Insulating walls and floors

The main walls are responsible for 35% of the heat loss from a typical dwelling. So upgrading them should significantly reduce a property’s energy consumption. Most homes built in the last 75 years or so have cavity walls which are relatively easy to insulate. But traditional solid walls in older buildings can also be upgraded by lining them with insulation either internally or externally. Only the main external walls need upgrading, so less work is needed with terraced properties.

In most cases you won’t need to apply for planning consent – even cladding the outer face of the walls is now classed as ‘Permitted Development’. However the Building Regulations set minimum standards for thermal efficiency where walls, roofs and floors are renovated, although contractors can normally self certify any major works.

Cavity wall insulation

This is one of the most cost-effective types of insulation, typically paying for itself from savings in energy bills in as little as 2 or 3 years. The work can be carried out with virtually no disturbance by specialist contractors. Insulation is pumped into the cavity via a series of small holes drilled in mortar joints in the outer face of the wall which are sealed up afterwards. Once the works are done, a ‘CIGA’ guarantee should be provided by the contractor along with a Building Control completion certificate. However, not all homes are suitable for cavity insulation, such as timber frame buildings and those with solid walls.

External wall insulation

Applying insulation to the outside of the walls can be very effective in boosting thermal efficiency. This has the additional benefit of improving the appearance of bland-looking properties, but is not normally suitable for older buildings with attractive period brick or stonework facades. It’s also important that traditional solid walls are able to ‘breathe’ so that moisture is free to evaporate away, and covering the outsides can potentially trap damp.

The works typically involve fixing rigid insulation boards to the outer face of the walls with special adhesives and fixings, followed by 2 coats of render. Making walls thicker involves a lot of work modifying window and door openings, roof overhangs, rainwater downpipes, plus any lights, pipework and satellite dishes. But it’s the need for scaffolding that makes this a relatively expensive option. External wall insulation is often applied to complete blocks of houses or flats being refurbished together.

External solid wall insulation also carries significant risks, and should only be carried out by a competent firm who have undertaken a full professional survey of the property. This survey should include a review of the existing building ventilation, as preventing the walls from ‘breathing’ can have serious adverse effects on the internal air quality of the building. Where an installation is carried out, any areas that are missed or badly detailed can form a ‘condensation trap’ which will result in internal dampness and mould growth.

Installation should never be carried out to solid walls during winter when the walls are damp as this will trap moisture in the structure which will then make its way into the building. Be wary of cheap quotes for external wall insulation – badly designed and implemented work can result in major damage to your building and indoor air quality.

Internal wall insulation

Lining the inside of the main walls in your home can achieve worthwhile improvements in thermal efficiency and can also be a suitable project for DIY. The main drawback is the disruption involved in removing and later re-instating things like skirting boards, shelving, radiators, sockets and switches. As with external insulation, getting the detailing right can be tricky, for instance around windows. Inevitably lining the walls entails a small loss of floor space, and it may not be suitable for rooms with period features or in buildings where walls are damp.

The works typically involve fixing insulation quilt or rigid boards to the walls with special adhesives or applying insulation to a timber framework, followed by plasterboarding. But lining the walls has the added advantage of providing a smooth new surface to decorate. As with external insulation the consequences of creating ‘cold spots’ can be severe, and the building ventilation should be reviewed before any works are carried out.

Insulating floors

The ground floors in a typical home account for around 15% of the building’s total heat loss. This is less than the amount lost through roofs and walls, but is still significant. Solid concrete floors are relatively difficult to insulate because laying a new insulated ‘floating floor‘ on top of the existing surface raises the floor level, with all kinds of knock-on effects to doors and stairs.

However, upgrading the thermal efficiency of traditional suspended timber floors can be relatively straightforward. The simplest and cheapest option is to carry out draught-proofing. This should be a straightforward DIY project that involves sealing gaps between floorboards, to skirting boards and around pipes with silicone sealant or rubber strips. However, where you’re planning to strip and polish the floorboards it‘s an excellent opportunity to insulate them. This can either be done from above, by temporarily removing all the boards, or from below if you can get access to the void below by removing just 2 or 3 boards. There are a number of different ways floors can be insulated, for example rigid insulation can be wedged between joists and secured on battens, or mineral wool quilt batts can be held in place with plywood strips or sheets of breather membrane stapled underneath to the joists.

Where insulation is added to suspended floors it is critical that the sub-floor ventilation is adequate. A floor void of at least 150mm should be maintained and ventilated by air bricks at frequent intervals on opposite sides of the void. Ensure any walls within the floor void do not prevent airflow, and that any existing air bricks have not been blocked. Alternatively, boarding over floors with hardboard and laying a thick carpet can work wonders in banishing cold draughts.