The written version
How Thermal bridges works
These labs are a visualisation aid to help you picture how a house works. They are not technical advice: the models and figures are simplified, so please don't rely on them for design, specification or buying decisions. Ask a qualified designer, engineer or installer about your own home.
Insulation only works where it is continuous. Wherever a floor, a lintel, a window frame or a steel beam crosses the insulation line, heat takes a short cut, and the inside surface there runs cold enough for mould. This lab is a museum exhibit of a two-storey slice of a cavity-walled house and its roof, cut open between a frosty garden and two heated rooms, so you can watch where the heat goes and where mould grows.
The tour, in words
What a U-value means
A U-value is the heat, in watts, that passes through one square metre of a wall for each degree of difference between inside and out. This wall (plaster, 100 mm of aircrete block, 150 mm of mineral wool in the cavity and a brick outer leaf) comes to 0.185 W/m²K. At 20°C inside and 0°C outside, each square metre loses about 3.7 W. The insulation holds 81% of the wall's resistance; the blocks and bricks hold very little.
One unbroken line
Draw the house in section and you should be able to trace the insulation all the way round without lifting your pen: floor insulation into the cavity, past the upper floor, through the window frames, over the lintels and into the roof. Wherever the pen has to jump, heat takes the short cut. Designers call this the pen test.
Where it breaks
A one-piece steel lintel, a floor slab whose insulation stops short of the wall, a window set out in the brick, a steel balcony beam through the wall and loft insulation that stops short of the eaves each give heat a path round the insulation. Each one is measured as a linear thermal transmittance, ψ, in watts per metre of junction per degree. With these poor details the 3 m wide specimen loses about 278 W at a 20°C difference, against 124 W if its walls and windows had no junctions at all.
The thermal camera
Seen from inside with a thermal camera, the plain wall sits at about 19.1°C. The bridges show as cool stripes and spots: with the poor details and 0°C outside, the window head is at about 14.0°C, the top corner of the bedroom under the eaves at about 14.4°C and the inside of the wall round the balcony beams at about 11.0°C. More insulation in the cavity hardly changes these numbers, because the heat is going round it.
Mould and condensation
Mould can grow where a surface stays at about 80% relative humidity, and water condenses where it falls to the room air's dew point. At −3°C outside and 60% humidity indoors the dew point is 12.0°C and the mould line 15.4°C. The poor window head, at 13.1°C, sits at 93% surface humidity, the corner under the eaves at 90%, and the cold patch round the balcony beams, at 9.7°C, is below the dew point. That is why BRE's Information Paper 1/06 asks every junction in a home to keep a temperature factor (fRsi) of at least 0.75. The benchmark assumes ordinary indoor humidity: in a room kept at 80%, even the plain wall reaches about 85% at its surface, and the answer is ventilation as well as good details.
Fix the details
Lap the cavity insulation below the floor and use insulating blocks at its base, use a thermally broken lintel (ψ 0.02), set the window in line with the insulation with an insulated cavity closer, stand the balcony on its own posts or fix it with thermal-break connectors, and carry the loft insulation over the wall plate to meet the cavity insulation. In the same damp, cold conditions the coldest junction is now 16.8°C and no junction goes above 73% humidity at its surface. The specimen's heat loss falls to about 142 W.
How much it matters
Scale the specimen up to a whole two-storey detached house with the notional fabric. With SAP's cautious defaults for details nobody has checked, the junctions add 47 W/K: 41% of the heat lost through the fabric, or about 940 W at 0°C outside, like a small heater left on. With SAP's reference ψ-values, the ones the notional house assumes, they add 12 W/K, or 15%. This lab's standard details are the same apart from the balcony, where a real certified connector takes the place of the reference value: 13 W/K, or 16%. With the best details it is 12%. Thicker insulation makes the share bigger: at 250 mm with the poor details it is 44%.
What the regulations ask
For a new home in England, Approved Document L (2021 edition, incorporating the 2023 amendments) sets limiting U-values of 0.26 W/m²K for walls, 0.16 for roofs, 0.18 for floors and 1.6 for windows and doors. Those are backstops: meeting the energy targets usually takes something close to the notional dwelling, with walls at 0.18, floors at 0.13, roofs at 0.11 and windows at 1.2. In this wall a U-value of 0.18 takes about 155 mm of mineral wool, 100 mm of PIR or 175 mm of wood fibre; the limit of 0.26 takes about 95 mm of mineral wool.
Junctions need drawings, an on-site check before they are covered up, and photographs. Their heat loss comes from ψ-values calculated by a competent person (following BRE's conventions in BR 497 and the temperature factors in IP 1/06), from an independently assessed database of details, or from the cautious defaults in SAP's Table K1, such as 1.00 W/m·K for a steel lintel and 0.32 for a ground floor. The 2021 edition also allows a single default y-value of 0.20 W/m²K for the whole house. For the house in this model, SAP's reference ψ-values (the notional house's) work out at a y-value of about 0.04 W/m²K, or 0.05 with a certified balcony connector in place of the reference value. The old government Accredited Construction Details are no longer a route.
The Future Homes Standard edition of Approved Document L was published on 24 March 2026 and applies to building work from 24 March 2027, with transitional arrangements. It keeps the same limiting U-values, and the notional house keeps the same walls, floors, roofs and windows. Its list of ways to assess thermal bridges no longer includes the blanket default y-value: junctions are calculated, taken from a database of assessed details, or given SAP's default ψ-values. Compliance is shown with SAP 10.3, or with the new Home Energy Model once that is approved for use.
The parts
The wall
- Plaster
A coat of plaster on the blockwork, or plasterboard fixed over it. It adds very little insulation. A wet-plastered wall also makes a good airtight layer, because the plaster seals the joints in the blocks.
13 mm of plaster: R 0.03 m²K/W. This is the surface the thermal camera sees.
- Aircrete blockwork
The inner leaf carries the floors and the roof. Aircrete blocks are full of tiny air bubbles, so they insulate several times better than dense concrete blocks. That helps most at junctions, where the main insulation has to stop or turn.
100 mm at λ 0.15: R 0.67 m²K/W. Approved Document L suggests lightweight blocks in the inner leaf to cut bridging at junctions.
- Cavity insulation
Fills the gap between the two leaves and does most of the work: in this wall it holds over 80% of the total thermal resistance. The lower its conductivity (λ), the thinner it can be for the same U-value.
For a U-value of 0.18 in this wall: about 155 mm of mineral wool (150 mm gives 0.185), 100 mm of PIR or 175 mm of wood fibre.
- Brick
Keeps the weather off. Rain that soaks through the brick runs down its back face and out at weep holes, so insulation that fills the cavity has to be a kind that sheds water.
102.5 mm of brick: R 0.13 m²K/W. It does little to keep heat in.
The junctions
- Ground floor junction
Where the wall meets the floor. Heat finds a way round the end of the floor insulation, down through the inner leaf and out under the cavity insulation. The fix: take the floor insulation right up to the wall, lap the cavity insulation well below the slab, and use insulating blocks at the base of the inner leaf.
ψ 0.32 W/m·K if nobody has checked the detail (SAP default), 0.16 in the notional house. This model takes the best detail, with insulating blocks, as half that: 0.08. Approved Document L asks for cavity insulation at least one block (215 mm) below the underside of the floor.
- Upper floor junction
The first-floor joists hang off the inner leaf, inside the insulation, so the cavity insulation can run straight past them. A gap in the batts at the floor zone undoes that. In a timber frame, where the floor zone cuts through the wall's insulation, Approved Document L asks for insulation of the same thickness inside the floor zone.
ψ 0.14 W/m·K by default; 0 in the notional house, where the insulation is continuous.
- Lintel
The beam over a window or door. A one-piece steel lintel with a base plate under both leaves carries cold straight through to the window head inside, a classic place for black mould. Approved Document L suggests separate inner and outer lintels with insulation between them, or a lintel with a thermal break and an insulated core.
ψ 1.00 W/m·K by default (SAP 10.2), 0.05 in the notional house, 0.02 for the best thermally broken lintels.
- Window reveal
Round the sides and bottom of a window the cavity has to be closed. Close it with blockwork and set the window out in the brick, and heat walks round the end of the insulation. An insulated cavity closer, with the frame in line with the insulation, keeps the line unbroken.
ψ 0.10 W/m·K per metre of sill or jamb by default, 0.05 in the notional house. Approved Document L asks for insulated cavity closers in every new home.
- Balcony connection
A cantilevered balcony has to be held by beams fixed back into the floor. Steel conducts heat over a thousand times better than mineral wool, so a beam straight through the wall is the worst bridge in this specimen. A thermal-break connector puts a load-bearing insulating pad in the way; a balcony on its own posts needs nothing through the wall at all.
ψ 1.00 W/m·K by default where supports pass through the insulation (SAP 10.2), about 0.35 with a certified thermal-break connector, 0 when the wall insulation is unbroken.
- Eaves
Where the wall meets the roof. The loft insulation lies on the ceiling and has to reach over the wall plate to meet the cavity insulation. It is often pulled back from the eaves, or squashed by a ventilation tray, which leaves the top corner of the room cold. That corner, where the ceiling meets the outside wall, is one of the commonest places for black mould in a bedroom.
ψ 0.12 W/m·K by default (SAP 10.2), 0.06 in the notional house. Keep a gap for air under the roof with an eaves tray, and carry the insulation over the wall plate below it.
Around them
- Ground floor
A concrete slab with a layer of insulation and a screed on top. Its U-value here is 0.13 W/m²K, the notional dwelling's figure. Its insulation has to meet the wall's insulation, or the floor edge becomes a bridge.
Approved Document L sets a limit of 0.18 W/m²K for the floor of a new home.
- Window and balcony door
Even a good window loses several times more heat per square metre than this wall. Its U-value here is 1.2 W/m²K, the notional house's figure; the limit for a new window is 1.6. Where it sits in the wall decides whether its frame continues the insulation line or interrupts it.
The frame is the insulating part of the window line: set it in line with the cavity insulation.
- Roof and loft
A pitched roof of tiles on battens and rafters, with a cold, ventilated loft. The insulation lies on the ceiling, 300 mm of mineral wool between and over the joists, so the loft above it is nearly as cold as outside. Air drawn in at the eaves keeps the roof timbers dry.
Its U-value here is 0.11 W/m²K, the notional dwelling's figure. Approved Document L sets a limit of 0.16 for the roof of a new home.
- Ground and foundations
Below the damp-proof course the wall stands on foundation blockwork and a concrete strip footing. Heat leaving through the floor edge has to get through the ground as well, which slows it a little.
Approved Document C puts the damp-proof course at least 150 mm above the ground outside.
The exhibit
- Inside
Two rooms, one above the other, heated to 20°C by a radiator under the window. Warm air rises off the radiator and cools as it slides down the window and the outside wall. Every breath, shower and pan of pasta adds water vapour to the air, which is why the indoor humidity is a control.
This specimen: a 3 m wide slice of a two-storey cavity wall and its roof, with a window, a balcony door and a balcony, cut open through the middle of the window and the door so you can see the section.
- Outside
A winter night, from a mild 15°C down to a hard frost at −10°C. Heat flows out through the wall all the time the inside is warmer. The colder the night, the faster it goes and the colder the inside face of each junction gets.
Thermal bridges matter most on cold nights: that is when their inside faces run coldest and damp settles on them.
- Mould
Mould spores are everywhere. They grow on a surface that stays damp, and a surface gets damp when it is colder than the room: the air touching it cools, and its relative humidity rises. Above about 80% for days at a time, black mould takes hold. Below the room air's dew point, water condenses on the surface itself. So mould grows in the same places a thermal camera shows cold: round the window head, along the skirting, in the top corner of the room.
BRE's Information Paper 1/06 asks every junction in a home to keep a temperature factor (fRsi) of at least 0.75, which keeps the surface dry enough at ordinary indoor humidity. In a damper room, ventilation has to do the rest.
- Thermal camera
Sees infrared, which every surface gives off according to its temperature. Pointed at a wall from inside on a cold day, it shows thermal bridges as cool stripes and spots: the places mould will grow first.
Surveyors use them to find missing insulation, cold junctions and air leaks. A few degrees of difference show up clearly.
- Insulation samples
Three common insulants. The number that matters is the thermal conductivity, λ, in W/m·K: the lower it is, the less thickness you need for the same U-value.
Typical declared values: mineral wool 0.034, PIR 0.022, wood fibre 0.038. In a masonry wall PIR boards usually go in as a partial fill, with a clear cavity outside them, and wood fibre usually goes in timber frame or outside a wall under render. Here each fills the same cavity so you can compare.
The model is a masonry cavity wall: U = 1 / (Rsi + Σ d/λ + Rse), with junctions added as ψ × length. Poor details use SAP 10.2's default ψ-values, standard ones its reference values (a certified thermal-break connector for the balcony), and the best ones certified thermally broken details where they exist. The temperature factor for each junction is representative of details of that kind; a real detail needs its own calculation. The coloured section in the lab is drawn from those surface temperatures and the plain wall's temperature profile as an illustration, without a full heat-flow simulation. The whole-house figures use a 9.6 × 6.5 m two-storey detached house with the notional dwelling's floor, roof and windows, and leave out ventilation.
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