Without some form of destructive investigation, identifying the likely content and thermal performance of an existing timber frame wall will generally rely on the approximate date of construction and measurement of wall thickness.
Broadly speaking, over the years, the construction industrys reaction to tighter Building Regulations has been to introduce a partial fill of mineral wool insulation into the stud void of a timber frame wall and then increase its thickness to keep in line with minimum requirements. In 1976 for England and Wales the minimum requirement for walls was 1.0W/m2K. It dropped in 1982 to 0.6W/m2K and in 1992 to 0.45W/m2K. By 2002, it was 0.35W/m2K, prompting many to increase stud depth to allow deeper insulation to be installed between studs.
Whichever method is chosen for upgrading walls, care must be taken that the timber structure is not subjected to interstitial condensation, which occurs when warm, moist air from inside a building penetrates into a wall, roof or floor construction and meets a cold surface. This causes the air to cool, lowering its capacity to carry moisture, resulting in condensation on the cold surface. Formation of interstitial condensation can raise timber above the decay threshold of 20% moisture content.
Choosing the correct materials which have been third-party approved for use in timber frame construction and achieving a successful condensation risk analysis of the wall, incorporating the proposed changes, will help to ensure that the wall can perform as required.
Provided that the disruption of removing the plasterboard wall lining can be accepted, stripping the walls back to the stud framework and increasing the amount and/or performance of insulation in the timber frame stud void is the most satisfactory way of improving the thermal performance, since it effectively repeats the current normal constructional procedure and keeps the same wall footprint (see Figure 1). The type of replacement insulation specified must be suitable for use in timber frame walls and its essential that a new vapour control layer is applied to the wall prior to new plasterboard being fitted.
Another method of improving insulation is simply replacing the original plasterboard with an insulated lining board, leaving the original insulation in place (see Figure 2). While this method increases wall thickness by only a few centimetres, this encroachment into the living space may be undesirable as it may require alteration to floor coverings, window frames/ boards, electrical sockets, radiator pipes, or even baths and kitchen units. The two methods noted above can be combined to deliver better thermal performance.
Another variation, which can be combined or used as an alternative, is to remove the plasterboard and vapour control layer before planting battens of the required depth on the inner face of the studs (see Figure 3). This effectively increases the stud depth, providing additional space for insulation to be installed. A new vapour control layer and internal lining are then installed.
When increasing the insulation, attention must also be given to the possibility of increased build-up of heat in electrical cables, requiring de-rating of electrical circuits. Also, if electrical cables come into contact with expanded polystyrene it can cause a chemical reaction, so special precautions must be taken to avoid this.
One solution, which assists in maintaining a continuous vapour control layer and separating services from the insulation, is to form a vertical services void immediately inside the vapour control layer by planting vertical battens over it onto the studs. Reflective vapour control layers can further enhance the thermal performance of these walls, but only when installed with the reflective foil surface facing an adjacent unventilated airspace.
Adding insulation to the internal face of a timber frame wall without removing the existing lining material may seem simple at first glance. However, the location of the vapour control layer in relation to the insulation means that interstitial condensation could form. The vapour control layer could become a 'cold' surface, attracting condensation which would be trapped in the wall structure. This method of upgrading cannot be recommended, except for buildings where the preliminary investigation shows that no vapour control layer was originally included, or a successful condensation risk analysis of the proposed construction has been undertaken.
On timber frame structures, externally applied insulation can be considered. However, this will entail removal and replacement of the original cladding to allow fitting, which is both labour intensive and costly if not required for other reasons.
When fixing timber battens, insulation or cladding to the outside of the timber frame, it is important to identify stud positions. Fixings which penetrate through the external sheathing, without hitting a stud, could be accidentally pulled out.
Vertical timber battens fixed directly over the original stud positions, with insulation installed between them, provide a simple and cost-effective method of increasing the insulation depth and therefore U-value of the wall. The insulation is retained by friction fit between the battens and by the breather membrane which covers them both. Wall ties, or further vertical/ horizontal battens, can then be installed to provide a drained and ventilated cavity, plus support for the external cladding.
Installing insulation boards which cover the entire external wall provide continuity of insulation. True warm wall construction only has insulation installed externally, leaving the whole of the structure warm and free from risk of interstitial condensation. However, unless the timber frame building being upgraded is particularly old, it is likely to have some insulation between the studs already. The behaviour of moisture in a wall with insulation between studs and externally is different, as not all of the structure will be the same (warm) temperature. This hybrid wall may create a condensation risk a particularly important factor for a successful wall is the breathability of the external wall insulation.
Whichever type of insulation is used, careful workmanship is important to ensure a tight fit between the insulation and surrounding materials. Gaps provide routes for air movement and thermal bridges.
On one final cautionary note, timber frame construction relies on a clear drained and ventilated cavity to maintain a timber moisture content below the decay threshold of 20%. Filling the external wall cavity of a timber frame building with insulation removes the cavitys ability to perform these tasks and risks timber decay. Therefore, full fill cavity wall insulation should not be used for upgrading thermal performance in timber frame structures.
Figure 3: adding battens and insulation internally and externally
- For further information email [email protected] TRADA will publish a Wood Information Sheet on this topic in the near future. Visit www.trada.co.uk/Bookshop.