Is this a typical example of how we can miss a trick through fear of thinking something is new?

This is so easy; follow the steps below to lay a structural insulated floor suitable for any surface finish.

  1. Dig out the floor area to the required depth
  2. Pour in and compact aggregate
  3. Blind with sand
  4. Cast concrete slab
  5. Overlay sheets of foam insulation
  6. Overlay with polythene sheet
  7. Apply top wearing screed then
  8. Tile and grout or use chosen floor finish

That sounds simple enough and it is in fact a very common way to install a floor and no one would be phased by it, now read it again but with a few added bits of information to give a more complete picture

  1. Dig out the floor area to 415mm
  2. Line with a 140gsm non woven geotextile membrane
  3. Pour in and compact 460mm loose volume of Technopor foamed glass structural insulating aggregate, compact in 3 passes to 350mm to achieve U value of 0.22W/m2K at a compressive strength of 270kPa and thermal resistance of l=0.075W/mK
  4. Overlay with DPM*
  5. Cover with a 65mm screed of site mixed binder, FenXA hydraulic lime screed at 1:3 ratio with washed sand 4 – 7mm to compressive strength of 5 N/mm² ≤ fc ≤ 15 N/mm²
  6. Apply top wearing screed of PEDES high resistance hydraulic lime protection mortar in 2 5mm passes to achieve compressive strength of (EN 1015-11) ca. 5 N/mm² – OR
  7. Using PEDES hydraulic lime mortar mix bed down and grout tiles or chosen floor finish

*not required if the floor is to breathe

It’s worth pointing out that both scenarios require the same tools, same mixing equipment, and same skill level, there’s really not much difference…or is there?

The cost of the second option is around 24% cheaper than the first – had the description been for block and beam it would be around 3% cheaper**. So what’s the difference, apart from the over use of scientific descriptions and terms (see point below), there are only 2 main practical differences….and they are

  1. The first scenario is based Ordinary Portland Cement (OPC) the second on lime, both are bagged products , both are added to the same additional ingredients, sand (aggregate) and water, both are mixed on site and both act as the binder for the pour.
  2. The second scenario combines the insulation with the structural element so avoids having to use a separate layer of insulation


Technopor foamed glass insulation compressed

Technopor made from 100% recycled glass


It’s worth pointing out that scenario 2 also comes with added benefits it does not require reinforcing mesh as the aggregate is the structural element, it is a breathable structure (without the polythene DPM) it is more flexible, contains far fewer salts and free lime and does not require expansion or slip joints even with underfloor heating the floor base is non capillary water cannot be wicked through and is closed cell so will not absorb water.

Why then is scenario 2 not a default option for floors? could this be to do with language and some unexplained resistance to the use of lime? Well maybe, but it appears that there are a number of reasons that could help explain this.

Firstly scenario 2 is new to many and ‘new’ is not as easy to commit to as ‘old’. This is in fact a load of old bunkum and a complete misnomer as lime as a standard building material was all most builders used right up to end of the 19th and early part of the 20th century, so it does have quite a track record and a large number of buildings built with lime are still standing and occupied, a bit more on this later.

Secondly lime (today) has been largely manoeuvred into the artisan world of restoration and heritage where bearded individuals (I can say this as I have a beard) and otherwise well-intentioned practitioners develop their own blends and mixes from which they draw a distinction between themselves and their competitors. This means that confusion reigns as customers are given options that can be anything from a little different to widely different then become paralysed by a choice that they are ill informed to make.

An example of this is the over-technilisation (if that’s a word) used when describing the use of lime, always referring you back to the suppliers technical department for design advice or insisting on the uniqueness of their product. Most modern lime building products are factory made engineered and thoroughly tested products with a full range of accreditations behind them from EN certification to CE marking. Of course installation should be to manufacturer’s instructions but for any good builder this will be easy, the products described above are ‘just add water’ products with exact ratios given for different applications. Unfortunately this is further compounded by an element of the supply chain that subscribes to the ‘myth is margin’ school where wrapping up a straightforward product in complex scientific language and over specific instructions with the ever present fear of catastrophic failure if the advice is not precisely followed manages to marginalise an otherwise versatile and proven low risk building material and increase the price into the bargain!

Thirdly is the lack of mainstream construction material manufacturers who use lime. OPC is the default binder for mortar, concrete, pug, render and for a host of construction products etc. and it is very good at what it does, but that does not mean it is necessarily better than lime. Lime has benefits that are clear and lead to a number of applications where using it would actually be better than OPC. These would be typically for breathable structures, repairing older monolithic walled buildings either brick or masonry or for those wishing to use a more environmentally sensitive method of construction.

Insulated, breathing lime floors offer tremendous possibilities in providing a potential alternative to inappropriate damp proof treatments and the use of concrete flooring in a wide range of building applications.

Breathable lime floor


One common and persistent problem is rising damp in traditional buildings with solid floors, often being exacerbated by the occupants’ choice of floor covering. Usually the chosen solutions – either lifting and relaying flagstones on a damp-proof membrane (DPM) or replacing with a concrete floor – simply divert moisture sideways into the walls, creating a damp problem and the likelihood of salt efflorescence (soft white mould looking substance on the plaster surface) this usually leads to a sequence of remedial treatments comprising injection damp-proof course, installation of wall drains, re-plastering with waterproof render, or dry lining, and so on. The resultant harm to the character and performance of a historic building will be familiar to many. Often a DPM is inserted beneath the floor of an old building and often at the insistence of Building Control. This can result in dampness in the walls of a building that previously suffered from no perceptible damp problem either in its walls or floors. This is not solving damp problems simply moving the problem elsewhere.

The principle of a breathing lime floor is simple. A base layer of loose-fill, coarse, non-permeable structural aggregate is laid on a compacted sub-base, usually simply bare earth with a layer of geotextile on top. When using a product like Technopor (see scenario 2 above) the floor benefits from a material with no capillaries (or micro-pores) and a closed cell structure (each hole is a tiny cul-de-sac), so the layer of aggregate is unable to draw water upward by suction or capillary rise (the normal means by which rising damp moves). It is therefore an effective barrier against rising damp i.e. a DPM.

Floors constructed without a DPM will allow the floor to “breathe” with moisture from beneath the screed being evacuated through the screed as vapour preventing the build up of moisture below the floor and subsequent movement of salt laden water into the walls. Where stone, paving or tiles are then to be laid over the screed the use of a breathable bedding mortar will ensure the performance of the screed is maintained.

It goes without saying that if you are using lime in the floor with a breathable structure other elements of the building should be built sympathetically using the same principles, this means plaster, mortars, renders, paints etc.

The main lessons to draw from this little pop at our construction industry seem to be clear and simple.

  1. Lime works: when included in building materials lime creates versatile and proven products
  2. Lime products are essential components of sensitive (and sympathetic) refurbishments ensuring that the building shell works as one….as designed.
  3. Unlike OPC lime breathes and is more flexible – it also sequesters carbon over its life
  4. Modern lime products are highly engineered, highly tested quality performance products
  5. The way we think we have always done it may not be the way it’s always been done, previous generations of builders did get it right…… not sure? go and look at St Pauls!
  6. It is never as difficult to use these products as you might be led to believe

And finally, because I haven’t mentioned it yet, lime products offer good value, do not assume they cost more, and anyway it’s all about performance: ask yourself if you think something is expensive……………compared to what?



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