Insulating a Concrete Floor

| April 8, 2015

The concrete floors of most modern houses should all have been insulated during construction, normally using a thick layer of dense polystyrene laid in sheets between the hardcore base and the finished surface. In older houses, this insulation layer may be missing from concrete floors, and it is definitely worth thinking about adding some if your home is feeling cold.

There are many companies who specialise in providing and laying insulation, but the process is not cheap, running from a few hundred to several thousand pounds depending on the area you need insulated. There is no reason why you can’t add an insulation layer to your concrete floors yourself, you just need to know the possible pitfalls to look out for.

Laying Insulation Boards on Top of Concrete – Pros and cons

You can buy insulation boards to lay on top of a finished concrete floor, as well as those designed to be embedded in the floor. Here are some things to consider before using this method.

Pros of Insulation Boards

  • Easy to handle – Insulation boards are fairly light, easy to manoeuvre and easy to cut to size.
  • Rapid Room Warming – Placing insulation boards above the concrete floor will mean the room warms quickly.
  • Multiple Uses – Insulation boards can be laid over both concrete and suspended wooden floors.
  • Levelling Floors – A layer of insulation can help to level badly finished concrete floors.

Cons of Insulation Boards

  • Thickness – Many types of insulation board are at least 50mm thick, raising the level of the floor.
  • Not a finished surface – You will need to lay a chipboard layer over the insulation boards, adding to the height.
  • Room Alterations – You will need to think about raising skirting boards, trimming doors and maybe moving sockets.
  • Cost or Materials – Good quality insulation boards aren’t cheap. You will also need to buy the chipboard and a damp-proof membrane.

Laying a Floating Floor

A floating floor is when you lay boards on top of a solid concrete floor, either to level the surface, to allow the addition of insulation, or both. Here is the easiest and most popular DIY method of laying a floating floor.

Step 1 – Preparing the Floor

Assuming the concrete floor you want to insulate is in a finished room, you will probably need to remove the skirting boards. You can read more about removing skirting boards here. Once that is done, check the floor is sound, dry and clean. If the concrete is cracked, repair it. If it is wet, find out the source of the moisture and remedy it before continuing. If the concrete is crumbling or flaking, it may be worth sealing it with concrete floor sealer.

Step 2 – Laying Insulation Boards

Measure the width of the room and cut the insulation board to the correct length. If the room is wider than the board is long, you obviously don’t need to cut the first one. Lay the first sheet of insulation in the corner of the room furthest from the doorway. Push it tight into the corner. If the board is not long enough, measure the gap between the end of the board and the wall and cut a section of board to fit. Lay the next board against the cut end, so that the joints are staggered rather than in a line across the room. Make sure the boards are pushed tightly together as you lay each one.

Step 3 – Laying the Damp-proof Membrane

With the room completely covered with insulation board, you can move on to the next step. A damp-proof membrane isn’t essential, but it is advised (the cost is minimal, so there is no real reason not to add one). The membrane should be cut slightly longer and wider than the room, allowing it to lap up the walls slightly. If you need multiple sheets of damp-proof membrane to cover the floor, overlap the joints by at least 300mm and seal using waterproof tape.

Step 4 – Laying the Chipboard

Using tongue-and-groove chipboard, lay the first sheet in the corner of the room furthest from the doorway. You will need to leave an expansion gap of around 10mm between the chipboard sheet and the wall all the way around the room. Apply appropriate glue to the tongue of the first chipboard sheet and slot the second into place. As with the insulation board, stagger any joints where sheets have been laid end-to-end. You shouldn’t need to use any nails or screws in the boards. Wipe away any excess glue from the joint and lay the rest of the boards.

Step 5 – Fitting Skirting Boards

You can now fit skirting boards around the room. If the overlapped membrane is protruding too much, trim it to 20mm above the level of the chipboard floor. Make sure it is behind the skirting boards before you attach them to the wall. The skirting boards will hide the expansion gap you left around the room. If electrical sockets need to be raised, it is best to employ a qualified electrician.

Insulating a Concrete Floor – Alternative Options

The method shown above is the best way to insulate a concrete floor without digging it up and laying it again with insulation added below the finished surface. But it isn’t cheap, and it can cause additional problems such as raising the floor level too much.

A cheaper and easier option for insulating a concrete floor is to line it with chipboard sheets, and then carpet over the top. Using a high-quality rubber underlay will further add to the feeling of warmth. This will in no way be as warm as using proper insulation, but it will make a noticeable difference.

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Category: Concrete and Render, Flooring, Insulation

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