Insulated Foundation Vs Traditional

What’s the Big Difference?

Unless you’re building a high-rise or multi-storey building, choosing a foundation type should be relatively simple – right?

Unless ground conditions are extremely uneven or usual in some way, it’s unlikely that a deep foundation will be needed. The loads are also going to be low in most cases, particularly with timber frame, so the choice remains between two shallow foundation types; a strip and a raft.

Strip Foundation

Strip foundations are most traditional, where the walls are supported by a continuous ‘strip’ of foundation directly underneath. In order to achieve adequate u-values, designing a strip foundation to be efficiency, requires strict attention to detail in terms of insulation detailing and ensuring a thermal bridge-free design where possible.

Insulated Raft Foundation

Raft foundations on the other hand are essentially reinforced concrete slabs of uniform thickness that essentially cover the entire footprint of a building. They spread the load of the build over the entire foundation area, and as the name implies, the foundation essentially ‘floats’ on the ground like a raft on water.

The concrete slab is poured into a ‘bowl’ of insulation, insulating it from direct contact with the ground. The edges are usually continuous with the wall insulation in an attempt to eliminate cold bridging.  This works particularly well with a pre-insulated and airtight Closed Panel timber frame system.

Raft foundations are often chosen over strip foundations where ground conditions are poor and settlement is likely. A raft foundation allows for speed and cost savings in comparison, with less excavation and concrete required.

NSAI Approved

Passive U-Value

Quick Install

No Cold Bridging

20% Cheaper

No Dry Time

Reduced Heat Loss

Complete System

Finished Floor

What Makes a Passive Slab Superior?

While a Passive Slab looks like a raft foundation, it is actually a system in itself as the ring beam supporting the walls are separate from the floor slab. The ground preparations are essentially the same for both, in that the site is stripped and made level with a layer of stone to the build up required, covering the entire footprint of the house.

A raft design offers improved thermal efficiency as the level of insulation under the edge of the slab remains consistent. EPS 300, with its high compressive strength, is used in conjunction with concrete and steel, while EPS 100 is used in three layers for the floor insulation. Depending on the design, there can be one or two ring beams involved, to carry an inner or outer leaf, for example a timber frame structure with a 100mm outer block skin.

Which Is Cheaper?

Another factor, of course, is cost. On average, a Passive Slab system saves approx. 20% in comparison to a traditional foundation, without taking into account the time and effort involved in preparing for a traditional strip foundation.

Insulated foundation systems may appear to cost more initially, however they require much less soil and ground excavation than that of a traditional strip foundation; speeding up the install time as a result. There is also the reduced cost of concrete. While a lot more insulation is applied, this is offset by speed, reduced groundwork preparation costs and approx. 50% less concrete.

The price is a given and remains unchanged. Exact quantities of materials can be costed from the design, avoiding wastage on site. When compared against a traditional strip foundation where the site is excavated to hard ground at an unknown depth, trenches dug and filled with concrete of an unknown amount; the savings and peace of mind are reassuring to say the least!

Are My Ground Conditions Suitable?

An Insulated Passive Foundation system is suited to all ground conditions. With an insulated foundation system, the load from the walls is spread out, allowing the foundations to be built where ground conditions are softer or more clayish. This also simplifies and reduces the reinforcement design in many cases.

Where ground conditions are pour, the system can be designed like a traditional raft, whereby ground beams and ribs are incorporated within the slab. In the case of very poor ground conditions where the ground has been filled, the raft can bear onto standard piles, while maintaining a complete thermal break between the cold ground and the raft. In any case the system is tailor designed and engineered to the requirements of your chosen superstructure such as a Closed Panel timber frame system and indeed your ground conditions, shown within your Site Suitability Report submitted with planning.

What About Ground Preparations?

As part of our package, we provide a complete design and engineering service. Our detailed drawings outline the build up required in advance of our installers arriving to site. Removing the soil is simple and straight forward. Trenches aren’t required as with a traditional strip foundation. Likewise, the preparation works of the sub-base and levelling of the stone only take a day or two. Essentially, following design and engineering, your site can be taken from a mucky mess to a clean finished floor in 1-2 weeks.