Floor Structures & Tranquility Floors

(and how to insulate them)

There are various structural floors that these floors will be laid on, so as the options for each are different here are some general forms of construction with drawings to help show what should be done. This advice comes from a long history of designing low energy buildings – see the Home Page. Both new [or modern] build and existing buildings are covered. As a general rule, all floors laid on the ground floor must have one (and only one) damp proof membrane in them – somewhere.
(Drawings referred to in the text below are also available here... as an image file)

  • drawing showing board laid between skirtings
    Drawing 1

Ground Floors on Concrete:

The best solution is to have what is called a floating floor, where the floor is not fixed down – see drawings 1 & 2. This is much more comfortable, taking some of the strain off knee joints and simultaneously providing great ground floor insulation. Properly laid you would not know it is floating.

The drawings show this preferred solution but it helps to have a fairly level concrete slab to start with. Run a straight edge over the concrete to determine if you can lay straight off it or not. If you can, proceed to ‘Setting up the floor’ – if not read on:

  • drawing showing board laid between skirtings
    Drawing 2

What to do if the concrete slab is uneven:

Well, there is one generally accepted solution and another tested at Tranquility where less costly solutions are always thought through and tried. The one is technically the right way to do it but expensive; the other which works perfectly in Tranquility where one floor is laid in this way is extremely cheap but not ‘approved’.

  • The approved system involves marking the low points (having run a spirit level over it) and using a floor levelling compound – which is like sloppy cement when mixed. Make sure the concrete is very clean before applying it, and follow the instructions. Essentially you pour it over the lowest places and it flows before setting to make a very level floor. But it is expensive.
  • The other way of doing it, which is less precise, involves levelling the slab with very dry sand. It must be extremely dry to allow it to ‘flow’. As above, use a long straight edge to mark where the low spots are. Take an appropriate amount of the dry sand and ‘spread’ it through each low area. Level it as well as possible. Now take a board of chipboard and loose lay it across the area sanded. By shuffling it around it will gradually spread the sand until it is extremely level. Walk on the board to make sure it does not bounce anywhere. Continue across the floor until it is all level, and check by loose laying several boards to make certain. When you are happy with the result proceed to lay as below. Note: there isn’t anywhere the sand can go unless very slowly it becomes even more level

The issue to be aware of, if a floating floor was not in the building design from the outset, is the height of the door bottoms which will define how much insulation you can lay. (The doors can be shortened slightly if that makes the difference).

  1. Flooring grade Polystyrene can be bought in many thicknesses, so most required thicknesses can be accommodated. The total height being laid above the concrete is going to be:
    1. The final floor – 12mm
    2. The Chipboard – either 18mm or 22mm, the 22 being more rigid
    3. The insulation – from 20mm to 100mm
  2. If there is the height available for 100mm of polystyrene so much the better, but lay it in two 50mm staggered layers and preferably tape each layer together. If not, whatever there is space for is good. Cover the concrete with the tight fitting slabs of polystyrene.
  3. Now lay a 1200 gauge polythene sheet over the polystyrene but lap it up the walls so it makes a sealed moisture proof tray when trapped behind the skirting boards. It is supplied in a 4m width, so if more than one piece is required they must be taped together with the appropriate waterproof tape.
  4. Next fit the Tongue and groove chipboard floor which is glued to form a single membrane.
Drawing 2-1
drawing showing board laid between skirtings

Hint: Leave about a 10mm gap between the edge of the chipboard and the walls. Cut some wedges for the first walls to be laid against to provide this space. Have some PVA adhesive ready which can be put into a kitchen detergent bottle for ease of application. Now cut the first board to have sawn edges against the walls but T & G for the others. Offer up the next board and when happy lift it and squeeze a thin continuous line of PVA along both top and bottom of the groove to make sure it is fully glued. Lay it and to make sure it fits tightly; simply jump on it so when you land your feet push the board to where it needs to be. Use a damp cloth to wipe the surplus adhesive from the top of the board and to make sure the adhesive is continuous. Continue covering the whole room, but the off cut from one run will start the next. Fit the wedges as you go finishing with a tightly laid well fitting continuous membrane floor. Leave for a few hours to dry hard then remove the wedges.

  1. Lay the Tranquility floor and finish it (N.B. follow the detailed instructions on How to lay a Tranquility Floor)
  2. Fit the architraves around the doors and then the skirtings to complete the room.

First Floors on Joists: see Drawings 3 & 4

As long as the existing floor is level, the Tranquility floor is simply laid on top, but if for some reason you are going back to the joists, simply lay chipboard – it can be 18mm so long as the joists are not more than 450mm between centres or 22mm if wider. So long as the board joints are on top of the joists it doesn’t need to be glued, but if the house is old and the joists irregular it would have to be glued well.

But you still have two options to consider. Do you remove the skirtings and architraves or lay between them?

  1. Drawing 3
    drawing showing board laid under skirtings

    The good practice solution is to remove the skirtings etc after which it is plain sailing, quick and gives by far the best result. But it does require them to be removed and re-fitted. It is an opportunity to fit improved skirtings, but if there is nothing wrong with what you have this will use more scarce resources the planet struggles to provide.

  2. Drawing 4
    drawing showing board laid between skirtings

    The alternative is to lay the floor between the skirtings, but this requires an expansion gap to be left between the floor and those skirtings.
    drawing showing additional mini skirting
    This can be left technically visible – though with most of the floors in Tranquility laid this way, almost nobody ever notices – or there are two solutions to conceal it: the first involves fitting a thin neoprene strip into it, but this visually highlights the gap; the second requires a small section of timber to be fitted to the skirtings to cover the gap, but this usually becomes untidy where the skirtings meet the doors.

Ground Floors on Joists: will normally apply to existing buildings.

The underside of the floor must be ventilated or the joists will rot, so there will be air bricks in the outside wall of the house. Such floors usually allow between a good draught and a howling gale to come up through them, so laying a floor is the perfect time to resolve this issue and to insulate it – see Drawing 3. The only direction for the long term price of energy is up, so all floors should be sealed as well as possible against all air leaks and a total draught proof situation can be achieved.

In order to do the job properly, the floor boards will need to be lifted as adding insulation on top of existing timber floor boards (if there were space) would cause them to rot. So there are a number of solutions and options but the golden rules are:

  1. Either remove all the floor structure and fill the void so you construct a solid floor which then isn’t ventilated or
  2. Insulate soundly under the floor and put a sealed vapour proof membrane over the insulation and under the new floor.
Drawing 5
drawing showing void under flooring

Filling the Void: see Drawing 5.

This requires everything to be removed and the room(s) will look like building sites at their worst. This solution wouldn’t make sense if the depth of the void was too great but up to around 250mm is fine, and starting from bare earth would not be a great idea:

  1. Remove everything to get back to the solid structure beneath the void.
  2. Level the bottom following the rules above ‘What to do if the concrete slab is uneven’.
  3. Determine what depth to build up to before the new floor is laid and follow the guidance above under ‘Ground Floors on Concrete’.
Drawing 6
drawing showing void under flooring
Drawing 6-1
drawing showing insulation under flooring

Insulating under the floor: see Drawing 6.

  1. Lift the floor boards and lay some scaffold boards or sheets of chipboard to work off.
  2. The bottoms of the joists must remain in the air flow along with about 1/3 of the depth to keep them dry. Decide what insulation depth is available and fix battens to the sides of the joists the insulation depth below the top of the joists.
  3. Cut the insulation you are going to use (polystyrene is best) to fit snugly between the joists and fit it so it is continuous. It must be tight up under the new floor as leaving gaps in a howling gale allows the cold air to move freely under the floor.
  4. Lay a 1200 gauge plastic membrane across the top of the insulation and joist tops – as above to run up the walls behind the skirting when fitted. This is hugely important for this installation. If more than one sheet is required they must be securely taped together with waterproof tape.
  5. Now lay the T & G chipboard floor, gluing as above and fixing down with annular ring shank nails securely to the joists. Fixings should be at 150mm centres along the joists but make certain the nails find the joists or there will be a hole through the membrane that isn’t sandwiched between the board and joists.
  6. Lay the Tranquility floor (N.B. follow the detailed instructions on How to lay a Tranquility Floor) and when finished fix the skirtings through the membrane to the walls. Stainless helical fixings are a wonder as there is very little finishing to do before decoration.