In my last blog, I focused on some basic principles of Building Science that apply to most of our existing homes. I highlighted the following physical issues that can impact on thermal comfort and indoor-air quality. I outlined principles that can reduce space heating energy demand, by upgrading the building fabric’s elements that enclose all the external envelope of the house, such as the roof, external walls, ground floors, windows and external doors. This blog explored principles of science that can combat heat-loss, draughts, discomfort and poor indoor health conditions, such as:

• Thermal Insulation.
• Prevention of draughts and uncontrolled wind infiltration
• Achieving air tightness of the building envelope.
• Principles applying to moisture control, humidity and vapour diffusion.
• Indoor-air contaminants and how they can affect health.
• Ways of achieving effective fresh air-change and energy efficient methods of Heat Recovery Ventilation.    

This blog hopes to focus on thermal insulation and energy efficient solutions to a range of typical external elements.

 Properties affecting Thermal Insulation performance:

Most Irish houses are very poorly insulated, which makes them difficult and expensive to heat and often cold, damp, draughty and unhealthy. Space heating of our average house consumes about twice the European average in energy, 99% supplied by imported fossil fuels.  The total energy consumed by space heating, water heating and electrical uses like lighting, household appliances, and electrical goods. At today’s fuel costs, it amounts to over €2,000 per average household, each year.  The average Irish house performs at a poor ‘E’ in BER rating and emits about 8 tons of CO2 into the atmosphere, each year on average. This is over twice the greenhouse gas emissions of the average European home.  Space heating comprises about 70% of the energy demand of the average Irish home.  This varies with floor area, volume, external surface area of enclosing elements, the level of insulation, wind tightness, air tightness, control of ventilation, efficiency of heating system, fuel used, etc. The difference in performance between a ‘G’ BER rated house and similar sized  ”Passive House’ is thirty times the heat loss. Once the indoor air temperature needs to be maintained warmer than the ambient out-door environment during the heating season, heat from the interior spaces can be lost by conduction, convection, radiation and evaporation. A vacuum is the best insulator of all. That’s why a thermos flask works so well in retaining its heat or chill. The thermal conductivity of different materials varies considerably from highly thermal resist to, highly conductive.  I will probably shock most readers when I show the colossal difference between common materials as I have listed below. On a relative scale for a given thickness, a high performance insulation product has a thermal conductivity of  = 0.20Wm, platinum polystyrene, = 0.33, glass or mineral wool =0.4, dry timber 1.4, medium density concrete= 1.4, steel = a shocking 20 and aluminium an incredible 150Wm. So, for a given thickness, aluminium transmits heat by conduction at a rate of 7,500 that of a very high performing insulation. You can therefore understand how poorly aluminium windows, fitted without a thermal break will perform and create heat loss or how a structural steel element forming a cold-bridge within an external element, transmits heat at a rate of up to 1,000 times that of the high performance insulation. The  ‘U value’ is the thermal transmittance and depends on thermal conductivity and the thickness of the material. The rate of conduction loss of heat depends on the insulation type and thickness, applying to each of the external elements and how cold-bridges are prevented. Different insulation types can trap air or at various levels of performance. Some are ‘breathable’ and vapour permeable, some natural materials are hygroscopic, which allows vapour to be absorbed, stored or released under different humidity conditions. The thermal performance of insulation also depends on how dry it is, as water is a high conductor relative to air. It also depends on how tight it is fitted, so as to prevent convective ‘thermal-looping’ through and around gaps, located at joints and junctions. The convection loss depends on the way cold air is prevented from infiltrating through the external fabric and how draughts are eliminated. This also includes losses caused by convection pressures of cold zones within the house, driving cold air towards warmer zones of the house, where warm moist air is less dense. It is pushed and displaced by internal draughts created by the pressure of cold denser air. As the warm air migrates towards the cold zones, its relative humidity increases, often reaching 100% RH saturation at the ‘dew point’, causing condensation. This can happen on the internal cold surfaces or ‘interstitially’ when driven by convection or diffusion into the external building element, causing heat loss and dampness.  If this dampness is sustained over time, it can cause decay of organic materials like wood, corrosion of steel, or create unhealthy growth conditions for mildew and moulds that emit toxic spores. Heat loss by radiation occurs when solar short-wave rays and UV transmits through a transparent medium, like glass. The longer-wave length, infrared radiation of heat can be trapped or reflected back by reflective, polished or bright emissive surfaces, where dark, matt, rough surfaces absorb heat more effectively. This is how greenhouse gas concentrations in the atmosphere, balance the Radiative Forcing that controls Earths climate. It works in a somewhat similar way when solar radiation transmits through glass and longer wave heat rays, get trapped as they pass through the glass. Under certain conditions, materials like polished aluminium foil can act as an insulator, by reflecting the radiant heat and reducing radiation emittance. Evaporative loss occurs when a liquid (water moisture) changes phase and cools as it evaporates to form a vapour and visa versa when a vapour condenses. When a phase change occurs, there is considerable latent heat released or absorbed in the process, such as from solid (ice) to liquid (water/moisture) to gas (water vapour).

Next week I will continue to discuss ways of how heat is transferred from inside to outside of an enclosed building.

-Duncan Stewart