Insulation: The Key Considerations

One of the simplest ways to keep heat in and save energy in the home is with efficient insulation. But what type of insulation should you use, and where? Laura Pank investigates
by Laura Pank
28th November 2012

The message about dwindling fossil fuel supplies and the consequent need for everyone to save energy and cut down CO2 emissions has surely reached every corner of the UK.

One of the simplest, easiest and cheapest ways to reduce the carbon footprint of a house is to build it with very high levels of insulation in walls, roofs and floors – warmth is kept in and energy consumption on heating is consequently reduced. In the case of existing homes, we can add insulation, or extra layers of it.

Building Regulations

The government’s current Building Regulations, Part L 2006, lay down the latest guidelines for making our homes more energy efficient. But rather than ruling that walls, roofs and floors must have a certain amount of insulation, each new home now has an overall Target Emissions Rate (TER) of CO2 to achieve. This is based on a list of elements that include insulation, but also takes into account heating, lighting, windows, building materials and so on.

In England and Wales the TER is calculated at design stage by the designer and submitted for approval to the local Building Control department. The method by which it is calculated is known as a SAP, or Standard Assessment Procedure (in practice, most people use one of the approved  SAP computer programs). The SAP is then checked again at completion of the building work to make sure it complies.

While it’s permissible to have a degree of trade-off between, say, the heating and the insulation – a superefficient boiler might mean you could reduce the degree of insulation – there are specified U-values laid down that must not be exceeded, and these are the guidelines to follow whether you are dealing with a new build or bringing your existing house up to minimum standards of insulation. So, given this rather complicated holistic approach, how do you translate a recommended U-value into the level of insulation you should fit? The first step is to look at the different areas of your home that you need to insulate. But of course, you don’t have to stick to the minimum guidelines – the more insulation you can fit, the better.

Where to fit insulation


You lose between a quarter and a third of the heat in your home through the roof, and yet this is the easiest area to fit insulation. The cheapest and most convenient type to use here is mineral or glass fibre. It comes in a rolled up soft slab, sandwiched between two thin sheets of plastic so you don’t have to handle the prickly fibre itself. It is best to wear protective clothing when you’re fitting it – gloves and a gauze face mask – so you don’t inhale the fibres.

If you are renovating, chances are that you already have some mineral fibre insulation in your roof space, but it may have become thin and flattened over time. The latest recommended thickness is 270mm. If you can get it in, add more – up to 450mm.

Mineral fibre roll is easy to cut with scissors: measure and cut it slightly larger than the space between your ceiling joists, cutting extra bits for any small spaces such as the loft hatch cover, then squash it in for a good fit. Lay another layer at right angles over the joists. It’s very important that you allow air to circulate in the roof space above and moisture to escape. If you have a water tank in the loft, insulate the tank itself, but don’t lay any insulation on the floor underneath it. Lay boards over the insulation to give you access to the tank, and insulate pipes with foam tubing. Note that different insulation methods and products are suitable for a loft that is used as a room. In this case you should insulate between the rafters to create a ‘warm’ roof.


Cavity walls If your house was built after 1920, it is likely that it has cavity walls – in other words, that is there is a gap of between 50mm and 100mm between the two layers of external walls. The idea of the gap is to give increased weather resistance, and filling it with insulation will greatly reduce the amount of heat escaping through the walls, cut down condensation, and keep the house cooler in hot weather. Since the 1980s houses have been routinely built with insulation – generally quilt or boards – in the cavities.

If you have cavity wall insulation installed retrospectively, the method is to drill small holes in the mortar of the exterior brickwork and then inject mineral wool, foam or polystyrene balls, pumped down a tube from a lorry. When the operation is complete, the holes are filled unobtrusively. It takes half a day or less and costs about £500 for a three bedroom semi. You also need to take care that any air vents are kept clear of insulation, and that the work comes with the standard 25-year CIGA (Cavity Insulation Guarantee Agency) guarantee. The Energy Saving Trust calculates that cavity wall insulation can pay for itself in less than two years in terms of fuel saved, and grants are usually available if you use a recommended installer (visit

Solid walls:Insulating solid walls is even more beneficial than insulating cavity walls because you lose even more heat through them in winter. There are two approaches: on the outside, or on the inside. External wall insulation involves applying a weatherproof cladding of synthetic render over rigid insulation between 50mm and 100m thick to the walls, which can cut up to 25 per cent off your annual heating bills. It’s a job that is best done when you have other external work in hand because it is expensive, around £1,800 for a semi-detached house, and payback time is five to six years. The EWIA, External Walls Insulation Association, maintains a list of proven systems and installers.

Internal wall insulation involves attaching insulation board, thermal laminates or wooden battens in-filled with mineral wool insulation, to the inside walls of the house. The effect is to reduce the amount of heat that passes through the walls and also cut down on condensation, making rooms warmer and drier in winter and cooler in summer. The drawback is that installing the insulation retrospectively involves removing radiators and making good around windows, doors and skirtings and in the process reducing the size of your room by the thickness of the board or lining. Phenolic foam board costs more than fibrous board, but takes up half the space for the same insulating power. Installed costs of insulating internal walls start at about £40 per m² and annual savings are reckoned to be £270-£340.


If you have wooden floors, you can lift the boards and lay mineral wool insulation beneath, held in netting suspended between  the joists. To achieve a good U-value of 0.22, aim to fit about 150mm of mineral wool. This should cost about £100 for the ground floor area of an average house, and you can expect to make savings of £40-£50 a year in heating costs. Be careful to leave underfloor airbricks clear of insulation to maintain a good airflow.

Unless you are laying a new floor, it’s not practical to insulate a solid floor because the only way to do it – laying boards on top of the floor – would affect the doors, stairs, skirtings etc. However, if you are laying a new solid floor such as ceramic or marble, or if you are installing underfloor heating, a layer of thin cork board is a good choice of underlay. Also important, and both highly effective and inexpensive, is to seal up draughty gaps between the floor and skirting with a silicon sealant that you can apply yourself from a £5 tube.

Multifoil Insulation

Multifoil is a sandwich of metallic films with wadding and mesh. Triso Super 10 from Actis is 30mm thick and is made up of two metallic films with reinforcing mesh, three wadding layers, eight foam layers and six internal reflective films. The main  advantage of multifoil is its thickness – or rather lack of it – because it makes it much easier to use than many other types of insulation, particularly if you need to retro-fit insulation into tight and awkward spaces.

However, multifoils do not perform as well as other insulation materials in conventional ‘hot box’ tests (where air flows over a material from the hot side to the cold side to test thermal resistance). Multifoil manufacturers argue that this is because the hot box measures how a product conducts heat, as conventional products stop heat by conduction, while multifoils stop heat transfer by radiation. Multifoil manufacturers believe that ‘in situ’ testing is more accurate.

One option is to use a multifoil together with a PIR board polyisocyanurate), to cover the rafters and insulation. But Judith Knox at Actis Insulation says some local authorities do authorise multifoil use on its own. Triso Super 10 has been measured under real conditions by Timber Research and Development Association Technology Limited (an independent consultancy usually known as TRADA) and certified as equivalent to 230mm of mineral wool, according to Actis Insulation.

Main image: Knauf’s space blanket loft insulation is easy-to-install

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