Your chosen TrustMark registered installers should first ensure that any existing ventilation measures which are unaffected by the planned work, such as air bricks and ventilation gaps at eaves, are clear of obstructions. They should then consider how the insulation measures you are having installed may impact existing ventilation throughout your property, the likely relative humidity in the different spaces within your home and what new ventilation measures may be needed to bring this to an acceptable level. This work may include carrying out pressure tests on your home and should be performed by a suitably trained individual.
They should also consider potential restrictions on your property which may prevent a successful retrofit. For example, for some listed properties it may not be possible to fit solid wall insulation due to limitations on external alterations. This might prevent the use of external render and insulation, whilst the presence of particularly porous materials in the walls may make them unsuitable for internal wall insulation. The advice and guidance of an appropriately qualified retrofit professional should be sought in such circumstances, particularly for much older properties.
Assessing Condensation Risk
To help installers, when calculating a U-value for the area being insulated (a measurement of how effective the entire construction will be at preventing heat loss) our technical services department also provides a Condensation Risk Analysis (CRA). The CRA is calculated under the British Standard BS 5250: 2011 +A1: 2016
. It allows the risk of interstitial or surface condensation to be estimated based on a number of factors including:
• 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.
The CRA analysis can allow alterations to be made which minimise the risk of condensation. This can include adjusting the thickness of the insulation layer and where it appears in a construction. Vapour control layers may also be introduced to prevent water vapour from moving from the warm to the cold side of a construction and condensing.
Care with detailing and installation of insulation
Once they are satisfied with above, the installers should then carefully plan how the insulation is going to be fitted, including creating detailed drawings for areas such as junctions. This planning is important to limit so called ‘thermal bridges’. These are created when materials which penetrate through a construction are less resistant to heat transfer than the surrounding construction – as a result, they act as a route for heat to transfer out of a space. Common examples of potential thermal bridges include roof rafters and junctions around windows and at room corners. If these areas are not addressed, they can create cold spots which allow condensation to form.
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 and therefore reducing surface temperatures and increasing the risk of surface condensation and mould growth.
To prevent this, the installers should look to insulate these areas whilst also ensuring effective ventilation is incorporated where required. For example, if they are planning to install insulation internally in a room-in-the-roof, insulation will typically be fitted between the rafters with an air gap of at least 50 mm between this layer of insulation and the sarking boards
. This allows air to ventilate out through the eaves rather than condensing on the cold sarking boards. A further layer of insulated plasterboard will then typically be fitted below the joists to prevent them acting as thermal bridges.
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.
These steps may seem like a lot to consider, but TrustMark installers must all undertake dedicated training around insulation retrofits to gain their certification, and so should be both familiar with these requirements and skilled at dealing with them. By adopting this thorough process, your installer will be able to ensure that the proposed insulation measures are appropriate for your home and are properly designed and installed - preventing issues with damp and saving you money for years to come.