Heat Loss - Walls

 
"The heat lost from the walls of an uninsulated house would typically be of the order of 33% of the total losses."
 
This section of the web site examines heat loss through the walls of your home. Its purpose is to look a little closer at a vital part of your home that protects you from the elements and keeps you warm. With an increased awareness of how your home is built you will be able to assess the options open to you.
 
In this section you will find.
 
      • A discussion of wall construction types.
      • Methods for insulating walls.
      • A worked example of the heat lost by a radiator.
      • Where to find grant information.

Wall Types

Construction techniques have changed over the years and continue to do so. Very many homes are of an age that they have solid walls, made of brick or stone. But from an insulation perspective (or others for that matter) this is not a very good design.
 
It is possible to install insulation and many systems are available. However, this section does not try to identify them all because it is a large and specialist area. Sufficient to say that options exist and this section will hopefully begin to sign post the way.
 
Here is a broad summary of wall types and insulation methods.
 

 Wall Type

 Insulation Method

 Single Brick  Dry Lining, external insulation
 Double Brick/Stone - No Cavity  Dry Lining, external insulation
 Double Brick - Cavity  Cavity insulation
 

Insulation Methods

Dry Line

Dry lining involves attaching insulation to the inside walls. There are a growing number of systems available for doing this but needless to say all involve a significant amount of work. For example, it will be necessary to remove skirting, architraving, move electrical fittings etc. Some years ago this involved fitting wooden battens to the walls and infilling with insulation. New developments have improved this to the point where the insulation can be bonded directly to the walls and consists of all the components required. i.e. insulation, vapour barrier, surface ready to decorate etc. One disadvantage of drylining is that it reduces the thermal store that the walls once provided. That is, a house with drylining will heat and cool more quickly which this could have a negative impact on comfort.
 
For further background information follow this link on dry lining systems.
 

External Insulation

It is also possible to attach insulation to the outside surface of walls. This has the advantage of being less disruptive than dry lining and does not reduce living area. Many councils have adopted this system for their housing. However, its suitability does depend on the external appearance. That is, it probably would not be a good solution for an old attractive cottage constructed of brick or stone. Further information on external wall insulation can be found here.
 

Cavity Insulation 

Modern homes invariably are constructed with external walls comprising of a brick outer skin and block inner skin seperated by a cavity that has two purposes; protection against damp and insulation. 
 
Until relatively recently the air gap was considered sufficient insulation but with rising energy costs, building regulations dictate that the cavity must now provide far more insulation.
 
This can be achieved at the time of construction by installing insulation in the cavity and optionally using insulating blocks to make the inner skin. For buildings without insulation it is still possible to insulate the cavity but this requires a different technique. For this
insulation is blown into the cavity by means of holes drilled through the outer skin.
 

Discussion

To state the obvious, heat loss through a wall occurs because the outside temperature is colder than the inner. However, the heat loss process is more complicated than it might first appear. It is dependent on conduction through the solid materials aswell as the air flow on the inside and outside of the wall (convection and forced cooling by wind and moisture).
 
In fact the air flow on the outside i.e. weather conditions, can have a considerable influence on heat loss through the walls. Whereas still air is a good insulator, in reality it is more likely to be moving i.e.windy, and could include rain or snow which will affect the heat loss considerably. For an analogy consider how cold it feels to walk out of the sea and onto a windy beach!
 
Green insulation
As well as the systems mentioned above, one further insulation technique that could be considered is to protect exposed aspects of your home with plants. These could be used to provide a wind break to slow the air (trees or shrubs) or to provide a protective layer on the wall itself (climber).
 
Reduce heat loss by insulating radiators
Heat loss increases with temperature difference between the inside and outside surfaces and so having hot radiators on external walls makes the heat loss worse. Consequently, consider insulating behind these radiators or having them moved if you now have double glazing. (Radiators were traditionally placed under windows to counter draughts. However, this is not generally as important with modern double glazed windows).
 
For a demonstration of the value of insulating behind your radiators please read on.

Technical Stuff

This section is going to demonstrate the value of insulating behind a radiator. It uses calculation techniques found in the U-Value section should further explanantion be required. If you are not of a technical mind then please ignore the calculations and skip to the all important answers. The calculations are included to illustrate that this widely offered advice can be justified with readily available information. By showing the calculations you may want to try some 'what-if' scenarios of your own!
 
In the 1970's many houses were built with decorative wall sections consisting of pebble dashing and slightly recessed. To achieve this the cavity was eliminated. However, this wall section was often under a window with a radiator on the inside wall. The calculations below examine the heat loss taking place.
 

Calculation of the Heat Loss

 

Wall Component

 Conductivity (U-Value) (W/mK)

 Thickness(m)

 Conductance (W/m2K)

 Resistance (m2K/W)

 Outside Air Layer

     

 0.04

 External render

 0.50

 0.018

 27.78

 0.04

Double thickness brick without air gap.

 0.84

 0.16

 5.25

 0.19

Dense Plaster

 0.57

 0.013

 43.85

 0.02

 Inside Air Layer

 -

 -

 -

 0.13

Totals

 

 0.19

 

 0.42

Overall U-Value

     

 2.38

 
 
The overall U Value of 2.38 W/mK can now be used to calculate the actual heat lost. To do this requires four additional pieces of information
 
  1. The size of the radiator i.e. area in m2. A typical radiator size would be 1.2m by 0.6m (0.72m2)
  2. The temperature of the radiator. 80degC is a frequently quoted flow temperature but the average temperature over the radiator surface will be lower than this. Let's say 70 degC. 
  3. The thickness of the wall. (This can be derived from the above table). The wall thickness used was 0.191m ( 7.5in).
  4. The outside temperature. The average winter temperature is approximately 4degC.
Based on the above information the heat loss is calculated as follows.
 
Heat Loss = U-Value x Temperature difference between inside and outside x Area
 
Heat Loss = 2.38 x 66 x 0.72 = 113W
 
So what is the approximate heat output of the radiator? Using a very useful table obtained from here  http://www.simplifydiy.com/plumbing-and-heating/radiators/power the radiator output is 1440W.
 
That means that 8% of the radiators heat is going through the wall!
 
With a gas price per unit of 3.245p/unit and a boiler efficiency of 90% the loss is costing
 
Cost per Hour = 0.113 x 0.03245 / 0.9 = £0.00407/hr
 
If the heating is on for approximately 10 hours a day. (Weekdays might be less. Weekends more.)
 
Cost per Day = 0.00407 x 10 = £0.0407/day
 
If the heating season is taken to be October to March
 
Cost per Heating Season = 0.0407 x 180 = £7.32
 

The Benefits of Insulation

Let's look at how a thin piece of insulation changes the losses.
 

Wall Component

 Conductivity (U-Value) (W/mK)

 Thickness(m)

 Conductance (W/m2K)

 Resistance (m2K/W)

 Outside Air Layer

     

 0.04

 External render

 0.50

 0.018

 27.78

 0.04

Double thickness brick without air gap.

 0.84

 0.16

 5.25

 0.19

Dense Plaster

 0.57

 0.013

 43.85

 0.02

 Insulation

 0.022

 0.012

 1.83

 0.55

 Inside Air Layer

 -

 -

 -

 0.13

Totals

 

 0.203

 

 0.97

Overall U-Value

     

 1.03

 
 
Heat Loss = 1.03x 66 x 0.72 = 49W
 
That's a saving of 43% or in financial terms £3.14 per year per radiator.

Grants

The walls of a building account for one of the greatest heat losses and so the government encourage insulation through grants. Further information can be found at the Energy Saving Trust