Insulating Foundations
Energy-efficient building design starts from the ground up.
Properly insulated foundations can prevent a building from losing heat through the ground, which helps to maintain consistent indoor temperatures. This reduces the need for heating and cooling and improves overall comfort for building users.
In this post, we will review the ways of improving traditional foundation design and the benefits of specifying insulated foundation systems for new build houses.
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Insulated foundation systems can virtually eliminate thermal bridging at the wall-floor junction.
Traditional Foundation Arrangements
The choice of foundation is influenced by a number of factors such as ground conditions and building design. One of the most common types of foundation in the UK is the traditional strip foundation. Strip foundations consist of concrete-filled trenches which are positioned below the building’s load bearing walls. In a strip foundation arrangement, the foundations of the external walls are independent from the ground floor slab. This means that the ground floor insulation can be positioned either underneath or on top of the ground floor.
Traditional strip foundation detail adjacent to a ground bearing floor slab
(See Detail Library detail MG1A for the full design)
When floor insulation is laid beneath the slab, the concrete is exposed to the room which means that the thermal mass of the slab can be utilised. The slab can absorb heat during the day and slowly release it at night, which helps to maintain more consistent temperatures.
When the insulation is laid above the ground floor slab, the thermal mass of the concrete is isolated from the room and it can’t regulate the room temperature in the same way. In these cases, the room heats up and cools down more quickly. Insulating above the slab can work well with underfloor heating as it helps to direct heat upwards into the room, rather than being absorbed into the concrete.
External wall and ground floor junction details showing options to insulate either above or below the ground bearing floor slab
(See Detail Library details MG1A & MG2 for the full designs)
In both scenarios, the junction between the ground floor and the wall foundation is an area that can be prone to thermal bridging if it’s not properly detailed.
Limiting Thermal Bridging
The building regulations require the specification of ever increasing amounts of insulation in order to reduce heat losses through the building fabric. The high performance of the wall and floor increases the likelihood that heat could escape at the junction between the two (taking the path of least resistance). Heat loss through the junction increases the risk of condensation and the potential for moisture related problems. As such, proper detailing of the wall, ground floor and foundation to avoid thermal bridging is increasingly important.
The diagrams below show strategies for limiting thermal bridging through the wall foundation and floor junction as recommended within Approved Document Part L V1 of the building regulations in England.
Diagrams showing the Building Regulations guidance for limiting thermal bridging at ground floor junctions detailed within paragraph 4.17 of Approved Document L1 (at the time of writing)
The key principle is to connect the external wall and ground floor insulation so that the room is isolated from the cold foundations. In a masonry cavity wall construction this can be difficult to achieve. It’s recommended that the cavity insulation be taken down past the floor level and the use of a lightweight concrete block be considered for the inner leaf to reduce thermal transmittance.
Concrete has a relatively high thermal conductivity and so it is crucial that the perimeter of the ground floor is detailed to avoid heat loss through the cold concrete foundation. The inclusion of a continuous thin vertical strip (minimum thickness 25mm) of rigid insulation between the external wall and the perimeter of the slab or screed should be included to reduce the potential for heat to escape through the wall foundation.
Reducing thermal bridging between the strip foundation and floor slab intersection of a masonry cavity wall construction
Insulated Foundation Systems
The principles discussed so far look to reduce thermal bridging at the wall foundation and floor junction when working with a traditional strip foundation arrangement.
To improve thermal performance further, an insulated foundation system can be specified as an alternative structural solution. An insulated foundation system has insulation underneath the wall foundation as well as beneath the ground floor slab. This approach allows the whole building to have a continuous thermal envelope. As the insulation is part of the structural system, it must be designed by a qualified engineer and inspected at stages throughout each project.
Insulated foundation systems generally use a very rigid and durable type of insulation called EPS (expanded polystyrene) insulation in blocks of various profiles. These insulated blocks are arranged above a waterproof membrane / radon barrier in several layers to create the formwork for a reinforced concrete floor and foundations. Denser EPS 300 insulation is positioned at the building perimeter, below internal load bearing walls and / or party walls. EPS 100 sheets are placed below the remainder of the floor. Depending on the design, there may be ring beams included to carry an outer leaf.
With systems such as the KORE insulated foundation system, it’s possible to achieve U-Values as low as 0.10 W/m²K. In addition to increased thermal performance and virtual elimination of thermal bridging, the system is quick to erect, allows less digging and uses less concrete. The system can also incorporate underfloor heating into the concrete.
KORE’s Insulated foundation system virtually eliminates thermal bridging by wrapping the entire foundation in a continuous layer of EPS insulation
Insulated foundation system detail on the Detail Library (junction with externally clad timber frame wall)
Insulated foundation system with strip foundation supporting outer leaf detail on the Detail Library (junction with brick faced masonry cavity wall)
Stacks of KORE EPS insulation laid above a radon barrier ready for setting out the Self-build, Co Clare project for Long Life Structures
Detailing Insulated Foundation Systems
Check out the Detail Library for examples of construction details for insulated foundation systems. Some of our recent details show the junction between insulated foundation systems and masonry, timber frame, SIP and CLT walls.
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Written by Emma Thackstone. Emma is an architect. At the Detail Library, Emma helps the Detail Library with drawing new details and carrying out technical research.
Image Credits
Source URL: https://www.kore-system.com/case-studies/clare-self-build-foundation-case-study/
Image URL: https://www.kore-system.com/wp-content/uploads/2023/05/Crusheen-mid-installation-aeriel-view.jpg
Project Engineer credit: Tanner Structural Designs Ltd
Photographer credit: Unknown
Source URL: https://www.kore-system.com/products/kore-insulated-foundation/
Image URL: https://www.kore-system.com/wp-content/uploads/2022/01/KORE-Insulated-Foundation-System-2020.png