Why it is important to limit condensation in the home?
Where moist (humid) air meets with a cool surface, it can turn to condensation either on the face of a structure (surface condensation) or inside the construction (interstitial condensation).
Whilst condensate might not damage some materials on which it occurs, if a sufficient amount accumulates it can run or drip onto other materials where it might cause damage.
A combination of high levels of humidity over long periods, together with warmth, a source of nutrition and oxygen can also support the growth of mould on surfaces or within constructions, presenting a potential hazard to health as mould is closely associated with respiratory allergies. Mould can also damage the building fabric, surface finishes, fittings, clothing and furnishings, particularly in unheated spaces such as lofts or parts of rooms sheltered from heating systems, such as cupboards or wardrobes placed against external walls.
Interstitial condensation, which is condensation occurring within or between layers of a construction can (if not controlled) cause deterioration of the building fabric, through corrosion, rot and decay. It can also reduce the thermal resistance of some materials, potentially significantly, which can adversely affect energy efficiency performance.
What are the potential causes of damp and condensation?
There are several common causes of moisture in buildings that can lead to condensation and damp problems:
• ground moisture and rising damp
- ground bearing floors and walls in contact with the ground can be affected by moisture transport into the porous building structure through capillary action (which draws moisture into small spaces). This can typically be identified through areas of cold/damp plaster or flaking paintwork near the base of walls, rot on skirting boards. It usually results from a damaged or absent damp-proof course;
• rainwater and penetrating damp
– where moisture enters the construction particularly through defects such as broken tiles or bricks, missing pointing or damaged guttering. These issues are particularly common in properties located within a severe or very severe wind driven rain exposure zone
. This can usually be identified by isolated patches of cold/damp plaster or bubbling paint work.;
- flood water can be absorbed into the fabric of a building, causing damage and creating a health hazard. In addition, as that water is released it can create an acute increase in the risk of condensation. The homeowner should be able to advise if flooding has occurred;
• spills and leaks
- damage or defects in the external envelope and defective joints or fractures in water or waste pipes can lead to damage to water-sensitive materials and finishes and is likely to increase the risk of condensation. Prompt attention is required to remedy such damage and defects;
• construction moisture
- water incorporated in materials, such as mortar, plaster and render is released as a construction dries out. This is most common on new build or extended properties;
• moisture generated by occupants and their activities
, which are liable to change during the life of the building. This may require additional ventilation to be added.; and
• air-borne moisture and flueless heating appliances
can also contribute to moisture in buildings and potential damp problems. Again, this may mean that additional ventilation such as an extract fan is needed in areas such as kitchens or bathrooms.
What to do if condensation or damp issues are identified?
As a TrustMark registered installer, it is your responsibility
to carefully pre-assess the building for any damp or issues which could lead to problems in the future and ensure these are effectively addressed before work is carried out. Ventilation should be carefully considered as part of this process.
Under the Green Homes Grant it is possible for homeowners to claim for a number items
which can support this. These include:
• specialist services from damp experts and structural engineers;
• modelling to evaluate moisture risk;
• air-leakage testing (both before and after measures are installed);
• repairing and treating damp; and
• repairing and improving ventilation
Once the causes of any damp are addressed, the construction should be given time to fully dry before any insulation is installed.
Understand when insulation retrofits may not be appropriate
Whilst insulation measures can help to greatly reduce the energy performance of a building, there are some cases when it may not be appropriate to add insulation. Common examples include retrofits on particularly old projects which may be constructed from unusual or particularly porous materials or have planning limitations which prevent a successful installation. If you are uncertain, it is important to seek advice and guidance of an appropriately qualified retrofit professional, particularly for much older properties.
It is also important to keep in mind that under the Green Homes Grant scheme, any work completed on park homes, high rise buildings or period properties which are protected (such as listed buildings) must be completed under PAS 2030: 2019 following the guidance in PAS 2035: 2019.
Learn more about PAS 2035.
How to address condensation risk when designing insulation measures
In order to avoid interstitial condensation in any element, materials with the highest vapour resistance should be located on the warm side, and those with lower vapour resistance on the cold side, of any thermal insulation. Constructions which incorporate an impervious external weathering layer (such as sheet metal or glass) contravene those basic
recommendations: in such constructions, provision should be made for ventilation
behind/beneath the impervious layer to allow any moisture to disperse to atmosphere.
The position of thermal insulation in relation to the thermal mass of a building determines
whether or not that mass is warmed by the heating system; designers should assess the
relative position of structural layers, thermal insulation layers and air and vapour control Layers in order to help avoid damaging condensation.
Surface condensation, which occurs on an internal surface within a building, can be largely controlled through attention to detail at junctions to help reduce cold spots due to thermal bridging. It can also be controlled through adequate heating to raise the dewpoint, coupled with ventilation to control and reduce humidity levels.
To assist installers in identifying suitable construction arrangements, our technical services department offers a Condensation Risk Analysis (CRA) with every calculation from our U-value calculation service. This is completed to BS 5250: 2011 + A1: 2016
and allows condensation risk to be estimated based on a number of factors:
• the individual components within a construction (e.g. brick, size of cavity, block, insulation, plasterboard);
• the order in which they appear;
• climate data for the building’s location; and
• an estimated relative humidity based on factors such as how many people live in your home and its size.
What to consider when detailing and installing the insulation
In addition to carefully planning the order of constructions, it is also essential to create detailed plans for challenging junctions to ensure thermal bridges are effectively addressed. Common examples include junctions around windows and doors and around joists or rafters which penetrate into the wall construction. Leaving any area uninsulated whilst insulating others can also act in a similar way as a thermal bridge, increasing the heat losses through any uninsulated area thereby reducing surface temperatures and increasing the risk of surface condensation and mould growth. As such, it may be worth considering insulating all elements at the same time or increasing ventilation levels.
Finally, the installation itself must be completed with care and attention. Insulation boards and vapour barriers should be cut and fitted accurately with no gaps or variations in the materials used compared with those in the plans. As older properties are rarely square, this will usually mean that insulation needs to be measured and cut specifically for the area it is being installed on (particularly when it is fitted at junctions or between rafters or joists).
You should have received training on the design and installation of insulation measures as part of the PAS 2030 certification process. If you have appointed any non-TrustMark certified subcontractors on a project, you must also ensure their work is compliant. If you are unsure then it is advisable to seek specialist training or to have detailed drawings produced by an expert.