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Step-by-step: Suspended Timber Floors

Ground floors can be incredibly draughty and cold and make a major contribution to the discomfort of inhabitants. Research has found that tackling insulation in suspended timber floors can lead to significant reductions of heat loss from floors. 

Improving the airtightness and energy efficiency of suspended timber floors is, therefore, a really good idea!

Green Building Store supplies a range of Pro Clima airtightness products suitable for improving the airtightness of a suspended floor, which work alongside insulation measures.

Which insulation?

We don’t supply insulation ourselves but would recommend a fibrous ‘breathable’ insulation such as wood fibre batts (semi-rigid), sheep’s wool, rockwool or mineral wool. Fibrous insulation can fit the space under a floor more snugly, reducing air movement. Its breathable properties help reduce the risk of rotting timbers by allowing water moisture to pass through more easily.

How to get at the floor

The next big question is whether to insulate the suspended timber floor from below or above. Both are possible but have their own challenges!

From below

To insulate a suspended floor from below you are going to have to enter the crawlspace and add all the insulation and airtightness products while crawling around in a dusty space. It’s not a brilliant job but you will get a lot of benefit, so here is our step-by-step guide:

  1. Install Solitex Fronta WA membrane & insulation

Solitex Fronta WA is a windtight and vapour open membrane that is more commonly used as a wall lining membrane. It works by allowing moisture to go out into the ventilated crawlspace BUT stopping air  getting into your insulated floor.  Solitex Fronta WA needs to be stapled to the underside of the joists to hold the insulation in place.

The Solitex Fronta WA  membrane comes in a roll of fabric 1.5 metres wide. Cut a length of it and install at right angles to the run of the timber joists. Staplegun the membrane into position, with staples, positioned every metre or so.

Install it in increments, one strip at a time. You may choose to run the line of the membrane at right angles to the line of the direction of the joists so that you apply one strip of Solitex Fronta WA at 1.5 metres wide across the length of the room which gives you a ‘shelf’ to then put in the insulation. Then you do the next strip and put in the insulation and staple it in position really well and then work your way across the room.

2. Seal all joints and overlaps between membrane strips with Tescon Vana tape

It is important to tape up all the overlaps of the membrane because you want to make sure that it’s fully sealed and no air can pass through into insulation. Use Tescon Vana tape to seal the overlaps, ideally with a continuous strip of Tescon Vana tape as far as you can go.  These taped joins form an extra-strong surface which can be useful if you need to staple/ screw any dowling into timber joists.

3. Seal the Solitex Fronta WA membrane to the perimeter wall

We now need to make sure the membrane is sealed to the outside perimeter wall of the room. There are a couple of options for this.

  • Tescon Vana: The membrane can be taped to the wall with Tescon Vana tape, which offers a quicker solution, with the benefit limited for time and crawling around wanted to see where I had done.
  • Orcon F: Alternatively you can use Orcon F adhesive, which is cheaper but maybe more fiddly. You need to ensure that the adhesive beading is continuous around to ensure a complete seal.

It is very important to prepare the walls before using either of these options.

Usually, we will be attaching the membrane to brick and block directly. In order to prepare the surface, it needs to be cleared of dust and debris.  We don’t want the adhesive to be stuck to the superficial surface of the dust and debris, we want it to stick to the stable surface of the wall.

To help with adhesion we usually recommend that you use Tescon Primer. it’s got the consistency of PVA glue and you just apply with a brush. You just paste a brushstroke width around the perimeter as it’s just a connection point for the Tescon Vana tape or Orcon F adhesive.

Alternatively, you could add a parge coat around the perimeter of the wall. A parge coat is a sand and cement solution which can be trowel applied and would create a  smooth connection point for the Tescon Vana tape or Orcon F adhesive. It also has the added advantage of stopping air movement from behind the brickwork.

4. Bracing the membrane if needed

Depending on the weight of the insulation and the span of the room, you may want to brace the membrane. To brace the membrane you can just use bits of dowling or wood which are then screwed to the undersides of the joists where you feel that the insulation sagging most. If it’s not a big space and it’s not sagging much, you may not need to brace it.

From above

If you are able to take up all the floorboards then our animated Warmer Homes film shows the way the Solitex Fronta WA membrane, insulation and airtightness measures can be added from above. Taking up the floorboards is obviously more disruptive to the household but definitely easier than crawling in a dusty crawlspace!

The importance of ventilation under the floor

We need to make sure that the ventilation under the suspended timber floor is adequate to stop moisture build-up and damage to joists and floorboards. Make sure that the air vents in the crawl space below the membrane and insulation are not blocked and that there is plenty of ventilation to help remove any moisture under the floor.

Risk factors

It is important to remember that all changes made to a building, including airtightness and insulation measures, potentially create risks.  When insulating and making a ground floor more airtight there can be risks of moisture build up and consequent rot in timbers.  This will depend on the wall type, existence of DPC, level of underfloor ventilation before and after the works have been undertaken and nature of the subfloor.  Particular risks are in embedded joist ends, joists bearing on sleeper walls and joists immediately adjacent to damp masonry. 

For a detailed understanding of the implication of undertaking deep retrofit we recommend the AECB CarbonLite Retrofit course which looks at these types of risk factors in greater detail or alternatively working with a building professional who has undertaken the course, or an equivalent, or who has a proven track record of high quality deep retrofit.

Additional measures for radical retrofits

If you are improving the airtightness and insulation of your suspended timber floor as part of a wider strategy to make your home as energy efficient as it can be (for example modelling the retrofit using PHPP), then we would also suggest the following additional measures.


Applying a parge coat of sand and cement behind skirting boards and running down the wall under the floor will make a huge contribution to stopping air movement coming through the floor. If you can get to that area we would always advise adding a parge coat because it is just a needless source of air movement. 

Adding an additional vapour control barrier

As we know, hot air rises so the danger of moisture-driven air entering the floor from above is minimal. By using the Solitex WA we’re stopping air movement going through insulation but you might want to add an extra vapour control layer on top of the floorboards (Intello Plus) to stop airborne moisture entering the floor from the house and also acting as an extra airtightness layer. This obviously adds cost to the project so I would only suggest that if you’re considering a radical retrofit and doing a major improvement to energy efficiency. There would obviously need to be a further hard flooring/ floating floor on top of that.

Your airtightness products shopping list

Products listed, in order of use, from the ground upwards

Energy efficient retrofit

Radical retrofit

If you want an additional vapour control layer on top of floor boards on the warm side of the floor (NB the Intello Plus would not be needed if a floating floor is being added).

  • Solitex Fronta WA
  • Tescon Vana tape to seal overlaps and membrane at perimeter of room
  • And/ or Orcon F adhesive can be used as a cheaper option for joining the membrane at perimeter
  • Tescon Primer to prepare walls
  • Intello Plus membrane for additional vapour controlAbi Anderson, Retrofit Coordinator and Department Sales Manager (Airtightness & specialist insulation), Green Building Store
    These guidelines are intended  to give a general understanding of the key products, costs and principles involved in elements of retrofit. Information in this blog is provided in good faith and in response to overwhelming demand from customers for practical information.   However, any use of this information is entirely at your own risk, no guarantee is given as to its suitability  and Green Building Store does not accept any liability whatsoever for building performance or safety, regulatory compliance or any other outcome which may result from its use or implementation.
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