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Insulation reduces heat transfer, thus

  • Reduces heat loss in winter, saving on heating costs and improving comfort
  • Reduces rate of heat gain in summer, saving on ac costs.
  • Prevents freezing of pipes
  • Reduces run cost of hot water systems

Main DIY Applications

  • Wall cavity insulation (for either original wall cavities or a retrofitted cavity behind plasterboard)
  • Solid wall insulation (fitted to wall surfaces)
  • Loft insulation
  • Pipe insulation to prevent freezing or heat loss
  • HW cylinder insulation
  • Underfloor insulation

Forms of Insulation

Still air is a good insulator, and is the basis of more or less all insulation products currently used in housing. One way or another, each insulation product traps air, and its the air that does the job.

This means that if 6" of insulation is compressed to 4", it has the insulation value of 4" of material. In practice the change in size of the trapped air bubbles means that the value is affected slightly less than this.

If insulation gets waterlogged, its insulation goes down greatly.


Cavity walls insulate much better than solid (non-cavity) walls, and are an early example of domestic trapped air insulation. Walls with no cavity can have a trapped air cavity attached by fitting battens and plasterboard - but there are better options now.

Large cavities allow some air movement, hence foamy and woolly insulation products give better insulation than a cavity alone.

Old cavity walls are also often open cavities, and in some cases end up with wind howling through them, much reducing their insulating effect.

Battens and other barriers to air movement improve an empty cavity's insulation a little, but nowadays denser insulation products are available that provide much better insulation value, and are well worth using.

Sheet or slab

Sheet or slab insulation is the most dense and effective type of insulation, and is used where insulation value per space needs to be maximised. This primarily means for roofs and solid insulation for walls.

Tough rigid sheets are used under floors, where walkability is required. Polystyrene also gets used in this situation, but tends to sag over time from foot traffic.


Roll insulation is far quicker to fit than sheet, and is mainly used for loft insulation. It is also used in wall cavities when its lesser insulation value is sufficient in the given space.

Loose fill

Loose fill is ideal for odd shaped or inaccessible spaces where the insulation will stay in place due to gravity alone. The biggest use for loose fill is for filling old cavity walls.

Insulation that doesn't insulate

Insulation is mainly used to insulate, but not always. Insulation material is often fitted into interior stud walls to damp sound transmission. Fibreglass & rockwool are the favourites, they're cheap, fireproof, readily available and acoustically absorbent to some extent.

Insulation materials

Expanded Polystyrene

now we know what polystyrene is shipped in

Low cost insulation available in

  • slab form
  • as small beads for loose fill & beanbags
  • shredded, cheaper than bead but less able to shed water
  • larger chunks for packaging

Mainly used for:

  • Roof insulation
  • Wall insulation
  • Floor insulation

Polystyrene is flammable, and produces toxic smoke on burning. This problem is made worse when painted with oil based paints.

Sometimes used for cavity wall insulation

  • Like any retrofit cavity insulation, it can saturate with water and cause damp problems
  • Beads are generally glued as they're injected, but in many cases the glue is omitted, and beads then pour out of any hole made in the wall
  • No longer the favourite cavity filler

Polystyrene in wall cavities is relatively well protected from fire, and is not considered a fire safety issue. However if PVC electrical cable is fitted in the wall cavity, as is occasionally done, polystyrene cavity insulation in contact with it causes the cable to leach plasticiser and become unsafe.

Sheet polystyrene is readily cut with any sort of knife, even a breadknife.

Extruded Polysyrene

Usually in slab form with edge rebates, this closed-cell version is much stronger than expanded polystyrene. It will bend without breaking, it can be sawn or drilled cleanly. It does not compress or deform much and can take minor loads. It is almost completely waterproof having a moisture retention content of a fraction of 1%.

  • Used in inverted roofs between the membrane and the capping slab
  • Used under floor screeds, especially for underfloor heating.
  • Used by RailTrack as lineside insulation, I hear, so shows that it's tough.

Slabs of 600 by 1200 sold in thicknesses of 25 to 200mm. (example StyrozoneN300)

Ceiling tiles

Insulating decorative ceiling tiles

  • Dangerous in a fire
  • Spreads fire rapidly
  • Generates thick toxic smoke when burnt
  • Gloss painting them makes them even worse for flammability
  • Ugly
  • Best removed on safety grounds

Polystyrene veneer

A thin crack covering layer on walls a few mm thick

  • Primarily used to stop condensation, and sometimes cover bad cracking
  • Dents very readily
  • Grim news in a fire
  • Best removed on safety grounds


Fibreglass insulation 5662-8.jpg
  • Comes in roll form, as loose fill and as semi-rigid batts
  • Lasts the life of the building no matter what life throws at it.
  • Waterproof
  • Insectproof
  • Verminproof
  • Fireproof
  • Springs back to shape if squashed
  • Lightweight
  • Loosefill fibreglass can be blown about in a draughty loft
  • Produces prodigious loose glass fibres
  • Tiny spiky glass fibres cause splinters
  • Concerns about its safety are occasionally expressed on the basis that it releases miniature sharp spikey non-dissolving airborne fibres, somewhat like known carcinogenic forms of asbestos. Fibreglass gradually dissolves in the lung, so doesn't pose the same risk
  • Dust mask should be used during handling
  • Gloves are also recommended to reduce splinters
  • Compressing a layer to improve insulation should be avoided. Squashing 6" down to 4" gives you about the insulation value of 4" of fibreglass, and the pressure can damage plasterboard.
  • Much used for loft insulation
  • Not a great choice in a windy position, wind can blow through it


A mineral fibre very similar to glass fibre, but denser, so much less airborne fibre.

  • Popularly used for loft insulation & internal stud walls
  • Comes in roll form, as loose fill and as semi-rigid batts


Celotex 4644-4.jpg
  • Rigid foamed boards
  • High R value per thickness
  • Good choice for lining walls
  • Kingspan & Celotex are the leading manufacturers
  • Seconds with minor defects are cheaper
  • Survives temperatures upto 400C


Cellulose insulation 1626-3.jpg
  • Low cost
  • Loose fill
  • Used in cavities
  • Plant fibre
  • Made from ground newspaper
  • Better performance than fibreglass
  • Slumps a bit over time, so not suitable for wall cavities
  • wikipedia
  • Appliable wet or dry - dry is more common for diy

Wet sprayed cellulose insulation:

  • Sticks itself in position
  • Same insulation value as loose fill
  • Doesn't slump
  • Must dry out before being boarded over



Used under timber floors

  • thermal insulation
  • sound absorbing
  • slight springiness
  • 3mm & 6mm common
  • breaks easily

Neoprene pipe insulation

Neoprene pipe insulation 5662-4.jpg
  • Foam-like tube pre-slit down one side
  • very easy to apply
  • Tape the slit shut or its much less effective
  • Corners in pipes are dealt with by cutting the 2 lengths of neoprene at 45 degrees
  • Tape the end gaps between lengths of insulation, and any other gaps or joints such as at corners


Felt pipe insulation 5665-3.jpg

Felt strip is used as pipe lagging

  • Its wound around the pipe(s) and secured with tape or a cable tie (don't squash the felt much)
  • Brown, hairy and untidy
  • Best used for pipes out of sight
  • Good for insulating grouped pipes, where foam jackets don't fit
  • Can be used for pretty much any shape and size of pipework
  • Foam tube is much quicker for insulating single pipes

PVC Cladding

  • Mainly used on gable wall ends
  • Keeps rain off the wall as well as insulating
  • Can make a messy wall look ok too, but not the world's favourite finish
  • Sometimes seen used internally, but tends to become hard to clean over time due to surface roughening, and not a popular finish even when new.

Timber cladding

  • Timber cladding is used as an interior and exterior finish
  • Provides a small amount of insulation
  • Several timber designs available
  • If fitting it anew, sheet insulation can go under the cladding

Insulating render mixes

Cement mixes containing polystyrene beads, leca or vermiculite

  • Fireproof
  • Paint to prevent water ingress and freeze/thaw damage
  • You can't have it all, to get fair insulation value the mix needs to be high in insulation beads, making it much weaker than concrete

Shredded Paper

  • Sometimes used as loose fill loft insulation
  • Waste paper is treated with fire retardant and chopped or shredded.
  • Recycled
  • Not damp tolerant


Cardboard 1in 4473-3.jpg
  • Very low cost insulation material
  • Rapid return on investment
  • Fitting used cardboard results in zero extra manufacturing energy use.
  • Cardboard wall insulation is popular in the US, but seems to have had little takeup here in the UK.
  • Its flammability can be resolved by painting it with a mixture of borax & boric acid.
  • Its damp susceptible, and can support mould if damp
  • Should not be used in situations where damp may occur.
  • Available free


Papercrete samples

A relatively novel building product, low density papercrete has good insulation properties and is made mostly out of waste paper and cement, with a number of possible additions including expanded polystyrene and fire retardants.

It can be cast into blocks, applied by tyrolean sprayer or poured into formwork.

Its not on general sale, its diy only. It requires lots of paper and a paddle mixer. It has a good track record so far, but is not BR approved for new builds.

Sheep's Wool

Sometimes used instead of fibreglass or rockwool.

  • Non-irritant, no protective clothes required.
  • Costs more than fibreglass & rockwool
  • Cheap if you or a nearby sheep farmer has wool they can't sell, unwashed wool is not allowed to be used
  • Insulation value low compared with PIR
  • This can result in excessive depth requirements in new builds, affecting ceiling height etc
  • Life 50+ years
  • Biodegradable
  • Chars, but doesn't spread fire
  • Treated against insects


Straw has long been used as insulation, and is another low cost option.

  • Susceptible to damp & mould, but less so than cardboard
  • Nest material for birds
  • Can contain unhatched insect eggs
  • Flammable, which can be resolved by plastering over the straw to deny the admittance of air for combustion.
  • Low cost
  • Used to be available in slab form, this is occasionally seen in use.
  • Insulation value per inch isn't great, works best where it can be applied thickly
  • Insurers may require MICC Cable (also see Terminating MICC/Pyro) in straw bale house construction

Clay beads

LECA expanded clay beads are occasionally used as house insulation.

  • Non-flammable
  • Unaffected by damp
  • Used as loose fill, mainly under floors


Similar to expanded clay beads

Bubble wrap


Only rarely used in houses, wrapped round water tanks or used as an insulating window blind.

  • Degraded by UV light over time.
  • Flammable
  • Used as greenhouse secondary glazing
  • May last ok in dark places eg lofts & cupboards.

Foamed Concrete

  • Lightweight concrete blocks have insulation value
  • Insulation is one of the reasons for their use as the interior leaf of exterior walls
  • They don't survive outdoors unless rendered.
  • The light weight makes construction quicker


  • Aerogel is a very low density glass/air gel rated at 0.013 W/mK
  • High temperature resistance
  • Its steep price restricts it to applications where high insulation value per size & high temperature rating are necessary. This does not include house insulation.
  • Sold by Spacetherm.


Bottles were historically used as floor slab insulation. A layer of bottles was laid before pouring the concrete, resulting in a degree of insulation plus reduced concrete use. The bottle shape produces lots of miniature arches in the concrete, which are a near ideal shape for loadbearing. For effective insulation there are now far better products.

Bottle walls and windows are used in alternative building, and create a pretty effect indoors, especially when mixed bottle colours are used. Bottle windows are quite usable for summerhouses etc. The bottles are used more or less like bricks, with thick cement mortar between them. The mortar provides most of the structural strength.

Bottles can be used for outbuilding floors where the budget is extremely tight, but insulation not needed. They displace some of the concrete.

When cement meets glass, ideally flyash or similar pozzolan can be added to the concrete mix to prevent the slight risk of ASR (alkaline silica reaction) causing cracking.


  • A whole lot of bottles needed
  • Bottles can be vandalised, but in practice such buildings are normally well appreciated and not vandalised
  • In principle, differential thermal expansion and ASR can both crack bottle walls, in practice they work ok though

Expanding foam

Expanding foam & tape tube extension 4661-4.jpg


Underlay 4514-3.jpg

Carpet underlays aren't used primarily for their limited insulation value. However on an uninsulated floor they do reduce heat loss to a degree, and that heat's worth money.

Perhaps some time someone will calculate what the payback is.


Cork 2824-2.jpg
  • Formerly used for insulation, but not much
  • Prone to rot & coming unstuck over time
  • The main use is for floorcovering


  • An obsolete insulator with safety problems
  • Sometimes needs removing for safety, but more often not
  • See Asbestos


Multilayered bubblewrap & foil type products are moderate insulators with debated insulation claims.

Unsatisfactory Insulation Materials


  • Too thin to give much insulation
  • Flammable
  • Reduces draughts

Polystyrene ceiling tiles & wallpaper

  • Too thin to give much benefit
  • Very flammable, spreads fire & produces toxic smoke
  • Dents very easily


  • Traps air but is thermally conductive


  • Degrades quickly from UV exposure

Spray-on Roof Insulation

  • Traps water, prone to causing rot
  • Makes the roofcovering non-reusable

Free insulation options

  • Cardboard sheet
  • Waste polystyrene
  • Straw
  • Bubblewrap for water tanks & cylinders
  • Scrap clothing as pipe insulation


The more popular choices for each job


Deep insulation is usually fine in a loft, so the ideal properties are low cost per insulation value, and easily laid roll insulation. The 2 main candidates are

  • Rockwool
  • Fibreglass

Less often used:

  • Shredded or chopped fire retardant treated paper
  • Sheep's wool (available in batts as Thermafleece or rolls from

Where insulation exceeds the depth of the joists and no floor is fitted, best practice is to fit the insulation in 2 layers, with the 2nd layer at 90 degrees to the first. This minimises any gaps and helps ensure the joists are completely covered too.

Where a floor is to be fitted, don't fit excess depth of insulation and squash it down. Any benefit of the extra depth is lost, and the ceiling may be bent or cracked by the excess pressure, or even come down.

When a floor will be fitted and its desired to use more insulation than the current depth of the joists, there are 2 options.

  1. The best is to add more timber along the top of the existing joists to deepen them, gluing and screwing it in place. This provides space and greatly [& stiffens the floor stucture].
  2. The simpler option is to insulate the existing depth, fit the floor, then lay out more roll insulation on top of the floor. This can be rolled back or just left in situ when boxes are placed there.
  3. Fit little supports for the floor, so it sits higher than the joists. Can be done with pieces of timber or little stands can be bought for the job.


The choice of roof insulation is mostly determined by depth. Lower cost insulators need more depth for BR compliance.

  • Polystyrene sheet - cheap (EPS or XPS), XPS can be used externally in an inverted roof design, as it is waterproof.
  • Polyisocyanurate - where depth of insulation needs to be minimised, not cheap
  • Sheep's wool - requires a whole lot of depth to meet BR

Cavity Wall Insulation

Fitted to existing original wall cavities:

  • Rockwool
  • Fibreglass
  • Polystyrene bead

Fitted to retrofitted cavities:

  • Polystyrene slab, expanded (EPS) or extruded (XPS)
  • Fibreglass
  • Rockwool batts
  • Polyisocyanurate

Solid Insulation for Walls

Fitted to the surface of solid walls (and occasionally to cavity walls) to increase insulation level further.

  • Polyisocyanurate (PIR)
  • Polystyrene foam backed plasterboard sheets may also be used, but don't provide very good insulation per thickness.
  • Cheap roll insulation may be used, eg rockwool, if battens are fitted and plasterboarded. However sheet PIR without studs gives much better insulation.
  • Hard surfaced insulating sheets. These have an extremely thin reinforced cement surface, so almost all their depth is insulation, but they're expensive
  • XPS or extruded polystyrene. This is tough and waterproof. It does not retain moisture, unlike PIR and EPS. It can be nailed to solid walls with long plastic spikes. It doesn't tear or split when fixing, and you can router pockets in it for back-boxes. You can fix plasterboard over it with even longer spikes but it's better to dry-line in front with self-standing partition. Thicknesses from 20 to 200mm buyable.

Tanks & Cylinders

The standard options for hot water cylinder insulation are:

  • jacket retrofitted - an improvement
  • foam coated cylinder - much better

Its also possible to create a container around the cylinder and fill the gap with insulation. In a typical airing cupboard this only requires hardboard across the front, and the space may be filled with any type of standard building insulation, or simply with free garbage of any insulating type, such as parcel packaging, cardboard & bubblewrap.


Neoprene foam tube is most common. Its very quick & easy to apply.

Felt strip is also used. Its slower to apply and looks lousy, but can be used for pretty much any shape and size of pipework.

If you're penniless, a scrap clothing wrap is perfectly effective. Avoid leaving any gaps and tape it in place well. Use a tape that won't fall off, or string or cable ties.

R & U Values

  • R is a measure of resistance to heat flow, ie insulation
  • U is a measure of heat conductance
  • R value = 1/ U value

U is measured in watts per square metre per centigrade (W / m^2 C)

  • The temperature figure is the temperature difference between the 2 sides of the insulation rather than ambient temperature
  • W/m^2 k is more widely quoted, but has exactly the same value as above
  • Beware of American R figures being quoted in completely different units

To determine a figure for multiple elements in series (which is how most walls are composed), just add up the R values of each element.

Insulation Values

The following values are a good guide, please check your chosen insulation product for its precise value.


  • Brick: R = 0.03 per 25mm
  • Single brick (4") R = 0.12
  • Solid brick 9" 0.44
  • Concrete 0.015 per 25mm
  • Limestone R = 0.017 per 25mm
  • Cavity wall: 0.53
  • 4" AAC wall: 0.78
  • Lime plaster R = .05 per 25mm
  • Unventilated Cavity R = 0.18
  • Outside air bounday layer = .03
  • Inside air boundary layer = .12
  • Aerated cement block


  • Softwood typically 0.25 per 25mm
  • Timber cladding typ 0.44 for 25mm

Insulation materials

  • Marmox R= 0.75 per 25mm
  • Kingspan R = 1.08 per 25mm
  • Celotex R= 1.31 per 25mm
  • Thermafleece (treated sheep's wool) R = 0.65 per 25mm
  • Expanded polystyrene slab 0.63 - 0.88 (loose fill value is lower)
  • Extruded polystyrene similar
  • Fibreglass loosefill R = 0.45 per 25mm
  • Fibreglass batt 0.55-0.76 per 25mm
  • Straw bale 0.26 per 25mm
  • Vermiculite loose fill 0.4 per 25mm
  • Pearlite 0.48 per 25mm
  • Rockwool loosefill 0.44-0.65 per 25mm
  • Rockwool batts 0.52-0.68 per 25mm
  • Cellulose 0.52-0.67 per 25mm (for both dry & wet sprayed)
  • Polyurethane panel 0.97 - 1.2 per 25mm
  • Polyurethane expanding foam: same
  • Foil faced polyisocyanurate panel 0.97 - 1.2 per 25mm
  • Cotton batt:
  • Aerogel 1.76 per 25mm
  • Vacuum 5.28 per 25mm
  • Cardboard 0.52 per 25mm
  • Wood chips and similar products R=0.18 per 25mm

Note that some insulation types have R values that deteriorate a bit over time due to compaction or loss of their insulating gas component. This is primarily true for polyisocyanurate(PIR), gas filled double glazing and vacuum panels.


  • Window, single glazed: 0.18
  • Window, double glazed: 0.35
    • can be improved to over 0.5 with noble gas fill & glass coatings
  • Note thicker glass makes no noticeable difference to R value
  • Window, triple glazed: 0.52
  • Thermal lined close fitting curtains & pelmet: 0.3
  • Roller blind: 0.18

Other materials:

  • Earth R=0.58
  • Weatherboard wall: 0.55
  • 12" mud wall 0.27
  • Chipboard R=0.17 per 18mm
  • Asbestos cement board 0.018 per 8mm

For more info see Wikipedia

Cold Bridging

A common issue with insulation is cold bridging. This is where there are patches that aren't insulted like the rest. Causes are:

  • Gaps left in the insulation
  • Places where insulation changes from interior to exterior
  • Places where existing building materials prevent fitting of insulation. Some examples are:
    • insulation sheet fitted into a timber frame, leaving the frame itself uninsulated
    • Cavity wall insulation unable to insulate around windows, due to lintel above, and existing masonry on all sides
    • Plasterboarded battens with insulation fitted between battens

Cold bridging also has the potential to cause a damp problem. Where previously either no condensation occurred, or it occurred over a large area and evaporated fairly quickly without incident, after insulation condensation can concentrate onto relatively small cold bridges and cause mould or fabric damage. This isn't normally a problem, but can be.

Wind and insulation values

Any gaps that admit wind allow heat loss to simply bypass the insulation in that area. Thus good insulation requires plugging any gaps. Depending on the situation this can be done with loosefill insulation, injected expanding foam, pressed in foam strip, or for opening doors and windows, brush strip or foam strip.

Wind can blow through some insulation types, eg fibreglass loft roll, much reducing their insulation value. These types are not well suited to use in windy positions.

Testing insulation values

Some unpopular insulating materials don't have a known insulation value, they've not been tested. If you want to use nut shells for example, you can if you wish set up an experiment to determine their insulation value.

A stack of the following may be used, in this order:

  • heat source (eg spotlight)
  • top metal plate to spread heat out evenly
  • an insulation material of known value (R1)
  • insulation material under test (R2)
  • bottom metal plate to spread heat out evenly

Thermometers or thermocouples are added at the 3 insulation boundaries, at

  • top metal plate
  • bottom metal plate
  • the interface between the 2 insulation layers

Thick high insulation value material is added around the 4 sides of the stack. The stack bottom is left uninsulated.

The stack is left with the heat source on for hours to stabilise, then the 3 temperatures are read.

The insulation value of the unknown layer is determined from the ratio of thermal resistance (insulation) values. Where the resistances are R1 and R2:

middle interface temp - bottom plate temp = (top plate temp - bottom plate temp) x R2/(R1+R2)

Most accurate results are obtained when the insulation values of the 2 layers are in the same ballpark, and top plate temp is high. For accuracy, avoid using results where the middle interface temp is close to top or bottom plate temp, or the top plate temp is low.

Self testing is only sufficient for applications where BR approval is not required. To obtain BCO approval with non-standard materials requires the test results and building insulation design to be signed off by an engineer. Other aspects of insulation also need to be considered in this

  • ability to survive damp
  • rotproofness
  • fire performance
  • longevity
  • insectproofness
  • and rarely toxicity


With 69,000 house fires in 2001, the performance of insulation in a fire affects loss of property and life. There are 3 main fire performance possibilities:

  • Fireproof materials such as fibreglass and mineral wool act as a fire barrier, if they remain in position.
  • Fire retardant materials burn away when flames reach them, but don't spread the fire further. They don't act as fire barriers.
  • Flammable materials such as untreated cardboard can ignite and spread fire. These products should generally be treated before use with a fire retardant, although there are examples where method of use provides another means to prevent spread of fire (eg plastered strawbale construction).
  • Some materials produce toxic smoke in a fire, eg polystyrene

Cladding applied for external wall insulation has been implicated in some major fires including Grenfell Tower (June 2017).

See Also