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Screed and floor insulation: your questions answered

Rob Firman, technical and specification manager Polyfoam XPS

Rob answers frequently asked questions regarding the specification and installation of insulation layers in ground floors.

CAN floor insulation be installed in multiple layers?
Sometimes the thermal performance requirements of a ground floor require a thickness of insulation greater than a manufacturer can produce in a single layer. On other occasions, insulation is the preferred option for making up a significant floor depth. Whichever is the case, the solution requires multiple layers of insulation to make up the required depth.

There are also times when there is a requirement to make efficient use of material. For example, one project required a total insulation depth of 225mm, and we were asked if it could be done using three layers of 75mm Polyfoam Floorboard insulation, under a flowing screed.

Every thermal insulation manufacturer has their own view on whether their products suit multi-layer arrangements. That’s mainly because different types of insulation behave in different ways. Any curvature and/or flex in rigid insulation boards can be exacerbated by using them in multiple layers, and that could cause insufficient screed coverage in places.

As an arguably more stable and often flatter insulation board, with greater compressive strength, extruded polystyrene (XPS) can be considered a better option for multi-layer build-ups than other rigid materials. Ultimately, though, the suitability of any such solution depends on the quality of installation.
Multiple insulation layers should be laid with staggered joints and shouldn’t be trafficked too soon. Doing so could introduce dents or bumps that risk the screed failing.

In-situ crushing resistance tests – why would a floating screed fail?
BS 8204, the British Standard code of practice for screeds, includes an ‘in-situ crushing resistance test’. It’s a method for checking screed performance onsite, and involves dropping a weight down a tube, onto the screed, four times.

The test works well with bonded and unbonded screeds. However, where a floating screed sits directly on an insulation layer, it effectively tests the screed and insulation as a composite. The test wasn’t designed for that. As strong and rigid as the insulation may be, it is still relatively compressible compared to a concrete slab below the screed. It can result in greater indentation in the screed, and the screed struggles to pass the test as a result, leading to questions over its specification.

It’s a flaw in the standard, and one that many insulation manufacturers are unaware of. That lack of awareness might lead to a lack of confidence if the screed manufacturer or installer challenges a test result. Better communication between the different parties involved in the design and construction of floor build-ups is the key to making floor system solutions more of a reality.

What is the best insulation to use with underfloor heating (UFH)?
With UFH, hot-water pipes are laid to a specific design and attached to the floor, then encapsulated by screed. Screed distributes heat evenly through the floor, and a flowing screed may even be force dried using the UFH system. The screed may be chosen specifically to have a low thermal resistance, improving the speed with which the effect of the underfloor heating is felt.

As the aim is to gain the maximum benefit from UFH, the pipes should be installed on a layer of insulation. The insulation restricts the flow of heat energy into the floor by conduction, meaning more heat goes into warming the room, improving efficiency. The plastic clips that secure the pipes in position can also be pushed into the insulation.

In a floating screed arrangement, an insulation layer used with UFH can fall into one of two categories.
The first is where it’s installed above the concrete slab or floor deck, and acts as the primary layer of thermal insulation in a ground floor construction. The thickness is determined by the U-value target.
The second is where it provides thermal resistance for the benefit of the UFH only and is usually of a nominal thickness.

In a ground floor, it’s supplementary to the primary thermal insulation layer, which is below the ground floor slab. In multi-storey residential developments where UFH is provided at every level, there’s no U-value to be achieved at the intermediate floors but the heating should be as efficient as possible.

Extruded polystyrene insulation (XPS) provides a flat, strong and dimensionally stable base for a floating screed, and can be specified in conjunction with underfloor heating at ground floor level or throughout a high-rise development.

The insulation under a floating screed needs to be capable of bearing the loads imposed on it. Otherwise, cracking – and even collapse – of the screed is possible. The high compressive strength of XPS insulation means it can bear significant loads.

Do floor insulation manufacturers offer warranties?
Clients and flooring contractors might seek warranties from individual manufacturers covering their respective products. A manufacturer of an individual component will not, however, guarantee the complete floor.

Insulation manufacturers typically advise that their products will last for the life of the building when installed in accordance with their guidance, third party certification and best practice. A screed manufacturer might offer a warranty to cover a defect in the screed material, but not to cover defects resulting from incorrect specification or installation.

The reason for this is that floor build-ups are not subject to system testing in the way a flat roof construction is. In any kind of system build-up, the performance of products and materials as a whole is taken into account. Without that ‘bigger picture’ view in the flooring industry, it is impossible for a manufacturer to commit to guaranteeing a complete floor.

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