What’s the difference between buying XPS by the m2 rather than m3?
In answering this question, we should start by taking a broader look at the history of how £/m3 came about, and the many subsequent options that are now available in the UK Inverted Insulation market today.
Originally only Dow Construction Products supplied the XPS inverted market in the UK: Roofmate SL. A bulk volume producer, Dow thought in m3, manufactured in m3 and sold m3. As the only XPS on the market, the Lambda value was a given, the thickness required to achieve a U-value straightforward and working out a price per m2 based on the m3 simple.
Now jump forward a few years. New players enter the market with new products.Polyfoam and Kingspan offer an XPS and Jablite introduce an EPS solution. These are all the same as Roofmate SL aren’t they? No, they are not!
Each has a different Lambda value, meaning the insulation thickness for a given U-value is different. Some even achieve different declared lambda values dependent on thickness, with the thinner boards being more thermally efficient than the thicker ones; creating a need to take great care when comparing manufacturers, products and thickness.
Then along comes Climate Change Legislation. This led to the withdrawal of products such as Roofmate SLX, Kingspan H350, Efyos Artic, etc. all made using HCFC or CFC blowing agents, which are more thermally efficient but damage the Planet. Hence them being banned from January 2020.
Today, most inverted roof insulation is manufactured using more Climate friendly gases such as C02, but still, have wildly different thermal efficiencies in their finished product.
So now we have a market with numerous manufacturers, differing blowing technologies, various thicknesses, and all competing to achieve a given U-value. This is why buying inverted roof insulation in £/m3 is not the most competitive solution for the roofing contractor, especially if manufacturer A needs to supply 2 layers of insulation to achieve a U-value, whilst manufacturer B can supply a single board thickness to achieve the U-value. Here, only considering the £/m3 price would completely ignore the additional installation costs of a 2-layer system.
For example, if we look at 0.15W/m2K U-value, the most common current U-value requirement, an XPS with a design lambda of 0.038W/mK will need to be 250mm thick and cost circa £120/m3 or £30/m2. An XPS insulation with a design lambda of 0.028W/mK will look more expensive with a £150/m3 price, but because it only needs to be 175mm thick the m2 rate would be £26.25m2. What’s more, the 0.028W/mK lambda insulation will be a single layer installation.
Would a £3.75m2 saving help you win a project? If it would, think twice before buying insulation by the m3.
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