More satisfactory is the use of some form of lightweight infill panels which add little weight to the frame and which can at the same time incorporate some thermal insulation to the walls, which will certainly be desirable. A method used at the Avoncroft Museum at Bromsgrove, when re-erecting timber-framed building using both traditional techniques and modern materials, has much to recommend it. The wattle is replaced with new, formed of impregnated hazel onto which a daub (of eight parts of clay, one part lime and one part cow-dung) is applied to the face. This coat is then scratched to form a key for a skim coat consisting of equal parts of sand and lime with some cement added in the proportion ЪУг. ЪУг. 1, and the panel is then limewashed. The daub is applied to both sides of the wattle, but in domestic work, where increased insulation is required, it can be omitted from the inner face and replaced with a woodwool slab cut and fitted and wedged in position, care being taken to provide an adequate air space between the wattle and the woodwool slab. The woodwool slabs are then plastered and decorated in the usual way. Woodwool slabs have long been used as a replacement for wattle-and-daub. They can be obtained in various thicknesses from one inch up to four inches and in various lengths, depending on their thicknesses, from six feet up to twelve feet six inches; however, they can be obtained only in two-foot widths and so, unless the studs are closely spaced, have to be joined. The woodwool slabs are generally fixed to a treated softwood frame screwed around the openings and the panel finished off with rendering externally and expanded metal lathing fixed to the softwood frame and plastered internally. Additional insulation can be incorporated between the woodwool and the internal plaster.
Another and perhaps the simplest method is to introduce treated softwood battens of the appropriate size screwed around the opening to the timber-frame. Onto both sides of this framework, expanded metal lathing is nailed or stapled with additional treated softwood noggings introduced if the span exceeds two feet. Between the two layers of expanded metal lathing a layer of compressed fibreglass quilt insulation can be introduced and the panels then rendered externally and plastered internally in the usual way. The disadvantage with all these methods is, as with wattle-and-daub, the problem of shrinkage between the studs and panel; however, this can be overcome by the introduction of a compressible preformed butyl strip which can in addition be pointed up with a flexible waterproof mastic filler.
A modern material gaining popularity is polyurethane foam sheeting, a material which has many advantages: it can fit any size or shape of panel without joints and has the lowest thermal conductivity rate of any commercially available insulation material. It is wedged into position, the gap being filled with polyurethane sprayed foam, securely fixing the sheet in position. Because of the excellent bond of polyurethane to timber, it overcomes the problem of water-penetration at the interface of the timber. The panel is then rendered with one of the specially developed renders, such as Cemfil, and plastered internally with two coats of gypsum plaster.
A small lead flashing should be provided to form a sill at the foot of all replacement panels dressed between the rendering and infilling and projecting about an inch and turned down to form a drip. The underside of the lead should be painted to resist tannic acid attack from the oak.
Brick nogging too often needs to be replaced, because of deterioration of either the brick or more commonly the mortar. The panels should be carefully removed and rebuilt, re-using if possible the original bricks for these are often of a fine quality as regards their colour and texture, although some are somewhat porous. Galvanized or non-ferrous ties should be screwed to the face of the studs and built into the joints of the brickwork to secure the panel. The joint between the brickwork and timber frame should be raked out and the gap filled with a waterproof mastic filler to prevent the ingress of water. Even where the panels are sound, it may be necessary to remake the joint between the brick and timber; this can be remade in mortar but the problem of perimeter shrinkage still remains and it is advisable for the joints to be caulked with a compressible preformed butyl strip which gives a base for mastic filler. When rebuilding brick panels, the temptation of incorporating patterns, such as herringbone and diagonal courses, should be avoided and panels should be renewed as existing, for the custom was not devised from a desire to add interest but to encourage the bricks to exert an outward pressure against the timbers while the mortar was soft.
Although in most cases daub will need to be replaced because of the decay of the supporting wattle, often, when the wattle is still sound, the old daub can be repaired. As we have seen earlier, the specification for daub varies greatly in different areas, and one of these can be adopted in repair. We have seen that at Avoncroft a mix of clay, lime and cow-dung in the proportion of 8:1:1 is used. Another found to be serviceable is a mix of lime putty, sharp sand and cow-dung in the proportion of 4:1:1 well mixed and bound together with chopped straw or long hair. All daub should be mixed as dry as possible to reduce to a minimum any shrinkage that may occur. Any shrinkage cracks should be filled with a similar material but without the binding. In addition old wattle-and-daub panels often shrink: these should not be removed if sound and can be wedged in position and any gap between panel and timber filled in with daub. Daub panels should always be limewashed.
As we have seen earlier, historically, exposed oak timbers were either left in their natural state or limewashed over in conjunction with the wattle-and-daub panels, and the blackening of the framing so common in the West, Lancashire and Cheshire was a later rather than an early idiom. Today there is a growing number of black-and-white buildings in these areas in which the tar-like materials are being removed, restoring the timbers to their natural state, for timbers so treated prevent them from breathing, moisture becoming trapped and decay developing.
The tar or bituminous paint can be removed in a number of ways. One satisfactory method recommended by the Society for the Protection of Ancient Buildings is an application of strong caustic soda, the surface thoroughly washed down with clean water, but a small trial area should be made in the first instance. This treatment is followed with the application of two or three coats of limewash over the oak, which after two or three days can be removed with a hard brush. A blowlamp should not, of course, be used. In those areas where traditionally timbers have been blackened, they can, for aesthetic reasons, be treated again, perhaps with a proprietary timber stain which allows the timber to breathe. Gloss paint and all tar-like products should not be used, nor should linseed oil, another common treatment, which tends to remain sticky, attracting dust and dirt as well as discolouring the timber. It is, however, probably best to leave the timbers in their natural state but, if treatment of some kind is thought to be needed, then a clear preservative fungicide can be applied after the removal of dirt. Oak can be satisfactorily cleaned with soap and water and a little household washing-soda followed by several rinses of warm, clean water.
All English oak is full of shakes, but though some appear almost alarming few are of much structural importance. It is true that in external timbers they can afford a lodgement for water but old oak is not sufficiently porous for water to soak into it to any great extent, and they should be filled only if water is being directed through the cracks to the internal face of the building. Hard fillers should be avoided, and in particular cement mortar, a material frequently seen, as these do not adhere to the timber and being inelastic do not expand and contract with the timber. A compound of oak and resin glue in the proportion of 1:1 or one of the many proprietary fillers on the market would be suitable. The filler should be kept back from the face of the timber and the base weathered to permit rain to run off. If the shake is large, oak slips should be bedded in the filler; the timber should be old but sound to avoid shrinkage.
Although oak is best left in its natural state, when repairs to the frame have been carried out, with either new or old wood, the natural weathering process can be expedited by the application of limewash over both old and new timbers, the excess being removed from the timber with a stiff wire brush the following day. Alternatively a coat of sulphate of iron can be applied, which produces a mid-grey effect. These treatments are also suitable for timber frames which have formerly been plastered.