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Can I DIY internal wall insulation?

Internal wall insulation is insulation of the internal surface of external (heat losing) walls, so it is an alternative to external wall insulation. If you are having a new kitchen or bathroom this is an unmissable opportunity to insulate the walls before installing the new fitments. Likewise if you plan to redecorate  this is good time.

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Details of warm batten internal wall insulation method at window reveals

Details of warm batten internal wall insulation method at window reveals

Two internal wall insulation methods

There are two main approaches to installing internal wall insulation. The first uses rigid foam insulation exploiting its rigidity to reduce or eliminate the use of battens or studs. Often this is insulated plasterboard. The second involves building a framework in wood or steel which allows the use of soft insulation and is appropriate if the wall is very uneven.

For the first type, rigid insulation, you can use a plasterer/builder or DIY. Plasterers usually use insulated plasterboard (plasterboard with insulation ready glued to it and a vapour barrier between them).

Always ensure that the wall is dry before you start. If it is damp, remove the cause – look for:

  • leaky gutter or downpipe
  • dripping overflow pipe
  • cracked or missing bottom slate
  • rising damp from a damp proof course bridged by soil or paving
  • poor pointing letting rain into the brickwork

1. Insulated Plasterboard / Rigid Insulation

Insulated plasterboard is attached to the wall either with dabs of plasterboard adhesive or screwed or nailed to battens of wood treated with preservative. The battens method avoids the joint cracking which is common with the adhesive method, but on the downside the battens waste a bit of space and could rot if the vapour barrier is poor.

Do NOT assume that a plasterer will specify board with a suitable insulation value. Most will specify the cheapest insulated plasterboard with a thin amount of expanded polystyrene and often no vapour barrier. YOU should specify a U value (heat loss rate) of 0.3 or better (less).

The plaster skim coat will hopefully be enough to seal against vapour sneaking through gaps at the top and sides, but all plasterers like to leave an unplastered gap at the bottom. An easy solution is to fill the gap with expanding foam, so get the plasterer to leave a gap of 20mm so you or he can get the nozzle of the foam gun in. Buy a proper separate foam gun, about £15-20 at Screwfix. Many plasterers and builders have no grasp of the importance of a vapour barrier.

Insulation boards pinned to wall ready for battens in warm batten internal wall insulation method

Insulation boards pinned to wall ready for battens in warm batten internal wall insulation method

2. Warm Batten Method of internal wall insulation

I am not keen on the insulated plasterboard method, and for the last room I did I easily persuaded the plasterer to use separate insulation boards and plasterboard, with the battens between the two.

I call this the warm batten method, as the battens are on the warm side of the insulation so cannot rot. This also allows a much better vapour barrier as you can inspect the sealing (expanding foam) around the perimeter before attaching the plasterboard.

This method can be done by a plasterer or builder, or you can DIY it a bit more easily than the methods with insulated plasterboard.

    1. First specify separate insulation boards and plasterboard (a considerable cost saving as the manufacturers charge a premium for bonding insulation to plasterboard). PIR and PUR insulation boards (Celotex and Kingspan) include aluminium foil coverings which act as a vapour barrier, a necessary feature, and extruded polystyrene is impermeable. If using expanded polystyrene you must add a separate vapour barrier. Most plasterers will suggest the cheapest and thinnest insulation. This is a completely false economy as most of the cost is in labour.
    2. Except where the thickness is not possible you should specify a U value (heat loss rate) of 0.3 or less as recommended in the Building Regulations. This can be achieved with 100mm of EPS (expanded polystyrene), 80mm of XPS (extruded polystyrene) or 60mm of PUR or PIR (Kingspan or Celotex) foam. The U value of an uninsulated solid wall is usually around 2.0, so 0.3 gives a reduction of 1.7 which will reduce the wall heat loss by 85%. Where thinner insulation must be used (window reveals and beside stairs) specify PUR or PIR insulation as these have the best insulation per mm thick.
    3. Position the insulation and vapour barrier against the wall leaving a 15mm gap all round (see below). Then place 25mm battens on top of the insulation in positions ready to accept the plasterboard. Drill and screw through the battens, the insulation and the plaster into the brick, and fasten with long screws tipped with rawlplugs. Use an SDS drill and bits, not an ordinary hammer drill. Now the most important bit…
    4. Seal all around to prevent warm moist room air getting behind the insulation.

I do this by (a) using aluminium tape to join insulation boards to each other and (b) using expanding foam to seal all around the perimeter (Aluminium tape does not stick so well to old surfaces). I leave a deliberate 15mm gap all round to accept the nozzle of the foam gun. If your contractor does not agree, contact the manufacturer of the insulation who should confirm what this article says.
    5. Then either fill in between the battens with 25mm foam insulation boards, or leave parts empty where you want to run services. Then attach plasterboard and skim or else fair the plasterboard joints, and finally replace skirting etc. Note that the vapour barrier is set back 25mm so is much less likely to be damaged by services and picture hooks. It is OK to have the vapour barrier within the insulation so long as it is no more than half way back; my standard spec is 40 or 50mm of PIR foam then the 25mm between the battens for a total of 65 or 75mm.
    6. Anticipate where you will need to attach fittings such as skirting, picture rails, curtain rails, radiators and kitchen or bathroom fitments, and put (extra wide) battens there. Try to minimise the number of penetrations for electric sockets, radiators etc. If your wall is a bit uneven you can pack it out with spacers of non rotting material such as solid plastic or insulating foam.
    7. For a really thorough job when insulating upstairs rooms, insulate the wall between the floorboards and the ceiling below, and where you have a solid ground floor carry the insulation down below floor level as far as possible to minimise heat loss at the floor edge.
Internal glasswool wall insulation between studs.

Internal glasswool wall insulation between battens

Using soft insulation between battens

If you are using soft insulation such as sheepswool or fibreglass wool or rockwool, then you need to build a composite framework of wood and foam (wood on the warm side), install the insulation within the frame, add and seal a vapour barrier (suitable plastic sheet) and then fit the plasterboard to the frame. If the wall is reasonably true and dry just attach the (100mm) composite battens to the wall horizontally or vertically.

Insulating interior walls that are damp or uneven

If the wall is uneven or incurably damp then you need an air gap between the insulation and the wall: first attach floor and ceiling studs, leaving a small gap behind, then fit vertical studs between them plus a few horizontals to combat bending verticals and slumping insulation.

You might attach the verticals against the wall at one or two points using foam or solid plastic spacers sloping away from the frame. The studs will need to include at least 2” by 2” of wood to be strong enough. Ventilate the air gap to the outside.

If you live in an area of severe driving rain, i.e. the most exposed parts of the West coasts of Scotland or Ireland, then cavity insulation is not recommended and IWI is a good alternative for cavity walls; you can attach it directly to the inner leaf which is kept dry by the outer leaf and cavity.

Internal wall insulation problems and challenges

Reduced Floor Space

The room will be a bit smaller, but this is usually not noticeable, unless the room is very narrow with an outside wall on the side, or has the staircase against an outside wall. While many terraced houses are narrow they are not usually tight front to back, so insulating the front and back walls internally rarely makes a noticeable size reduction. Insulating the kitchen wing will make the room significantly narrower, but you probably won’t notice it as the room is usually too narrow to eat in anyhow.

Extending reveals and replacing coving and cornices

Internal wall insulation involves extending internal window sills and reveals, and removing and refitting skirting boards, window architraves and picture rails. Any period coving must be hacked off and replaced – a major cost. It sometimes also involves adjusting wiring and radiators.

Risk of interstitial condensation if done badly

Remember that with internal wall insulation it is essential to seal the room-facing surface of the insulation to stop water vapour getting into the back of the insulation or into the wall and causing damp. See the separate article on vapour barriers for more.

Internal Wall Insulation Costs

Internal wall insulation is usually cheaper than external wall insulation, substantially so if you do not have period coving. The price for professional work is about £50 per m2, which gives typically a 6 or 7 year payback. If you need to replace period coving, the payback rises to about 12 years.

© Martin Normanton, Walsall Ecohouse, Jan 2013.

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Diagrams:
1. “insulation and 3X glaze” shows details of warm batten method at window reveal. In this case 25mm fibreglass insulation was used between the battens, but 25mm of Kingspan or Celotex is better. The window reveal has been extended to accommodate the insulation thickness (shown but not labelled) in line with the edge of the original sash frame. On this window the secondary glazing was itself double, so the overall effect is triple glazing.

2. “Wall insulation elevation” shows the insulation boards (warm batten method again) pinned to the wall and sealed with expanding foam (not Aluminium tape in this case) ready for the battens to go on. It also shows how polyurethane insulation was extended into a trench 600mm deep to reduce heat escaping from the edge of the solid floor.

Further Information:
Visit Martin’s SuperHome for more.

Also see:
internal wall insulation at green open home events in Mar/Sep
insulation vapour barrier
insulating a solid wall
underfloor insulation

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