The most expensive way to insulate a solid wall is to apply the insulation to the outside. The expense is caused by the need to provide a strong waterproof facing to the insulation instead of the plasterboard used internally. The advantages and disadvantages of external insulation are many.
Assuming you have decided to go for external insulation, you start by attaching Tanalised or treated softwood battens to the wall exactly as described for internal insulation. The battens should be at 600mm centres. If you want to have very thick insulation you can start with battens fixed horizontally to the wall, and then nail to these counterbattens which run vertically to carry the cladding. The spacing of both lots should still be 600mm centres.
At the base of the wall the external cladding must stop at least 150mm above ground level.
The insulation can be expanded or extruded polystyrene, both of which can be used below ground level, or resin-bonded glass fibre or mineral wool, which should stop 150mm above ground level. Once the insulation has been pushed in between the battens a layer of breather paper should be stapled over the battens and insulation. The paper should be laid horizontally, starting from the bottom of the wall, and the subsequent layers of paper should overlap the lower ones by 100mm. The purpose of the breather paper is to throw off any rain that penetrates the cladding.
Doors and windows will present problems, as will the eaves and gables of roofs. If your roof has a good overhang there is no difficulty but if the edges are flush as they frequently are in Victorian terrace houses, you will have to devise a lead flashing which will protect the top of the insulation from rain. The best solution is to extend the roof over the top of the insulation, which is easy if you are planning to re-roof the building. For this reason it is perhaps wise to use a lot of insulation, at least 100mm if you apply it externally, otherwise it hardly seems worth the trouble.
At windows and doors there are two possibilities. The first and best is to take out the window and door frames and replace them on the outside of the new wall. If this proves difficult you will have to build a frame of battens round the existing openings and try to make a watertight seal where the new cladding meets the old frames. At the sill you could use two layers of plain tiles laid in mortar with staggered joints on top of the new battens. The new sill must have a good slope on it to throw the water away from the wall as quickly as possible. An alternative method might be to remove the existing sills and cast new larger concrete ones using a mix of one part by volume cement to two parts sand and three parts aggregate (maximum size 10mm). You can see why external insulation is expensive.
There are several possibilities for cladding the external insulation but we will describe only two. The easiest to put up is Tanalised (or treated) softwood boarding, preferably shiplap pattern, as this is easier to use than feather edged boarding. The boarding, which should be at least 16mm thick, is nailed to the battens with 50mm sheradised lost head nails. Start with the bottom board and spend a lot of time making sure that it is fixed perfectly level, as it will then form a base for the other boards. You should check the boards frequently with a spirit level to ensure that they are horizontal. Joints should be made only at battens, and the joints in one course of boards should not coincide with the joints of the boards directly below.
The timber can now be painted or preferably stained with a preservative stain or even creosote, which will not peel or flake like paint.
An alternative for cladding the insulation is cement rendering on a special metal lath underlay. The metal lath, which provides a key for the render, is nailed to the timber battens which have had breather paper stapled to them as before, with 38mm galvanised plasterer’s nails through its reinforcing ribs. These ribs are formed in the expanded metal sheet and allow the lath to span up to 600mm between supports. The sheets of laths should also be wired together with 1.22mm galvanised wire at 100mm intervals, with the wire ends stuck through the lath so that they do not poke out through the rendering.
The lath should stop at least 150mm above ground level and be terminated with a render stop bead, a metal strip which makes it easy to form a neat drip edge to the rendering. The stop bead should also be used above window and door openings. At corners, pieces of bent expanded metal should be used, tied to the lath with wire as above, to carry the rendering round the angle without cracking. The rendering should be applied in three coats, all of a mix consisting of one part by volume (a shovel full) cement, one part hydrated lime and six parts sharp sand.
Rendering is slightly easier than plastering because it does not have to look so flat, but it must be fairly smooth and free from cracks or else it will let the rain through. If in doubt, have a plasterer do the job, or at least ask someone to show you how to do it. If you are determined to do it yourself, mix up a small amount and practise putting it on to the float and then on to the wall — somewhere where it will not matter if it looks a bit rough. A lot depends on mixing the render to the right consistency before you apply it, so be prepared to spend a day getting the hang of it before you tackle the whole wall. When it is finished you can paint the rendering with external paints as sold for painting brickwork. Timber cladding is generally easier than rendering but painted rendering looks more conventional and may be easier from the point of view of obtaining planning permission, especially if you live somewhere like Suffolk where rendering is a traditional finish.
External insulation is probably best used if you are doing a thorough renovation of an old building. Once you have to take the tiles or slates off a roof because the timbers need repairing and once you have to replace existing rotten and broken windows and even possibly doors, it is much easier to design a scheme that incorporates the insulation on the outside surface of the wall.