A third of the heat lost from a house is through the walls according to the Energy Saving Trust. An uninsulated, 225mm solid brick wall will have a U-value of 2.23W/m2. Adding just 50mm of PUR (Kingspan or similar) insulation will reduce that to just 0.4W/m2. Bear in mind that the current building regulations call for a maximum U-value of 0.35W/m2 so we could go a lot further.

There are two basic kinds of solid wall: brick, which is typically 225mm thick, and stone which is typically a lot thicker, and works in a different way.

Stone walls

A typical stone wall will be constructed of two skins of stone with the gap between filled with rubble. Thinking used to be that the rubble fill was just a cheap form of construction, but the reality is that it is designed to work in a particular way. Rainwater will penetrate the outer skin and some of that water will find its way to the rubble-fill. Because the rubble is relatively loose the water drops through it to the ground and away, preventing the inner skin becoming damp.

In addition, the wall is kept dry by air movement – wind acting on the outside skin and air moving through the wall. Interrupt those flows and the wall can become damp.

The way to insulate stone walls is with a breathable insulation, preferably set off from the inner skin to leave a 25mm air gap. Sheep wool, hemp or cellulose are all good insulators for this application and 100mm will reduce the U-value to about 0.35W/m2.

Bear in mind that walls are designed, they don’t just happen. And they are designed to work is a specific way. Interrupting the way they work with non-breathable materials like PUR insulation, gypsum plaster, damp-proof course, will stop them working properly.

My own house has 450mm stone walls that someone has proudly plastered. The result is that the walls get damp and it is slowly blowing the plaster off the walls. I now have a long-term project of stripping the plaster and either re-plastering in lime or leaving it as stone, depending on the look my wife decides on.

Brick Walls

Rainwater will penetrate less far into brick than stone. Wind on the brick tends to effectively dry the wall. The critical issue is the dew-point. When warm air meets cold air it condenses and moisture is released. In an uninsulated brick wall that point will be towards the outer surface of the wall. Insulating internally tends to draw the dew-point into the wall. What needs to be avoided is drawing it in to the inner surface of the wall.

There are 3 ways of doing this:

1.    External insulation
Google external insulation and you will find dozens of options. This works particularly well if your house is rendered as they lend themselves to render finish.

2.    Thin inner insulation

Options include Therma-coat or Sempatap. These are, respectively, an insulating paint and a neoprene-like foam.

3.    An insulated stud wall set 25mm off the internal surface.

All of these will work but what is best in any given situation will depend on the situation.

What is the impact?

Insulating walls is effective in its own right, but best done as part of an overall insulation upgrade. Heat, like water, tends to take the path of least resistance so insulating the walls can mean that the heat escapes more quickly through the roof.

To put some figures around it; if the house has a heating bill of, say, £600 per year, insulating the walls in the way described will reduce that to around £430. Upgrading all the insulation to a similar level could see the bills drop to less than £300.

Photo by Chris Loxton

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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