Heat Transfer

heat transfer in an extension

In order to discuss the energy efficiency of residential structures, here’s a quick revision of the mechanics of heat transfer. You may recall, from your Physics class way back in time, that there are 3 ways to transfer heat energy:

  • Conduction, where heat energy transfer occurs by direct contact
  • Convection, heat energy transfer occurs through a moving material (such as air)
  • Radiation, where radiation transfers the heat energy using electromagnetic waves.

Heat energy will flow to achieve equilibrium between points of temperature difference. This is why the heat that we generate in our houses during the cold months is constantly seeking to escape to the lower temperature outside. Obviously, this energy flow reverses during spells of hot weather as the heat energy attempts to flow into the house.

Thermal Efficiency
If you could minimise the rate of heat transfer occuring in your house, it would become thermally efficient. A 100% thermally efficient house is one that doesn’t lose any heat. A 50% thermal efficient house is typified by a house where central heating radiators are blasting out heat. All of the heat energy produced is escaping via uninsulated walls, single-pane glazing and draughty doorways and windows.

Heat transfer via radiation is relatively insignificant for the walls and roofs of a house. Putting radiated heat to one side, we will now examine heat transfer by conduction and convection.

Heat Transfer by Conduction
We want to find which building materials or systems are poor conductors of heat. In order to compare the different material or systems, we need a measure of thermal conductivity and that brings us back to the U-value.

The U-value
The U-value is the rate of transfer of heat through a structure (which can be a single material or a composite), divided by the difference in temperature across that structure. U-values are written as the amount of heat (expressed in Watts) passing through a square metre (m2). You will see this unit written as W/m2K.

Low U-values mean that only a small amount of heat is being transferred. Consider the U-values of the following materials:

  • single-glazed window, 5.0
  • double-glazed window, 1.6
  • triple-glazed, 0.8.

To calculate the U-value of a composite system, we need to introduce another unit of measurement, the thermal resistance R. Lets look at a typical composite system in a timber-frame structure, the external wall system, and see how this works.

This wall system has several components:

  • Cladding
  •  Battens
  •  Breather membrane
  •  External panel
  •  Insulation
  •  Internal panel
  •  Services channel
  •  Internal plasterboard

Each component has an R-value, a measure of its thermal resistance, which is dependent on the component thickness and the type of material used as a component. We would expect a thick slab of insulation to have a higher thermal resistance than a thin slab of insulation. Similarly, for the same thickness, we might expect a higher thermal resistance for a higher quality of insulation material.

When we add the reciprocal of all the R-values together, we get the composite U-value. For example, a timber frame wall system as described above with an overall wall thickness of 300mm might deliver a U value of 0.29W/m2K. If you want to know more about calculating these values, there is plenty of information on the internet.

Improving U-values
There are several ways in which we can improve the U-values:
1.Upgrade the material
Using material with a higher insulation value is perhaps obvious. However, the increase in material cost needs to be justified within the overall house construction budget.
2.Increase the thickness
Increasing the thickness also impacts on the overall house construction budget. An additional concern is that thicker walls mean less usable space in the rooms.
3.Improve the construction method
Most types of insulation are inefficiently applied on-site. Mineral wool tends to slump in a timber frame, rigid insulation is seldom cut to be a perfect fit between studs and cavity-fill insulation may not be an airtight fit to the inner block skin. Because of these site imperfections, the actual U-value tends to be higher than the theoretical U-value.

A superior improved construction method is off-site fabrication. The high levels of Quality Control in a factory environment means that it is more likely that the design specification will be met when a timber-frame closed panel is fabricated. This means that the U-value will be as designed.

You can improve the U-value further by using Structural Insulated Panels (SIPs) instead of the closed panel system. SIPs are essentially a sandwich of two sheets of OSB (oriented strand board) bonded to a central layer of insulation, usually polyurethane. If you specify SIP as your construction method, you have some reassurance that the U-values will be very close to the design values.

4.Minimise thermal bridging
Thermal bridging describes a situation in a building where there is a direct connection between the inside and outside through an element that is more thermally conductive than the rest of the building envelope. As a result, there will be wasteful heat transfer across this element. There are several ways in which suppliers can minimise thermal bridging. You should ask the supplier how they are achieving this.

Heat Transfer by Convection
The most thermally efficient external wall system in the world is a waste of money if it’s installed in a draughty house. This is because the draughts will just take the heat away in winter. The house must be airtight and this requires a perfect seal to all gaps. This is possible but it requires a consistently high level of workmanship from the contractor erecting the structure.

When assessing a Suppliers construction system, you want to know that the quality of workmanship will be high on your project. While you cannot attend the site each day to ensure all gaps are being sealed, there is a way that you can check the final result. You can test the finished house for airtightness.

An airtightness test involves attaching a large fan to a door aperture and sucking the air out, attempting to create a vacuum. You can then quickly locate any air leakages so they can be sealed. Once all obvious air leaks have been sealed, a specific pressure reading can be achieved.

You can commission this test for your own completed house. By specifying the achievement of a certain pressure reading as part of your contract with the Package Supplier, this motivates the supplier to ensure the required level of workmanship on your project.