Recently, a friend of mine asked for some help with an underfloor heating system. He was renovating the property that he was planning on moving into and had received a quote of £15,000 for a system installation (without a floor on top), and we decided this was a bit of a rip-off.

Any underfloor system you install will add floor height, unless you dig down. This was not a problem for us as the door frames of the house were around 2 inches above the floor level. If your door frames are flush, then you might want to consider whether it makes sense for you.

Overlay isn’t the easiest thing to get your head around when coming from a place of zero background. There are lots of options, and I’m not convinced by the thermal outputs of most of them.

I had hoped that we might be able to use an egg crate system which allows for 16mm pipes at a 100mm spacing; however, the floor was suspended, and after calculating the dead load capacity of the floor, it was not feasible. Egg crates enable snail rather than serpentine laying of pipes (so they spiral in and out); the advantage of that is an even temperature across the floor. The egg crate system would weigh around 50kg/m² with a very minimal 25mm screed, so it was decided that the only approach was to use overlay.

Since I knew very little about overlay, I watched a few videos on underfloor heating and then rang up an underfloor heating company and asked for advice. I had already persuaded my friend to lay tiles on top of any system to improve heat conduction and the efficiency of the system, so they suggested using the cement-coated overlay panels. It was either these or foil-coated. They suggested the cement-coated panels are the more premium option and offer better thermal conduction, though I’m sceptical. I don’t really see how lots of heat would travel through such a thin layer of cement, but perhaps I’m wrong.

The cement coating meant that we could use tile adhesive to stick tiles directly to the top of the board without requiring a screed, so that seemed like the logical choice.

Heat Outputs- 20mm panel

Heat Outputs – 20mm Panel

ZL-CPS300-16 Datasheet Download

The website lists the following heat outputs at 50 degrees C, while the datasheet just lists 80W/sqm:

• Under tiles: 97w/sqm
• Under laminate, wood, engineered: 65w/sqm
• Under carpet, vinyl: 51w/sqm

Which is right? I don’t know.

Wait, you didn’t do a full heat loss calculation?

I didn’t do a heat loss calculation as it was irrelevant to the build. The entire purpose of this underfloor heating installation was to get it in as soon as possible for the lowest price and the best reasonable specification. We weren’t actually doing a full system design. It was more about trying to reduce the cost of the future heat pump installation while maximising comfort.

I had done some basic calculations; I knew the system we planned to install had around 70sq/m of area and would produce around 3kW of heat at 40 degrees C with tiles.

Fixing the boards down

I was told by the underfloor heating company to fix these down with tile adhesive – that is if you are putting tiles on top of. If you are using laminate on top of them you should use mechanical fixings. https://theufhgroup.co.uk/products/tekwarm-ufh-self-tapping-screws-and-washer-for-20mm-board-set-box-of-100-pcs

I had laid tiles before, but nothing this size. There was a small learning curve in applying the tile adhesive – this won’t apply to you as you won’t make this mistake.

After watching a few videos, I chose the following tool. As you can see, it has large curved cutouts. Generally, cutout size is in proportion to the tile size, so the bigger the tile, the bigger the cutouts.

Since the tile trowel has ‘professional’ in its name, we believed it would elevate us to a higher standard of workmanship. Unfortunately, we had made an error, and having a ‘professional trowel’ didn’t make us professionals.

Another Important Point! Level the Floor Before Installing the Overlay!

After doing a fantastic job of installing the overlay, we patted each other on the back. Well done us. Then we got out the laser level and found out that our level plywood base that we installed onto was not, in fact, level in all places. But what is two centimetres of fall across runs between friends? It’s a 1 in 200 fall, so not much really.

We would have to rectify this error later. We had initially thought we could just level by using tiles and tile levelling pegs https://www.screwfix.com/p/no-nonsense-tile-levelling-system-kit-201-pieces/609PG?tc=KT8&gStoreCode=KT8&gQT=1 However, I would soon find this was not an option.

Time for Some Polypipe

The pipe we installed came in 300m runs and was relatively easy to install once we figured out the technique, which turns out isn’t carrying 300m of pipe and unravelling it by spinning it in mid-air. Rolling it is better—who knew! As you unravel it, you have to turn with the corners; this is fairly easy and logical once you figure it out.

Important!

How long should your runs be? Most documentation online states the maximum length that your pipe runs should be (leaving then returning to the manifold) should be 100m with 16mm pipe. This is a limit not a target, I wish I had thought more about it before installing.

We ended up with four equal length runs of 90m each. However, if I were doing it again I would have dropped down to 60m or even 50m. This is because the temperature difference between the flow and return would be less so the temperature drop across the floor would be less. It would require a larger manifold but there would be a reduction in pressure loss.

Now is the time to do a pressure test

You will need to connect up your pipes to the manifold and check for leaks. This part isn’t shown because it wasn’t done. My friend didn’t want to buy the manifold yet, what a gamble.

“Those Aren’t Tiles; Those Are Laminate”

A ordering error that made life a little challenging. While tiles had been specified, laminate had been ordered. This was a problem as it reduced the thermal conductivity. This was unfortunate, but since I had assumed we might need to put a radiator somewhere, I was not particularly bothered.

However, if I had known that we were using laminate, it would have meant we didn’t have to use tile adhesive to fix the overlay down instead could have used mechanical fixings.

“What Do You Mean There Is No Latex in Latex Screed!”

Since our floor wasn’t level and we wanted to maximise conduction of heat from the polypipe to the floor, we decided to lay a thin screed. While you might believe that ‘self-levelling’ screed is, in fact, self-levelling, it isn’t. So it won’t magically find the lowest point in your floor unless you are pouring a thick layer. It is, however, very liquid and easy to lay.

We spent a lot of time trying to figure out what the most appropriate type of screed to use was. The choices are either fibre-reinforced or latex-based. I couldn’t figure it out, so I rang up one of the companies that makes it, who told me ‘there is no latex in latex screed’. So I decided to make the executive decision and use my civil engineering background. I know that concrete has very little tensile strength, which means it is bad in bending unless it is reinforced with steel. I couldn’t see why a thin layer of screed wouldn’t be similar, in that it would have very poor tensile properties. Since we were going to try and do a thin layer across the floor with a bit deeper in places, I wanted some tensile strength in case the floor moved and expanded. So we went for fibre-reinforced rather than ‘not actually latex’ latex screed.

If your floor is flat, you can use dry screed overlay for thermal mass: https://www.insulationuk.co.uk/products/fermacell-r-flooring-2e11-1500-x-500mm-x-20mm

Get a Paddle Mixer

You will need a paddle mixer—a drill paddle mixer is not suitable. You need an actual one. For example: Energer ENB768MIX 1200W Electric Paddle Mixer 220-240V. I didn’t get this one; I borrowed a friend’s old DeWalt one, but they are similar.

I wouldn’t recommend a battery one as if you are mixing 40 bags of screed, then you won’t want to be waiting for batteries to charge. You are meant to mix each bag for 2 minutes, so you would need a lot of batteries.

Factor in the Pipe Cutouts

In the overlay, there are gaps where you haven’t put pipes. You can either fill these with pipes, or they will fill with screed. The amount of screed that they will require to fill is significant, so remember to calculate it. We didn’t, so had to make a few trips to Screwfix.

Remember to Prime the Overlay Boards

If you are going to put a screed over your overlay boards, then it is recommended that you prime them. This is apparently to improve adhesion and reduce cracking.

Tips on Laying a Levelling Screed

Three people is optimal. If you are doing it from bags, then you will need three people and two buckets. One person to put down the screed and roll it with a spiky roller. This helps remove air bubbles and lets the fibres line up (if using fibre-based screed).

Another person will need to mix with the paddle mixer, and the other prepare the water and add cement when mixing. You don’t need to do it this way, but it worked well for us.

Next Up: The Floor

That’s it! The next photos are putting down a floor. We faffed around debating what type of overlay to install and how it would affect the conduction from the floor to the laminate but decided it didn’t really matter. So we went for a cheap option from Screwfix. This was a budget installation after all! I wanted to minimise the insulating properties of overlay there were some options which had cutouts, but I wasn’t convinced it was any better.

Total approximate costs

Note: Cost per sqm calculated based on 70 sqm total area for all materials.

Underfloor Heating Systems Terminology