IMPORTANT: On 1 September 2017, Celotex took the precautionary measure to temporarily suspend the supply of Celotex FR5000, Celotex CG5000, Celotex CF5000 and Celotex SL5000 while we investigate the results of recent tests (Parts 6 and 7 of British Standard 476). In addition, we have recently identified a compliance issue relating to our calculation and testing of the declared lambda value of products in the 4000 and 5000 ranges and the Crown-Bond and Crown-Fix products within the Crown Flat Roofing range. Due to this issue, the suspension of the 5000 range will continue and now includes the FI5000 and GD5000 products. Materials relating to the 5000 range products are for information only. Please note that all products in the TB4000, GA4000 below 100mm, PL4000 and CW4000 ranges manufactured after 15 December 2017 will be marketed from January as Celotex 3000 with a declared lambda value of 0.023 W/mK.

Please use the link below to read our full statement regarding declared lambda:

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Please use the link below to read our latest statement regarding Class 0:

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Underfloor heating is now becoming an increasingly popular method for homeowners wanting to improve the insulation of their home. Not only does underfloor heating provide consistent background heat – the application can also eliminate the requirement for radiators and in turn reduce heating bills.

 However, before the installation process can take place, there are a few decisions to be made that will determine whether the system will provide the required levels of insulation.

This post will cover some of our top considerations, all of which should help you to decide upon the best insulation and heating application for both new-build and existing homes.

1) Which underfloor heating type is most appropriate?

First things first, there are two types of underfloor heating to decide upon: wet or electric.

Whilst electric systems can be a popular choice when it comes to insulating a single room, such as a bathroom or conservatory, this can prove expensive when applied to larger insulation projects.

Wet underfloor heating on the other hand, which involves heat being generated via a boiler through pipes into the floor, costs almost half of the price to run than electric systems.

This benefit tends to outweigh the extra costs for installing wet underfloor heating, and in most cases makes it more suitable and energy-efficient long-term.

2) Which flooring materials provide the best heat output?

Both wet and dry under floor heating can be installed within most floor constructions, depending upon the amount of space available to raise the floor level and how much weight the flooring can take. Predominantly, you need to be aware of which flooring materials provide the best heat output when combined with an underfloor heating system.

Suspended floors are a popular choice for renovation projects due to their little height buildup. For timber suspended floors, a plate or foil system would be used as long as it is well supported and has no movement. Typically, this involves an aluminum heating plate being installed on top of the joist and below the deck.

Wood flooring, being an inherently stable material, means that expansion and contraction will be minimal, particularly with engineered board. According to Build It magazine, optimum results will only be achieved with this material if the wood is less than 22mm thick, has a width-to-thickness ratio of no more than 4:1, has a moisture content of 6-8% and the floor temperature does not exceed 28C.

However, the above suspended floor options do not conduct heat as efficiently as other solid flooring, such as ceramic tiles, concrete, slate and stone flags.

Within these solid-flooring options, screeded construction acts to diffuse the heat across the surface, providing an even temperature at floor level. As a result, any screeded system will provide a much higher W/m2 output than any plate or foil system, making it the most appropriate option. Screeded construction also allows pipes to simply clip on top of the insulation.

3) How do you prevent movement in screeding once heat is applied?

Another point to consider is how different materials expand and contract once heat is applied, and how this movement impacts the flooring over time.

Whilst screed does expand and contract with heat, in the screed, this should be absorbed by an expansion gap and edge insulation. Build It magazine suggests that any tiles fitted on top, which may well expand at a different rate to the screed, should be laid with an expansion gap around the perimeter of the room. This gap can usually be covered by a skirting board, with flexible adhesive and grout applied.

4) What is the appropriate type and depth of screed?

We tend to see two types of commonly used screed: sand and cement-based screed or flow (liquid) screed. Whilst sand and cement screed takes longer to heat and cool down, flow (liquid) screeds takes significantly less time due to its’ restricted thickness of 40mm.

There are pro’s and cons to this – whilst flow screed takes less time to circulate heat, thinner liquid screeds can give the end user the feeling of hotspots within the floor as opposed to an even temperature; additional pipe work may also be required. In comparison, thicker screeds, despite taking longer to heat, are known to retain heat for longer.

The minimum British Standard is 65mm of hand-mix (3:1 sand/cement) or 50mm of liquid screed.

5) Where should the insulation be positioned?

In line with the latest Building Regulations, it is required that all new builds have insulation positioned below the concrete slab. However, underfloor heating should also implement insulation above the slab and directly below the pipes to reduce the effects of downward heat loss. We commonly see floor build-ups on new builds with insulation only above the slab.

6) How much insulation is required?

As an industry, the general consensus is that 100mm of PIR insulation is used for underfloor heating applications. However, meeting the required U-value for each project and performance of different boards can mean this varies depending on the product used. Using a high performance (and denser) board such as Celotex FI5000 can result in the thinnest 75mm board being used. Not all PIR boards perform in the same way so ensure that the product used meets the required specification.

7) What is the maximum temperature that underfloor heating should reach?

British Standards require that surface floor temperatures must be below 29◦C, so underfloor heating suppliers and contractors should conduct heat loss calculations as part of the design process.

Getting the right amount of pipework into the floor and in the right areas should provide between 40 W/m2 and 55W/m2 of heat. Too much can overheat the floor, whereas too little would mean the system could be running constantly, increasing your heating bills.

8) Which type of insulation is best to use?

 Correct insulation forms an essential part of any well-designed underfloor heating system; therefore choosing the right type is crucial to ensuring as much heat as possible is absorbed into the house rather than into the ground below. Insulation below the pipes should therefore be much greater than the insulation created by the floor covering.

Celotex FI5000 is a premium flooring product targeted at under screed applications, to minimise insulation thickness and downward heat loss. This premium performance Celotex PIR solution provides reliable long-term energy savings for building structures and has excellent dimensional stability. Pipe retaining clips may also be inserted directly into the Celotex FI5000, which has an enhanced compressive strength of ≥175kPa.

Further application benefits include the built-in vapour control layer, which enables direct screed without the need for an additional separating layer. This means that the easy-to-cut boards saves the installer both time and money.

To find out more about how Celotex FI5000 can be used in your next underfloor heating project, view the product datasheets here.