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Internal Wall Insulation

Internal Wall Insulation

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Most of the houses we live in now will still be homes decades from now. However, it will be difficult for us to meet their current heating demand with only renewable energy. For us to make a zero carbon future possible, we must therefore take measures to improve old houses. Effective external and internal wall insulation techniques are a vital part of this.

For some older houses that have very thick stone walls, internal insulation may be more suitable than external cladding. It may also be appropriate if your house is in a conservation area, with restrictions on changing the appearance. Sometimes it’s necessary because narrow streets and pavements leave no space for external cladding. Internal insulation can be practical as part of a spread out retrofit, where you are upgrading your home in stages.

Inside and out

You may find that a combination of internal and external wall insulation will work well. This could be partly to satisfy planning permission requirements, but also for practical reasons.

For example you could use internal wall insulation at the front if you can’t change the existing façade. But you could switch to external insulation for a mostly blank side wall. Internal insulation may be impractical if a staircase runs alongside the external wall – because it would make the stairway too narrow.

You must overlap the insulation when externally-insulated and internally-insulated walls meet. If you don’t you’ll get a ‘cold bridge’ at this corner – a weak point in terms of heat loss. To prevent this, continue the internal insulation layer around the corner to give an overlap of around 300mm to 600mm. This will leave a step in the wall, so you could add a bookcase or some other detail to give a tidy finish.

Choosing insulation materials

Conventional insulation panels are slabs of mineral wool or plastic foam. At CAT we promote low-impact alternatives made from natural fibres because these involve lower pollution and energy use in manufacture. Natural materials should also be easier to recycled or dispose of at the end of their useful life. In addition, natural materials tend to promote breathability and this is often vital for older homes.

Natural low-impact internal wall insulation options include wood fibre boards, hempcrete, or perhaps fluffy insulation batts within a timber framework.

Rigid wood-fibre insulation usually comes as a package – including screw & plug fixings, mesh, membrane, and renders. Using a system like this should ensure the materials are compatible, so you don’t get cracks and damp ingress.

Hempcrete is a mix of hemp fibres with a lime binder. We used it at CAT to form the walls of our WISE building, but it’s also great for renovation projects. The hemp and lime mixture can be sprayed on, or hand-filled behind wooden shuttering, with timber uprights for structural support. Experimentation is going on into the use of other fibres (such as miscanthus grass) and other binders (such as clay).

You could perhaps use fluffy insulation like sheeps wool or hemp batts within a timber framework. You can’t just put these materials against a cold wall – you need an approach to protect from damp. A layer of cork is a possible solution.

Risk management

Some types of plastic foam insulation can be thinner than natural materials for the same insulation value. However, there are risks from using an impermable layer on an internal wall. Moisture build-up within the wall could eventually result in damage to the building fabric and the insulation.

Check if the insulation you plan to use is certified for use without a vapour control layer (VCL). Foil-backed plasterboard is sometimes used to give a vapour control layer on the warm side of the insulation. However, it’s hard to get continuity to this layer with all the joints, and this could cause problems. It may be better to have a separate membrane as the VCL, and then a standard plasterboard or other finish.

A thick insulation layer can bring the dew point of the wall too far inwards. This leads to the risk of interstitial condensation – where moisture condenses somewhere within the wall. It is possible to make a technical case under building regulations to justify an insulation layer slightly below standards. For example to propose the use of a breathable material that will not cause damp problems in an old building. As part of a well-planned retrofit, this can still perform well. It may also be worthwhile to put in some monitoring to keep track of damp levels.


When using boards you should fix them over a continuous plaster coat for a good air-tight finish. This is much better than the standard ‘dot and dab’ method of attaching boards. The dot and dab approach leaves lots of air movement in the air gap, which leads to heat loss.

If the existing internal finish is a breathable lime plaster then this could be left as the continuous layer. However, you’d really want to remove gypsum plaster and add a new breathable layer before adding internal insulation.

To get to really good levels or air-tightness, you’ll need some good quality tape to use at edges and joints. Although this tape is expensive, it’s useful where strength is needed – at window reveals, corners, and so on. At some flat junctions you could perhaps use a cheaper foil tape (the type for sealing insulation to pipes).

Adding a continuous insulation layer internally will involve cutting away the perimeter of any intermediate floors. Otherwise you’ll get air leaks and heat loss from the ‘thermal bypass’ due to the uninsulated strip left between floors.

You need to pay careful attention to where floor joists run through the insulation layer and into the cold wall. Seal with tape where the joists go into a wall, then add the insulation layer cut to shape around the joist, and then tape the insulation layer to the joist.

Windows and doors

When adding internal insulation, you must continue the layer within the window reveal and up to the frame. Otherwise the surround will become a thermal bridge (a path for heat to escape), vulnerable to condensation, damp and mould.

You won’t have much space available in a window or door reveal, and may need to use different insulation. In an older house, cleaning back the plaster can free up a useful gap. If you’re replacing windows as well, then installing a slightly smaller one might help in some locations.

Sometimes the shape or position of a window will leave very little space for insulation. In these cases, a small amount of a very thin but higher-specification material could be appropriate. For example a composite of plasterboard with high-performance ‘aerogel’ insulation.

Check with the installers to see how the detailing at windows and doors will be carried out. Make sure that the approach being taken will avoid thermal bridges.

Funding and Support

To find out more about funding that you may be able to claim for energy saving measures, check with your local authority. At the moment, support is usually for households on low incomes or those claiming certain government benefits, but it is being expanded to more people in April 2023.

See the Government’s Help to Heat summary page for an overview of the funding schemes. In Wales ask Nyth/Nest scheme (0808 808 2244) or Warm Wales about energy grants for households. In Scotland or Northern Ireland try either Home Energy Scotland or NI Energy Advice.

Finding professionals

External insulation may well be part of wider ecoretrofit, so see our page on whole house eco-retrofits for more about that process. As mentioned there, you can find local eco-builders using the member listings of the AECB, who promote sustainable building and retrofit. Another listing site for green builders is the Green Register. More widely, the Government’s TrustMark scheme now covers professionals that carry out energy efficiency work including retrofits.

Further information

CAT’s short courses offer lots more advice and hands-on experience with eco-building and renovation. See also these blog pages by our expert course tutors: key advice for a successful eco-refurbishment, and how to improve traditional buildings.

See also a case study of internal insulation on an old stone house by Judith Thornton, who has worked and taught at CAT and is now researching new low carbon building materials. She’s using a miscanthus-lime mix, or a cork-lime plaster in some places.

Information about conventional solid wall insulation options and prices is available from the Energy Saving Trust. Historic England publish some free guides on the sympathetic insulation of solid walls in old buildings.

Related Questions

What is breathability?

This can be a confusing term for two reasons. Firstly because it refers to moisture not air and secondly because the exact physical processes which go on in a building are complicated (and still being researched). However the metaphor of breathing captures the important idea of continual movement. Moisture in itself might not matter, as long as it doesn’t take up permanent residence in your walls.

Broadly, breathability is the ability to buffer moisture – to soak it up and then release it – thus avoiding a situation where water vapour is retained by the fabric of a building. The technical term for this property is hygroscopic. Natural materials tend to be more hygroscopic than synthetic ones. The term “vapour permeable” also gets used in this context, referring to materials that allow water vapour to pass through at a certain rate.

A breathable construction will be made up of a gradation of vapour permeable materials, with the materials towards the inside being less vapour permeable than the materials towards the outside. A rule of thumb is that the material on the inside (warm side) should have five times the vapour resistance of the material on the outside (cold side). Without using a very high resistance material such as polyethylene or foil-backed board, this can be achieved with a suitable vapour-check layer on the warm side of the insulation. This layer slows down the passage of moisture to a safe level.

The construction also needs to be airtight on the inside to ensure the vapour-check layer works as intended. So it’s important to not put gaps in the vapour check layer for services (wires, etc), and instead have a thin space for these services between the vapour check and the internal finish (e.g. plasterboard). The final layer of external cladding might be waterproof (such as tiles) but there must be a ventilated gap between this and the sheathing board that actually completes the breathable construction.

In a UK-type climate the interior of a house usually has a higher relative humidity (RH) than the outside because it contains people exhaling, cooking, and washing. It’s a very general rule that, left to themselves, things will always migrate from an area of higher concentration to one of lower concentration, so there will be movement of water vapour towards the outside (or, to be more technical, the interior has a higher vapour pressure).

At times of high interior RH, water vapour will enter the walls, lowering the RH (both high and low RH cause problems and the ideal range is 40-60%). At times of lower interior RH, almost all of the water vapour will migrate back into the interior to be taken away by air currents. Any water vapour that might have managed to penetrate the wall deeply will be drawn out towards the exterior because of the lower permeability of the outer layers, so no harm done. Any water coming in from the outside will be unable to get very far as it is pushing through layers of decreasing permeability. The breathability of materials interacts with both temperature (which affects RH) and ventilation (one of whose functions is to carry away water vapour).

How do I choose between insulation materials?

There are basically three different types of insulation material:

  1. Organic – those derived from natural vegetation or similar renewable sources, which tend to require a low energy use in manufacture (a low ‘embodied energy’). Examples are sheep’s wool, cellulose, cork, wood fibre, and hemp.
  2. Inorganic – derived from naturally occurring minerals which are non-renewable but plentiful at source. Likely to have a higher embodied energy than organic materials. Examples are mineral/glass fibre, perlite and vermiculite (from volcanic rock) and rigid foamed glass.
  3. Fossil organic – derived by chemical processes from fossilised vegetation (oil) – a finite resource. Fossil organic insulation materials such as expanded polystyrene and polyisocyanurate or phenolic foam are highly processed, resulting in a high embodied energy.

Which is best?

If possible it is better to choose insulation materials that have not been heavily processed as this will reduce the carbon footprint and environmental impact of your home. But it is far better to install cheaper inorganic or fossil organic materials with the right physical properties and a low thermal conductivity than to install nothing at all.

In many cases, organic insulation material can be applied instead of inorganic or fossil organic, but there are exceptions. For example, there is not an organic insulation material suitable for cavity wall insulation.

Think also about the ease of installation. Loose fill insulation is quick to put in in lofts, but cannot be DIY installed in anything other than a flat place. Rigid boards and batts will come in certain sizes, but need to be cut to shape if you have some unusual spaces. Some materials can be cut with a knife, but a few will need a saw. Some mineral wool now comes in a thin foil or plastic wrap, to protect from the fibres. You should still wear a face mask when installing any type of installation, as small fibres of any kind are best not inhaled.

How could I insulate a flat roof?

The first two of the following four methods involve adding insulation to the outside of the roof, so will be suitable if there is little headroom underneath, or if access is difficult. The second two involve adding insulation underneath the roof. If insulating on top of a flat roof, make sure that it still drains well so that water does not pool on top.

Upside Down or Inverted Roof

An ‘upside down’ roof uses waterproof insulation on the outside of the building structure. The insulation is laid over the existing waterproof membrane and held down with something – which could be pebbles, turf (for a green roof), paving slabs, etc. Suitable insulation materials will tend to be a bit more expensive, and include cork, foamed glass and closed-cell plastic foam. You’ll need to check that the structure can bear the weight of the insulation and finish. This option keeps the existing membrane, but there is a risk that water will percolate through the insulation and so cool down the roof deck – causing condensation.

Warm Roof

A warm roof will have the insulation laid over a vapour control layer (itself over the roof deck), with a membrane laid over the insulation and suitable finish on top. You’ll need to check that the structure can bear the weight of the insulation and finish. If you are replacing the roof membrane anyway, then this will be a better solution than the ‘upside down’ roof (above), as it will keep water above the insulation and so keep the roof decking warm. You could still keep the existing membrane underneath the insulation if it would be difficult to remove.

Cold Roof

The insulation is put between the roof joists. A ventilated gap needs to be retained between the top of the insulation and the roof decking, to avoid condensation build-up. It can be difficult to get adequate ventilation, so this method is often not recommended.

Internal Insulation

A method similar to dry-lining of walls can be used, with a plasterboard/insulation board added to the underside of the internal roof, below the joists.

How could I insulate a pitched (sloping) roof?

A ‘cold loft’ roof, with the insulation at ceiling level (laid flat in the floor of the loft) is generally the most economical, and easy to install. However, if you want to insulate in the slope of a roof in order to make use of the space, then here are some tips.

The most economical way of achieving a good thickness of insulation in the roof slope is to have two layers of timber, one supporting the roof finish and another supporting the insulation and ceiling finish. Batts or loose fill insulation is generally better between rafters, because it will give a tight fit. Otherwise rigid material has to be very precisely cut to fit well.

To reduce cold-bridging, timber I-Beams or I-Joists can be used when adding new beams or building a new home.

In an existing roof (with rafters supported on roof beams), the second layer of timbers (ceiling joists) can be hung off the rafters using hardboard, ply or timber ‘hangers’, or nailed crosswise to them, or they can span between the roof beams. This technique can also be used with flat roofs.

An air space of 50mm must be left between the insulation and the tiling felt, unless the felt is of a low-vapour resistance type. If using a breathable membrane, with insulation up against it, then above the membrane you would put counter battens (top to bottom) as well as the standard battens (side to side), for adequate ventilation beneath the tiles. Sometimes, a breathable membrane is used with only standard battens, with the membrane slightly draped between rafters to allow ventilation – in this case an air gap of about 25 mm would be needed between membrane and insulation.

Pitched roof insulation
Pitched roof insulation

Thin wood-fibre boards (22 or 35mm thick) can be used as an alternative to a membrane under tiles. When re-roofing, the fibre-board is laid over the rafters, and then counter battens (in line with rafters), and then standard battens to fix the tiles/slates to. Thicker wood fibre boards can also be used, to give more insulation and achieve a lower U-value (to minimise heat loss).

A ‘warm roof’ will have waterproof insulation on the outside of the structure (so the main timbers are on the warm side of the insulation). It’s a useful way of upgrading an existing roof when internal room height is at a premium. On a sloping roof, the tiling battens are supported by rigid insulation and fixed through to the rafters by special screwnail fixings. The insulation must be waterproof, such as cork, foamed glass or closed-cell plastic foam board – these will tend to be more expensive than the standard insulation materials for internal use.

You can also read more at the webpage of the Energy Saving Trust, on Greenspec’s pages about ventilated and unventilated roof insulation options, and in a guide from Historic England.

How could I insulate a solid floor?

Traditionally, solid floors were laid directly onto soil. This relies on the ground underneath being kept dry, usually by it being higher than the ground outside the building, and by having adequate drainage.

The most common method now used is to have a thick concrete slab laid on a damp-proof course (e.g. a polythene membrane). A layer of polystyrene insulation is then finished with sand/cement screed and tiles or board.

For a low-impact alternative to the above you could look into using recycled aggregate in the concrete (rather then newly quarried material), and perhaps using stabilised earth as the screed. You could also consider using recycled polythene or bitumen for the damp-proof course.

A solid floor of stabilised earth or limecrete should have a solid insulation material below it, such as cork, perlite or foamed glass, with recycled polythene vapour check and damp-proof membrane (DPM) below this.

Try to achieve at least 150mm of insulation for a solid floor. Insulation should be placed around the edge of the floor, and the floor finish supported on some sort of rigid insulation. Possible materials include cork, perlite (volcanic glass), lightweight expanded clay aggregate (‘Leca’), foamed glass (slabs or granules), fibreboard, mineral wool boards, or plastic foam of some sort. A vapour check layer will normally be required to prevent condensation occurring within the insulation layer.

Another possibility is a hemp & lime (or ‘hempcrete’) floor. Lime has a much lower environmental impact than cement, so if you can use it place of cement in mortars or concrete you will be reducing the ’embodied energy’ of the floor and the carbon emissions from construction. The hemp provides the insulation. See for example the details of how we insulated the WISE building at CAT.

If you are redoing a floor, then you may have the chance to consider underfloor heating. Because it runs at a much lower temperature than standard radiators, wet underfloor heating is more efficient and provides a more comfortable type of heat. It’s particularly appropriate for use with heat pumps, as these need to supply low-temperature water to run efficiently.

You can also see some information and examples on the Energy Saving Trust and Historic England websites.

How could I insulate a suspended timber ground floor?

If you can access the floor from below via an unheated cellar or basement it will be easier, otherwise you’ll have to lift the floorboards (which requires care to avoid damaging them).

225mm of a renewable or mineral fibre type of insulation is a decent amount. Make sure you keep good ventilation to the underfloor space beneath the insulation – with vents at either side for air flow.

Batts or loose fill insulation is generally better between joists because it will give a tight fit. Otherwise rigid material has to be very precisely cut to fit well.

Renewable insulation will need to be protected by a breathable membrane to protect it – if the floorboards are not well sealed. See the website of the supplier of insulation you’re using (or call their advice line) for advice on the type of membrane that would be needed. Natural insulation materials include hemp, sheeps wool, recycled textiles, recycled paper (loose fill), and wood fibre.

Loose fill insulation can be carried between the joists on a membrane or netting nailed to floor joists or on a low-vapour resistance board (for example softboard, a fibreboard bonded by heat rather than glues – this is good for Warmcel insulation made from recycled newspaper).

For other insulation materials (e.g. standard mineral fibre types) it will be worth looking on the website of the manufacturer as they’ll usually have guidance sheets on how the material should be installed and what limitations there may be.

You can also see some information and examples on the Energy Saving Trust and Historic England websites.

How can I reduce condensation in my house?

Condensation is due to excessive moisture, cold conditions, cold surfaces or inadequate ventilation. It can cause mould, heat loss and building damage. To address these issues, the room should be properly insulated and adequately heated (to keep the surfaces warm).

So do take all feasible insulation and draught-proofing options, and look into improving single glazing with either replacement glazing or with secondary glazing (a cheaper option).

Condensation may still occur on replacement windows, as they’ll still be the coolest surface. New windows will be more airtight than old ones, so warm moist air will be no longer be escaping through cracks in the frame and around the seals. This means that existing damp issues may become more pronounced. Many windows will include trickle vents in the frame, to allow a small amount of ventilation, but to keep the house warm and dry you may need to take a few other measures to avoid producing lots of moist air.

Drying clothes indoors can easily cause problems of damp and condensation, leading perhaps to mould, etc. So if you need to dry indoors, it should be in a room that can be shut off and ventilated (perhaps with heat recovery, as mentioned below).

The bathroom and kitchen in particular should be able to be ventilated in a controllable way, to stop moist air circulating into the rest of the house. For example, after a shower or bath, leave the bathroom door shut and the window open for a while until moist air & condensation on the window/mirror has cleared. Do the same when cooking if you can; if your home has an open plan layout at least stop the moist air circulation where possible (e.g. to the upstairs rooms).

To avoid heat loss from a room like a kitchen or bathroom, where lots of moist air is regularly produced, you could consider a heat recovery extractor fan. It may be worth getting a slightly bigger heat-recovery fan unit than you need, as they can be a bit noisy if they are operating on full power. This may be fine if are just switching it on for a short while to clear the bathroom, but it could be obtrusive in the kitchen. If a fan unit is not supplied with more solid covers over the plastic slatting, it could be worth fitting something if you live in a house that is a bit exposed to the wind (as they could let in draughts when not in use). An openable wooden casing could be fitted quite easily.

If drying clothes indoors is not an issue, and you’re already careful about venting away moisture from bathroom & kitchen, then excessive condensation may be from some other cause, such as a water leak somewhere (e.g. from a pipe under the floor or in the loft), or water penetrating the structure from outside (such as rainwater coming in cracks in masonry, or if gutters are broken). If problems persist, it would be worth investigating these issues, as over time they’ll cause damage to the building.

Do I need to retrofit a new damp proof course?

A damp proof course (DPC) is a layer of material between the foundation and the rest of the building which prevents water from getting into the building fabric. Traditionally these were two overlapped layers of slate or lead, but some old buildings do not have them at all. Modern buildings tend to use DPCs based on bitumen or polythene (perhaps recycled plastic), and they’re standard and cheap to fit at the construction stage. For a timber frame building with minimal foundation you will only need a small amount of DPC material. A DPC is important for this type of construction because timber can soak up moisture and will rot if not able to dry out easily.

Do I need to add a damp proof course to my old house?

There is a concern that you must fit a DPC to satisfy your mortgage company. However, many won’t actually insist on this for an older property if there’s no evidence of a damp problem. There are several possible causes of damp and if you can identify the cause and solve it, perhaps with some feedback from a professional, this is often enough.

‘Rising damp’ is often just a catch-all term for damp issues that could be solved in other ways. Whereas retrofitting a chemical DPC can turn out to be a waste of money, quite apart from the environmental impact of the chemicals. If it doesn’t solve the problem you may find that you’re not covered by the guarantee. A subsequent drill test which shows that there’s no ‘rising damp’ could be taken to indicate that the DPC worked and you just have a different problem – despite this other problem probably being the case all along!

So what should I do?

First check your roof, gutters, pointing and ground levels (or get a professional to do this). Drainage around a building is important. If possible, dig out anywhere where the ground level is higher outside than in. In some cases, digging down 200-300mm and putting in stones or gravel will help drainage around the site. Fixing these problems will cure most damp problems.

Another possible cause of damp is condensation, which generally occurs on the coldest surfaces. This can be lower down walls, so looking like ‘rising damp’. To address condensation your home needs to have adequate insulation, heating and ventilation. See our information pages on these topics for more advice.

If walls have been affected by long term damp, then even when the problem has been solved it will usually involve re-plastering internally. This is because the original plasterwork will be affected by hygroscopic salts, which could result in continued damp problems. A building may take a year to dry out.

What if there are still damp problems?

There can be a problem if a DPC has been bridged – for example with render. Water can potentially move up the render where it can’t move up the brickwork or masonry. This could also cause problems with frost damage. A DPC must extend through the full thickness of the wall, including pointing, rendering or other facing materials.

When retro-fitting a DPC in an older property make sure that any original lime-based plaster or render that was removed gets replaced with the same sort of material. Lime-based materials are breathable and flexible, unlike strong cement renders. The latter can cause serious cracking problems where the old plaster meets the new.

To accurately test for moisture a company would need to do proper testing, such as a drill test (possibly time consuming and costly), rather than just using a damp meter. If you’re convinced you have rising damp then get a proper drill test done by an independent local surveyor.

Further information is available in this article on the misdiagnosis of rising damp and solutions to damp problems, written by Tim Hutton. See also Abbey Independent Surveys and Mike Parrett, who has investigated thousands of buildings without finding rising damp. In every case, damp was attributable to something else.

What is a U-value, and what is thermal conductivity?

The U-value is a measure of how many watts (representing the rate of flow of energy) pass through one square metre (m²) of a construction detail (such as a wall) for every degree difference in temperature between the inside and the outside. Temperature is measured in kelvin, and 1K = 1°C (degree centigrade).

As an example, a U-value of 6.0W/m²K for a single glazed window means that six watts will be escaping through each square metre of glass when the temperature difference is one degree. If it is 20°C in the house and 0°C outside, then the heat loss is 20 x 6 = 120 watts per square metre.

U-values are generally used to describe the thermal performance (heat loss) for a section of construction that involves several materials – such as a wall made up of timber, insulation, board & render.

Thermal conductivity

For individual materials, such as a type of insulation, you’ll come across the term ‘thermal conductivity’, also known as a k-value or λ-value (lambda). This is the rate at which heat flows through a particular material, and good insulation will have a low thermal conductivity. It is measured in watts (heat flow) per metre (depth of material) per degree difference (inside to outside), so the unit is W/mK.

Most natural insulation materials (hemp, wool, recycled paper or textile) will have a thermal conductivity of about 0.035 to 0.040 W/mK, which is similar to the performance of conventional mineral wool insulation. Foil-backed plastic foam insulation boards are slightly better, with thermal conductivity about 0.023 W/mK. So about 100mm of the plastic foam board will give equivalent insulation value to about 150mm of the various woolly types.


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