I know what you are thinking, polished concrete flooring is always going to be cold, right? A huge rock hard piece of concrete running right the way through your home is probably going to be as cold as ice. If you are anything like myself, that mental image I kept was hardly inviting and I would suspect far from comfortable for many home owners.
Fortunately this scenario is now far from accurate. Maybe a decade or so ago we might have had such a scenario where polished concrete flooring was a really cold surface and would not be suitable for use in a domestic environment.
But as technology has evolved, things have moved forward for the better and underfloor heating systems are becoming way more popular, and with so many brands to choose from now, under-floor heating systems for concrete are now a viable and are easily sourced heating solution.
So please move past those visions of an grey iceberg running through your home or commercial unit, polished concrete floors are now warm and tactile. Plus, they are in my opinion far more aesthetically pleasing than any porcelain tiles, wood flooring, resin flooring or vinyl floor finish.
Things To Consider When Installing Concrete Over Underfloor Heating
Using concrete polishing over water fed heating systems is somewhat different to more traditional flooring options like wooden flooring, tiles over a sand and cement screed or an anhydrite liquid screed with encased heating pipes.
Firstly you have to account for the extra thickness associated with a concrete floor as opposed to a regular floor screed. Where a cementitious floor screed is usually in the region of 50mm and 75mm in thickness, a concrete substrate should always be a minimum of 100mm thick when used over underfloor heating systems.
This additional thickness of flooring material helps the sheer mass of the floor to cope with the heating process. Even though the expansion and contraction of the slab is minute in measurement, it is definitely a factor that needs to be accounted for and extra thickness and mass goes a long way in reducing potential cracking.
A perimeter joint made of a compressible (usually blue) foam tape should be used around the floor area, as opposed to expanded polystyrene insulation boards. Celotex and Kingspan insulation boards are for reducing thermal bridging; not for creating a movement gap as they are not flexible enough to allow for any lateral expansion of the concrete installation.
Concrete likes to sit quite happily in square or rectangular shaped bays, usually around 6m x 6m. As soon as you change the area by making an L shape bay or reducing the width by going through a doorway there is always going to be a greater risk of the concrete cracking. Crack inducement joints will help to control this.
So don’t forget to either form crack inducement joints with screed rails or use a diamond blade to saw cut joints at junctions where the concrete will naturally want to crack; doorways, a change of direction, columns/posts etc.
When we mention cutting a joint in the floor, we recommend that the cut goes at least 30% through the thickness of the whole concrete if not 50% through. We wholeheartedly agree this sounds like a very scary prospect, given that all the in-floor heating pipes are embedded somewhere beneath the surface and you don’t know exactly where.
Why are we placing joints in the floor? Basically, you are trying to convince the slab to crack along these nice new straight lines where they can be hidden beneath a colour matched flexible sealant and not diagonally across your newly laid and polished floor.
As we always advise our clients, all concrete floors for polishing should be installed by concrete specialists to ensure the floor will be suitable for further grinding and polishing works, after the installation of underfloor heating. Professional concrete installers will understand all the materials required and how their product should be laid over the radiant in-floor heating pipes.
Along side the concrete contractors, you should also be employing highly competent and experienced heating engineers to ensure there are no leaks in the pipework, as repairing said leaks will be difficult in a concrete substrate.
These same heating engineers should also give you a certificate or written proof that the heating system installed does not exceed a surface temperature of 27-29 degrees Celsius which will be reliant on the heat of the feed water and pipe spacing.
Depending on the heating system being used, the temperature is controlled either via a mixer valve or a flow valve to reduce the water temperature from around 80’C (too hot for underfloor heating pipes) to 50’C.
As a consequence of using trade professionals to install your floor, there will be in an increase in the polished concrete floor cost per square metre or per square foot, but we feel this slight increase in cost is more than worth it for a poured concrete floor that you will actually like the look of and enjoy living with. Much better to do this than to have to settle for a less premium flooring option such as ceramic tile.
For extra heat efficiency of your water fed underfloor heating system, you should also increase the amount of insulation you use below your pipes to help retain as much heat from the aggregates and sand as is physically possible. Once the concrete flooring is warm it will retain heat very nicely due to the huge amount of thermal mass inherent within a concrete floor. The extra insulation goes along way in reducing unnecessary heating cycles.
Ensure the insulation boards are laid on a compressed layer of sand to prevent the boards from moving and to level across a usually less than flat over-site concrete slab. A damp proof membrane is necessary over any aluminium foil faced insulation boards to prevent a reaction between the alkaline concrete mixture and the foil. If this is not done you’ll experience a lot of more air bubbles in your installation. Tack the membrane to walls to help prevent movement whilst installing the concrete.
The Right Type Of Concrete For Underfloor Heating Systems
From a technical standpoint, a concrete with a hardness of approximately C35 or 35 newtons with a water to cement ratio of less than 0.40, would offer a great starting point for the ideal concrete mix to use over heating. You may need to include a water reducing agent within the mix to improve workability during the installation phase.
These two figures ensure a higher quality of concrete mix and reduce the overlay porosity of the surface. A low water to cement ratio also helps reduce the potential for cracking both after initial installation and when the heating is commissioned. You also want to avoid getting a soft concrete surface as it will not polish so well.
We would suggest using large aggregate sizes. 20mm stone well graded downwards will cope better than 10mm stone when heated. Heated polished concrete floors with larger stones tend to crack less and retain more heat than those with smaller stones, but this statement is dependent on the water to cement ratio and overall strength of the concrete, over-site construction and type of insulation used. Without those fundamental qualities the large stone would offer next to no benefit.
It has been said that concrete floors are somewhat of a rigid sponge with 85% solid mass and 15% air trapped within the matrix of capillaries which form inherently during the installation and curing process.
As we already know, the secret to great insulation is trapping air right? So ideally we need to remove as much air as possible from the concrete floor slab and reduce the overall insulation tog rating of the actual concrete mass. This ensures the heat passes through that 100mm of build up directly to the surface.
We can reduce the amount of trapped air by using a subtle amount of cement super plasticiser and a de-foaming agent within the concrete mix design. These two ingredients help draw the millions of trapped air bubbles upwards through the slab and form them into large bubbles so they can be ejected from the surface whilst the concrete is still in its plastic unset stage.
Benefits Of Underfloor Heating
The ancient Romans knew all about the joys of living with warm floors and in our modern times we have enjoyed them for at least a decade under tiled floors. However it has to be said that the combination of underfloor heating systems and polished concrete is an even greater partnership.
Not only do polished concrete floors retain a large amount of heat for long periods of time, thanks in part to their large levels of thermal mass, you can also reduce the temperature to minimum in a lot of cases and still achieve wonderful levels of comfort whilst consuming considerably less energy.
If your underfloor heating is well planned and installed, you should find that heat is nicely distributed around the floor area, in a more uniform fashion than radiators or convection warmth usual from wall mounted heating systems.
A warm floor encourages you to actually want feel that warmth with your feet. By removing your footwear at an entrance to a home, you are both embracing this wonderful new source of heat and in turn reducing the amount of cleaning and maintenance required to your polished concrete floor. This is because any exterior dirt and debris will be left at the door and not be brought in on your shoes. Less cleaning, now that’s got to be a real benefit for everyone!
Underfloor heating for concrete flooring can be water fed or electrically wired. It is becoming more popular for some floor slabs to have electric heating installed in areas where there are half price or reduced rate electricity tariffs during the night hours.
Most systems are however water fed and powered by an efficient gas boiler. More environmentally friendly options are available however with ground sourced heat pump becoming a real favourite with our clients over the last few years.
Example Construction Of Concrete Floor Slab
Okay, this section is likely to be divisive amongst those who either lay floors or build houses as it may seem somewhat over fussy for something that is largely unseen. But floors we have worked on which have turned out the best, have the most efficient heat transfer and the least amount of cracking where concrete is arranged like this.
From top to bottom
- Concrete mixed and laid in accordance with CARRcrete spec for InfinityFloor
- A252 Steel reinforcing mesh on 40/50mm castles
- Water fed underfloor heating pipes, clipped into insulation
- Damp proof membrane laid over insulation and tacked to walls
- At least 100mm of quality insulation boards laid in a staggered bond
- Layer of compressed sand to prevent insulation boards from moving
- Over site concrete
The Correct Way Of Commissioning Your Underfloor Heating System
Always check with your heating engineer for their recommended method of commissioning a heating system beneath a concrete floor slab.
In general, you should always bring the heat up slowly and gently. Allow the heating to circulate with a maximum of 2 degrees water temperature per day until you reach an absolute maximum of 29 degrees concrete surface temperature.
Allowing your water temperature in your underfloor heating system to exceed 50 degrees will increase your chances of cracking and encourage failure of many floor sealers on the market, along with an accelerated risk of deterioration of the polished concrete surface itself.
Are There Alternatives To 100mm Concrete Over Underfloor Heating?
If you do not have the floor height to achieve a full 100mm of concrete or a specific colour of concrete surface is required. We would recommend a concrete overlay is used over a standard sand:cement screed.
This flooring system has many benefits in that the screed can be laid relatively easily in a short amount of time to a depth of 50-75mm. The overlay system is then applied between 2-10mm in depth, thus creating far more opportunities to fit within your available floor height.
A major benefit in the use of a screed and overlay over standard polished concrete floor underfloor heating is that the reduction in flooring thickness also lends itself to lower energy usage and a reduction in warm up time over a thicker concrete substrate.
It is very important to note that the screed needs to be left to cure and dry out thoroughly before the concrete overlay or microcement can be installed over the top. This is to ensure the screed shrinks and/or cracks before covering, allowing for the undertaking of remedial works and repairs.
Most sand and cement floor screeds dry out at a rate of 1 day per millimetre of thickness unless you use specialist additives. For example a 75mm screed will take at least 75 days to dry and be suitable for covering. You should never use underfloor heating as a means to dry the screed out, this will simply weaken and soften it.
The use of a rapid hardening and drying cement can greatly reduce the turnaround times of screed curing. There are many cements products (usually based around high alumina cement) on the market from brands such as Ardex and Mapei.
If a concrete veneer or overlay system is used over radiant floor heating, there are a greater range of controllable finishes and colours available unlike regular readymix concrete which is pretty much a case of ‘you get what you are given’ unless you pay extra for a bespoke mix.
We manufacture our own concrete overlay flooring solution called PROtop and MicroFloor. When densified and sealed both of these premium options provide a robust and durable floor surface that is easy to clean and maintain.
PROtop is mixed with aggregates to enable a stone finish to be ground and polished, the thickness of this overlay product is determined by the size of the aggregates used. For example if you want to see 10mm diameter stones visible, the overlay is most often 8-10mm in thickness after grinding and polishing.
Microfloor is a microcement trowel applied finish which can be used on floors and walls and is often only 2-3mm in thickness. Ideal for covering existing rigid and deflection free surfaces to create a bespoke power trowelled finish.
Sorry, we cannot advise on resin floor or wooden floor finishes with in-floor radiant heat as they are outside of our scope of expertise, we work purely with concrete products due to the rustic and organic aesthetic that concrete gives us with each floor we work on. We’ve never seen the appeal of resin or indeed any floor that looks artificial or mass produced, when each concrete floor is unique and different from the last.
Please do not hesitate to contact us at CARRcrete if you need further advice and guidance on polished concrete, overlay systems and underfloor heating best practices. We look forward to hearing from you!