How to Insulate Old Houses

We all know that energy efficient systems and improved thermal performance in old houses will save on running costs, maintain comfort and reduce CO² emissions —and if you are able to achieve this by using materials that are ecologically friendly, you will be making an additional positive contribution.

However, adding insulation to old houses can often be extremely tricky, particularly where the walls are concerned. Modern materials and techniques can often be incompatible with traditional construction and the use of the wrong insulating materials could well cause serious harm to the building fabric. Most modern houses are built from hard, strong, impervious materials. To exclude moisture they rely on physical barriers such as damp-proof courses and membranes, cavity walls and cladding. Historic and traditional houses are completely different. Many have solid walls, and most have porous fabric, which both absorbs and readily allows the evaporation of moisture. This is often known as the ability of the building fabric to ‘breathe’. A more technical term for it is vapour permeability.

So if you are planning on a project of this sort, and if the house is listed, you should first check if you are likely to be allowed to do it. Approved Documents L1 and L2 of the Building Regulations treat listed buildings and those in Conservation Areas as special cases which should be considered on their individual merits. However, if any of the work you propose will affect the character of your house, you will need listed building consent. (Visit www.communities.gov.uk for the latest Building Regulations).

Whatever your plans, a key consideration with old houses, whether or not they are listed, is to try to minimise the amount of intervention to the historic fabric. It is also wise only to undertake work that allows the fabric to revert to its original state. “This makes sense in case the technology changes,” says Douglas Kent, technical secretary of the Society for the Protection of Ancient Buildings (SPAB).

Modern Techniques

This poses particular problems for owners of traditionally built houses, because it is feared that all too often they will not fit into the parameters of the computer program to be used by the Energy Assessors.

Many old cottages would fail a modern airtightness test and organisations such as SPAB fear that sealing up buildings of this sort to make them airtight could lead to high levels of condensation, mould decay, and health problems for the inhabitants.

SPAB also fears that the introduction of Energy Performance Certificates could see a rush to insulate old houses in unsympathetic ways that would produce minimal environmental gain. They instance double glazing as a prime example of this.

Douglas Kent of SPAB says: “We fear that instead of allowing old buildings that lack cavity walls to ‘breathe’, owners could be encouraged to insulate the fabric in misguided attempts to save energy. Old buildings might therefore become warmer, but also damper.

“At SPAB we also fear that the calculation methods being used by the assessors will overlook the thermal mass benefits of many old walls that are able to store and re-release heat gradually.”

His advice to owners of old cottages and other period properties which gain poor scores in EPC tests is not to panic. “The listed building status that many of these old houses have should give them some legal protection,” he says.

The Main Solutions

1. Pitched Roofs: Here, rigid urethane insulation (a) is fitted between rafters, on top of a dry-lining board (b). Breathable sarking membrane (c) can be added over the battens, but must have air space between it and the insulation.
2. Floors: Insulation can also be fitted below the floor slab if renovations allow. Here, a rigid phenolic board (a) fits between the hardcore (b) and the slab (c), onto which screed (d) is then poured.
3. Interior side of external wall: For those unable to fix insulation externally, an insulated dry-lining board might be a good option, fixed to timber battens.
4. Walls (exterior): For owners of homes where external walls can be re-rendered, a rigid phenolic insulation core (a) can be attached onto a bedding compound (b). A mesh is then installed, allowing a new render finish (c).

Outside Walls

There is little doubt that insulating from the outside is the easier option. Using this method will cause far less disruption to the building. Its other advantages are:

• It is easier to apply. 
• It is better from the thermal mass point of view: it acts like a duvet outside the house, absorbing heat in summer and reducing the cooling-down rate of the walls after the summer heat disappears. This will become more important as global warming increases, because it slows down the rate of overheating inside houses.
• It is likely to prove a better means of cutting down on air infiltration and thermal bridges than insulating internally. 
• Outside the house there is likely to be one long interrupted wall. If insulating internally, that wall will be broken up by partitions,which makes the task more difficult. There will be heat losses through those partitions, especially if they are built of masonry.

However, when insulating from the outside there can be problems if the roof does not overhang the walls, given that they will be thicker than they were before the insulation was applied. There may also be problems to overcome if the window openings are set back any distance. In addition to this, in old cottages there tends to be heat loss in the lower section of walls where they come into contact with the ground.

You may also find that if the house is listed you might not be permitted to apply exterior insulation at all, or only on one or two walls.

Inside Walls

Most experts will advise against insulating old houses on the inside. The reasons are as follows:

• Loss of space due to the increased thickness of the wall.
• Problems that arise when decorations such as skirting boards are removed.
• Electrics and plumbing may have to be rerouted.
• Disruption to the occupants.
• Problems with internal walls. Where these join the external wall there will be problems with heat escape around the edges. This will reduce the effect of the insulation, so the answer is to create ‘returns’ around the edges where the internal walls join the front wall. This is tricky, and means a ‘step’ in the internal wall.
• There is also the problem of condensation, and this is a complex issue.

The great fear is that by adding insulation to the inside of vapour-permeable solid walls (solid brick, earth or masonry), you will create condensation within the wall. This is known as interstitial condensation. A small amount does not matter in masonry but if the build-up is too high then there can be rapid decay — especially in timber frame buildings.

It is vital to ensure that any condensation in a house with solid, vapour-permeable walls travels from the inside to the outside. As a rule of thumb, the vapour resistance on the warm side of the insulation (within the room) should be at least five times higher than the vapour resistance on the cold side.

Check this with the supplier of your interior insulation or through a building consultant who specialises in energy rating and conservation. The manufacturer or your supplier should be able to run a check to make sure that the moisture will travel in the correct direction. All manufacturers of building materials publish vapour diffusion values for their products. By entering these values into an approved software program, the condensation risk can be assessed.

It is highly likely that you will need a vapour ‘check’ behind the insulation on the ‘warm’ side. This is to ensure that the relative humidity stays below 100 per cent (at which point condensation will occur) at all interfaces within the structure. If there is timber within the wall or any other material that could be damaged by moisture, the calculation should show that the relative humidity will stay below 90 per cent to allow a margin for error.

Materials

Wall insulation can be achieved by using a variety of materials including wood fibreboard, phenolic or urethane board, sheep’s wool, mineral fibre, newspaper, cork or hemp, or a hemp and lime mix. All these materials have their own advantages and disadvantages. Wood fibreboard is popular at present because it is entirely vapour permeable. The material – some systems are tongue-and-groove and others butt-jointed – is first screwed into position against the external wall, before being lime rendered and then colour washed.

These boards have good sound-insulation properties as well as being able to achieve a U-value of 0.27.

If you want to use entirely natural materials, it is possible to use one of the hemp or sheep’s wool-based breathable insulation products. These can be used externally by being attached to the wall and held in place by a wood fibreboard, which is again finished with a lime render and limewash.

Reader Solutions - The 18th century cottage

When Steve and Ava Howard bought their 18th century former farm worker’s cottage in a mid Suffolk village, it was almost uninhabitable and close to demolition, particularly because it was unlisted. The daub in the walls was in such a poor state that the laths cracked and broke when they started repairs to the frame. Because the building was unlisted they were permitted to replace the daub with what Steve refers to as “the modern equivalent.” His system involved using a wood fibreboard on the outside, covered with a lime render.

“It’s a really amazing material to work with because it is so pliable,” says Steve. “It is ideal to apply to the exterior of an old timber framed cottage.” In order to conform with the thermal insulation regulations, they used cavity bats on the inside and finished off with a plasterboard and one-coat plaster.

Reader Solutions - The 200-year-old tollhouse

After 19 rather draughty years in their 200-year-old former tollhouse, Anne and Chris Willoughby, who live near Eye in Suffolk, decided to insulate their house with Diffutherm wood fibreboard.

They were as keen to protect the solid-walled house against noise – it is on a busy B-road – as against draughts, and the installation has been successful on both counts. However, Chris is sceptical about the number of years it will take them to regain their £1,000 capital outlay. “Last winter – the first after the insulation was installed – we used three quarters of the amount of heating oil we normally use, but then it was a warm winter,” he said. “I think we shall save about £300 a year in heating bills, and it has certainly greatly reduced the traffic noise.”

Insulating a Victorian terrace
Penny Poyser and Gil Schalom are the owners of a Nottingham eco house. Although not immediately obvious as being a green home, the semi-detached property is the perfect demonstration of how you can retrofit high levels of insulation to ordinary period houses. At the front of the building the insulation is fitted inside, so the façade is unaltered. The material used was 100mm ozone-friendly insulated dry-lining. On the side and north-facing rear wall where it is less important to show the period brickwork, 150mm exterior wall insulation has been applied to the rear face.

Further reading:

• Solutions for Insulating Walls
• How to Insulate Your Loft
• Choosing Insulation
• Natural Insulation
• Everything You Need to Know About Insulating Your Home
• Insulating a Wall Cavity - DIY or Professional?
• Can I Externally Insulate my Cottage?