Drylining with thermal board for low cost wall insulation

Drylining with plasterboard is a relatively low-cost method of getting top-quality walling, and for most applications it is not a difficult task to construct a suitable timber framework. But in certain instances it may be possible to dispense with even this.

Drylining with thermal board

With its backing layer of expanded polystyrene, thermal board is particularly useful for drylining ‘cold’ external walls and for improving the thermal and acoustic insulation of internal walls. With minor variations in the actual methods of fixing employed, it can be used as an alternative to standard wallboard and can be mounted in the same ways.

But when the thermal board is used specifically for its insulating qualities, on walls that have already been properly finished, there is often no need to go to the elaborate lengths of providing a support system. Instead, a special adhesive can be applied directly to the wall and the boards are pushed firmly against this while secondary fixing takes place.

Obviously this method can be used only on smooth-faced concrete, masonry or plastered walls which are dry and perfectly level.

Start the preparations by removing any skirting boards marking fixing positions on the floor adjacent. Any wall fittings that are removed and which have to be replaced—may have to have their fixings relocated to accommodate the additional depth of the cladding. For example, the fixing boxes for electrical switches and sockets will have to be removed and then reset only partly into their wall recesses. To avoid cumulative errors, mark and cut the corresponding apertures in the thermal wallboard just prior to fitting the relevant section on the wall.

Suitable apertures can also be cut for heavy fittings such as a cistern or sink. If these are removed, they must be remounted on the support wall in a way that does not strain the plasterboard cladding or the wall itself.

If at this stage, it looks as if the window sills will need replacing, remove these now. However, it may be possible to plane the edges square and then extend them by gluing and screwing on new fillets of timber so that they satisfactorily overhang the new wall depth.

When you have dealt with the fittings, remove old wallpaper and other surface decorations. Old paintwork need not be removed, but it is a good idea to scour this with coarse sandpaper or a wire brush, so that you provide a good key for the adhesive. Run over the whole wall with a damp cloth afterwards to remove dust and grime to improve adhesion.

Fixing thermal board

Thermal board is available in widths of 1200mm and, as is usual for walls, the boards are fixed vertically. The next stage is therefore to mark up the wall, bearing in mind the measurement of the boards.

Start the marking out procedure from a suitable wall or door and arrange for a cut edge to coincide with an internal corner. Allow the thickness of the board plus 2mm when determining a new window reveal’s measurements.

Mark the wall at 1200mm intervals and at intermediate positions half way between.

You can then cut the board lengths to the correct height. Use a fine-blade saw and cut the board with the plaster surface uppermost. Arrange to have the cut end nearest the floor so that any irregularities can be concealed after- wards by the skirting.

The adhesive is then applied to the wall, enough for one board at a time. Take the first of the marked 1200mm centres as a guide to where the edge of the board will be fixed and trowel on adhesive 50mm inside this line. Use a notched applicator—which you could make from an offcut of hard-board or plastics—to spread the adhesive in a band some 200mm wide for the complete vertical length of the board. Then apply adhesive along the opposite vertical, again 50mm inside the marked line. The last vertical band of adhesive should be applied along the intermediate line, approximately half way between the two edge bands. Finally, apply adhesive to the horizontal gaps at the top and bottom to complete coverage.

To fix the first board, push it hard against the adhesive and check that the vertical edges are plumb. If necessary, use a foot lift or packing board to keep the board tight against the ceiling while the secondary fixings are made. This job must be done immediately, and you may find assistance useful at this stage.

Drill holes to coincide with the top, middle and bottom of each of the vertical adhesive bands. Use suitable plated screws and Fischen-type plugs to fix the board to the walls at these nine points, allowing for at least a 25mm wall penetration to be sure of a strong enough fixing.

Then, without breaking the surface, screw in the heads to form a slight recess at the fixing point. These

Apply adhesive to the wall for the next board. Lightly butt this against the fixed board when you position it, but ease off slightly to leave a 2-3mm gap. Use plugs and screws to fix the board in place as before, then repeat the fixing sequence for the remaining boards, cutting holes and trimming to shape only as you come to each individual board.

To complete a wall, you will probably have to line a window or door reveal. Remove the polystyrene backing carefully when you come to deal with the butt joint at external corners. Adhesive alone is normally enough to fix thin strips of thermal board used in this way but use screw fixings as well if you are in any kind of doubt.

Filling plasterboard joints

If you are using plasterboard as a substitute for traditional plasterwork, the nearest you will get to the difficult skill of plastering is the process of jointing, whereby you conceal the tell-tale gaps between boards. But you must take extra care over this job if the plasterboard is going to be decorated only with a paint finish.

Plasterboard jointing entails using special paper jointing tape, a cementlike filler and finishing compound. Specially reinforced tape is used for external corners.

You will need to make, buy or hire a 200mm rigid-blade applicator and a narrow 50mm decorator’s knife for taping, although a paint scraper of similar appearance is nearly as efficient. An essential item is a purpose-made jointing sponge, a circular plastics foam sponge mounted on a wooden handle. This tool is used in various stages of jointing to remove excess material, ‘feather’ edges and to yield a smooth surface finish. All these materials and accessories can be obtained through your builders’ merchants.

The edge profile of the boards to a certain extent influences the jointing procedure. Bevelled edge joints are close-butted to produce a characteristic V-joint which can be filled with only a finger-smear of joint finish. With square-edged—-best used when the plasterboard is to be skimmed with plaster or covered with wallpaper—the joints can simply be taped. Use anaglypta or similar paper strips along with a suitable adhesive.

However, the process of jointing plasterboard is normally associated with tapered-edge board, and here it is possible to get really smooth joints that are virtually seamless.

Before you begin jointing, complete all fixings and remove all surface dirt and dust in the joint area. Mix up a little joint filler according to the directions on the packet.

Sprinkle the powder into water as you mix and aim for a thick, creamy consistency. The mix has a working life of about half an hour, so mix only as much as you think you may need to avoid waste. Do not use filler which has started to set, and make a point of cleaning your mixing utensils before preparing a fresh batch of material to prevent soiling.

Use the special applicator to apply a continuous band of the filler along the tapered recess where two boards join. If large gaps have inadver- tently formed during the cladding operation, these should be partially filled beforehand making sure that the filler is rammed well home.

Cut the required length of jointing tape and use a taping knife to push this into the band of filler—which acts as a bedding compound—so that it straddles it and acts as a reinforcing membrane. Make sure that there is sufficient filler beneath the tape to ensure good adhesion and avoid trapping bubbles as these may cause cracking and blistering later.

As soon as the tape is correctly in place, apply a second layer of filler to the full depth of the tapered recess so that the filling compound can be levelled at the shoulder of the taper by the scraping action of the applicator. Before the filler has a chance to set, moisten the jointing sponge and use this to wipe surplus material away from the edges of the joint band, taking care not to furrow or mark the remaining filler.

Rinse the sponge frequently during cleaning off stages, and clean the sponge well when you have completed the job. If you accidently damage the filler layer, let this dry off and then apply an additional dressing layer.

If, during the course of drying, the filling subsides to a depression along parts of the joint, this indicates that the filler is over-diluted or that you have left insufficient drying time between coats. This can be easily corrected in the future, and depressions can quickly be levelled with a further coat of filling compound.

After about an hour, when the filler has set, proceed by applying a layer of joint finish over the whole joint area. Mix this according to the instructions so that you get a consistency similar to that of thick cream. During subsequent evaporation, this consistency can be restored by again adding water, although the mix should really not be kept overnight.

Use the taping knife to remove odd tails of material that may have dried proud of the surface and joint area.

Use the applicator to apply a thin layer of the finishing compound, about 200mm in width , then immediately ‘feather’ the edges using a slightly dampened jointing sponge. Let this first coast of finish dry completely and then apply a second in a band about 250mm wide, taking special care over feathering the edges. , Dealing with edges and corners 3 Straight plasterboard joints are easily mastered but you may experience a I little more difficulty when jointing cut I edges and corners.

Cut edges should ideally occur only at internal angles, but they do some- times occur between two boards in the middle of a flat piece of wall. Start by abrading the edges lightly to remove paper burrs. Next brush on PVA emulsion sealer to reduce the suction effect of the exposed plaster.

Next fill the joint, taking care to force the filler right back to the frame.

When this is dry follow with a thin layer of finishing compound and bed the joint tape in this. Finally apply two layers of finish, allowing for drying, and carefully feather the edges to a smooth finish.

Internal angles are often the meeting point for cut edges, one of I which will be concealed by the butt join. In this case, start by packing the gap between the two boards with joint filler, then apply a thin layer of joint finish to each side of the join. Cut a suitable length of jointing tape and push this tightly into the corner, folding it as you go and using the applicator or a brush to force out trapped air bubbles.

After this, immediately apply a thin layer of joint finish—to a width of about 75mm on each side of the joint— and use a dampened jointing sponge to smooth the edges. Allow this coat to dry fully before applying the final coat, which should cover a band of 100mm on each side of the join.

External corners need a slightly different approach, using special corner tape with metal reinforcing strips along the edges. Start by using PVA sealer to dress exposed cut edges and to reduce the suction effect of the dry plaster.

Cut the tape to the correct length and crease it along its centre line, then apply a 50mm wide band of joint filler to each side of the corner and press the tape firmly into position carefully checking to see that the external corner edge forms a perfectly straight line.

Straightaway apply a broad band of joint filler—about 125mm—over both sides of the corner, feathering the edges with dampened jointing sponge. When the filler has set, apply a slightly wider band of finishing compound, again feathering the edges. When this is dry, apply a final coat of joint finish, about 200mm wide, on each side of the corner.

For maximum protection of corners it is advisable to use special, angled beading made of perforated galvanized steel.

Use 50mm bands of filler on each side of the corner to bed the angled beading, removing any material which oozes through the perforations when the strip is pushed home firmly. To prevent movement, permanently anchor the beading with two nails poked between the joint gap.

When this fixing coat has set, apply another coat of filler on each side of the corner, using the applicator and the feathering sponge to produce a reasonably fine corner. Leave the quarter-round metal bead at the extreme corner exposed to provide a resistant corner edge.

Finishing off and decorating

Nail and screw ‘spotting’ is carried out while the main joints are setting or between stages. The depressions are filled flush with the board surface with two thin coats, the first of filler, the second of finish.

When final coat of joint finish has dried, the uneven surface textures can be corrected by covering the whole wall with a thin slurry of joint finish—a mixture prepared by adding approximately 2.5kg powder to 2 litres of water.

You can use the jointing sponge for applying and smoothing out the slurry, working in a continuous circular motion.

Matched pre-formed plasterboard coving can be used to conceal the small gap between the walls and a plastered ceiling. Corners which remain exposed should be treated in the same way as internal wall corners, but first apply a narrow strip of PVA sealer along the contact area to prevent suction by the dry plaster. Care should be taken when fitting covings to give a neat finish.

To finish the plasterboard wall at floor level replace the old skirting or fit boards using the previously marked fixing points.

When the jointing and the top-coat of slurry have dried, proceed immediately to decorate the wall using only products specifically recommended for plasterboard. If you are papering the wall a special primer must be used if the paper is to be removed at a later date without damaging the plasterboard surface.

If you are painting, an initial coat of the same primer helps you obtain a more uniform paint finish over the different surfaces and reduces absorption by the plaster.


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