Between a Rock and a Hard Place

Rock and Hard Place
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Cameron Payne - Flat Roof Technical Manager

When specifying a new waterproofing system for a flat roof refurbishment, choosing the correct waterproofing membrane for the application is essential, but it is not the only factor to consider. 

One of the most important factors when refurbishing a flat roof is addressing the thermal elements. Work on existing thermal elements must comply with Regulation 23, paragraph L1(a)(i) of Schedule 1 of the current Building Regulations, ‘in so far as that is technically, functionally and economically feasible’. 

A ‘thermal element’ refers to insulated parts of the building such as a wall, a floor, or a flat roof. 

In the case of flat roof refurbishment, the insulation needs to be upgraded to meet the latest minimum requirements of a 0.18W/m²K U-Value. This applies if you are planning a major renovation, defined as more than 25% of the external building envelope, or more than 50% of the flat roof area to be renovated. Renovation of a ‘thermal element’ of a flat roof includes replacing the waterproof membrane. 

Most commonly, a polyisocyanurate (PIR) insulation board is used but this may not be suitable due it being combustible. An insulation board which is increasingly being used is stone-wool insulation. Stone-wool is comprised of stone mineral fibre and being derived from stone it has the benefit of being non-combustible. We advocate the use of this insulation material whenever possible. As well as being inert and non-combustible, stone-wool also offers superior acoustic properties versus other types of flat roof insulation; useful when sound reduction and/or absorption are priorities. 

Whilst we endorse the specification of non-combustible stone-wool insulation, there are several factors to consider if you are to avoid finding yourself caught between a rock and a hard place. 

Stone-wool insulation: 

Has a poorer thermal performance than other insulation types, such as PIR.

  • U-values achieved by an equivalent thickness of stone-wool insulation are significantly less than PIR insulation.
  • For instance, to achieve the minimum 0.18W/m²K U-value, a typical thickness of around 210mm of stone-wool is required, versus just 130mm of PIR.
Insulation Thickness Difference[57]

Requires deeper constructions to achieve U-Value thresholds.

  • The additional thicknesses can be a real issue if there are height restrictions on any areas of the flat roof.
  • Often, on refurbishment projects under windowsills and door sills that can’t be removed or raised (for reasons such as budget constraints or additional disruption), the additional thickness of insulation required to achieve the U-value can mean that adequate upstand heights are not achieved.
  • BS 6229:2018 states that upstand heights of the waterproofing system must achieve a minimum of 150mm above the finished roof level. Any less than that and it will compromise the guarantee.

Is substantially heavier.

  • Whether on a new-build or a refurbishment project, the structure supporting the flat roof must be able to take the additional weight loadings of stone-wool type products.
  • If an overlay is being considered, can the structure take it? Qualified and competent structural engineers will have to carry out surveys to determine the existing structure’s strength and integrity.

Has poorer compression strengths compared to other types.

  • Insulation compressive strength is defined as ‘the point at which the load causes the insulation to yield before it reaches 10% relative deformation’.
  • Stone-wool insulation materials typically achieve around 70-90 kPa which is much less than the compressive strength of PIR, at around 150 kPa.
  • This means that care must be taken when installing plant/machinery on a flat roof that has stone-wool insulation, to make sure that the point loadings are sufficiently spread out to prevent damage to the waterproofing system.

Is more expensive and currently is suffering from a lack of availability across the construction industry*.

  • Costs must be considered. Stone-wool type products are substantially more expensive than PIR, EPS and XPS insulation.
  • Compounding the high costs right now is scarce availability and subsequent extended lead-times, created by a lack of raw materials combined with high demand. This may have a significant knock-on effect on installation schedules. 

Awareness of potential issues is key to avoiding them and designing them out. Contact Bailey Flat Roof Technical Department to discuss your project and to utilise our expertise. 

*As of 10th March 2022