New basement constructions can offer highly valuable space for existing and new properties where site or planning restrictions prevent you from building up or out. When carrying out this work, it is important to ensure the basement is properly insulated to provide a comfortable internal space that is affordable to heat.
In this article, we’ll look at how best to go about insulating a new basement including:
- Where the insulation should be fitted
- What insulation products are best suited for basements
- What U-values you’ll need to achieve.
Should basement insulation be fitted internally or externally?
When digging out a new basement, we recommend insulating the floor and walls externally. This approach ensures that the constructions are on the warm side of the insulation, minimising moisture and reducing cold bridging. It also makes detailing more straightforward than on internal installations, making it easier to effectively limit thermal bridges.
What type of insulation is suitable for basements?
Insulation fitted externally on a basement construction must be:
- able to handle high compressive forces from the surrounding ground; and
- resistant to groundwater penetration.
Kingspan GreenGuard range boards meet both these challenges. The extruded polystyrene (XPS) insulation boards can be used for both the wall and floor applications in basements. They are highly resistant to water absorption and the three products within the range cover compressive strengths up to 700 kPa.
Learn more about the Kingspan GreenGuard range.
What U-value should the basement wall and floors achieve?
The required U-values for basement walls and floors will depend on the building type, the country it is located in and whether the installation is on a new or existing building.
For new build projects, the Building Regulations/Standards in England Scotland and Wales all set an area weighted limiting U-values, which the total floor area (including soffits) must meet on average. In all cases, no area of soffit should achieve a worse U-value than the threshold value shown below.
An area weighted Notional Dwelling/Building specification U-value is also provided. This specification is used by energy assessors and is applied to a notional building of the same dimensions as the actual building. From this, they can generate the performance targets for the actual building. Whilst not compulsory, they often form the starting point for building specification.
| England Approved Documents L 2021 | Wales Approved Documents L 2022 | Scotland Technical Handbook | Threshold |
| Notional Building Limiting U-value | Notional Building Limiting U-value | Notional Building Limiting U-value | |
Basement walls (W/m2K) | 0.18 0.26 | 0.13 0.18 | 0.15 0.17 | 0.70 |
Basement floors (W/m2K) | 0.13 0.18 | 0.11 0.15 | 0.12 0.15 | 0.70 |
Learn more about the energy targets in the Building Regulations and Standards
How to calculate a U-value for basement floors
There are some additional considerations when calculating the U-value for basement floors. As with all floor applications, the exposed perimeter area (P/A) ratio will need to be calculated.
Watch the film below on how to measure perimeter area ratio to calculate how much floor insulation you need.
The calculation will also need to take into account the depth of the floor and the thickness of the basement wall constructions. Our technical support team can assist with these calculations.
Tips for insulating a new basement
As basement applications are highly specialised and may be impacted by a range of factors such as the surrounding soil type and ground conditions, it’s always best to get advice from a professional.
Below are some best practice tips:
- Kingspan GreenGuard boards can either be laid above or below the damp proof membrane (min 1200 gauge polyethene) above the compacted hardcore. If fitting the insulation above the DPM then a polythene sheet (min 500 gauge) should be overlaid on the insulation to prevent the wet concrete penetrating the insulation during pouring.
- The DPM should be laid under the basement walls and a triangular shaped piece of Kingspan GreenGuard placed between the slab and the exterior of the basement wall to create a slope and the platform for the basement wall.
- A 30 mm min strip of Kingspan GreenGuard should also be laid vertically as an upstand around the perimeter of the floor slab internally, running from the bottom of the floor insulation to the top of the floor screed.
- The wall insulation should be fitted externally of the tanking membrane which is applied to the external walls.
- All insulation boards should be fitted break bonded with no gaps. If more than one layer is installed then joints should be offset.
- A cavity drainage membrane or layer of no fines gravel should also be fitted outside the insulation board and should be covered with a geotextile membrane to prevent fines from blocking the drain. Additional ground drainage may also be needed depending on the ground conditions.
For more detailed information, take a look at our guide on insulation for basements
On site training and support
Build your team’s knowledge with onsite training and support from one of our Product Technicians. The presentations cover the full range of Kingspan products and applications from flooring to roofing and everything in-between with both internal and external wall insulations.
