Housing Retrofit: Ground floor insulation

Insulation and concrete slabs

 

Solid ground floor insulation in context

The temperature difference between an internal space to ground is significantly smaller than the temperature difference between the internal space to outside air. In general, recent research (George, Geens & Graham, BFF, Spring 2006) has shown that solid ground floor insulation as an addition to well-insulated walls and roof, contributes very little to the building’s overall thermal performance. The designer should balance the extra benefit(s) attached to installing a new slab with the cost, marginal carbon gains and pragmatics involved.

Space heating reaction time

When considering the location of the insulation below or above the slab, it is useful to consider the likely heating use pattern of the occupiers.
- Where a quick heating response time is required, for example when a family returns home in an evening, insulation above the slab should be considered.
- Where heating is used over more prolonged periods, for example where a space is occupied throughout the day, the thermal mass provided by a slab over the insulation should be considered.

Compression resistance

When assessing the suitability of an insulation material, it is important to ensure that the material has adequate compression resistance to restrict the effects of the compressive load applied by any loading:
 

Dead load

Dead load is the weight of the floor finish upon the insulation material. Refer to BS 6399: Part 1: 1996 for more guidance.

Design load

Design load is the load applied in use. These loads are usually localised or point loads associated with the use of the floor (eg furniture) and result in increased localised loading acting on the insulation. Likely point loading should be considered in calculating the required thickness of floor finish, particularly where the insulation is located above the slab.

NB

The ground floor insulation examples illustrated below are for general guidance only. The performance of products and methods of fixing vary. When specifying, confirm with the product manufacturer their installation methods and expected performance.

 

Ground floor insulation methods

 

1 Upgrading an existing slab

• The simplest way of improving the thermal performance of an existing concrete ground floor is to add insulation and a new floor deck on top of the existing floor.

• Though simple, raising the floor level through added insulation will usually require skirtings and radiators to be removed and re-fixed and doors to be reduced in height. Other problems might include unequal step heights at staircases and the raising of step heights at external doors. It might be considered that these problems are irreconcilable with building regulation requirements and could rule out this method of insulation.

• Ensure that any remedial work is undertaken on the slab ahead of installing the insulation.

 

2 New slab – insulation above slab

• The floor finish over the insulation can be either screed or boarding. It is argued that a screed finish is more durable and can offer a modicum of thermal mass otherwise absent. Board finishes, on the other hand, can be quickly installed and provide a quicker thermal response time.

• Some manufacturers offer floor-finished insulation slabs where boarding is pre-bonded to the insulation. In these instances, a separate vapour control layer is usually not required.

• Provide a perimeter (including internal walls) insulation strip upstand, at least 20mm thick and with a thermal conductivity of at least 0.025 W/mK or greater, between the wall and the screed to reduce thermal bridging.

• Ensure that the damp proof membrane (either above or below the slab) provides continuity with the damp proof course in the surrounding walls.

• Ensure that the insulation boards are tightly butted.

• Provide a flexible sealant between the skirting and the floor finish to limit air leakage.

• ... and remember to ensure that the finished floor level matches the existing.

 

3 New slab – insulation below slab

• Provide a perimeter (including internal walls) insulation strip upstand, at least 20mm thick and with a thermal conductivity of at least 0.025 W/mK or greater, between the wall and the slab to reduce thermal bridging.

• Ensure that the damp proof membrane (either above or below the slab) provides continuity with the damp proof course in the surrounding walls.

• Ensure that the insulation boards are tightly butted.

• Provide a flexible sealant between the skirting and the floor finish to limit air leakage.

• ... and remember to ensure that the finished floor level matches the existing.

 

4 Suspended timber floor

• Suspended timber ground floors can be easily upgraded.

• A range of insulation materials can be fitted between the joists to make a quick and efficient contribution to reducing overall heat loss.
- Rigid slabs can be friction fitted from above against nailed timber bearings to the sides of the joists;
- Insulation quilts can be installed upon mesh or netting to suspend between or fixed to the underside of the joists.
- Blown insulation needs to be contained by mesh or boarding beneath the joists.

• One of the most important issues with timber floors, is their propensity for air leakage. Where old floorboards are relayed or new ones installed, careful attention is needed to ensure that all joints between boards are adequately sealed.

• Insulation should be taken right up to the edge of the floor and any space close to the outside wall filled with insulation to avoid any gaps. Insulate between the last joist and the wall.

• Ensure that there are no gaps between the insulation and the u/s of the floor.

• Insulate hot water pipes.

• Provide a flexible sealant between the skirting and the floor finish to limit air leakage.

• Where polystyrene insulation is specified, ensure that it does not come into contact with PVC cabling.

• Maintain sub-floor ventilation (see Approved Doc Part C for guidance)

• One of the problems of providing continuous ventilation to the underside of the floor, is that it can greatly restrict the efficiency of the insulation, particularly loose-fill and mineral fibre. One consideration might be to attach a breather membrane to the underside of the floor joists - but check first with local Building Control that this is acceptable.

• The insulation should not block the air bricks in the outside wall.

• A fire-resistant board should be used beneath the joists if the floor is above a garage or basement.

 

Determining U-values

• The heat loss through a ground floor varies with its size and shape. The Building Regulations require that ground floor U-values are calculated, in accordance with BS EN ISO 13370: 1998.

• The standard uses the ratio of the exposed floor perimeter to the floor area to take account of the variation in heat loss due to floor size and shape (P/A ratio).

• The measurement of the perimeter (P) and area (A) should be to the finished inside surfaces of the perimeter walls that enclose the heated space (unheated spaces such as porches or integral garages should be excluded).

• In the case of semi-detached and terraced dwellings and blocks of flats, the floor dimensions can either be taken as those of the individual dwellings themselves, or of the whole building. When considering extensions to existing buildings, the floor dimensions may be taken as those of the complete building including the extension.

 

Typical U-values (W/m²K)

U-value target

EST best practice for the u-value of an insulated loft is .... W/m²K, though much higher values are achievable.

 

Insulation material u-values for ground floor concrete slabs

 

Mineral wool (above slab only) *

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
80	0.13	0.19	0.22	0.24	0.26
100	0.12	0.17	0.20	0.21	0.23	0.24	0.25	0.25
150	0.10	0.14	0.15	0.17	0.17	0.18	0.19	0.19
210	0.09	0.11	0.12	0.13	0.14	0.14	0.14	0.15

Source: *Knauf Rocksilk Thermal Floor Slab

 

 

Polyurethane (PUR)**

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
75	0.11	0.15	0.18	0.19	0.20	0.21	0.22	0.23
80	0.11	0.15	0.17	0.18	0.20	0.20	0.21	0.22
90	0.10	0.14	0.16	0.17	0.18	0.19	0.19	0.20
100	0.10	0.13	0.15	0.16	0.17	0.17	0.18	0.18

Source: **Kingspan TF70 continuous and with no thermal bridging

 

 

Polyisocyanurate (PIR)***

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
75	0.11	0.15	0.18	0.19	0.20	0.21	0.22	0.22
100	0.10	0.13	0.15	0.16	0.17	0.17	0.18	0.18
150	0.08	0.10	0.11	0.12	0.12	0.12	0.13	0.13
200	0.07	0.08	0.09	0.10	0.10	0.10	0.10	0.10

Source: ***Celotex GA3000 & XR3000 based upon 65mm screed and 20mm perimeter upstand

 

 

Extruded polystyrene (XPS)****

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
75	0.12	0.17	0.20	0.22	0.24	0.25	0.26	0.26
100	0.11	0.15	0.17	0.19	0.20	0.20	0.21	0.22
140	0.09	0.12	0.14	0.15	0.15	0.16	0.16	0.17
160	0.09	0.11	0.13	0.13	0.14	0.14	0.15	0.15

Source: ***Celotex GA3000 & XR3000 based upon 65mm screed and 20mm perimeter upstand

 

 

Insulation material u-values for suspended timber ground floors

 

Mineral wool roll with conductivity of 0.044 W/mK*

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
100	0.16	0.21	0.24	0.26
150	0.13	0.17	0.19	0.20	0.21	0.22	0.22	0.23
200	0.12	0.15	0.16	0.17	0.17	0.18	0.18	0.18
250	0.10	0.13	0.14	0.14	0.15	0.15	0.15	0.15

Source: *Knauf Loft Roll 40

 

 

Mineral wool slab with conductivity of 0.037 & 0.035 W/mK (140mm)**

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
100	0.15	0.20	0.23	0.25	0.26
150	0.13	0.17	0.19	0.20	0.21	0.21	0.22	0.22
200	0.11	0.14	0.15	0.16	0.17	0.17	0.17	0.18
250	0.10	0.12	0.13	0.14	0.14	0.14	0.14	0.15

Source: **Knauf Rocksilk Flexible Slab

 

 

Polyurethane (PUR)***

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
75	0.16	0.21	0.23	0.25	0.26	0.27
80	0.16	0.20	0.23	0.24	0.25	0.26	0.27	0.27
90	0.15	0.19	0.21	0.23	0.24	0.24	0.25	0.25
100	0.14	0.18	0.20	0.21	0.22	0.23	0.23	0.24

Source: ***Kingspan TF70 between 50mm joists oc 400mm overlain with 18mm chipboard

 

 

Polyisocyanurate (PIR)****

 

thickness	Ratio of perimeter (m) to area (m2)
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
75	0.16	0.21	0.24
100	0.15	0.18	0.20	0.22	0.22	0.23	0.23	0.24
150	0.13	0.15	0.16	0.17	0.18	0.18	0.18	0.18
200	0.11	0.13	0.14	0.14	0.14	0.15	0.15	0.15

Source: ****Celotex GA3000 & XR3000 between 50mm joists oc 400mm overlain with 18mm chipboard

 

Publications

Building Research Establishment (BRE)

• Thermal Insulation: Avoiding Risks, C.Stirling, BRE Press, 2001
• Installing Thermal Insulation, BRE Press, 2006

 

 

Further information

 

• Insulation materials compared
• TRADA
• National Insulation Association
• British Urethane Foam Contractors Association

Ground floor insulation products on GreenSpec

 

 

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