Interstitial condensation


So why does it occur?

Interstitial condensation refers to condensation occurring within spaces (or interstices), and usually this means within stud walls, or the rafter spaces of a house with a room in the roof with rafter-level insulation. Sometimes it can occur within the structure of a wall if it is made of very porous material. Condensation is the same physical process whether it occurs on the surfaces of walls or ceilings or within walls or rafters - the difference is simply where it occurs. But once you understand where water vapour is coming from, and under what circumstances it can condense, then designing an insulation build up to avoid it is usually straightforward.

Condensation occurs when the air cannot hold any more moisture, and droplets of water begin to form on surfaces. Warm air can hold more moisture than cold air: it follows from this that if the temperature of moisture-laden air suddenly drops, or if too much moisture is entering a cold space, then condensation will occur.

Room in roof or Vaulted ceiling?

Where there is a room in the roof or vaulted ceiling the insulation is at rafter level. The insulation material can be packed between the rafters with the waterproofing underlay above it, or set above or below the rafters - or a combination of these - and it is in the gaps between these materials that interstitial condensation can occur.

12-15 litres of water vapour a day!!!

The 12-15 litres of water vapour per day produced in a domestic house through the usual activities of washing, cooking, etc. will migrate through the plasterboard into the rafter space unless a vapour barrier – typically polythene or very thin aluminium foil – is set underneath the plasterboard to prevent this. If unchecked, this water vapour will reach the cold side of the insulation and condense within the rafter space if the temperature and humidity conditions are such that a dewpoint occurs. So on the cold side you should make it as breathable as possible – using a breather membrane – or else ventilate if there is a vapour-impermeable underlay such as bitumen felt, so that this water vapour can be swept away.

As with roofing underlay in a cold loft, there may be a few days in the year when condensation is unavoidable because of weather conditions – though you won’t see this inside the rafters, of course - but the important thing is that it subsequently evaporates. It’s when moisture persists that mould spores get a chance to germinate and grow, rotting the timber.

So the principle should always be to have a vapour barrier on the warm inside and something very breathable or ventilated on the cold outside, and ideally the insulation should become progressively more breathable going from the warm to the cold side.

Where a silver multifoil is used over the rafters with insulation below, then condensation becomes unavoidable unless a very good vapour barrier is used underneath the plasterboard and not subsequently breached, e.g. by fitting downlights.

This also means that you wouldn’t use a silver multifoil vapour barrier (VB) in or just under the rafters with insulation beneath it, because even if thermal laminate with an integral vapour barrier layer is used, this does not block vapour anywhere near as well as the silver multifoils, and it also relies on perfect taping where the boards join.

If two layers of PIR board are used – some between the rafter and some below – a rule of thumb is that you should always have the thicker one on the cold side, else condensation could occur at the interface.

But what about a listed building requiring a fully breathable solution?

This could be problematic, as water vapour can rapidly pass through breathable insulating materials to the cold side, so aiming for a low U-value may be inadvisable. Sheep’s wool and hemp will provide a certain amount of moisture buffering (by absorbing and then releasing it), but carrying out a condensation risk calculation may be advisable. If less insulation is used, the fabric of the roof or wall is then warmer (by heat loss from the inside) and will help prevent condensation. Alternatively TLX Gold could be used instead of a breather membrane, to keep the rafter space sufficiently warm that condensation does not occur. Fortunately Listed buildings are exempt from the requirement to achieve 0.18 W/m²K.

What about stud walls, where a ply sheathing board is used on the cold side?

With modern new builds requiring very low U-values – typically 0.18 W/m²K as compared with 0.28 W/m²K for a new wall on an existing house – a vapour barrier is essential. With a historic timber frame requiring breathable insulation it may be worth considering using the more breathable wood fibre boards, or using TLX Gold on the outside to keep the board beneath warmer.

Should a vapour barrier always be used?

Generally in our climate we can usually assume that our houses are warmer than the outside temperature, and warm air can carry far more water vapour than cold air, and so (since higher concentrations try and move to areas of lower concentration) water vapour movement is from the inside outwards. (This is not the case in places like Florida, however, where there are hot humid conditions outdoors and movement is from the outside in). There is an exception, however; where you have a particularly porous wall – whether because of traditional porous building materials or porous mortar – where water can be forced into the wall by wind-driven rain, then an increase in temperature would drive water vapour inwards. A drop in the external temperature would mean that it would be trapped by an internal vapour barrier and condense. The advice here would be to not use a vapour barrier and not to insulate to the current requirement of U = 0.30 W/m²K for an existing wall.

Condensation risk calculationsInterstitial condensation calculation

A condensation risk calculation can be carried out to see if condensation is likely to occur – but this is based on an idealised model. It does not take account of gaps at the sides of the insulation board, or daily temperature fluctuations, the presence of junctions, gaps letting air through, or whether joints have been inadequately taped. In general just considering the guidelines above will provide your answer, though a calculation can prove useful if vapour-permeable Avoiding interstitial condensationinsulation materials are to be used in a fully breathable solution.

In the figure right, the blue line shows the temperature profile through a stud wall with plasterboard (no internal vapour barrier), breathable insulation, and an external sheathing board (layer 4), and the red line indicates the dewpoint conditions. The blue line touching the red indicates that condensation would occur just under the sheathing board.

View our case study demonstrating this in practice

A further example of this solution can be found HERE


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