If the only tool you have is a hammer then every problem begins to look like a nail

 What has the following analogy about bread got to do with hammers or even the comfort of your home? Well it’s all to do with choice, read on....

White breadImagine when you went to the supermarket all you saw in the bread isle was white sliced bread and white bread rolls from a range of manufacturers, that’s what you would buy. In this isle all the point of sale marketing reinforces the packaging message of ‘how good and nutritious the bread is’ accompanied by a dazzling array of deals and offers, the packaging carries logos and badges letting you know they are safe, assured and approved products. Also advertised are a wide range of recipes and different ways to be creative with bread to show how versatile and beneficial white bread is for your family’s health.

Imagine this is all you ever experience until one day by accident you stumble across a new isle with granary bread, wholemeal bread, artisan loaves, brown bread, fruit bread, gluten free bread, sourdough, pitta bread, crumpets....... in fact a wealth of new and exciting breads, why did no one tell you this was here? This is all good stuff. Now we all like a bit of good old British sliced white (nothing better for a bacon butty) but it’s not best for everything, this is all about balance and choice.

This simple analogy has a parallel in how we choose insulation, it’s about perceived choice and our ability to make informed decisions, we need real choice and we need to be equipped to compare on a like for like basis, at the moment most British builders, self builders and professionals never leave the white bread isle, you have to ask yourself why. The most common and widely sold insulation products in the UK are Mineral wool, Polyurethane rigid boards (PIR and PUR) and Polystyrene foams making up over 95% of the £900 million (approx) annual total UK insulation sales the domestic market is worth around 20% of this and of that by far the largest area of use is the retrofit market accounting for around 55%. [1]

Badly fitted mineral wool compressed Badly fitted mineral wool insulation

To pursue our bread analogy further there is another issue and that is Building Regulations. Now I must be clear it’s not the regulations themselves that cause an issue it’s their interpretation. In the bread analogy this would be similar to all dietary advice on daily recommended allowances being interpreted as the pinnacle of well being and health, so as long as the calories, fat, sugar, salt etc is not exceeded then that’s fine even if you do it by eating mainly one thing. Now we would all agree that that is not healthy eating. A similar approach can be seen in the insulation market but this time substitute dietary advice with building regulations. Building regulations are a minimum standard, a national safety net to ensure we don’t institutionalise and deliver poor building practices or unsafe working: what they are not is a quality standard. How many times have you heard that someone is ‘building to Regs’ often  manufacturer's promotional literature will have a section advising users on how to ‘reach’ Regs (allowing people to think this is the zenith of building achievement) and getting there was all you need to do. Lurking here is the triple whammy of a falsely limited choice, regulations being interpreted as the target for quality and performance and the use of limited metrics to measure compliance (this refers to U values and that’s another whole story)...it’s no surprise that in terms of building quality and performance we are not in the premier league.

Back to insulation: it is wise to bear in mind two things when choosing insulation first, published data on thermal performance is only applicable if the material is installed perfectly (the data is calculated using lab tests) and second just because it’s saving energy doesn’t make it necessarily sustainable. Of the two most common insulation materials mineral wool is most commonly used in lay flat scenarios and although can be misused and poorly fitted the issues are not as serious as the consequences of misusing rigid boards where the range of applications is huge, one of the biggest and well known manufacturers identifies 31 applications for a house; no wonder builders assume that it is some form of panacea.

Fitting and performance issues are particularly true of elements such as studs and windows which provide a parallel heat conduction path unaccounted for in the published performance values and in any house this is where much of the installed insulation will end up.


There is plenty of evidence to support the claim that there is a significant problem in the UK between the designed and as-built energy performance of buildings especially new homes[2]. What’s known as The Performance Gap. Researchers’ found that sample results revealed that measured whole building U-values were commonly recorded at over 1.6 times greater than that predicted[3] and in some reported cases there was an astonishingly wide gap of over 100%[4]. Poorly fitted or inappropriate insulation choice is undoubtedly part of the problem. The picture left clearly shows how poorly fitted insulation can leave gaps. Choosing the right insulation for the job is therefore vital in ensuring that designed performance can be met.

Flex2Using rigid boards between studs will never be as effective as a friction fitted batt - see
picture to the right.  Coupled with the shrinkage (inherent in rigid foam boards see appendices) the consequences for the thermal performance of the building longer term are obvious. Like the bread scenario this is not to say that one type of insulation is better than another rather that different types of insulation have different strengths and often a combination of materials chosen for their performance, ease of installation, breathability or thermal mass will provide a better outcome than trying to adapt one type of insulation to all circumstances. There is a dreadful tendency in this country to simply default to a very narrow selection of materials probably because they are so easy to buy and the alternatives are not.

snowy-housePerhaps a good example would be insulation in a roof. The roof of a building is a solar collector and it is also at the top of a building and so collects rising internal heat. Obviously keeping heat you have paid for in in the colder months is a sensible precaution but what about keeping heat out in the warmer months too? Part of this is to do with features not normally accounted for in the UK such as thermal mass, and phase shift; features such as this are acknowledged as a vital part of the decision making process when designing buildings, and we in the UK need to get better at this according to the RIBA Enterprises NBS[5]. If the only metric by which you choose insulation is the U value and not its handleability (if that’s a word) density (phase shift potential) , breathability or durability then don’t be surprised when it fails. With the roof example phase shift should be a key determining factor, for a new build the target U value set within Building Regulations is 0.11 W/m2K.
m_0071_Refurb_special_311Using wood fibre insulation boards will increase the phase shift  because the density is high, the U value of a typical wood fibre board is 0.38Km2K about the same as mineral wool and only marginally worse than a rigid foam, but the roof manages heat more efficiently moderating the internal environment and reducing the need for mechanical control, the structure is also vapour open so reducing the likelihood of damage or deterioration (mould) due to damp.

Using a 220mm insulated joist, wood fibre insulation between and a 22mm external wood fibre sarking board phase shift of over 12 hours can be achieved, other combinations can provide phase shift of over 24 hours, to see the full range of options please download our guide the "Steico Pitched Roof U Value & Phase Shift Calculator" by clicking the link then go to the "Technical Info" tab to download the Pdf.

Conclusions....and 10 reasons why change is slow

The answers are simple when publically available information on designing for performance, durability and health published by independent researchers, regulatory and professional trade bodies are considered they are....

  1. Insulation is not usually chosen by considering all the performance benefits, so performance failure is more or less guaranteed in the areas where demonstrable and proven better options are not deployed.
  2. The near universal availability of some insulation products masks choice and alternatives, it effectively trumps all other solutions.
  3. We do not choose the best insulation for each application – most buildings require a mix of insulation types chosen for their ability to perform over a range of performance criteria so for example you would expect PIR to be best in the sub floor and cavities whereas wood fibre will be best in roofs and stud walls. This is not normal practice.
  4. The existence the ‘Performance Gap’ is symptomatic of a malaise, but not one that we can address by doing more of the same.
  5. Alternative insulation materials like natural insulation are considered left field and peripheral by many in the trade even though they have many qualities required in certain applications a proven track record elsewhere and carry UK and EU accreditations of equal value to many leading synthetic insulation materials
  6. Changing from the readily available and cheap solutions to anything new is considered too expensive; alternatives are often discounted solely on the basis of price.
  7. We are not ready to sacrifice small amounts of space (<100mm) to better performance. We would rather thin and ineffective than thicker and effective.
  8. We are not good at assessing value based on performance
  9. The big insulation manufacturers dominate the market in terms of supply chain and messaging – no surprise that of the two insulation materials highlighted in this article both have brand names that are now generic terms for insulation like ‘Hoover’ for a vacuum cleaner. This suggests implacable market dominance.
  10. We don’t like change.


  • Limitations

Effectiveness of the insulation of a building envelope can be compromised by gaps resulting from shrinkage of individual panels. Manufacturing criteria require that shrinkage be limited to less than 1% (previously 2%). Even when shrinkage is limited to substantially less than this limit, the resulting gaps around the perimeter of each panel can reduce insulation effectiveness, especially if the panels are assumed to provide a vapour/infiltration barrier. Multiple layers with staggered joints, ship lapped or tongue & groove joints greatly reduce these problems.

Insulation performance can be reduced over the first 3 years due to gas exchange from the cells. This reduction of performance is usually included in the manufacturers declared value of thermal conductivity

R-Values (Resistance to Heat Flow) of polyisocyanurate will drop as some of the gas that has low thermal conductivity escapes and is replaced with air. This thermal drift occurs within the first two years after installation. An R-value of R-9 per inch will drop to an insulation value of R-7 and then remain unchanged unless the material is damaged. Typical R values of PIR insulation range from R-5.6 to R-8.

  • Some background on PUR & PIR

The first commercial applications for polyurethanes were developed in the middle of the 20th century. Since then they have been finding use in an ever-increasing number of applications in industries such as construction, automotive, refrigeration, furniture and footwear. Polyurethanes are extremely versatile and are found all around us in many everyday items.

Rigid PUR insulation products are made by reacting a liquid polyol component with a liquid polymeric isocyanate, Methylene Diphenyl di-Isocyanate (MDI), component in the presence of a blowing agent and other additives. The mixed components then react exothermally to form a rigid thermosetting polymer and since the blowing agent evaporates during this exothermic reaction a rigid closed cell low density insulation product is created. Excellent insulation is achieved because the gas trapped within the closed cell structure has a very low thermal conductivity and there is minimal heat conduction through the solid cell walls due to the low density, where approximately 97% of the volume of the foam is trapped gas.

Rigid PIR differs from PUR in that it is produced using an excess of the MDI component. In the presence of an appropriate catalyst the excess MDI reacts with itself to form isocyanurate which is characterised by greater heat stability. The resultant PIR insulation products exhibit increased fire performance and reduced combustibility and higher working temperature limits compared to PUR and when incorporated into building products, can meet some of the most demanding fire performance requirements such as those currently stipulated for some applications by the insurance industry.

  • What is PIR

Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is a thermo set plastic typically produced as a foam and used as rigid thermal insulation. Its chemistry is similar to polyurethane (PUR) except that the proportion of methylene diphenyl diisocyanate (MDI) is higher and a polyester-derived polyol is used in the reaction instead of a polyetherpolyol. Catalysts and additives used in PIR formulations also differ from those used in PUR

[1] AMA Building insulation products market report - UK 2013-2017 analysis

[2] Closing the Gap Between Design and As-built Performance: Evidence Review Report: Zero Carbon Hub 2014

[3] Bridging the domestic building fabric performance gap: D. Johnston, D. Farmer, M. Brooke-Peat & D. Miles-Shenton Dec2014 Centre for the Built Environment, Leeds Sustainability Institute, Leeds Beckett University

[4] Zero Carbon Hub Carbon Compliance for Tomorrow’s New Homes: A Review of the Modelling Tool and Assumptions. Topic 4: Closing the Gap between Designed and Built Performance. August 2010

[5] http://www.thenbs.com/topics/constructionproducts/articles/what-is-a-u-value_heat-loss-thermal-mass-and-online-calculators-explained.asp

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