Heritage Retrofit

38 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION Wood-wool insulation boards ready for lime rendering and daub as possible. The theoretically poorer U-value may not be as bad in practice and the greatest reduction in heat-loss is often achieved simply by creating a dry, draught-free structure. A modern material similar in concept to daub, but with more durability and better U-value, is a hydraulic lime/hemp mix that can be cast in-situ to form a homogenous breathable infill. If the frame and/or the panels are in poor condition and repairs would involve the loss of a high proportion of historically significant fabric, there would be a strong case for protecting the wall behind a shelter coat of lime render or other regionally appropriate material. This is usually preferable to creating a crude modern replica of the wall in band-sawn timber, and may provide the opportunity to insulate outside the wall line. INSIDE THE WALL LINE If the timber frame and infill are in sufficiently good condition, and are robust enough to cope with continuing exposure with limited interventions, insulation can be fitted to the inside face, either directly to the wall or with an air gap. However, this will have a serious impact on the appearance of the room, obscuring features such as window surrounds, skirtings and adjacent ceiling mouldings, and it will reduce the internal floor area. More significantly, there is an increased risk that moisture entering the wall will become trapped, even if all the materials used in the new lining (insulation, plaster and paint finish) are vapour permeable. If problems do occur, they are unlikely to become apparent until significant damage has occurred. The risk of driven rain penetration can be reduced by careful gap-stopping and the reinstatement of overhangs, but any intervention that restricts the passage of water vapour through the wall significantly increases the risk of condensation and/or water entrapment. For this reason, non- breathable rigid insulation such as PIR (polyisocyanurate) boards should not be used, even though they can achieve better U-values at relatively small thicknesses. Insulating inside the wall line also greatly increases the risk of condensation due to cold-bridging in those areas which, for various reasons, cannot be insulated. In particular, the ends of floor beams and joists built into the external wall are at greater risk of increased degradation. OUTSIDE THE WALL LINE For many reasons, fitting insulation to the outside face of a timber-framed wall is often the best solution, both in terms of hygrothermal performance and building conservation. • The wall is fully protected (assuming materials and detailing are correct) • Necessary repairs can be kept to the minimum structurally required, and can usually take the form of additional surface-fixed straps, etc. These repairs are reversible and involve no loss of historic fabric. • Air penetration through the wall can be fully controlled • Insulation can be continuous with all original fabric on the warm side, reducing the risk of cold-bridging and condensation • Keeping what thermal mass there is in the wall on the warm side also helps to balance diurnal variations • The historic significance and appearance of the interior is not compromised • The intervention is reversible. External insulation will alter the external appearance: the additional thickness requires changes to window reveals and other features, and conceals the timber frame. This often meets with resistance, both professional and public. However, there is a strong historical precedent and the benefits are considerable. Historically, render was usually applied direct to lath nailed to the frame, and it is widely held that this must offer good protection to the frame, simply because it is breathable. However, it is quite common to find widespread active Deathwatch beetle attack in timbers immediately behind lime renders, but rare to find it in exposed external timbers, suggesting that sometimes moisture content of a lime-rendered frame can be high enough to sustain fungal and beetle attack. When applying new or replacing old render, a vapour permeable membrane should be used and the lath set off the frame on counter-battens if possible. The recent development of relatively high-performance breathable multi-layer insulation quilts, effectively insulated breather membranes, has great potential as they increase wall thickness far less than most other breathable insulation materials. Although designed for use in roofs, these quilts have been successfully used to insulate timber-framed walls behind render or weatherboard. New materials need to be used cautiously until their long-term performance is better understood, but equally, they should not be dismissed out of hand. Furthermore, imported materials that perform well in cold dry climates may not work in wetter UK conditions. Perhaps the best advice is to question everything. In a surprising number of cases, what appears to be a timber frame is actually an agglomeration of paint, mastic and cementitious render repair concealing a severely degraded and structurally compromised frame. Sooner or later this will require such extensive repair/ replacement that protection with a lime render or other cladding would almost certainly provide a more effective and conservative solution while avoiding further loss. If the appearance of a timber-framed building is deemed desirable, this can always be applied to the face of the new render – there is a long tradition of what many now consider ‘fakery’. At least what remains of the frame and surrounding fabric is retained for future generations. RELATED REPAIRS If the timber frame is to remain exposed, the essential first step in improving the thermal performance is to ensure that the frame and surrounding fabric are in good condition, and consist of materials that allow the wall to breathe. A conflict arises where an alteration regarded as part of the building’s history is demonstrably causing damage. Brick infill for example, does not always cause problems, but can significantly increase the rate of degradation of the frame, particularly when bedded in cementitious mortar, where frames are relatively light, poorly constructed or weakened by decay, or where the bricks project outside the face of the frame, creating ledges that trap water. The use of inappropriate materials is not the only problem. The introduction of impermeable materials was usually prompted by the failure of earlier or original wattle and daub infill, which usually began to fail once the protection of big overhangs was lost. Although