Tech Articles

Insulation: What to specify and why


Insulation products have developed significantly with technological advances, with the development criteria remaining largely centred around performance. Legislation has acted as the catalyst for development, from the basic requirements under Building Regulations AD Part L, to compliance with government carbon reduction targets, driven through advanced programmes such as the Code for Sustainable Homes and BREEAM.

One might argue that legislation is driving technology. Consequently, technology has contrived to produce a range of products that are perceived to ‘just work’, presenting little apparent difference between them. But if specifiers can better understand how heat transfer principles influence insulation performance, and how insulation operates at its core level, there is the opportunity to insulate our buildings to a maximum and appropriate level with the confidence that specified performance can be sustained throughout the lifetime of the building.

The installed performance of an insulation product is greatly reliant upon not only the adherence of contractors to manufacturers and general best practice workmanship requirements, but also the suitability of the insulant specified to its installed location.


Building design professionals identify, talk about and specify insulation products most days of every week. As one of that group I know we generally understand the principles under which insulation materials operate and work, and we understand that these products manifest in a variety of ways: in terms of colour, surface finish and texture, core composition and, importantly, performance. Given that the specification of materials that insulate is a science-based decision, we will look at the necessity to understand not only the mathematical performance, but the peripheral factors that can influence the final installation.

Day to day we specify insulation products based upon a minimum requirement under the Building Regulations AD Part L in concert with manufacturers performance data. In order to specify insulation correctly, knowing that it ‘just works’ is not enough. We need to better understand the reasons why it works, and apply the correct technology to any given construction detail. In understanding more fully the processes that make insulation work, and indeed the factors that stop it from working so well, or not at all, we will be in a far stronger position to specify the correct material for the correct application, and provide the best performance set against other issues that relate to sustainability, cost, and location within the building fabric. The corollary of which will always be a compromise, but if assessed correctly and thoroughly by the specifier, it will result in the ‘best fit’ solution for any given situation, remembering no two situations will ever be the same.

It is the performance issue that is so easy for design professionals to get hung up on. After all why would we specify a mineral wool insulation, when a polyurethane foam will give the same insulation performance for around half the thickness? It is a question so often asked, but never really answered in full. This paper aims to explode some myths, and provide sufficient underlying detail to enable deduction of the particular answer that applies to a variable question. The variable will relate to the exact circumstances that apply to any given construction detail: in consideration of partner materials, location and position within the building envelope and building type. In other words we should find that there is not a one-size-fits-all solution for all insulation problems.


Today’s built environment specifiers are working under a new set of rules. The rules have been around for a decade or two, but remain largely unwritten. They nonetheless pressurise the decision-making process in the technical design of our buildings. We are of course referring to those pressures to be more green, to engineer a lower carbon environment and, overall, to move towards greater sustainability. No form of construction is truly sustainable, particularly when we begin to introduce materials that are subject to complex manufacturing processes, that either have a high embodied energy, and/or incorporate materials that are the product of an increasingly finite resource. So the emphasis is on ‘more’ or ‘greater’ sustainability. True sustainability can only be found at the end of the rainbow alongside that crock of gold.

To be fair to the insulation manufacturers, the larger ones at least, have put significant measures in place to reduce reliance upon raw materials, recycling pre- and post-manufacture, to reduce packaging and ensure that what packaging is used remains recyclable, and to reduce energy use in production and transport, and have zero waste to landfill policies. That not only bolsters good industry and public relations, but such an investment will also return premium dividends on long term operational costs. So lets not kid ourselves, it’s great news for the balance sheet too.

We generally find that the manufacturers sing with one voice with regard to their marketing rhetoric. This asserts that their insulation products will save far more energy / carbon, over the installation lifetime than it has cost to manufacture. An interesting argument which we might as well leave with the academics to work through, because as an industry we have no choice as to whether we insulate or not. The greatest conundrum lies with calculation of the ‘best fit’ for any given design jigsaw.

Installation vs Performance

In the pressured world of building design it is so often the case that we must look to performance to be top of our specification list. At the same time we must also acknowledge that performance measures are usually obtained from the insulant manufacturers figures that are wholly based upon lab test results. Such results are accepted across the board — by building designers and the legislative bodies such as building control authorities. But the clue is in the name — ‘Lab test’. So not an on-site test then? Something that actually has real value? In reality of course no two ‘on- site’ situations will provide exactly the same conditions, so testing can only be carried out to provide a comparison between different insulation products, using exactly the same conditions. As a result the only way manufacturers can demonstrate and illustrate performance is by the production of sales and technical literature describing the perfect installation, where joints are perfectly made, insulation is uniformly continuous, and all tolerances are millimetre perfect. Anyone who has ever been on a building site will know this cannot happen, and indeed if the truth be known, the manufacturers know it too. But, with a modicum of justification they will hold their hands up and say ‘What else can we do?’

We can take some comfort in the implementation of Green Deal assessments. The diktat here is to adhere rigidly to the ‘golden rule’. This says that the cost of the energy saving measures proposed must not exceed the projected savings made by the resulting use of less energy. In practice, in order to put in place a metaphoric buffer, Green Deal Assessors (GDAs) are adopting a very conservative line on projected savings. So much so that projected savings involving insulation will use calculations at 75 per cent of the manufacturer’s performance data.

On-site installation and performance will improve over time, as a result of training and education at site level. It is part of the bigger picture that relates to the contribution made by workmanship to the overall success or otherwise of construction projects in general. Contractors who do not make a considerable effort in this direction will not survive. The caveat being that this will not happen next week, next month and maybe not next year. Just incrementally over time.

The effect of water

Insulation and water do not mix. All insulation product types will be affected within a range from negligible, e.g. extruded polystyrene (XPS), to severely compromised, e.g. wool insulants. The degree of compromise will be related to the degree of contamination. So any environment where water vapour can exist without threat of rapid and total evaporation, or the presence of physical water droplets themselves, will reduce insulation performance. Once installed into the matrix of the insulant, water will conduct the energy that the insulation is trying to contain. The larger the water droplet, the greater the conduction.

If we take glass wool installed into a full fill designed cavity wall. if one of the masonry cavity sides has been exposed to rain immediately prior to installation of the insulant, there will be an immediate reduction in the potential insulation performance of the completed cavity wall. If the insulation itself has been allowed to become wet through, performance may well become negative.

The presence of water will almost certainly guarantee a significant performance reduction that is infinitely variable and somewhat proportionate to the degree of water contamination.

These are of course workmanship issues that may never go away due to their direct correlation to the ‘human factor’. The science associated with construction will always be threatened by compromise from issues relating to being behind on programme, late delivery of materials, or on-site labour variables. As much as we would like to, as building designers we cannot control every step in the process.

The detrimental affect of water on insulation starts out on the premise that water is always looking to change state, in that it is trying to evaporate. The misconceptions begin when we casually assume that water will only evaporate when it boils: not so. The next section will deal with the subject of air movement more fully, but we will acknowledge here that thermodynamics rule the performance of insulation and heat retention and/or distribution of heat throughout any given environment. Generally speaking the interaction of water, water vapour and moving air are the key players in insulation performance.

Water, water droplets, down to molecular level, contain energy. The energy can be kinetic, if the water molecules are in motion, but ever mindful of that old maxim that ‘energy cannot be created or destroyed’, ultimately that energy will manifest itself as heat. The heat will act as the catalyst for evaporation. The heat is ‘latent heat’. Latent means ‘hidden’, and of course parceled up in a water droplet this speck of heat is well hidden. However, the cumulative affect of tens of millions of droplets over time is a significant contributor to heat loss. Add to this ‘scale’, and that will provide an infinite variable that will defy absolute control.

Insulation performance / Heat transfer

For purpose of simple explanation, insulation products are designed to frustrate the transfer of heat across the material itself. There are only three methods of heat transfer that act in this instance: Radiation, Conduction and Convection.

Radiation: radiant heat can only travel in straight lines. Introduce a solid object between points A and B, and point B will no longer receive the radiant heat as the heat will have stopped as it encountered the new object. Radiant energy is also the only form of heat transfer that can occur across a vacuum. Any object whose temperature is higher than the ambient surround will release energy as radiant heat. Air molecules do not stay still long enough to be significantly affected by this energy, which is only ultimately received and absorbed by solid objects, i.e., not a gas. Water, while still a fluid, is affected to a greater extent than gas, although it still lags some way behind solids.

Conduction: is a process of energy transfer, of which heat production is the by-product. The process is inherently reliant upon physical contact. If there is no contact conduction cannot take place. Contact between two materials of different temperature causes the molecules within the cooler to become excited and jostle/move around. The greater the temperature differential, the faster the molecular movement. This movement equates to kinetic energy, but the kinetic energy will ultimately convert to heat. The greater the kinetic energy the more heat that will be produced as a consequence. However, conduction does not provide for 100 per cent energy transfer, as as an object gets warmer there will always be some heat dispersed through radiation and convection.

Convection: the transfer of energy via fluids (gases and liquids). It is this method that plays the greatest role in the liberation and transfer of heat in buildings. The most common propagation of this effect is from solid to gas, i.e. object to air, and then back again, as the air meets with the insulated external building fabric. However, typically it is not quite as simple as that.

The process is initiated by an energy transfer due to conduction, and complicated by the level of water vapour (for water vapour read water at its molecular level) that is supported by the air. The water molecules contain heat given to them through conduction from the warm surface from whence they came. This could have been a damp cloth on a radiator, a boiling kettle, or expelled breath. The water vapour and the air cannot be separated as gases. They will only part company when the saturated vapour pressure is reached, i.e. the quantity of water (albeit in vapour form) exceeds the level of heat available to maintain it as a gas (vapour), and therefore condenses. Condensation causes this latent heat to be released; the temperature to water vapour ratio alters, and once it has altered far enough the process will start again. This is an inherent micro-climate, and is not peculiar to buildings, artificial heat, or insulation. The world’s weather systems follow a very similar cycle.

If we could keep air still and dry it would perform as a highly efficient insulant. However, the laws of thermodynamics confirm that if air is heated, its molecular structure expands and becomes less dense relative to the air surrounding it, and so rises. As it progresses further from the heat source that caused it to rise, it begins to cool. The molecules contract and increase in density and sink back down. But it does not end there; air molecules are in a constant state of flux, dependent on the ambient temperature, and interference from any point, or background heat sources. This process of heat transfer is otherwise known as ‘convection’, but then as we have previously said, water vapour will compromise the simple performance of air as a heat transfer mechanism. Air will cool at a rate dependent upon the amount of water vapour saturation. The greater the saturation, the slower the cooling.







Thermal conductivity, thermal resistance and U-values

Insulation materials serve, in their own individual ways, to limit the flow of energy (heat) between two bodies that are not at the same temperature — fluid or solid. No single, or combination of insulants can stop this release of energy totally. But greater insulation performance is directly attributable to the thermal conductivity of the insulant. That is the rate at which a fixed amount of energy has been tested (under lab conditions) to transfer across a known thickness of the material. The direct inverse (reciprocal) of this measure is the material’s thermal resistance, which measures the material’s ability to resist the transfer of heat.

Thermal conductivity: often referred to as the ‘K’ or ‘λ’ (lambda) value is a constant for any given material, and is measured in W/mK (watts per kelvin meter) — the higher the λ value, the better the thermal conductivity. So for good insulators we are looking for as low a value as possible. Any material will have an ‘insulation value’, or thermal conductivity, for example: steel and concrete will have very high thermal conductivity and therefore very low thermal resistance. This makes them poor insulators, but on the other hand they make great ‘radiators’ of heat.

The λ value for any material will become higher with an increase in temperature. Although the temperature increase will need to be fairly significant for this to occur, and the temperature variants in most buildings are generally within the tolerances that would render any change in the lambda value negligible.

Thermal resistance: referred to as the ‘R’ value of a material, thermal resistance is a calculable product of both the thermal conductivity and the material’s thickness. The R-value is calculated from the thickness of the material divided by its thermal conductivity and expressed in the units m2K/W (square metre kelvins per watt). The greater the material thickness, the greater the thermal resistance of that material will be.

U-value: in construction terms, while a U-value may be calculated and attributed to a single thickness of any insulant (see the above table), or other material; it is more usual to calculate the U-value as a product resulting from the assembly of different materials in any given form of construction. It is a measure of the transmission of heat through a pre-determined area of the building fabric — this being 1m2. The unit measurements are therefore W/m2K (watts per square metre kelvin) and describes the heat transfer, in watts, through a square metre of building element (nominally wall, floor or roof) multiplied by the temperature difference on each side of the single material, or composite building element. This is used to calculate the heat transfer, or loss, through the building fabric. For example, if a wall had a U-value of 1W/m2K — with a temperature differential of 10°, there would be a heat loss of 10 watts for every square metre of wall area.

The larger insulation manufacturers produce huge amounts of technical and promotional literature incorporating a vast range of figures that can be confusing, largely because not all present their performance in the same way and, of course, the marketers have their input with an ever increasing diversity of branding and methods of product promotion. Insulants are primarily presented based upon performance. However, confusion reigns when there appears to be no consensus on ‘thermal conductivity’, or ‘thermal resistance’. One manufacturer may present one or the other, while another may cite both. But then thermal resistance results must then be related to a range of material thicknesses, which just means more numbers to look at.

While the manufacturers focus on product performance, most gloss over, or ignore altogether other key issues that can directly affect performance. As we have already discussed, without exception (so including XPS to some degree) water/moisture have a huge affect upon performance. Hence the specification of the correct insulation product within building areas that are likely to generate a cold and potentially damp environment, for example, under-floor voids. Any drop in insulant performance is immeasurable once construction of that element is complete and covered up, although lab tests may confirm the magnitude of performance loss relative to a given level of moisture contamination. As ever, site conditions cannot be recreated exactly, therefore building designers and specifiers must apply experienced judgment in any given site application.

How Insulation Works

We have previously discussed that insulation materials seek to limit all forms of heat transfer. Once the principles are understood it is reasonably straightforward to apply them to the various materials, and thereafter to understand why some materials ‘perform’ better than others. With premium performance comes the greatest cost. The lowest performing products will usually be the least expensive.

The main heat loss vehicle follows the ‘conduction and convection’ route. These two modes of heat transfer predominate in all insulation performance. Limiting air movement to something south of negligible is the initial aim.

Open cell products (e.g. mineral and sheep’s wool/expanded polystyrene (EPS))

There is no difference in the way that glass, rock and sheep’s wool products carry out the task of insulation. There is a partial misconception that this insulant traps air within its core. Air movement is minimised within the core, but it is not trapped as in unable to go anywhere.

The graphic below shows a sectional core image of a typical glass wool product overlaid with a representation of the millions upon millions (per square metres) of ‘open cell’ air pockets that are created during manufacture. At the same time as the manufacturing process forces air into the core of the glass fibres, a previously introduced binding agent is activated to form a matrix locking the composition together. This produces the ‘spring loading’ that is associated with mineral wool insulation, allowing it to regain its shape and thickness after compression.

The open cell nature of the matrix will allow air migration through its core, but the route is tortuous and so heat loss due to convection is minimal. The principle in operation is the formation of such small air pockets that air movement is brought to a virtual but not complete stop. In actuality each air pocket, or cell, is physically linked to its neighbours. This allows air to migrate between cells, hampered only by the glass fibre matrix that forms the pocketed construction.

A material will only be able to radiate heat that it is able to absorb. The glass strands and their binder are poor heat conductors. So heat loss via radiation is deemed to be negligible.

Dry air is a good insulation gas. So with open cell products, if we can prevent contamination of the core air by water vapour (using vapour control barriers), the ultra small air pockets will significantly limit air movement. The air contained within the pocketed matrix of the wool is gradually warmed by the heated air from the building through an initial heat transfer process of conduction, as the warm air comes into contact with the glass fibres, supplemented by convection, as the pocketed air is forced to move as warming alters its density.

Closed cell products (extruded polystyrene/chemical foam-boards)

The principles of insulation are not different across varying material technologies. All we are trying to do is prevent heat transfer for as long as possible, ever mindful of the potential side effects this may cause, due to the increased potential for the build-up of water vapour to ultimately condense unseen. This can be controlled to a significant extent by implementation of effective ventilation and vapour control membranes.

Closed cell technology utilises the controlled introduction of gases (blowing agents) during manufacture that can form a much more dense matrix of individual cells than we might equate to glass wool or EPS. The cells are formed as bubbles of the gas whose thermal conductivity is significantly less than that of air. Combine this with the inability of water vapour to readily contaminate the cells,* and this provides for a significantly higher performing insulant.

*The matrix of some chemical foam insulants may be susceptible to break-down over time by the presence of water, or water vapour.

The cell walls are extremely thin which limits conduction, but are gas tight. The dense cellular composition further limits the potential for gas movement, as it may only move within the confines of its containing cell, and not between cells. So as with open cell materials, the process of heat transfer from warm to cool sides is affected by a combination of conduction via the cell walls and limited convection via the cell gas.

Each cell is responsible for minuscule amounts of energy that would not be measurable on an individual basis. The material’s efficiency is therefore very high and effective over the area of an unbroken board. The installation performance is always significantly reduced by poor workmanship in board cutting and jointing.

In an effort to improve long-term performance, manufacturers face foam-board products in particular, with a shiny foil layer. This acts to minimise contamination by water vapour by acting as a vapour barrier, while also reflecting radiant energy back into the building. Taping of foil-faced board using a foil tape can improve vapour control, although it will have little impact upon a poorly constructed joint that is not consistently tight.


In the same way that water will always try and find its own level, heat will always seek to find a temperature equilibrium. Insulation materials are reliant upon their inherent properties due to their molecular make-up, to minimise the three forms of heat transfer — radiation, conduction and convection.

In reality the greatest quantities of building heat losses are consequential to air movement. Any moving body of air will extract heat from an object or surface over which it passes. The heat loss is proportional to the speed of the moving air, the amount of water present and the temperature differential between heat source and air. The faster the air movement over a heat source, the faster the heat transfer occurs. The presence of water droplets will act as an accelerant to this process, although control over water vapour saturation will usually need to be exercised to avoid problems caused by uncontrolled condensation.

Condensation may be controlled to a large extent by ensuring the water vapour in the air is contained within the warm internal environment created. Vapour control layers on the warm side of the insulation, effectively sealing the envelope to air migration between warm and cooler zones are the theoretical solution. Current materials technology and carefully monitored workmanship in assembling those materials, can achieve near zero air leakage through the insulated envelope, and indeed Passivhaus design is reliant upon this, while using controlled ventilation to remove contaminated air: design principles that are reliant upon workmanship in order to succeed.

Addressing the cellular construction of dedicated insulation materials, the intrinsic aim is to prevent the movement of gases within the insulation core matrix, in doing so the loss of heat consequential to that movement will also be reduced.

Although ‘open cell’ insulation products, such as wool all have a much greater migration of air across them that limit their performance. Their flexible construction gives a far greater advantage in terms of quality control of installation workmanship. Due to the nature of the material, jointing produces a very similar result to the material itself. Whereas all rigid board products carry an onerous installation premium penalty which requires manufacturer’s ‘lab test’ precision for jointing, which is so often seemingly unachievable on site.

Current affordable technology offers up a choice between open and closed cell matrices for insulation products that physically manifest in a variety of ways. Insulation materials with a more dense, self-contained cellular composition, will provide a lower thermal conductivity (λ value) and so a higher thermal resistivity (R value) to out-perform ‘open cell’ materials, which rely on maintaining dry air within their cores for ultimate performance.

There are open cell foamed products available that due to their core matrix composition have a higher thermal conductivity than their closed cell cousins, but have advantages with greater flexibility to accommodate building movement, and of course any deterioration of cell walls will not result in the liberation of the gas content.

In specifying insulation products the building designer should acknowledge the potential for water contamination, and the possibility of gas migration within the core matrix and the resulting compromise in performance, that could deteriorate further over the lifetime of the building, unseen and unchecked.

There are better performing technologies on the market with ‘aerogels’ and ‘evacuated panels’, but that performance is reliant upon the same principles of limiting heat transfer and for the time being has a limited specification niche, and remains largely cost prohibitive for the vast majority of applications.

This article was first published in Journal of Building Survey, Appraisal & ValuationSpring 2013, Volume 2, Number 1,  pp 16 – 26, published by Henry Stewart Publications, London.