Cos Kamasho, DIPG head of data services delivery at RICS, looks at insulation costs
The specification of insulation has come under scrutiny in the aftermath of the horrific fire at Grenfell Tower. The possible role played by the combination of composite aluminium polyester coated panels with foam insulation has led some clients to look at alternatives, especially in residential buildings.
Rigid insulation boards like Styrofoam, Polyurethane (PUR) and Polyisocyanurate (PIR) have excellent thermal properties, high strength to weight ratio and perform well when subjected to moisture. They are widely used with external cladding and walls, and the additional strength is also important on floors and flat roofs exposed to dynamic mechanical loads. Spray foam insulation is often injected in existing external cavity walls.
Fibre glass insulation is soft and flexible and tends to be used in tightly packed long rolls or individual batts. The spun fibres provide a good thermal barrier but are susceptible to trapping moisture which could lead to mould growth. Other forms of batts and rolls are made from mineral wool and natural fibres such as sheep wool and cotton.
For the same thickness, rigid boards are generally more expensive than most alternatives but offer higher thermal resistance. So to achieve the same U-value requires approximately twice the thickness of mineral wool batts compared to PIR boards.
The table below provides the latest sample cost breakdowns for various types of heat, sound and fire insulation products. These guides reflect the prices a developer might expect to pay on a medium-sized residential project for products in the low to upper-middle specification range. Prices do not include the contractor’s preliminaries, overhead and profit margin. The base date is January 2018 at UK mean location and prices are based on the BCIS Schedule of Rates Online Estimating Data.
The following rates are based on the UK average and represent typical prices at 2017 Q4. Please note that prices can vary significantly depending on the exact specification
Thermal insulation to basement
Expanded polystyrene and geotextile grooved boarding for external use on basement walls as insulation and protection to the damp proof membrane
100mm thick £21.55-24.24
130mm thick £27.62-31.07
150mm thick £30.84-34.70
Thermal insulation to floors
PIR rigid insulating board under floor bearing concrete beds or screeds
50mm/75mm thick £12.05-13.56/£15.83-17.812
100mm/130mm thick £19.45-21.88/£23.79-26.76
150mm/175mm thick £26.69-30.03/£30.30-34.09
Thermal insulation to walls
PIR rigid cavity wall batts/ slabs, built in as work proceeds, 50mm £11.36-12.78
75mm/ 100mm £15.14-17.03/£22.55-25.37
Fibre glass cavity wall batts/slabs, built in as work proceeds,
75mm/100mm £7.72-8.69 /£9.22-10.37
Expanded polystyrene bead injected into cavity wall, 50mm £9.86-11.09
Woollen insulation batts fixed between the members of timber studwork
Recycled paper insulation laid in poured in between studwork
Wood fibre insulation batts fixed between the members of timber studwork
600mm wide straw bale infill, fixed with wooden stakes, rendered both sides with cement lime on expanded metal mesh, painted with lime wash externally and emulsion internally £97.03-109.16
75mm thick overall insulation system to external face of solid masonry wall, 50mm thick PIR rigid insulation board fixed mechanically to wall, alkaline resistant reinforcing mesh, 20mm thick three coat render, microporous external emulsion paint £59.93-67.42
100mm thick overall insulation system to external face of solid masonry wall, 75mm thick PIR rigid insulation board ditto £65.20-73.35
Thermal insulation to ceilings and roofs
200mm glass fibre insulation quilt laid between roof joists £10.62-11.95
100mm glass fibre insulation quilt laid over joists or above suspended ceiling grid £7.08-7.97
Multi-layer foil quilt insulation fixed to sloping soffit of rafter (use in conjunction with traditional insulation) £10.83-12.18
PIR rigid insulation board with low emissivity composite foil facings on both sides, laid loose, 75mm thick £14.64-16.47
100mm/130mm thick £18.57-20.89/£23.26-26.17
Foil faced PIR rigid insulation board, bonded to 5.5mm WBP exterior grade plywood
Warm insulation to pitched roof, comprising 100mm PIR rigid insulation board fixed between rafters and 25mm to underside of sloping rafters £13.22-14.87
Warm insulation to pitched roof, comprising 130mm PIR rigid insulation board fixed between rafters and 25mm to underside of sloping rafters £14.99-16.86
Insulated plasterboard with tapered edges, fixed with galvanised nails to softwood, joints filled taped and flush jointed, nail or screw heads filled with joint filler, slurry coat to surface, ready for decoration, 2400mm high