Condensation Risk Analysis in London and Nationwide

What is a condensation risk analysis?

It is a calculation that shows whether moisture will condense in or on your walls, roofs and floors. There are two kinds of risk, and we check both.

• Interstitial condensation forms inside the build-up, where warm, moist air from the room meets a layer that is cold. You can’t see it, but over time it wets insulation, rots timber and corrodes fixings.
• Surface condensation forms on the inside face of a wall or ceiling where it is colder than the rest of the room. That is usually at a junction: a window reveal, a corner, the floor edge. Left damp, it turns into mould.

When you need one

Building Regulations Part C, requirement C2(c), says walls, floors and roofs must protect the building and the people using it from the harmful effects of interstitial and surface condensation. Approved Document C points to BS 5250 and BS EN ISO 13788 for how to show it.

Building control, warranty providers and structural engineers most often ask for a condensation risk analysis on:

• internal wall insulation on solid brick or stone walls,
• warm flat roofs and roof terraces,
• timber frame and steel frame walls,
• rooms in the roof and insulated pitched roofs,
• floors over garages, passageways and other unheated or open spaces,
• any build-up with an unusual layer order or a vapour-tight finish, such as cladding, tiling or a metal liner.

Retrofit is where we see it most. Adding insulation changes where the cold layer sits, so a wall that has been dry for a hundred years can start getting wet inside.

How we check interstitial condensation

We run the calculation to BS EN ISO 13788, following BS 5250:2021, the current code of practice on managing moisture in buildings. It works month by month through a typical year (the method is often called Glaser):

1. We set up the build-up layer by layer: thickness, conductivity and how easily water vapour passes through each layer.
2. For each month, we work out the temperature and the vapour pressure through the build-up, using the outside climate and the inside conditions for the building’s use.
3. Where the vapour pressure reaches saturation, condensation forms. We work out how much, and whether it dries out again by the end of the year.

The result is one of three:

• No condensation at any point in the year.
• Some condensation in winter that fully dries out in summer. It is usually acceptable, as long as the amount is small and the material at that point can take it.
• Condensation that builds up year on year. This fails, and the build-up needs to change.

If it fails, we don’t leave it there. We tell you the change that fixes it and rerun it to show it works. Usually that means a vapour control layer on the warm side, moving the insulation, a more open membrane on the cold side, or a different board.

The monthly method has limits. It doesn’t model driving rain, moisture built into new masonry or air leaking through gaps. If your build-up needs more than this method can show, for example some internal wall insulation on old solid walls, we tell you before we start.

Surface condensation and mould at junctions (fRsi)

Junctions are where surface condensation and mould start, because the insulation is thinner or broken there. We check them with the temperature factor, fRsi. It shows how warm the coldest point on the inside surface stays compared with the room and the outside. The higher it is, the lower the risk.

BRE IP 1/06 sets the minimum (fCRsi) by building use, for example:

• 0.75 for homes, residential buildings and schools, to avoid mould,
• 0.50 for offices and shops,
• 0.80 for kitchens, canteens and sports halls,
• 0.90 for swimming pools and other high-humidity buildings.

We work out fRsi in our 2D and 3D junction models to BR 497, the same models we use for PSI value calculations. You get a pass or fail for each junction. We are members of Elmhurst’s Psi-Value Competency Scheme.

Build-ups we see most often

• Internal wall insulation on solid walls. The old brick ends up colder than before, so any vapour that gets past the insulation can condense on it. A continuous vapour control layer with taped joints and sealed service holes is usually the answer, but the calculation shows how much control the wall needs.
• Warm flat roofs. The insulation sits above the deck with a vapour control layer under it. Getting the layer order or the VCL wrong puts condensation on the timber deck.
• Timber frame walls. Vapour control on the warm side, a breathable membrane on the cold side. If the outer layers are tighter than the inner ones, moisture gets trapped in the frame.
• Floors over garages and open passageways. Cold from below, so the layer order matters just as much as in a roof.

What you get

• A calculation sheet for each build-up: the layers, their properties and the month-by-month result.
• A clear statement for each one: no condensation, condensation that dries out, or a fail.
• For a fail: the change that fixes it, with the build-up rerun to show it passes.
• An fRsi result and a pass or fail for each junction, where you book the junction checks.
• A summary for building control or your warranty provider.

What we need from you

• Each wall, roof and floor build-up, layer by layer, with product names and thicknesses, from inside to outside.
• What the building is used for (home, office, school, kitchen, pool and so on).
• The town or postcode, for the outside climate.
• For junction checks: the construction details or drawings of the junctions.

Price and turnaround

Priced by the number of different build-ups and junctions. Send the details for a free quote. Turnaround: 5 working days once we have everything we need. Booking U-values or PSI values with us at the same time? We price them together.

Our experience

More than 17 years in energy assessment, with a mechanical engineering background. We are members of Elmhurst’s Psi-Value Competency Scheme, and we model junctions and build-ups to the BRE conventions (BR 497, BR 443 and BRE IP 1/06).

Questions we get asked

Do I need a condensation risk analysis?
If building control or your warranty provider asks for one, yes. It is most common on internal wall insulation, warm flat roofs, timber frame walls and floors over unheated spaces. Send us the build-up and we’ll tell you if it needs one.

What is the difference between interstitial and surface condensation?
Interstitial condensation forms inside the wall, roof or floor, where you can’t see it. Surface condensation forms on the inside face of the room, usually at a cold junction, and leads to mould. We check the first with BS EN ISO 13788 and the second with the fRsi value.

The result says condensation forms but dries out. Is that a pass?
Usually, yes. A small amount that fully evaporates over the summer is normally acceptable, as long as the material where it forms can take it. We say so in the report, layer by layer.

Will a vapour control layer always fix it?
Not always. It only works if it is on the warm side, continuous and sealed at joints and service holes. In some build-ups, moving the insulation or changing the outer membrane works better. The calculation shows which.

Is this the same as a damp survey?
No. A condensation risk analysis is a calculation from your build-up details, done at design stage or before you insulate. It doesn’t diagnose damp that is already there, such as rising damp or leaks.

Related: U-value calculations · PSI value and thermal bridging calculations · Retrofit energy assessments · SAP calculations and EPCs · SBEM calculations · Energy statements

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