Our Guide to Internal Wall Insulation

Roughly 35% of heat loss in a building occurs through walls, windows, and doors. In most modern houses cavity wall insulation would be seen as the best solution to limit such heat loss. But in older homes with solid walls, or modern homes not suitable for cavity wall insulation, a great alternative is internal wall insulation, attaching insulation products to the interior of your outside walls. Internal wall insulation can help prevent heat loss in your home, saving you money on energy bills and benefiting the environment.

In this guide from Building Materials Nationwide, we’ll take a thorough look at the two types of internal wall insulation, tackling both rigid insulation boards and insulated batten walls with plasterboard facings. We’ll look at the benefits and drawbacks of each, the costs of materials, and the complexity of the installation process.

Who Needs Internal Wall Insulation?

Internal wall insulation is most commonly used in houses with solid brick walls, or houses whose cavity walls are unable to be insulated, either due to high amounts of wind-driven rain in their area, or damage to the external walls. In these cases, homeowners have a choice between internal and external wall insulation. The two main reasons that people would opt for internal wall insulation are due to the cheaper application cost, or because their home is a listed building or in a protected area, which would prohibit exterior alterations. A drawback of internal wall insulation is that it is generally less effective than external solid wall insulation.

Sorting Out Dampness Before Insulating Your Internal Wall

If your internal solid walls have problems with dampness you will need to deal with this before insulting them. Insulation can further contribute to dampness, causing damage to both the brick wall and your insulation. If you have issues with dampness, you should redo your wall with damp-proof materials and repaint it before installing insulation.

It is vital that your house has good ventilation and air conditioning, and that your exterior wall has a high level of vapour permeability, to further reduce the chances of issues with dampness arising over time.

Using Solid Insulation Boards for Internal Wall Insulation

Plasterboards backed with a solid insulation board can be fitted directly to your internal brick walls. These insulation boards include dry lining, coming in thicknesses between 10mm and 60mm. They will be attached to your brick wall using either plaster or adhesive, before being held in place with extra fixings. The joints between boards will be sealed to make them airtight. This method only removes a small amount of floor area from your rooms. Solid insulation boards will only work on a flat wall, as they need to be airtight. If your wall is uneven you should build an insulated batten wall instead.

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Building a Batten Wall for Internal Wall Insulation

By securing a timber or metal stud framework to your solid walls, you can build a single-facing stud wall, in which you can utilise a variety of traditional insulation materials between the studs. You can use batts, slabs, or rolls of insulation, made from mineral wool or sheep’s wool, for great-quality insulation. This wall can be faced with insulated plasterboards, or with rigid insulation boards to further increase the insulation potential of your internal brick wall. This method will use a greater proportion of your floor space than solid insulation boards. A benefit of batten walls is that they can provide additional ventilation by creating an air gap, which can reduce the risks of dampness.

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The Best Internal Wall Insulation Options

If you are opting to use solid boards for your internal wall insulation, a quality entry-level choice of insulated plasterboard is the British Gypsum Gyproc ThermaLine Basic. For a more premium model, the Warmline PIR Insulated Plasterboard offers premium thermal insulation for a slightly higher price tag.

If you are looking to build a batten wall for internal thermal wall insulation, this is a time-consuming but not costly process, with 25mm timber battens and boxes of screws and wall plugs coming in at low prices. Depending on the risk of continuing dampness in your property, it may be advisable to install a more costly metal stud wall. There are several types of insulation suitable for a batten wall, including batts, slabs, and rolls. An environmentally friendly choice is the Knauf Earthwool FrameTherm Roll, which is designed specifically to provide great thermal performance to external brick walls when fitted between timber studs. Another option would be the Thermafleece Cosywool Natural Sheeps Wool Insulation, which boasts moisture regulation and excellent thermal wall insulation.

This batten wall can be topped with the insulated plasterboards shown earlier, allowing it to be decorated. It can also be covered with a traditional insulation board, for increased heat retention. An option would be the Ecotherm Eco-Versal PIR Insulation Board, which offers great insulation and vapour control. Another option would be the Celotex General Application Board, which are multipurpose boards manufactured from rigid polyisocyanurate.

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How to Install Internal Wall Insulation

Installing internal wall insulation to your existing walls is a complex process, that would be above the capabilities of most amateurs. There are additional elements to keep in mind, firstly you must address any issues with dampness, but secondly, any radiators or electricity sockets would need to be removed and reinstalled on top of either your solid insulation or your stud wall. If you were to get professionals to insulate the internal walls of your house, you would be looking at a cost of about £7400 for a typical semi-detached home, based on an estimate from the Royal Institute of Chartered Surveyors. If necessary, you can start by insulating a single room, or one side of your house. Any internal thermal wall insulation you have installed on your solid walls will bring benefits to your property.

If you feel like you have the experience and time to insulate your internal walls, then the process for attaching the solid boards includes glueing them to the wall with a specialist adhesive or with mechanical fixings. The gaps between boards will need to be sealed with mastic and then covered with a skim of plaster. The boards should cover the entirety of the floor to ceiling, to ensure the vapour barrier of the insulated plasterboards. Any gaps in insulation can cause cold bridging in your home.

If you need to build a batten wall, we have a handy guide on our website. Earthwool and Thermafleece insulations are designed to friction fit within a stud wall, securely nestling into place. For looser materials, there are insulation fixings available that are drilled into a wall and used to hold the insulation in place. Boards on top of this must be attached to the wooden studs using plasterboard nails.