Thermal bridges and mould: why condensation forms in corners and what a thermal bridge costs

Where mould appears and why

Mould needs damp, not water running down the wall: it grows where the relative humidity right at the surface stays above 80 % RH for days. Room air cools next to a cold surface and its relative humidity rises; with 20 °C and 50 % RH in the room, a surface reaches 80 % RH at 12.6 °C and 100 % RH, the dew point, at 9.3 °C. The dew point is explained on the page on relative and absolute humidity. The cold spots of a house are its thermal bridges: places where heat escapes more easily than through the rest of the wall.

Why mould grows in the corner of a room

The corner between two outside walls is a geometric thermal bridge: inside it has a small surface that receives heat, outside a large one that loses it. In an old uninsulated wall the corner has f_Rsi ≈ 0.61: at −5 °C outdoors, 10.3 °C on the surface and 94 % RH. The same corner insulated outside has f_Rsi ≈ 0.86 and stays dry. Corners at the floor and at the ceiling under a cold attic are colder still, because three surfaces meet there.

Mould on window reveals: causes

The reveal is the thin strip of wall between the window and the facade. When the window sits in the middle of the wall and the facade insulation stops at the edge of the opening, heat bypasses the frame through the reveal. Without reveal insulation the calculation gives f_Rsi ≈ 0.71, at the mould limit at 20 °C and 50 % RH; with 3 cm of insulation on the outer reveal, 0.83. The concrete lintel above the window and the sill below behave the same way.

The plinth, the balcony and the roof junction

The plinth (the bottom of the wall, next to the ground) is cold when the facade insulation stops above ground and the concrete footing links the warm slab to the cold ground: f_Rsi ≈ 0.53 in this page’s calculation. A concrete balcony slab passes through the insulation like a cooling fin; a concrete ring beam or cornice at the roof junction does the same unless the insulation wraps it.

Mould behind furniture

A wardrobe against an outside wall keeps the warm air away from the wall: the surface behind it is a few degrees colder than the rest of the wall, even without a thermal bridge. The checker above computes the open wall; behind furniture and heavy curtains reality is colder.

The temperature factor f_Rsi

f_Rsi says how warm a detail stays on the inside, whatever the weather: 1 means the surface is at room temperature, 0 that it is at the outdoor temperature.

What is f_Rsi

f_Rsi = (θ_si − θ_e) / (θ_i − θ_e) where θ_si is the lowest inside surface temperature, θ_i the indoor and θ_e the outdoor temperature. For a plain wall with no thermal bridge, θ_si = θ_i − U · R_si · (θ_i − θ_e), so f_Rsi = 1 − U · R_si. An old wall with U 1.2 W/(m²·K) has f_Rsi = 0.70: at −5 °C outdoors and 20 °C indoors, 12.5 °C on the surface. A Passive House wall with U 0.15 has 0.96: 19.1 °C. For shaped details (corners, reveals, balconies) f_Rsi is computed numerically, to EN ISO 10211.

Why R_si 0.25 and not 0.13

R_si is the resistance of the air layer against the wall on the inside. For heat loss (the design U-value) 0.13 m²·K/W is used. For the mould check EN ISO 13788 and DIN 4108-2 require 0.25 m²·K/W: in corners, behind furniture and curtains the air moves less and the surface is colder than 0.13 suggests.

The 80 % RH and 12.6 °C limit

Mould is taken as possible when the relative humidity at the surface reaches 80 % RH (EN ISO 13788; per Passipedia, for 12 hours a day). DIN 4108-2 does the sum with 20 °C and 50 % RH indoors and −5 °C outdoors: the surface must stay above 12.6 °C, that is f_Rsi ≥ 0.70 (Passipedia, thermal bridges). The owner of this site designs houses at 23 °C in winter: with 23 °C and 50 % RH the limit rises to 15.5 °C and the minimum f_Rsi at −5 °C becomes 0.73, slightly stricter.

The Passive House criterion: f_Rsi ≥ 0.70

The Passive House Institute requires f_Rsi ≥ 0.70 with R_si 0.25 for components certified for the cool-temperate climate (Romania’s), the “hygiene” criterion (PHI, criteria for opaque construction systems). That is a separate thing from the comfort criterion for windows, which limits the installed window U-value so there is no cold draught by the glass.

Typical details: where mould grows

Eleven details at −5 °C outdoors and 20 °C with 50 % RH indoors. The plain walls from the formula; the others from a 2D calculation made for this page, to EN ISO 10211, on typical sections described under Method. They are typical values, not a calculation of your house: the same detail can come out better or worse with other materials and dimensions.

What to do

Condensation on walls in winter: what to do

Two levers: a warmer surface and drier air. On the construction side, insulation outside, continuous over the detail: around the corner, along the reveal up to the frame, over the ring beam and cornice, down the plinth below ground level. On the use side: keep indoor humidity between 40 and 55 % RH (heat recovery ventilation, see the ventilation calculator, or short airing several times a day), keep wardrobes off outside walls, heat every room and do not leave bedrooms cold with the door shut.

Fixes for corners and reveals

The corner of an old wall is solved only by insulating outside; insulating inside moves the problem to the junctions with floors and internal walls. At the window: 3–4 cm of insulation on the outer reveal, with the frame set as close to the insulation as possible, and for new windows the frame set in the plane of the insulation.

Balcony thermal bridge: solutions

In a new house the balcony stands on its own posts or is tied to the slab through a structural thermal break, which carries the load through stainless steel bars and insulation instead of concrete. In an existing house the slab is wrapped in insulation on all faces, at least for the first metre next to the wall.

The plinth

The facade insulation continues down the plinth in a material that tolerates damp (extruded polystyrene), at least 0.5–1 m below ground. In this page’s calculation f_Rsi rises from 0.53 to 0.77 with the insulation taken down 0.9 m.

What a thermal bridge is and what ψ is

A thermal bridge is a linear (an edge, a junction) or point-like part of the envelope where heat escapes more easily than through the elements next to it.

What is a thermal bridge

Thermal bridges are geometric (corners, where the shape makes the outer surface larger than the inner one), structural (concrete, steel or masonry passing through the insulation: balconies, ring beams, lintels, footings) and installation bridges (the window frame against the insulation). They do two things: a colder inside surface (mould, Part A) and extra heat loss (Part B).

What ψ is

ψ (psi), the linear thermal transmittance, in W/(mK): how many watts each metre of thermal bridge loses for every degree between inside and outside, on top of what the walls next to it lose. The loss a year: Q = ψ · l · G_t with l the bridge length in m and G_t the heating-season degree hours in kKh. For Bucharest, the site’s typical year gives G_t = 61.9 kKh to 20 °C (172 days averaging 12 °C or less).

How to calculate a thermal bridge

ψ is computed numerically to EN ISO 10211: the section of the detail is divided into small cells, each with its material’s conductivity, and the heat flow is solved. From the total flow you subtract U · length for the walls next to the detail; what is left is ψ. EN ISO 14683 gives simplified methods and higher default values for when there is no calculation.

External dimensions and negative ψ

ψ depends on how the walls are measured. PHPP (the Passive House planning tool) measures outside, so at a corner the area in the calculation is larger than the area heat actually flows through and ψ comes out negative: −0.56 W/(mK) at the corner of the old walls, −0.09 at the insulated corner, in this page’s calculation. With internal dimensions a corner has a positive ψ (the SAP 10 default: 0.18). Use the values below only with their own convention.

Thermal-bridge-free Passive House: what it means

The Passive House Institute calls a detail “thermal bridge free” when ψ ≤ 0.01 W/(mK), with external dimensions (Passipedia; PHI criteria). The loss through details is then negligible and, offset by the corners with negative ψ, can be left out of the balance. Passipedia reports that without this care the remaining thermal bridges added up to 14 kWh/(m²·a) to the heating demand in real projects.

Typical ψ values

What a thermal bridge costs

The thermal bridges of a typical 150 m² two-storey house in Bucharest, with example ψ values chosen within the ranges above: “typical” is an insulated house without care for the details, “bridge-free” one with the details solved. G_t = 61.9 kKh; heat from a condensing gas boiler (efficiency 92 %), gas at €0.065/kWh (the cost calculator’s prices of 28 October 2026, converted at the BNR rate of 5.3495 lei/€ on 28 October 2026). “m² of wall” shows how many square metres of insulated wall with U 0.25 W/(m²·K) lose as much.

Work out the thermal-bridge loss of your house

Enter the length and ψ of each thermal bridge. The yearly loss is ψ × length × degree hours; the cost, with gas and a condensing boiler. The starting values are those of the typical house in the table above.

Thermal bridges in PHPP and certification

In PHPP each thermal bridge enters the energy balance as ψ × length, next to U × area for the walls, roof, floor and windows, with external dimensions. For Passive House certification the details must either meet ψ ≤ 0.01 W/(mK) (certified systems or calculation) or be calculated and entered with their own ψ; either way f_Rsi ≥ 0.70 at R_si 0.25 for Romania’s climate. Semper Supra calculates a project’s critical details (balconies, plinth, windows, the roof junction) to EN ISO 10211 as part of the PHPP model: ψ for the balance and f_Rsi for mould. See Semper Supra’s services.