How to prevent condensation in roof spaces

The simple science behind why condensation occurs is that warm air can hold more moisture than cold air. Condensation forms in the roof when warm air from the living space of a property enters the cold roof space above the insulation.

A warm living space with a temperature of 20°C has the capacity to hold 20 grams of water per cubic metre of air, but a cold roof space at 10°C can only hold 8 grams. So, when the warm air rises up into the roof, 12 grams of water per cubic metre of air needs to go somewhere. When a surface is cooler than the dew point temperature (when the air is fully saturated with water vapour), condensation will occur as droplets, typically on the underside of the roof underlay.

Condensation occurs in roof spaces when there is either too much warm air circulating upwards from living spaces, or when there is inadequate roofing ventilation.

Older homes tend to have a degree of natural ventilation due to less accurate building methods, therefore offering more opportunity for moisture to escape. As new build homes have a much better thermal performance in a bid to reduce energy consumption, roof spaces tend to be colder and the risk of condensation is increased. This is the reason why Building Regulation Part C, Clause 6.2c states that, “Roofs should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial Condensation”. 

In a correctly constructed roof, condensation may occur on a temporary basis due to extremely cold weather, when people are keen to keep warmth in and cold out by limiting air exchange in the home. This is a common occurrence and this condensation usually dissipates from the underlay within a few days with no harm done.

However, when this condensation builds up it can make other roof elements such as rafters and insulation wet, causing water staining on the ceiling or structural damage to the roof. 

There are two ways to protect against condensation. You can prevent the warm air from reaching loft space in the first place, or you can ventilate the roof space to remove the air before condensation forms. 

We can try to prevent water vapour from reaching the roof space as much as possible by opening windows, installing window ventilation systems and changing our living habits. A well-sealed ceiling will minimise air leakage through to the roof space. BS 9250 gives guidance on how to minimise the air leakage through junctions in the ceiling such as loft hatches, ceiling lights, etc. This will not only reduce the risk of condensation but it will also improve the energy efficiency of the property. Achieving a well-sealed ceiling is easier to achieve in new properties than it is in existing properties.

Secondly, we can make sure that a roof is designed with appropriate ventilation methods so that the moisture that does get up there can be released before it builds up to harmful levels. BS 5250 sets out the minimum requirements of ventilation required for roof space ventilation, the requirements vary depending on the roof build-up such as warm roof versus cold roof, what type of underlay is being used etc. Once you know how much ventilation is required at high level and low level, you can decide which method and products you wish to use to. Each solution has benefits and drawbacks, some solutions are cheaper than others while some can be easier to install than others, so the several factors need to be taken in to account when deciding which to use. 

BS 5250 advises that “occupants often fail to use a building as intended…designers are advised to err on the side of caution and adopt robust fail-safe solutions”. Therefore, consideration into appropriate ventilation methods should be taken at the start of any roof project.

There is a wealth of roof ventilation products on the market, including eaves, concealed tile, ridge and top abutment ventilators. Roof geometry and pitch can dictate where these are best placed, but cross-flow ventilation is usually improved by ventilators installed just above the horizontal insulation at each side of the roof. Further high-level ventilators will help draw air in through lower ventilators creating a circulation path for air flow.

BS 5250 offers minimum requirements for roof ventilation. It recommends that with cold roof constructions (uninhabited, unheated loft spaces where the insulation is laid over a horizontal ceiling) there should be at least a 7mm continuous ventilation gap at the eaves when using a vapour permeable underlay with a normal unsealed ceiling.

Good external air flow is also required for ventilation to work well. Eaves to eaves ventilation isn’t as successful in homes with full loft areas or that are situated very close to adjacent buildings.  

In a roof space, condensation typically appears on the underside of roof underlay as it is a cold surface. Temporary condensation is nothing to be too concerned about when it occurs for a few days in cold weather, or when the building fabric of a new build home is drying out during the first 12 months following completion.

However, this build-up of condensation can be avoided through installing passive ventilation systems and choosing an appropriate underlay. 

Non-breathable underlay

Bituminous (Type 1F) underlays have been used for many years but are waning in popularity in favour of more modern “breathable” membranes. It acts as an extremely effective secondary barrier but does little for the release of any moisture or vapour. However, this traditional material can still be used as part of a well-ventilated roof space, providing that it is used in conjunction with appropriate ventilation systems, as with any kind of underlay. 

Vapour permeable underlay

Vapour permeable underlays have been used in European construction since the 1970s and were certified by BBA in 1982 for use in tiled ventilated cold roof constructions.

With widespread use in the UK, they allow water vapour to escape whilst providing a barrier to wind and rain, letting moisture out without letting it in. This underlay is a very effective supplementary method of reducing condensation levels when used in conjunction with an appropriate ventilation system.

One major difference between 1F and VPM is the need to tape and drape. It’s a common misconception that you can just lay this like a 1F felt, it is imperative that you follow the guidance for your roof type and location.

Air permeable underlay

A newer addition to the underlay market, air permeable membranes are promoted as having the ability to release even more moisture than their vapour permeable counterparts. Some manufacturers claim that it can be used in isolation as a complete means of ventilation, which is unlikely as it offers no method of circulating air throughout roof space. 

Be aware that this type of underlay is not yet acknowledged by the British Standard BS 5250: 'Code of practice for control of condensation in buildings', covered by housing regulations or supported by any recognised testing. This does not mean avoid them, but you must pay attention to manufacturer’s instructions and consider the impact of your choices on the airflow of the void.

Air permeable underlays do come with a higher price tag, and their extra permeability in comparison to vapour permeable products isn’t necessarily required if you have an adequate ventilation system in place, however it does provide an extra level of performance if a belt and braces approach is needed. 

Correct use of membranes

Any type of membrane needs to be correctly installed to perform as it should. It is essential to consult manufacturer’s instructions as not all underlays, even those marketed as “breathable”, are the same. Underlay manufacturers will also provide guidance on the suitability of their products for any given location.

A common mistake made when installing underlay is that many people don’t tape down laps to prevent them flapping in windy conditions. The permeability of a modern, lightweight underlay is rendered completely ineffective if it is not secured correctly to prevent wind and rain ingress.

We also advise that tiles should not be cut near exposed membranes. The dust particles can block the tiny holes in permeable underlays, affecting their ability to let moisture pass though.

In short, be more considerate with the use of permeable products and do not treat them like a 1F underlay. 

Underlays of any description should not be used as a sole means of ventilation, as this is not what they are designed to do. A quick fix, cost-cutting solution does not exist, with the combination of a suitable ventilation system and good quality underlay being the best way of successfully controlling condensation in a roof space, ensuring the building will perform as it needs to for now and the future.