Thermal insulation for buildings
Insulation products have developed significantly with technological advances. Legislation has acted as the catalyst for development, from the basic requirements under the Building Regulations Part L, to compliance with government carbon reduction targets, driven through advanced programmes such as the Code for Sustainable Homes and BREEAM.
Insulation products vary in terms of colour, surface finish and texture, core composition and, importantly, performance. The specification of materials that insulate is a science-based decision, but a successful specification relies on the specifier understanding not only the mathematical performance, but the peripheral factors that can influence the final installation.
Specification of insulation products is often based upon the minimum requirement of the Building Regulations AD (Approved Document) Part L and their relationship with manufacturers performance data, and it has been suggested that legislation is driving the production of a range of products that 'just work', presenting little apparent difference between them. In order to specify insulation correctly however, the specifier needs to 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, specifiers will be in a far stronger position to specify the correct material for the correct application.
The installed performance of an insulation product is reliant upon not only performance characteristics and the adherence of contractors to manufacturers and general best practice workmanship requirements, but also the suitability of the insulant specified to its installed location.
 How insulation works
Any object whose temperature is higher than the surfaces that surround it will lose energy as a net radiant exchange. Radiant heat can only travel in straight lines. Introduce a solid object between points A and B, and they will no longer directly exchange radiant heat. Radiation is the only heat transfer mechanism that crosses vacuums.
Conduction is reliant upon physical contact. If there is no contact, conduction cannot take place. Contact between two substances of different temperature results in a heat exchange from the higher temperature to the lower temperature substance. The greater the temperature differential, the faster the heat exchange.
Convection is 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, typically as the air meets with the external building fabric.
The process is actually initiated by an energy transfer due to conduction, and is complicated by the level of water vapour that is supported by the air. The water molecules store heat given to them through conduction from warm surfaces. 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 it 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. The world's weather systems follow a very similar cycle.
If air could be kept still and dry it would perform as a highly efficient insulant. However, 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, it begins to cool. The molecules contract and increase in density and sink back down. 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 'convection' is complicated by the fact that air will cool at a rate dependent upon the amount of water vapour saturation. The greater the saturation, the slower the cooling.
Insulation materials limit the flow of energy (heat) between two bodies that are not at the same temperature. Greater insulation performance is directly attributable to the thermal conductivity of the insulant. That is, the rate at which a fixed amount of energy transfers 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. Good insulators will have as low a value as possible. Steel and concrete have very high thermal conductivity and therefore very low thermal resistance. This makes them poor insulators.
The λ value for any material will become higher with an increase in temperature. Although the temperature increase will need to be 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
Thermal resistance, referred to as the 'R' value of a material, is a product of thermal conductivity and 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.
In construction terms, while a U-value may be calculated and attributed to a single thickness of any material, it is more usual to calculate it 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 describe the heat transfer, in watts, through a square metre of a building element (such as a wall, floor or roof). 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.
 Open cell products
Open cell insulation includes products such as mineral and sheep's wool insulation. Expanded polystyrene (EPS) insulants are technically 'closed cell' in their structure, but their performance is akin to an open cell material due to the linking across the structure of the air pockets that surround the blown cell beads that are the essence of its composition.
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 metre) 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.
Dry air is a good insulation gas. So with open cell products, if contamination of the core air by water vapour can be prevented (using vapour control barriers), the ultra small air pockets will significantly limit air movement.
 Closed cell products
Closed cell insulants include products such as extruded polystyrene and chemical foam-boards. Closed cell technology utilises the controlled introduction of gases (blowing agents) during manufacture that form a much more dense matrix of individual cells than 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. (NB 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.
The material's efficiency is very high and effective over the area of an unbroken board, but is 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.
 Installation vs performance
Insulation manufacturers produce technical and promotional literature incorporating a vast range of figures that can be confusing, and not all manufacturers present their performance in the same way.
However, this is not the same as an on-site test. 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 manufacturers illustrate performance in sales and technical literature by describing the perfect installation, where joints are perfectly made, insulation is uniformly continuous, and all tolerances are millimetre perfect. Anyone who has been on a building site will know this does not reflect reality.
To this end specifiers may take note of the implementation of Green Deal assessments. The diktat here is to adhere to the 'golden rule' 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 make sure of this, Green Deal Assessors (GDAs) are adopting a very conservative line on projected savings, and projected savings involving insulation use calculations at 75 per cent of the manufacturer's performance data.
In addition, while the manufacturers focus on product performance, they can gloss over other key issues that directly affect performance, such as 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.
Insulation and water do not mix. All insulation product types will be affected within a range from negligible, (such as extruded polystyrene (XPS)), to severely compromised (such as 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 within 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.
So, for example, where glass wool is installed into a full-fill cavity wall, if one of the masonry cavity sides has been exposed to rain immediately prior to installation of the insulant, there will be a reduction in the potential insulation performance of the completed cavity wall. If the insulation has been allowed to become wet through, performance may well become negative.
Today's built environment specifiers are under increasing pressure; to be more green, to engineer a lower carbon environment and to move towards greater sustainability. The larger insulation manufacturers have put significant measures in place to;
- Reduce reliance upon raw materials.
- Recycle pre and post-manufacture.
- Reduce packaging and ensure packaging remains recyclable.
- Reduce energy use in production and transport.
- Have zero waste to landfill policies.
Insulation materials are reliant upon their inherent molecular make-up, to minimise the three forms of heat transfer — radiation, conduction and convection. The greatest building heat losses are from 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 condensation.
Condensation may be controlled to a large extent by ensuring water vapour in the air is contained within the warm internal environment. 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 allow much greater migration of air across them, and this limits 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 rigid board products carry an onerous installation premium penalty to achieve manufacturer's 'lab test' precision standards of jointing.
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 any deterioration of cell walls will not result in the liberation of the gas content.
In specifying insulation products the building designer should consider 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 performance is reliant upon the same principles of heat transfer, and for the time being has a limited specification niche, remaining largely cost prohibitive for the vast majority of applications.
This article was originally authored by Mark Wilson MCIAT, with copyright transferred to Henry Stewart Publications for the purposes of publishing. It was the winner of our Chartered Institute of Building-backed article competition in June 2013.
A longer version of the article was first published in Journal of Building Survey, Appraisal & Valuation, Volume 2 Number 1, April 2013, published by Henry Stewart Publications, London.
 Related articles on Designing Buildings Wiki
- Accredited construction details ACDs.
- Approved document D.
- Aerogel insulation for buildings.
- Building fabric.
- Building regulations.
- Cavity wall insulation
- Code for sustainable homes.
- Conventions for U-value calculations (2006 edition) BR 443.
- Heat transfer.
- Interstitial condensation.
- PA ratio.
- Phase change materials.
- Polyurethane spray foam in structurally insulated panels and composite structures.
- Solid wall insulation.
- Sound insulation.
- Thermal bridge.
- Thermal comfort.
- Transparent insulation.
- U-value conventions in practice: Worked examples using BR 443.
 External references
- Welsh Government: External Solid Wall Insulation: A Planning Guide for Householders. October 2013.
- Rix Petroleum: Energy Saving Guide: Reducing your carbon footprint around the house. October 2013
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