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Is interstitial condensation a risk?

Interstitial condensation differs from other forms of damp that affect properties because it occurs inside walls, where it is impossible to see what’s going on. For this reason treating it is especially hard, and you could have this problem and not be aware of it.

On the other hand, it’s important to keep it in perspective: it’s not going to affect everyone, and it’s only worth investigating if you think, after reading this, that you fall within the risk parameters. Remember that, if the wall actively ‘breathes’, interstitial condensation is not nearly so likely to be a problem.

What is interstitial condensation?

Interstitial condensation manifesting as mould and rot on battons in walls and ceiling

Figure 1. Interstitial condensation manifesting as mould and rot on battons in walls and ceiling

Interstitial condensation can occur in solid and cavity wall properties, and happens when pressure and temperature differences force warm humid air through hygroscopic (water absorbing) materials until they reach a point cold enough for it to condense upon a surface. It is the result of the interaction between a complex set of factors. This includes:

  • the amount of steam & evaporation and the occupancy level in a given room
  • relative humidity, temperature and pressure both inside and outside (which varies all the time)
  • the structure and composition of a wall, floor or ceiling/flat roof (sloping roofs above ventilated lofts are rarely a problem)
  • and the surfaces (e.g. paint type), both inside and outside, of a wall.

Each case will be different, and the article below explores all these factors, some science, potential causes and potential solutions.

In general, walls, floors and ceilings need to be hygroscopic (able to absorb moisture from the air) in order to let the building ‘breathe’. The solution can never be simply to block off the possibility of moisture passing through the surface of a wall, ceiling or floor on the inside.

Understanding of this difficult subject is still evolving with on-going research, so in some cases solutions are not yet clearly apparent.

Some examples of interstitial condensation

Figure 1. In this case temperature and dew lines don't meet so there is no condensation

Figure 2. In this case temperature and dew lines don’t meet so there is no condensation

The example shown in Figures 2 and 3 here is of a completely filled cavity wall. When it is heated inside in the winter, warm, moisture-carrying air is being pushed out. The temperature (red line) decreases slightly up to the insulation, then decreases dramatically, and then less so as it approaches the cold exterior.

The point at which moisture could condense (the dew point) is at a lower temperature. It is represented by the blue line, which in the first instance, Figure 2, never meets the red line, and so condensation does not occur.

Interstitial condensation in a filled cavity wall - where temperature and dew lines meet condensation occurs

Figure 3. In this case temperature and dew lines meet and condensation occurs

In Figure 3, the relative humidity is now higher on the inside, and the temperature is lower. More moisture is being pushed out, at a temperature closer to the dew point, through the insulation, and where it hits the cold interior surface of the outer leaf of the cavity wall, it reaches the dew point and it condenses. It can then run down the wall, potentially cause rot or harm the insulation.

On the other hand, when the conditions change, it could evaporate back out through the outside leaf. In this case there would be no harm.

Interstitial condensation in a partially filled cavity wall

Figure 4. Interstitial condensation in a partially filled cavity wall

In Figure 4, which shows a partially-filled cavity wall in winter, the problem has a different cause. The relative conditions are reversed. Here it is moisture coming in from the outside, where the relative humidity (RH) is higher than the inside. It is caused by a warm front from the west arriving in the late winter, when the inside of the wall is cold. It brings with it much higher humidity. It is still condensing in a similar place, however: on the inside of the outer leaf.

Note that if the relative humidities and temperature differences are not so extreme, the dew line and temperature line may never meet, and so condensation does not occur.

The dew point can also be where the warm, humid air meets a sufficiently cold vapour-impenetrable barrier. One example is the end of a timber beam, which will be cooler where it passes through the brickwork. Here, persistent damp will cause it to rot.

Can I tell if I have interstitial condensation in my property?

One manifestation of interstitial condensation might be mould growing on the inside wall surface (Figures 1 & 6). Mould due to interstitial condensation might be distinguished by being over an entire surface instead of confined to a corner of the room. A building inspection, when a property is being sold, might also discover rot inside the building structure.

Relative humidity

So, when relative humidity reaches 100%, then the dew point is reached and moisture will condense from the air. It’s worth bearing in mind that relative humidity decreases as the internal temperature rises, therefore heat inside a room can alleviate the problem. Relative humidity is classified as follows in the Building Regulations:

Humidity class Building type Relative humidity at internal temperature
15°C 20°C 25°C
1 Storage areas < 50 – 65 < 35 – 50 < 25 -35
2 Offices and shops 50 – 65 35 – 50 25 – 35
3 Dwellings with low occupancy 65 – 80 50 – 60 35 – 45
4 Dwellings with high occupancy, high moisture, maybe heated with unflued gas heaters 80 – 95 60 – 70 45 – 55
5 Specialist buildings such as laundries, breweries, swimming pools > 95 >75 >55

Figure 5. Relative humidity at a given internal temperature in different building types The first stage in investigating the problem would be to try and identify the cause of the moisture arising.

Check the paint

If there is condensation on a wall surface, particularly in a wet room, check the paint surface. This may not be due to interstitial condensation, but to vinyl or gloss paint or a damp blocker having been applied. This will have made it vapour impermeable, i.e. humid air will not be able to pass through this barrier. Instead, it will collect on the surface as condensation. Unless it can dry out quickly, it can foster the growth of mould.

In this case, there is no choice but to take off this paint layer and replace it with vapour-permeable paint. Clay paint is recommended for this purpose. Although relatively expensive, it goes much further.

Is the problem constant?

One sign of interstitial condensation might be mould growing across the inside wall surface

Figure 6. One sign of interstitial condensation might be mould growing across the inside wall surface

Is the problem the same all the year round? Of course you may not be able to see it in order to determine this, if there is no outward manifestation. But whether the moisture moves inwards or outwards may depend on conditions which can vary at different times of the year (summer, winter) and according to which direction the wall faces (sunny or not) and the local climate e.g.:

  • driving rain, cold wind (winter)
  • warm wind driving rain (in spring).

If, for example, its occurrence coincides with the arrival of a warm front from the Atlantic in winter, when the interior of the wall is cold, moisture will travel from the outside through the wall to condense at the dew point.

In this case, insulation on the outside will help, for example a hemcrete render. Or the problem may be non-seasonal, instead being due to what happens on the inside of the wall, e.g. water vapour from:

  • a bathroom, laundry room, kitchen, etc.
  • a cold, unoccupied room in winter suddenly being heated up and full of activity, pushing moisture into the cold wall.

Could the activity be modified to reduce humidity? For example, relocating laundry drying to a well ventilated room, or installing effective extractor fans, or single room mechanical ventilation with heat recovery.

Other forms of damp within walls, such as from rising damp or rain ingress through cracks, are different because they don’t suddenly condense at a point within the structure. These would be treated in a different way.

Ideally, solving the problem of interstitial condensation involves the positioning of monitors within a wall with records being kept over a year to determine at what point and under what conditions the condensation occurs. But in the absence of this, how can interstitial condensation be tackled?

Check the ventilation

First, check whether the original ventilation bricks or chimneys/flues have been blocked. The same applies to trickle vents on windows and roof voids. Blocking of any of these will hinder passive stack ventilation within the building and contribute to the problem. All ducts or flues must extract air into the outside.

The type of ventilation needed depends on the property. There is obviously a balance between ventilation and maintaining a comfortable internal temperature. In well-insulated properties, it would be worthwhile investigating mechanical ventilation with heat recovery, either for a single room, or the whole house.

Whether heat recovery is employed or not, ventilation fans can be linked to a humidity sensor, only coming on when humidity is over a certain level.

Check the occupancy

This applies particularly to historic buildings. As in the example above, often they are occupied intermittently, so interiors can be cold and then rapidly warmed. Warm, moist air may then try to escape through walls where it may reach a cold point and condense. In this case, external wall insulation is the best solution.

Check for vapour barriers

Other kinds of vapour barriers besides impermeable paints may have been introduced, and these might be within the wall, particularly if wall insulation has been applied. In this case it might be advisable to contact the contractor to find out. Otherwise, it might be necessary to explore the insulation to see for yourself.

The importance of hygrothermal materials

To have the best chance of solving the problem, the layers of materials within the wall, floor or ceiling (wherever the problem is), all need to be both:

  1. hygrothermal and porous
  2. progressively more vapour resistant moving from the warm (inside) to the cold side.

A hygrothermal material allows more moisture in than can get out at any one time, and is able to store it without harm until conditions change so that the water can be released. (Imagine a porous stone or brick doing this; lime and hemcrete also achieve this easily).

The vapour resistance of a material is a measure of its resistance to letting water vapour pass through. It is measured by its µ-value (“mu-value”), also known as its “water vapour resistance factor”.

Here are some examples of the µ-value of materials (many more statistics on the hygrothermal properties of materials are here):

Mineral wool: 1
Polystyrene beads: 2
Loose-fill cellulose fibre: 2
Loose-fill expanded perlite: 2
Urea-formaldehyde foam: 2
Loose-fill exfoliated vermiculite: 2-3
Woodwool board: 5
Perlite board: 5
Wood fibreboard: 5
Render: 10
Timber: 50-200
OSB: 50
Polyurethane foam: 60
Expanded polystyrene: 60
Concrete: 100-130
Extruded polystyrene: 150
Polystyrene: 100,000
Polythene: 100,000
Metals (off the scale – not listed).

To convert a µ-value to the vapour resistance (MNs/g) of a given material, multiply by its thickness in metres, then divide by 0.2 (g.m/MN.s). E.g. for a material with a µ-value of 60 and thickness of 30cm, its vapour resistance is 60 × 0.3 m ÷ 0.2 g.m/MN.s = 1 MN.s/g.

Using this information, you can work out the progressive vapour resistance of the various layers of different materials of different thicknesses within a wall/ceiling/floor.

To go further, and have a good chance of successfully modelling its hygrothermal properties as it is exposed to different climatic conditions around the year, you would need to use modelling software. Two applications exist for doing this (and there are free versions of each):

But which way goes from warm to cold?

Applying external wall insulation can resolve interstitial condensation problems.

Figure 7. Applying external wall insulation can resolve interstitial condensation problems.

So far so good, but which way goes from warm to cold? From the inside to the outside or vice versa? Mostly, we think of the inside of a house being warmer than the outside, so we build so that the layers become progressively more vapour resistant from the inside to the outside. But in the British climate we have to accommodate for the fact that the external conditions change throughout the year.

In the summer, generally it is warm outside and the relative humidity of incoming air will not cause a problem. In the winter, it is cold outside, and sometimes the relative humidity can be high, but the materials with which this air might come into contact, on or inside the building skin, may or may not be colder than the air; condensation can only occur if they are colder.

In the spring and autumn there may also be issues. Especially in the spring, conditions inside the wall can be cold. Warm moisture can be pushed in from the outside if there is a warm front bringing rain. In this situation it may be drawn in and condense on a cold surface within the wall. This is not necessarily a problem. As long as the hygrothermal property of the wall is good enough, it will allow it to dry out later. (Modelling could help to determine this.)

Sufficient ventilation will also help. If it is a problem, then applying external wall insulation with a waterproof render and a gradient of high to low vapour resistance from outside to inside in a breathable construction, as in Figure 8 would be the ideal solution to preventing interstitial condensation all year round. But this solution is not always possible, especially on older and listed properties.

Older buildings

If a vapour resistant barrier is on the cold side of the insulation, then moisture coming in from the outside can condense on the outside surface of it.

Figure 8. If a vapour resistant barrier is on the cold side of the insulation, then moisture coming in from the outside can condense on the outside surface of it.

A recent report on retrofitting older buildings, Responsible retrofit of older buildings by The Sustainable Traditional Buildings Alliance (STBA), expresses concern about using internal wall insulation in buildings constructed pre-1919 for this reason: “preventing heat flow into walls which may be needed to help drive out latent moisture and thus prevent external surface or interstitial condensation.” It concludes: “As moisture responses in buildings will be location-specific, the appropriate type and amount of insulation, particularly of internal wall insulation, may need to vary in response to different regions, locations, orientations and building forms.”

This is essentially saying that to be as sure as possible, modelling would need to be done using the software above as part of the design work. Let’s take a closer look at what could happen.

Some types of insulation come with metal foil on one or both sides (e.g., PIR). These therefore have a high vapour resistance that is a barrier to moisture. If this, or any other high vapour resistant barrier, is on the cold side of the insulation, then a) moisture coming in from the outside can condense on the outside surface of it. Alternatively, b) moisture being driven out from the inside can condense on the inside surface of it.

In the first instance (a) it would therefore not be a good idea to put such insulation on the inside of a wall, especially with a foil layer flush against the existing wall. Therefore, if internal insulation is still to be used, an air gap could be left between the wall and this layer. Moist air coming in from the outside into this would have a chance to dissipate.

Some people advocate letting this space be ventilated, in order to better permit this to happen. Others say it defeats the point of the insulation, as an air current can suck warmth through the insulation from the inside of the building. If the gap were to be ventilated, it could be done for example using air bricks at the bottom, and vents into the roof or inter-floor space at the top; there would be many different types of opportunity in practice.

In the second instance (b), where the moisture is being driven out from a warm, relatively more humid interior, then this type of insulation should be avoided altogether in favour of a more organic solution such as woodfibre batts (in fact it would be preferable to use it in case (a) too, space permitting).

In either case, the ideal solution is to use external insulation, as in Figure 8, but unfortunately it can less frequently be used on traditional buildings due to planning constraints. This whole area is still a subject of on-going research.

Implications for the Green Deal

If you are trying to get Green Deal support for a retrofit on a property like this, you will need to be aware that the Building Regulations’ recommended strategies for dealing with interstitial condensation are largely inappropriate for traditional, solid walled structures. Yet these Regulations are applied to work done under the Green Deal. Your Green Deal provider should be to be made aware of this, and of the report of the Sustainable Traditional Buildings Alliance (STBA) mentioned above, in the hope that appropriate steps can be taken and support still received.

Cavity walls

Cavity walls that have been insulated provide a similar kind of problem, as we saw at the beginning. Condensation can occur within the cavity on the cold side of the insulation. Here again, one could make sure the cavity is ventilated, so that air can enter at the bottom and leave at the top. Yet these days we are aiming for more airtight construction. Adding ventilation into a wall seems to go against the benefits of doing this, so it should only be considered as a last resort. So, with cavity walled buildings, it is more likely that the external wall insulation could be used which, as we have seen, is always preferable.

Intelligent membranes

What about using intelligent membranes? An intelligent membrane is a particular kind of vapour control layer which changes its permeability according to relative humidity, pressure and temperature conditions, so it can vary between being a vapour barrier and a breathable membrane. It is generally taped into place over the insulation, on the warm side. It is certainly useful, but not as a complete solution. Firstly, in practice, it is very easy for a vapour control layer to be fitted poorly. There may be gaps, or even pinprick holes in it. Secondly, if the moisture is arising within the room, it still needs to go somewhere, and so adequate ventilation etc. must be provided, as described above. It is therefore not advisable to rely solely on the vapour control layer to prevent moisture getting into a wall, floor or roof.

A word about floors

Interstitial condensation can also occur in suspended floors. To prevent this, firstly infill between the joists so that there are no air gaps with a breathable material such as blown cellulose or woodfibre batts squeezed in to fit. Insulate underneath the joists as well, and beneath that layer insert a damp proof layer. On top of the joists put a vapour control layer, which will lap up inside of the skirting boards. Place flooring on top of this.

Getting technical

There are various standards for dealing with interstitial condensation: BS5250:1979 – Dewpoint Method BS5250:1989 – Appendix D contains a calculation procedure BS5250:2002 references BS EN ISO 13788:2002 CEN TC89 WI 29.3 Standard for ‘Assessment of moisture transfer by numerical simulation’ in preparation. For instance, Standard G4.1 says: “A floor, wall, roof or other building element of a dwelling must minimise the risk of interstitial condensation in any part of a dwelling which it could damage”. But these do not cater for every situation. First of all they do not cater for traditional, solid wall buildings. Secondly because of the “unknown unknowns”.

The unknown unknowns

When all is said and done, there is still much we don’t know about interstitial condensation. Although we know a lot about the thermal conductivity of materials, we know less about their vapour permeability under different temperatures, pressures and humidity levels. There is also no data about how air infiltration into a structure, caused for example by wind, which will affect pressure, alters the picture, including how it behaves in cavities.

Other variables include the water sorption values of materials (their absorption and adsorption properties, or hygrothermal behaviour), and how liquid water diffuses through materials. All of these need to be linked to specific climate data and user behaviour.

If any readers have particular advice for particular situations, we would love to hear their success stories and advice. Please respond via the interstitial condensation thread on the SuperHomes forum.

eco renovation - talk to homeowners

© David Thorpe Jan 2013. David is Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

Also see:
Do I need MVHR?
MVHR in a Victorian house
DIY internal wall insulation
Insulation vapour barrier


A Further View:

Interstitial condensation is not endemic

by John Doggart, Sustainable Energy Academy

The risk of interstitial condensation hasn’t stopped the widespread use of internal wall insulation in France. And in the UK there is growing evidence that this problem does not present where the installation and ventilation strategy is approached with care. This includes a vapour barrier where specified, generally on the internal side of internal wall insulation, well installed by trained operatives.

  1. France has no history of cavity wall construction, and since 1975 France has insulated approximately 10 million houses, the vast majority with internal wall insulation. There have been very few reports of problems according to Daniel Quenard at CSTB (France’s equivalent of BRE in the UK) at Jan 2012. CSTB does not view moisture difficulties as an endemic problem.
  2. Kingspan has supplied several million sq metres of wall insulation and report that they have not experienced endemic problems.
  3. When asked, SuperHomers did not report issues with mould in corners following internal wall insulation.
  4. A Brighton SuperHome with internal wall insulation is currently being monitored by Glasgow Caledonian University. A related graph shows the moisture content at joist ends. This shows that moisture content is steadily dropping after insulation, i.e. that the insulation has made the moisture position at joist ends better.
  5. Results from monitoring interstitial condensation at a Camden SuperHome from Jan 2011 – Dec 2012 shows that moisture levels are within acceptable limits. Here 100mm of polystyrene was the internal wall insulation used.
  • Membrane Brick: 7-14 (2011), rising to 9-16 (spiking to 19 g/m3) by Sept 2012 – Acceptable value
  • Membrane – Plasterboard: 7-15 (2011), rising to 8-15 (spiking to 18 g/m3) – Acceptable value
  • Floor, Insulation: 7.4-12.6 spiking to 15 g/m3 – Acceptable value
  • Floor, Foam: 0.3-0.6, declining to 0.2-0.3 with spikes to 0.5 g/m3 – Acceptable value
  • Floor, Space: 0.9 declining to 0.1 then vanishing from July 2011 to July 2012 and coming back at 0.1 g/m3 – Acceptable value

Install following instructions and ventilate

A number of factors can contribute to the successful installation of internal wall insulation:

  • a ventilation strategy that removes moist air (eg. humidity triggered MVHR units in kitchen and bathroom)
  • insulation fitted in accordance with manufacturers’ instructions

Reputable suppliers like Kingspan are setting up training schools for installers, to ensure that they know what to do, and it is recommended that trained installers are specified in contractual documents.

Ends | John Doggart Oct 2012. John is the Chairman of the Sustainable Energy Academy, co-founder of the Existing Homes Alliance and developer of the WHISCERS internal wall insulation system.