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An introduction to solid wall insulation

Posted by Tim Pullen on 3 December 2009 at 2:13 pm

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.

More information about solid wall insulation from YouGen

Insulation information page

How solid wall insulation cut my bills to a fraction of the UK average

Planning permission not needed for (most) external solid wall insulation

What is thermal bridging insulation, and should I worry about it?

Wise up when buying solar, double glazing and insulation

Find an insulation installer

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.

If you have a question about anything in the above blog, please ask it in the comments section below.

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6 comments - read them below or add one

EnergySavingGrants.orgComment left on: 19 May 2017 at 9:29 am

When installing External Wall Insulation, we would recommend that a breathable render such as silicone is used, as opposed to a non-breathable render such as acrylic. Although more expensive, silicone render will allow water vapour to pass through the walls, thereby reducing the risk of condensation build up within your property.

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vincenewmanComment left on: 28 April 2014 at 5:18 am

Deleted due to spam content

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pauletteComment left on: 16 September 2012 at 2:42 pm

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

I need a diagram or more explanation as not sure which part is the internal wall.  Do you mean leaving 25mm from the inside of the current brick wall and than adding batons on spacers attached to the brick wall to attach some insulation and then internal cover.  Thereby leaving the usual gap.

 So foam infill like icynene is not suitable for a solid break wall.  I would love to externally insulate the property but the roof hang in scottish old houses just is non existant. 

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EnergyLink Ltd

EnergyLink LtdComment left on: 12 July 2010 at 4:19 pm

I just wanted to mention that if you wanted to go down the grant route for insulation CERT (carbon emissions reduction target) funding has been extended until December 2012. Where income does have a bearing on the grant is if your household earns less than £16k a year after taxes you will get a 100% grant but over that you can get a grant of up to 70% (often depending on the size of your house)

There is comprehansive information at

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HeatandEnergy.OrgComment left on: 2 January 2010 at 6:10 pm

Although there are no nationally recognised grants for installing solid wall insulation, we have sourced some regional discounts fort the DIY supply of sempatap thermal, though this varies by region and is limited, the equivalent to a 25% discount! For assisatnce go to

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AnonymousComment left on: 17 December 2009 at 11:48 am

Thanks to a recent Government-backed energy reduction initiative, everyone – homeowners and private tenants – can benefit from Home Insulation Grants from 80% or up to 100% towards the total cost of loft insulation and cavity wall insulation, regardless of income
In a typical poorly insulated UK home, 35% of all heat is lost through the cavity walls and other 25% lost through the loft.
Poor insulation is costing you money – without you even realising it!!
Free insulation offer is available for average sized properties. If your property is exceptionally large for its type there may be a client contribution required.
For loft insulation, the existing loft insulation must be less than 3 inches. The vast majority of properties can be done free on the scheme.
FREE Insulation if you receive :

•Disability Living Allowance
•Attendance Allowance
•Pension Credit
•Income Support
•Housing Benefit
•Working Tax Credit (with household income of less than £16,040)
•Child Tax Credit (with household income of less than £16,040)
•Council Tax Benefit (not 25% single occupant rebate)
•Income Based Job Seekers Allowance

These are available for cavity wall insulation and to top loft insulation up to 270mm (11 inches)
These are not means tested and as long as you are an owner occupier or rent privately then you qualify.

you can book a Free no obligation survey or apply for FREE or Part funded grant here at :

Having a gap between the walls is as daft as having a space between your ears. Keep heat inside!!

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