Suspended Timber Floor Insulation: From Underneath Installation Guide (Method 2)

If the conditions are appropriate, and accessibility is adequate, insulating from below presents the opportunity to significantly reduce heat loss, reduce energy costs and improve thermal comfort without having to remove or damage internal finishes.

It should be noted that in geographic areas susceptible to flooding alternative solutions should be sought and this should be discussed with our technical team before starting any installation work. 

If the subfloor is exposed soil and no damp proof membrane (DPM) is installed on top of it, contact our technical team for guidance. 

Vents are installed in opposite external walls to provide adequate circulation of fresh air to keep timbers dry and prevent decay. Where tassel/sleeper/dwarf walls are present under the floor, they typically include ventilation gaps in the block work to ensure adequate air circulation. If the installation of under floor insulation results in this cross-ventilation being blocked or reduced, an additional ventilation provision should be provided.  This may include the removal of bricks, installation of additional external vents, or tapering the thickness of the insulation near the walls to maintain the original ventilation rate. Airtight sealing of membranes to tassel/sleeper/dwarf walls would use the same method as for sealing to external walls.

Check and note the following:

• Condition of the floor structure: ensure it is free from rot (dry and wet rot) especially at the abutment to any external walls, tassel/sleeper/dwarf walls 
• Evidence of existing dampness, staining or condensation on surfaces under the floor
• Evidence of infestation (e.g. woodworm bore holes or active infestation)
• Type, suitability and condition of floor timbers, substrates and any openings
• Type and condition of floorboards or pre-existing breather membrane 
• Any room space ventilation requirements.

If there are any issues or questions raised by the checklist, contact our technical team for guidance.

When working under the floor, it is the responsibility of the installer to undertake a comprehensive risk assessment before any works are undertaken. Always wear strong suitable clothing and a dust mask/respirator. It is also important to ensure the crawlspace is well ventilated in accordance with guidance set out in BS 5250. Use a portable light to see clearly and beware of any sharp points, nails and splinters on the subfloor and above your head.  Wearing a hard-hat or some form of head protection is recommended. Care should be taken in the event asbestos or other potentially harmful materials are present upon inspection.

Please note the following key materials needed and their roles:  

• Pro Clima Intello Plus intelligent airtight vapour control membrane
  Provides airtightness and vapour control
• Pro Clima Tescon Vana airtightness tape
  For sealing membrane overlaps and sealing damaged areas of Intello Plus and Solitex Plus
• Pro Clima Contega Solido SL plasterable tape
  For sealing membranes to perimeter masonry walls
• Pro Clima Tescon RP PRIMER / Tescon Sprimer
  To prepare rough/uneven surfaces prior to using Contega Solido SL to seal membrane to perimeter wall.
• Pro Clima Orcon F airtight adhesive
  For sealing Intello Plus to rough floor joists
• Pro Clima Solitex Plus reinforced breathable membrane
  Windtight breathable membrane applied under insulation to limit air movement through the insulation from below (thermal bypass) and to allow the assembly to breathe
• Pro Clima Aerosana Visconn Fibre (depending on floor detail)
  Airtight fibre reinforced paint. Aerosana Visconn Fleece may also be required depending on specific circumstances.
• Thermo Hemp Combi Jute natural insulation
  Natural breathable insulation for installation between joists
• Gutex Multitherm or Gutex Multiplex Top (optional)
  Breathable rigid woodfibre insulation for installation below joists for additional thermal benefit if space permits

When floorboards and skirting cannot be removed, installation of underfloor insulation from underneath (within the crawlspace) is the next most common method. This brief guide will highlight the method of sealing for airtightness and vapour control as well as offering some helpful tips.

When installing airtightness from underneath, Pro Clima Intello Plus intelligent airtight vapour control membrane is recommended instead of Pro Clima DA membrane. This membrane is available in 30m² or 75m² rolls and offers an effective and robust way of managing moisture within the floor build-up.

Please refer to fig. 1 and fig. 2 for diagrams of the finished floor build up.

First measure the gaps between the joists and make sure any loose material, dirt and dust is brushed off the joists and walls. Intello Plus should then be cut into strips and installed between each floor joist. The strips should be stapled with 8mm deep x 10mm wide galvanised staples to hold them up against the underside of the floorboards (see Fig. 1). If a pneumatic stapler is available for you to use, this will simplify and speed up the stapling process and lead to less risk of damaging the membranes while working in a confined area.

Each strip should be 100mm wider than the gap between joists. This gives 50mm of extra membrane at the edges for sealing it to the joists. The roll of membrane is 1.5m wide, so 500mm x 1.5m strips are easy to make and handle under the floor. Or, to reduce the number of overlaps or strips required, you can unroll a length as long as the joists and then cut that long-ways into three full-length strips.

The staples can be applied directly into the floorboards or into the tops of the joists to hold the Intello Plus in place. Please note, if Intello Plus is stapled directly to the floorboards and the floorboards are later removed, the membrane will be damaged and need replacing or repair.

Starting in one corner of the room, the first strip of membrane should be installed parallel to the joists as close as possible to the wall without it folding down onto the wall. Press the membrane between the joists so there is approximately 50mm of membrane extending down onto each joist and the membrane goes right into the corners. Then staple it to hold it in place. Lift the edge and apply a continuous 5-8mm bead of Orcon F Flexible Airtight Sealant to the joists approximately 25mm below the floor level and then lightly press the membrane onto the bead to make contact and create an airtight seal. It is important not to flatten the bead completely to ensure long-term elasticity and flexibility. This can be done by pressing either side of the bead rather than directly on it. If not using one long strip of membrane, each strip should overlap the previous strip by at least 100mm. Then seal the overlap with Tescon Vana Airtight Tape.

Once the membrane is sealed to the joists with Orcon F, the other edges (the short edge in contact with the wall) can be sealed to the perimeter wall with Solido SL Airtight tape as detailed in our "from above" installation guide. This method helps avoid tricky folds or cuts at all the wall/joist/floor corners.

If there is an OSB or Plywood board under the finished floor, the OSB or Plywood can act as a reliable airtightness layer in conjunction with Aerosana Visconn Fibre airtightness paint and Pro Clima sealing tapes. It may be more practical to apply an Aerosana Visconn Fibre airtightness paint directly to the underside of the OSB/Plywood.  Aerosana Visconn Fibre can be used to seal the boards to adjacent joists and adjoining walls to create a continuous airtightness layer. It is important to strictly follow installation guidelines for Aerosana Visconn Fibre. Where gaps of larger than 20mm occur a suitable Pro Clima airtight sealing tape such as Tescon Vana can be used to tape over the large gap or Aerosana Visconn Fleece can be used and then painted over with Aerosana Visconn Fibre (following the Aerosana Visconn Fleece installation guide precisely).

Once the Intello Plus airtight membrane has been installed, Thermo Hemp Combi Jute or Gutex Thermoflex natural, breathable insulation can be friction fitted between the joists. Measure the gap then cut the insulation 20mm larger than needed. The insulation can then be squeezed into place.  Note, it can be compressed by upto 10% without affecting the thermal performance.

To help hold the insulation securely before the weathertight membrane is installed, you can use twine or make 50mm wide straps from off-cuts of membrane and staple that between the joists to pin the insulation in place. 

Once the insulation is installed, Solitex Plus windtight breathable membrane should be installed with the blue side facing you perpendicular to the joists (across the joists) and stapled to the underside of the joists using 8mm deep x 10mm wide galvanised staples.  

The very first length of membrane should be positioned perpendicular to each joist and right up against the wall with as little gap as possible between the wall and membrane. Where the wall is not perfectly straight, you can cut the membrane around any obstacles and make it follow the contour of the wall.  You need to ensure the long edge of membrane does not fold down onto the wall at any point. When finished, you should end up with the start and end of each length of Solitex Plus folding down 300-500mm onto the wall, with the perimeter edges flat and close as possible to the wall (not folding down).

Including insulation between the joists will significantly reduce heat loss, nevertheless, the exposed length of the floor joists is a repeating thermal bridge where significant heat transmission can still occur. When one considers at least 11% of the average suspended floor is represented by the floor joists, insulating these joists will ensure complete thermal continuity over the whole floor area.

In such cases a breathable woodfibre insulation such as Gutex Multitherm or Gutex Multiplex Top may be mechanically fastened to the underside of the floor joist (see Fig.3 and 4). These may be held in place with an additional batten or a thermally broken mechanical fixing such as Fischer Termofix 6H-NT. 

These Gutex boards are inherently windtight and weatherproof at all tongue and groove joists. Butt joints (e.g. where the board is cut to fit) must be primed with Tescon RP Primer or Tescon Sprimer and taped with Tescon Vana.