Module 5: Internal solid wall insulation


When insulating solid walls it is not always possible to apply external insulation as there can be restrictions with access, altering the visual appearance of the external facade, such as listed building or conservation area restrictions, or issues with neighbouring properties and the depth of insulation required. Therefore, internal insulation is the alternative solution.

This can be a cheaper way to insulate as there are less components required and as they do not need to be weather tight, they tend to be cheaper. Insulating internally can also be a good way to improve or renovate internal finishes on external walls and provides a marked increase in thermal comfort instantly.

However, internal insulation can pose a risk to the fabric of the building if the wrong materials are used or if applied incorrectly. Typically, during the winter months there is a flow of moisture vapour from the interior towards the exterior. This vapour condenses when it meets cold masonry and will begin to accumulate. If this process occurs around timber window frames or joist or beam ends then moisture accumulation can lead to rot. It is therefore important to use materials such as wood fibre insulation, which is capable of storing and dispersing moisture, to ensure that any embedded timber remains dry.

Using cheap, impervious insulation materials might seem to be the answer as they would seem to prevent moisture flow from the interior of the building. However, they cannot prevent the flow of moisture through the timbers themselves or through breaks around partition walls, sockets, joints, etc, causing moisture to accumulate in the wall. Additionally, impervious insulation prevents walls from drying towards the interior which in the long term raises the overall moisture content of the wall. This accelerates the rotting process so whenever timber is present and the substrate contains materials that are porous, vapour permeable systems with good levels of capillarity (ability to move moisture around) such as wood fibre insulation should always be used.

With all internal insulation it is very important to ensure that there are no gaps between the insulation boards and the wall being insulated. Gaps can allow air movement and mould to grow in the voids which, if connected to other voids, allows mould to spread behind the whole insulation layer. Mould releases spores in to the internal environment which are very bad for the health of the occupants, the longevity of the building and the insulation itself and so it is very important to ensure no voids are present behind the insulation boards.

Finally, internal insulation increases the reliance on external finishes keeping driving rain out of walls. The lower heat flow through the wall reduces it's ability to dry. Therefore it is important to use only systems designed for internal use and to detail them carefully. Computer modelling, using software such as WUFI, gives reassurance that the proposed measures will work as required and can also identify scenarios where external facade impregnating creams may be required. This should be undertaken before installing any internal wall insulation to ensure that no long term issues are likely to arise.

Assessment of the structure

First an assessment of the structure should be made to determine whether insulation should be applied. Generally in the UK our prevailing weather comes from the West or South West and so any walls with these aspects should be checked carefully. If the external wall surface is rendered or clad, this should be in good order and there should be no cracks or leaks in it.

If the wall is not rendered or clad, all pointing must be in good order or replaced if not. If the wall is made of pre 1940’s traditional brick or stonework then it is likely that the wall will allow varying amounts of rainwater to penetrate, depending on the porosity of the brick, stone and mortar. In these scenarios, when you can’t change the external appearance with render, you can either insulate to a low level to ensure the wall continues to dry or you can apply an impregnating cream to the exterior to reduce penetration of rainwater. These creams reduce water penetration by up to 80% but only reduce the vapour permeability by 5-10%, meaning that although expensive, they can create a structure that can be safely insulated.

Types of insulation

There are many types of wood fibre internal insulation, each with their own properties and areas of use. Most systems are based on rigid insulation boards which require flat surfaces on which to be mounted. This usually involves applying a levelling coat of plaster to the wall to flatten it out. The wood fibre boards then require an additional coat, typically using a lime based plaster, to be applied to the boards prior to fixing them to the wall, ensuring that there are no voids between the wall and the boards.

If walls are very uneven, such as is common with poorly constructed brick terraces or random stone/rubble walling then there is a type of wood fibre insulation which can mould itself to the shape of the wall surface so long as the variation in the surface is less than 20% of the thickness of the board. This product is called UdiRECO and can save time during installation as there is not normally any levelling coat requirement. This avoids the vagaries of drying times and allows you to see where you are fixing the boards.

All these systems are mainly mechanically fixed to the walls and so the walls must be strong enough to take the fixings. Very soft, friable materials such as cob or very soft stone will often require either very careful fixing with longer fixings than usual or use of timber battening to provide a support for the boards.


Airtightness is very important for internal wall insulation as without good airtightness high levels of moisture can condense in the walls and cause damage to the insulation system and the building.

The airtightness layer should be formed on the inside or warm face of the system and is usually formed by the plaster layer. Penetrations should kept to a minimum which means that insulation should pass through suspended floors, behind partition walls and should have as few services running through it as possible.

Sealing of junctions with floor joists, structural beams and any openings should be carried out using airtightness tapes, such as Ampacoll Fenax, to minimise air leakage. Gaps between insulation boards and timber penetrations should be packed with flexible wood fibre insulation, such as SteicoFlex,to maintain the wood fibre insulation’s ability to transport moisture away from joist ends and other structural timbers. Expanding foam should not be used to fill these junctions as this prevents this from happening.


Each system has it’s own specific set of installation instructions so the below is a general overview. Please ensure when installing systems you closely follow the manufacturers instructions.

Prior to installation make sure of the following:-

  • all surfaces are dry, stable and will take the fixings
  • none of the wall areas to be insulated are below ground level
  • no moisture is penetrating the wall from the exterior
  • all roof/wall abutments are weather tight and will not cause water penetration

If there are gypsum plasters or vinyl/acrylic painted plasters on the walls they should be removed prior to installation as they hinder the moisture flow through the wall.

A levelling coat using Baumit RK38 should be applied to the walls if there is variation of more than 4-5mm in the surface. This should be ruled off to create a flat surface which is then left to dry. Once dry the boards can be attached to the wall, starting at the base.

If the wall has less than 4-5mm variation then a coat of the system plaster (usually a lime or clay based plaster) can be applied to the back of each Diffutherm or UdiTHERM NF board prior to fixing against the wall will ensure that each board is fully connected to the wall with no voids behind it. If the wall is completely flat, such as old lime plastered walls then there should be no need to apply the backing coat, although this should be check with your supplier.

If using the UdiRECO system, voids deeper then the capabilities of the board should be filled with a hydraulic lime based mortar/plaster and allowed to dry prior to fixing the boards.

Start the first row of boards at the bottom of the wall to be insulated in landscape orientation. If working on a suspended floor then the boards can be slotted down around the floor joists so long as this area is still above ground level. This will help to manage moisture around the joists ends, reducing the risk of any moisture damage.

Boards should be cut to size and checked in situ prior to either applying a backing coat plaster to them or to fixing. Once correct, position the board in the correct location and drill in 2 fixings to hold the board loosely.

Fixings are drilled directly through the surface of the board, approximately 100mm from corners and edges, to a depth of at least the length of the fixing plus 20mm. e.g. If you have 115mm fixings, holding a 60mm board then ensure the drill penetrates the board and wall to a depth of at least 135mm. For internal insulation 6 fixings per m2 is the normal number of fixings required but please consult system manufacturers guidance to confirm.

Fixings into masonry should penetrate the masonry wall by a minimum of 40mm for hammer type fixings but typically 65mm for the UdiRECO system fixings. For solid boards hammer type fixings, such as Ejot STR-U or Fischer Termoz, are the cheapest and quickest although on soft masonry such as soft brick, Thermoplan/Porotherm or soft stone, screw fixings such as UdiASSEMBLY Masonry fixings would be advisable.

Once the first board is held in position loosely subsequent boards can be added to the wall, following the same procedure. Vertical joints should be staggered by at least 300mm so it is advisable to install the boards in horizontal rows with the off-cut from one end being used to start the next row at the other end. Complete wall panels fully before moving on to adjacent panels.

Once a board has all it’s neighbouring boards added then the rest of the fixings for that board can be installed. This is to ensure that the tongue and groove connection can be made for each joint as if the boards are tightly fixed first it is very difficult to make that connection later.

Internal corner joints are simply a case of butting the boards tightly against each other and glueing if necessary but external corners require a little more detailing. External corners are a butt-joint which should be made carefully and also glued with specialised glue, such as UdiSPECIAL ADHESIVE. The boards on the first panel should be left to oversail the corner by 5-10mm so that they can be cut or planed back accurately when the boards on the adjacent panel are installed.

When installing boards up to window openings, horizontal and vertical joints should be kept 300mm from the corners of the opening. If this is not possible then the joints should be glued to prevent cracking.

When installing the top layer of boards up to an existing ceiling it may be necessary to cut the back part of the groove off of the bottom of the board to enable the board to be fitted. This allows a tight fitting against existing structures but does require the joint to be glued thoroughly.

UdiINRECO System

The procedure for installing this system follows the same principles as above but fixing into random stonework is different. To ensure each UdiASSEMBLY masonry fixing is drilled in to stone rather than mortar, the fixing guide should be used. This is a quick process of placing the guide against the wall, marking the preferred fixing positions and then transferring the markings to the surface of the board. This process is repeated for each board.

Each board is fixed loosely and the whole wall panel is boarded as above but as the rest of the fixings are installed the boards are pulled in tight against the wall. Using a vertical laser level or a long spirit level tighten each screw fixing to bring the boards into the correct position. Impact drivers should not be used for installing these fixings as they tend to damage the heads of the screws, preventing further adjustment.

Internal corners are treated as a simple butt joint however, as above, external corners need careful detailing. Insulation from the first completed wall panel should over-sail the corner but the full depth of the insulation (e.g. 100mm insulation should over-sail the corner by 100mm).

Once all of the fixings have been installed and tightened to the correct position then this corner should be cut in-situ to give an over-sail of the depth of the flexible backing on the board. E.g. for the 100mm board the final depth of the flexible backing will be approximately 60mm. This allows the adjacent panel to form a lap joint which can be glued prior to tightening the fixings.

Window & Door reveals

Window and door reveals are detailed similarly for both of the above systems. The boards should be finished flush with the edges of the openings and a 20 or 40mm wood fibre reveal board should be used to insulate the reveals.

The reveal boards should be bonded in place using the system plaster as an adhesive on to the masonry areas with system glue used where the boards lap on to the edges of the main insulation boards. Generally the heads are installed first with the sides second and sill last. This ensures that the boards don’t drop out of position whilst waiting for the adhesive to set.

Services and attaching fittings

Service runs through the insulation boards should be avoided if possible. However, 20mm chases can be cut into the surface of the wood fibre boards to allow plastic electrical conduit and back boxes to be installed. These should all be well bedded and sealed in the system plaster to maintain airtightness and no metal fittings (ducting/back boxes) should be used other than stainless steel.

Diffutherm and UdiTHERM NF boards will hold lightweight fixings such as small light fittings by screwing directly through the plaster layer with a 50 x 5mm wood screw or for medium weight items you can use the Fischer FID50 spiral insulation fixings. The surface of the UdiRECO board is quite tough and even medium weight fitting can be held by simply screwing in 5-6mm diameter wood screws.

However, to hold heavy weight fittings such as shelves, you need a fixing into the masonry behind. This can be achieved with products such as Fischer Thermax Fixings which incorporate a thermal break to minimise additional heat loss.

Applying plasters

The plaster layer forms the airtightness layer for most wood fibre internal insulation. It is very important that this is continuous so any penetrations must be well air-sealed prior to applying the plaster layer. All openings should use a render edging strip to maintain good levels of airtightness between the plaster layer and the frame of the opening.

The entire surface of the wood fibre boards should be plastered, even if they will later be covered by another layer, such as timber panelling.

The below video shows how the first coat of plaster is applied to wood fibre boards, including the reinforcing mesh. The video shows the plaster being applied to an external surface but the application process is the same for the interior.

Once this coat has cured (at a rate of 1mm of plaster thickness per day) this can be finished with a fine lime finish plaster, such as Baumit Klima Glatt or Baumit RK70. Clay finishes such as Baumit Viton Fein can also be applied.