Insulating solid walls from the inside

If you can see both headers and stretchers on the outside of your house, your walls will probably be solid and therefore rather more difficult to insulate. Before you try to insulate them you must be sure that they are protected by a damp proof course. If you cannot find one or if you are doubtful, it is worth obtaining a free survey and quote from a damp proofing firm; for addresses see under ‘Damp Proofing Contractors’ in the Yellow pages. A good firm will offer a guarantee of their system, and your local authority may give you an improvement grant to cover half the cost. The local authority environmental health department, which deals with improvement grants, may suggest some names of reputable firms, or you can try asking other people for their experiences.

You will have to chip off the damp plaster before the damp proofing is carried out, but if you plan to insulate do not replaster as the insulation can be put over the unplastered wall. If you qualify for a grant this will contribute towards the cost of replastering, or of the battens and plasterboard if you insulate the walls after the damp proofing. Only when the wall is successfully damp proofed can you think of insulating it.

Internal Wall Insulation

The cheaper method is to insulate solid walls internally but this will cause a fair amount of disturbance to the inside of your house and it may be best to work on one room at a time. Before insulating internally you must decide what to do about chimney breasts and fireplaces. If the chimney is to be kept in use the insulation should be stopped either side of the fireplace. Any chimney breast on the floor above a stove or fire in constant operation will be a useful emitter of heat and can also be left uninsulated.

If the fireplace is not used you can carry the insulation across it. In this case the stack should be ventilated to prevent any condensation in the closed off flue. To avoid excessive air changes in the building use two air bricks (225mm x225mm), one in the external wall at the bottom of the chimney, and another, if the stack above roof level is demolished or capped off, at the top. Of course you can remove the chimney completely but in old buildings chimney stacks act as useful buttresses and their removal may weaken the walls.

All internal wall insulation starts with the temporary removal of skirting boards. The best way to add internal insulation is to leave a ventilated cavity between the insulation and the brickwork. You can do this by building a freestanding framework of softwood studs nailed to a sill on the floor and to a plate at ceiling level, leaving a gap of 25mm between the rear of the studs and the wall. The studs should not be less than 50mm x 75mm (which would give 75mm of insulation) or 50mm x 100mm: anything thinner than 50mm x 75mm will not be strong enough to span from floor to ceiling without bending if you lean against the wall.

It is essential to use timber which has been vacuum treated with preservative (e.g., Tanalised). This is a commercial process so you can buy the wood already treated. A reasonable job can also be made by giving the wood three good coats of timber preservative if you cannot buy it already treated. If you use wood with no preservative it is very likely to rot once it has been built into the new wall.

The studwork is started by laying the sill on the floor at least 25mm from the wall surface. If the floor is timber you can nail the sill to it, using nails long enough to locate the sill but not go through the floorboards in case there are wires or pipes below; 63mm nails would be suitable for a 50mm thick sill. If the floor is concrete you could try to fix the sill down with screws and plugs as described for fixing battens to walls. If you cannot fix the sill to the floor lay it in position and measure the length required for the end studs. These measure the same as the height from the top of the sill to the ceiling less 50mm which is the thickness of the plate at ceiling level.

Screw the studs to the wall, ensuring that they are vertical and that the gap behind the studwork is maintained. They must be fixed very firmly; use at least five screws per stud. Skew nail the sill to the bottom of the studs with 100mm nails, using a block of wood behind the sill to prevent it from moving when the nails are driven in.

Now cut the plate to length, put it in place on top of the studs, and skew nail it to them, again using a spacing piece. Cut the uprights to fit between sill and plate; if they are a tight fit you can put them in at an angle and th,n tap them into a vertical position with a mallet. They should be skew nailed at top and bottom and should finish up at 400mm or 600mm centres.

Frame round window and door openings with vertical and horizontal timbers.

To prevent moisture vapour collecting between the new studwork wall and the old wall, the gap should be ventilated by drilling holes through the existing wall to the outside. These holes should be about 20mm diameter and at about 900mm centres with one row near the top of the cavity and one row near the bottom.

You can mortar pieces of PVC pipe into the holes for the sake of neatness, and plug them with little pieces of glass fibre insulation to keep out mice. The best way to drill the holes is to hire a hammer drill, the type which delivers very fast hammer blows to the bit at the same time as it rotates, and to use a long, large diameter masonry bit.

You will have to build out the existing frames of doors and windows to take up the new wall thickness and some methods for doing this are shown in the illustration (right).

The insulation must then be placed between the studs, but if there is a cavity behind them there will be a problem fitting the insulation without blocking the cavity. The solution is to use a type of glass fibre insulation called flanged building roll: this is designed to fit between studs and has a flange at each edge to fix it to the timber.- If you use the polythene-faced type, the polythene will form the necessary vapour barrier to keep moisture vapour out of the wall. It is important to fix the studs at the right spacings to fit the roll, and they should be at 400mm or 600mm centres.

If you use the wider spacings you will need to use thicker plasterboard for the internal lining of the room, 12.7mm rather than 9.5mm; but you will achieve a better insulation value because less of the wall will be taken up with timber, which is not as good an insulator as glass fibre. The insulation is unrolled over the studs and the polythene flange is then stapled to them.

It is also possible to use expanded polystyrene to insulate the wall. In this case the material should be cut to fit tightly between the studs and then pushed very carefully into place. Make sure that the polystyrene does not block the cavity; it should be flush with the face of the studs. You can use your hand at the free end of the insulation to position it but the other side must be pushed into place very gently. The expanded polystyrene should have a flame retardant additive (i.e., Type FRA).

The wall should be clad with plasterboard once the insulation is in place. Plasterboard is usually used because it is cheap and gives a low flame spread to the surface of the wall. If you have used flanged roll insulation with a vapour barrier you can use plain plasterboard; otherwise use foil-backed plasterboard which will form a vapour barrier. If you choose to use some other sort of cladding you will have to check the fire regulations, and you must staple a vapour barrier of heavy gauge polythene (with the joints folded to make them vapour tight) over the studs and insulation before putting up the cladding.

If you use plasterboard, nail it to the studs with 38mm long galvanised plasterboard nails spaced about 150mm apart. The sheets of plasterboard should be measured and cut to make a tight fit at floor and ceiling. To cut plasterboard score along the paper with a sharp knife and then bend the cut off piece back to break the plaster core. Finally, cut through the paper on the other side. Plasterboard can also be cut with an old panel saw. Where two sheets of plasterboard meet at a stud try to keep the nails well back from the edges of the board and do not hammer too hard or you will damage the edge. Finally, for a good finish, the surface should have a skim coat of plaster.

The joints between the sheets of plasterboard and the holes made by the nail heads can be filled with special fillers and tapes supplied by the manufacturers. These are supposed to allow the boards to be decorated directly, but the finish is not nearly as good as that given by a skim coat of plaster. It is worth paying a plasterer to do the skim coat, especially as it allows you to use up oddments of board and also means that you do not have to worry if the plasterboard does not go up very neatly; the plaster will cover all mistakes.

If you have not the space to leave a ventilated cavity between the insulation and the old wall, you can apply the insulation direct to the wall surface. In this case the first step, having removed the skirting boards, is to fasten ‘breather paper’ over the whole wall surface. The paper will allow moisture vapour in the wall to escape through the brickwork, if it is porous enough. It will also prevent any damp or rain that seeps through the brick wall from wetting the insulating material. Fix the paper with 100mm overlaps —you will find it easier to put it up vertically like wallpaper than horizontally. The paper can be fixed to the wall (with the side marked ‘outside’ facing out) using the short galvanised clout nails sold for holding roofing felt; all you need is enough nails to prevent the paper falling off the wall while you carry out the next part of the job.

Now screw Tanalised softwood battens to the wall. The battens should be 50mm wide and as thick as the insulation you wish to use. Standard sizes of softwood are 50mm, 75mm and 100mm thick. The 50mm x 50mm battens should be screwed directly to the brick or stone wall using 100mm no 12 screws and plastic wall plugs (we have found that the type called ‘Plasplugs’ are the easiest to use).

To put up the battens you will need an electric drill with a 6mm diameter masonry drill long enough to stick out at least 110mm from the drill chuck. Put the batten in place on the wall against the breather paper, check that the

timber is vertical, then drill straight through the wood and into the wall as far as the drill will go. Take a plug and push it into the hole in the piece of wood until it is flush with the surface. Then put the point of the screw into the hole of the plug and tap gently with a hammer to push the plug through the wood to the bottom of the hole in the masonry. Using the biggest screwdriver you can find, tighten the screw and the batten will be held in place. Each batten will need about four screws evenly spaced. This method is much easier than trying to mark the positions of the plugs on the wall separately. Put the battens at 600mm centres on the wall and make certain that they are all vertical, and parallel to one another.

If you are planning to use 50mm x 75mm or 50mm x 100mm battens so as to have more insulation, the battens will have to be counter-bored. This involves drilling holes about 20mm in diameter through the battens with a brace and bit to a point about 50mm from the back of the timber. If you do not do this the screws will not be long enough to go through the timber into the wall. Once counterbored, the battens can be fixed as before, the only difference being that you will need a longer masonry drill as the drill chuck will not be able to go through the counterbored holes.

Once the battens are up you can put in the insulation. This can be mineral wool, glass fibre, expanded polystyrene (Type FRA), extruded polystyrene which is a better vapour barrier than expanded polystyrene, expanded polyurethane if you can afford it, or any other material that is available in sheet or blanket form. Cost is perhaps the best guide to your choice and you will also be limited by what is available locally. Glass fibre or expanded polystyrene are the most likely choices because they are cheap and easy to come by.

If you buy glass fibre in rolls 1200mm wide you can use a saw to cut these (before you unwrap them) into 600mm rolls, which can be fitted between the battens. Glass fibre is very itchy to handle and it is best to wear gloves with gauntlets — bee-keepers’ gloves are very good. The manufacturers say that the stuff will not cause lung cancer because the way it is made, by squirting molten glass through tiny nozzles, ensures that the diameter of the fibres is constant, and greater than the diameter of natural fibres like asbestos which are known to cause respiratory disorders. All the same it is quite a good idea to wear a face mask or a farmer’s respirator just in case.

Having put the insulation in place you can fix up the plasterboard. This should be foil-backed to form the essential vapour barrier to the insulation, and 12.7mm thick if the battens are at 600mm centres. If you plan to have electric sockets or light switches on the insulated wall they should be surface-mounted and the holes where the wires come through the plasterboard should be sealed with mastic. If you made sure that the battens were at 600mm centres, vertical and parallel to one another, you will now find that the plasterboard can be nailed up quite easily. Take care to buy plasterboard 1200mm wide. Finally, you can put the skirting boards back (after the plastering if you have it done) and the job is finished.

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