer on what would make the difference.
Kingspan and Kooltherm are registered trademarks of the Kingspan Group plc. Product figures are from Kingspan’s published literature and BBA Certificate 16/5299; health figures from UK Government damp and mould guidance; airtightness example from the Scottish Government’s Building Standards Technical Handbook. Calculations on this page are simplified illustrations.
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