Insulation – Making the Right Choice

The obligation to insulate buildings has become very clear. With issues of rising energy prices over the longer term, security of energy supply based on Ireland’s reliance on imported fossil energy, and the potential for fuel poverty as heating costs rise in poorly insulated existing buildings, it is actually not just a positive investment but a critical one. With the various grants which are now available and coupled with rising energy costs there has never been a better time to insulate.

Before we delve into the amount of insulation recommended in each building element, it’s important to make a clear decision as to which insulation we should use.


All thermal insulation generally performs on the same basic principle: heat moves from warmer to colder areas. Thermal insulation separates the heated space from the cooler unheated space and it is the resistance of the thermal insulation that determines how long it takes for heat to transfer to the cold side. Therefore, on cold days, heat from inside a building seeks to get outside. On warmer days, the heat from outside the building seeks to get inside. Insulation is the material which slows down this process.

There are many forms of insulation on the market, which generally contain pockets of gas (e.g. air or pentane to name two), which resist the transfer of heat. At some stage, it is inevitable that heat will escape, but effective thermal insulation will slow down this process significantly.

Thermal insulation significantly reduces the energy required to heat buildings. In fact, if sufficient quality insulation is correctly installed in buildings, such as those built to the Passiv Haus standard, this can offset the requirement for any conventional heating system! Such buildings require a very low energy input for space heating, 90% less than that of conventional buildings. There are examples of this in over 10,000 Passiv Haus’s which are now constructed throughout the world, a number of which are built in Ireland.

In recent times insulating materials have played second fiddle to some of the more visual, plug on, “sexy”, renewable energy solutions. However, no matter what form of energy is used to meet the heating and cooling demands of a building, if it is not sufficiently or properly insulated, energy loss is inevitable.

Insulation should be an absolute priority at the early stage of any construction project. While budget is central to any building project, when choosing the type of insulation you require it is important to look beyond mere price comparisons, and evaluate the range of benefits each insulation material provides and its suitability for the job in hand. Insulation should be viewed as a critical investment as oppose to a cost. After all, it is much more costly to re-insulate a building, compared to getting it right first time. So when choosing an insulation material, what are the key performance characteristics one should seek? It’s surprising to some people the impact an insulation material can have on not only the energy efficiency of a building, but also, its impact on comfort levels and building health.

The following are a number of key performance indicators one should consider when selecting insulation:

  1. Thermal Resistance
  2. Acoustic properties
  3. Heat Storage Capacity
  4. Vapour Resistance(breathability), Hygroscopic behaviour
  5. Fire Resistance
  6. Ecological properties (Manufacturing, impact on the environment and disposal)

Thermal Resistance:

The primary characteristic of any effective thermal insulation is that it is a very poor conductor or heat. The Thermal Conductivity of an insulation indicates its ability to conduct heat (denoted as “k” value). This is measured in units of Watts per meter of surface per degree Kelvin (W/mK). Metals such as copper are excellent conductors ,with a thermal conductivity of 401W/mK while sheepwool insulation has a conductivity of only 0.038W/mK. Therefore Copper conducts over 10,000 times more energy compared with Sheepwool!

The U-value, more correctly called the “overall heat transfer coefficient”, describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions.

When we know the conductivity and thickness of an insulation material, then the “U” or Thermal Resistance (denoted as the “R” value in m2W/k) can be calculated. The “R” value is the inverse of the “U” value. A “U” value is measured in W/m2K. (i.e. 100mm of sheepwool (thermal conductivity of 0.038W/mK), has a U value of 0.38W/m2K).

The lower the U value, the slower heat is lost – and the less energy you need to keep your home warm. The following illustration compares the thermal resistance (“R” value) of various materials. The higher the “R” value, the more effective the material is at preventing heat transfer.

Figure 1: Thermal Resisitance comaparison of various materials at 100mm thick (the higher the better)


Acoustic properties

The two primary types of sound transfer are airborne and impact sound. Most thermal insulants are used to reduce the transfer of airborne sound (music playing, people speaking etc). Fibrous insulation materials with a high density are generally the most effective at reducing the transfer of airborne sound based on their flexibility and inherent ability to absorb the transfer of sound within its fibrous structure.

Heat Storage Capacity

Heat Storage Capacity (HSC) is an often overlooked characteristic of insulation materials. HSC is particularly important in areas of a building which can be exposed to large variations of high temperatures during the day and low temperatures at night (i.e. living spaces in attics.).

An insulation with effective HSC characteristics will assist in ensuring that the temperature within the living space will not fluctuate dramatically from being too warm during the day to being too cool at night during summer months. The key characteristics which determine the HSC of insulation are that it has a high density, high specific heat capacity and low conductivity. Materials with the most effective HSC are made from woodfibre, cellulose and Hemp whereas materials with poor HSC are generally foam based insulants and low density manmade fibre insulation such as fibre glass or mineral wool.

The two parameters which can be identified as defining the effectiveness of these insulation materials are their contribution to amplitude damping and phase displacement.

Amplitude damping is the relationship between the exterior temperature variation and the variation in interior temperature. For example, if the external temperature variation from day to night is 30 °C and the interior temperature variation 3 °C, the value of the amplitude damping is 10 (30 °C/3 °C) In other words: The temperature variation is suppressed by the construction component on its way from the exterior to interior to one tenth. Depending on the construction, usage and living requirements, a minimum amplitude suppression of 10 to 15 is desirable.

The phase displacement is the time span between the highest external temperature and the highest interior temperature. One aim of thermal protection in summer is to retard temperature penetration of a roof or a wall to such an extent that the highest temperature of the day only reaches the room side when the outside temperature is so low that the heat can be driven out by ventilation. The target here is a phase displacement of 10 to 12 hours. A portion of the heat stored in the construction components is then returned to the exterior of the house.

Whereas two insulation materials may have the same U value for a given depth, they can have completely different HSC properties.

Vapour Resistance(breathability), Hygroscopic behaviour

Vapour resistance describes the resistance of a given material to the transfer a water vapour. This is particularly important where insulation is to be installed between timber elements (i.e. in the roof/ceiling of a block house or walls of a timber frame building).

In the event that moisture penetrates a building envelope with timber components it is desirable for this moisture to dry out as quickly as possible to prevent mould growth and structural defects. Where insulation materials with a high vapour resistance (such as closed cell foams or foils), are installed within a timber structure this may inhibit the ability for vapour to diffuse rapidly to the cold external side.

One of the key units for measuring the vapour resistivity of any material is its vapour resistance relative to still air measured as the  µ (pronounced as mew) value. The lower the  µ value the more vapour diffuse it is. When we multiply the  µ value by the thickness of the insulation material, then we identify the vapour resistivity of the material. For example, Hemp natural insulation has a  µ value of 1 where as polyurethane foam has a  µ value of 100.

Insulation materials can also contribute to regulating the humidity within the construction and the living space. The Hygroscopic behaviour of an insulation material is it’s ability to absorb and release water vapour as the relative humidity of the air changes. This can assist in stabilising internal humidity and can have a dramatic effect on comfort levels as well as indoor air quality by reducing the risk of mould growth and dust mites.

Fire Resistance

It is also important to consider how an insulation material reacts if exposed to fire. Does it contribute to the spread of the flame? Will it give off a poisonous gas. Will it assist in protecting the structural integrity of a building. Has research been conducted on it’s fire resistance qualities? Does it comply with relevant European or Irish fire class standards?

Ecological properties (Manufacturing, impact on the environment and disposal)

The built environment not only contributes about 50% of the CO2 to the atmosphere in Ireland, but it is one of the largest contributors of waste as well. UK research has shown that in some cases for every 3 houses constructed, the equivalent of one building goes to landfill in terms of waste! Therefore when selecting an insulation material, its recycling properties should be considered. The raw materials used to manufacture an insulation material should also be a priority for those espousing to install an ecological insulation.

Whereas man made fossil fuel based insulation materials tend to not only require vast amounts of fossil fuels to manufacture, natural insulation such as Hemp or Woodfibre not only require much less energy in production, but also absorb huge volumes of CO2 from the atmosphere during their cultivation. For example, research has shown that for every m3 of Hemp insulation produced, 13kg of CO2 is absorbed. In this way some natural insulation reduces the carbon footprint of a building even before they are installed!

Figure 2: The Life Cycle of Natural Insulation: Thermo Hemp

The introduction of Building Energy Ratings (BER’s) in Ireland and more stringent requirements for higher levels of quality insulation has led to a significant influx of a vast array of alternative insulation materials. In a slowing market consumers have more time to research the various insulation products available on the market. Characteristics which were, until now, considered supplementary, are now seen as a critical part of deciding which insulation to use in their homes. Living health, durability, and comfort should be considered just as critical as thermal performance.

The range of natural insulation products currently available vary from wood fibre mats, hemp, sheep’s wool, woodfibre softboards and cellulose/recycled paper products. Some of these products are displayed below:

Figure 3: Various Forms of Natural Insulation: Hemp, Thermafleece, Woodfibre mats

Each of these materials has a unique combination of key characteristics which helps to create a healthier, comfortable, energy efficient, durable construction.

When considering the energy efficiency of buildings one should not under-estimate the positive contribution which natural insulation materials offer buildings on so many levels. Structures which are energy efficient, durable, healthy, ecological and sustainable in every sense of the word should be designed with some form of natural insulation, the benefits of which are not only in the money we save, but in the environment we live, particularly as it is estimated that we spend up to 90% of our lives in buildings.

Whether natural or man made insulation is used in buildings, the airtightness of the building envelope is integral for efficient thermal performance. Insulating buildings to high levels without due consideration for airtightness leads to well insulated but draughty buildings. Unfortunately there are many such buildings throughout the country today, some only built over the last 10 years.


The Government has undertaken a commitment to reduce carbon emissions by 80% by 2050 and is developing calculation tools and strategies to try and ensure that this commitment is satisfied. This will no doubt have significant implications for our buildings, both existing and futher proposed construction.

In 2008 Part L of the Building Regulations was revised to achieve a 40% improvement in primary energy consumption and CO2 emissions in new dwellings; a 60% improvement in primary energy consumption and CO2 emission standards are expected to follow this year.

The follow up to this insulation article will provide guidance and recommendations for insulating buildings either internally or externally and how to meet these demands. I hope that this article has helped to clarify some of the primary features one should inquire about when considering which insulation to use.

By Niall Crosson BTech, MEnSc, MIEI

Insulation images provided courtesy of Ecological Building Systems

February 2010