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Insulation properties

cork oak

A cork oak. The bark (cork) is stripped about every 8 years and regrows

The qualities of insulation types

Assuming that the question of designing in enough insulation into your self build project has been dealt with the question is about what kind of insulation materials to use. Here the issues are –

  • the U-value (how well it insulates)
  • resistance to moisture
  • compressive strength if needed
  • is it self supporting?
  • is it easy to fit?
  • must it be fire resistant?
  • must it also be acoustic insulation?
  • is it vermin proof?


This is probably the first criterion because some insulating materials are not suitable for use if they are in contact with a damp surface (such as a wall which is partly below ground). These include wool, cork, mineral fibre (although mineral fibre batts may be OK), and recycled paper fibre. Instead you can use foamed glass or suitable grades of polystyrene or polyurethane.

There is also the issue of whether insulation might trap moisture. For instance if foam insulation is sprayed between joists then it needs to be breathable so that water can escape without rotting the timber.

Compressive strength

Insulation which goes beneath a concrete floor slab or needs to support a roof finish must have the required compressive strength. These tend to be foamed glass, polyurethane and polystyrene.

Self supporting

In some situations (such as the normal cavity wall) the insulation needs to be self supporting over a certain distance so mineral fibre batts would be used rather than quilt.

Ease of fitting

This is most important because poorly fitted insulation regularly shows up as a problem. At its worst it can cause whole areas of a surface to have virtually no insulation value at all and possibly lead to interstitial condensation. Even very small gaps of a few millimetres can cause massive problems……more on fitting problems

There are five main types of situation where poor fitting can happen

  • poor jointing
  • gaps on the face of the insulation
  • areas missing
  • slump and settlement
  • insulation causing bridging of cavities

Poor jointing and gaps on the face

– this is a particular problem where solid sheets of insulation such as polystyrene are used. In some cases it only takes a gap of a few millimetres between sheets to completely spoil the efficacy of the insulation. Take for example a traditional cavity wall which gets insulated using expanded polystyrene sheets and imagine that the sheets fit closely but have slight gaps because they were not cut perfectly or mortar snots hold them away from the inner leaf.

As can be seen, the inner leaf is subjected to a stream of cold outside air and has very diminished insulation. Something similar happens with mineral fibre, especially at joints and edges. If a number of such cavities link up then this can form a huge source of escaping heat. There are some interesting pictures of badly fitted mineral fibre insulation on this pdf download from Kingspan. However it would not be difficult to find equally awful examples of badly fitted expanded polystyrene, polyurethane or phenolic board insulation. It is interesting to speculate about why quality control and the culture around domestic building practice is so poor in most of the UK

Areas missing

this tends to happen when there are areas which are inaccessible or where a spray in product such as warmcell is being used and it is physically impossible to reach the area. This can easily happen at the eaves or in complex areas where there are extra noggins. This problem needs addressing at the design stage.

Slump and settlement

this can occasionally happen with insulation such as Warmcell, polystyrene bead and any other poured or pumped insulation. In the case of Warmcell in floors it should be packed in quite tightly so that if it settles slightly then it is still in contact with the lower surface of the floor itself.

Bridging of cavities

if the wall design includes cavities then it is most important that they don’t get accidentally bridged by the insulation. This can happen particularly with mineral wool being badly fitted and the result is that moisture may find its way over from the outer leaf to the inner one.

Fire resistant and acoustic insulation

In many situations the thermal insulation may also serve as fire resistance and acoustic insulation. This is often a complicated matter which is dictated by the building regulations. Suffice to say that if you want to alter the type of insulation from that shown on the drawings which have received building regulations approval then you will probably need expert advice from an architect, building technician (or your building inspector if you are on friendly terms).

Vermin proofing

Problems with vermin have almost disappeared from the public conciousness because of the long history of building mainly with masonry walls and avoiding timber. There is something of a change taking place at present and if timber frame is being used then there needs to be design consideration for excluding vermin. The rule of thumb is that if you can get a pencil into a hole then a mouse can get in. Hence the size of the openings in air bricks. If mice get into fibrous insulation they can burrow for long distances and you may have no way of knowing the routes they are using, let alone controlling them. In theory they may be able to get right round the house. The points of entry in a timber house are likely to be around the sole plate or up through a suspended ground floor. It is worth checking the insulation you intend to use to see if it discourages rodents. For instance Warmcell has been tested and the boron fireproofing also discourages mice. Mineral fibre is a positive paradise for rodent nest building. If in doubt it pays to cover any susceptible areas with plasterer’s lath. This is galvanised expanded metal which will not allow mice through the openings.


Cork, hemp, flax, mineral fibre and cellulose fibre are all in good supply and present little threat to the environment. At present hemp is mainly produced on the continent though there are plans to produce it in the UK.

Expanded polyurethane and expanded polystyrene are of course both oil based and cause considerable CO2 pollution in their manufacture.

Wool may cause concern because of the damage sheep do to the countryside, especially upland areas, and may also be a problem from a vegetarian standpoint as their rearing is intimately bound up with the meat market. Sheep, along with other ruminants, produce a considerable proportion of the world’s CO2 emissions. There have also been historic failures with wool insulation due to moth infestations.


There is a long history of concern over the use of mineral fibres (being inhaled by operatives) in the UK including some local authorities banning their use in the past. However, although many people find working with them unpleasant there doesn’t seem to be any firm evidence of health risks.

The Building Regulations have an Approved documents part D dealing with urea formaldehyde and its potential toxicity.

Insulation values

Isonat hemp/recycled cotton insulation

The table gives the approximate insulation value (or K value) of various materials in column E. The lower the K value the better the material is as an insulator. However the best insulating materials cannot always be used in particular situations such as where there is dampness, where they need to be load bearing to some extent or where they need to be self supporting. These factors are shown in columns B to D.

The K value of materials can vary considerably depending on several factors such as moisture the material may have absorbed, how compressed it becomes and the way it is made. Manufacturers tend to test their materials under ideal conditions and there is some doubt as to how reliable many of the values are in practice.

Timber is included in the list to show how timber members may contribute to thermal bridging. Structural timber’s thermal conductivity has traditionally been ignored in calculations, particularly for studwork.

Both aerogel and vacuum panel insulation are still in the early stages of deployment.see more

Vacuum panels, due to their increased insulation properties (maybe 4 or 5 times better than traditional insulation) can be used where space is very limited. This is particularly useful in situations such as where

there is limited headroom such as attic conversions

a thicker insulation would necessitate the incorporation of a step – such as the insulation of a room beneath a terrace accessed from a room above

the internal insulation of buildings where loss of floor area might be a major problem

insulation of historic buildings where major changes cannot be made to the appearance

A  B  C  D  E  F
 type  tolerate damp?  (16) compressive strength  (11) self – supporting? (8)  K value (4) embodied energy (7)
loose cellulose  no no can be  0.035  0.94-3.3
cellulose batts  no low yes  0.035  0.94-3.3
cotton/denim (15)  no low can be (3)  0.039  ? but low
mineral fibre quilt  no low can be (3)  0.044  16.6
mineral fibre batts  yes low yes  0.046  18
hemp no low can be (3)  0.04  ? but low
HempWood no low yes  0.038  ? but low
wood fibre board some makes low yes  0.04  ? but low
hemp/lime walls yes good yes  0.09  ?
cork limited (1) good ? (2) yes  0.04  4
wool no low some makes  0.04  20.9
expanded polystyrene yes depends on density (2) yes  0.035  88.6
e.p. beads yes low  no  0.035  72
foamed glass yes good yes  0.056  27
wood wool slab yes good yes  0.083  ?
softwood timber limited good yes  0.14  (9)  7.4
hardwood timber limited good yes  0.16  (9)  ?
densely baled straw no low yes  0.06  ? but low
phenolic foam (12) yes good yes  0.02  ? but quite high
PIR (13) yes can be good yes  0.02  ? but high
aerogel (5) ? low yes  0.009  ?
vacuum panel (5) ? ? yes  0.009  ?


(1) Cork will tolerate moisture for short periods.
(2) OK with the correct grade (so for instance expanded polystyrene can be obtained in grades strong enough to support concrete slab floors).
(3) Some quilts have a support backing built in, especially those for acoustic insulation.
(4) the K value, measured in W/m.K. The lower this is, the better the insulation.
(5) these are both emerging technologies so some values are missing. Spacetherm is one of the commercial products based on aerogel.
(6) e.p. bead values depend on the source of the bead (post consumer waste, manufacturer’s waste etc).
(7) measured in MJ/Kg. Values mainly taken from the Inventory of Carbon and Energy (ICE) database.
(8) This indicates whether the insulation will stay where placed rather than slumping
(9) perpendicular to the grain
(10) parallel to the grain
(11) This is to do with whether the insulation will support something more than itself, e.g. a concrete floor
(12)Phenolic foam is what people often mean when they mention Kingspan.  Kingspan make a wide range of insulating materials so it is fairly meaningless to simply say ‘Kingspan’. Other manufacturers also make phenolic insulation. The Institution of Civil Engineers has published a report “The environmental impact of phenolic foam insulation boards
(14)Timber is a very difficult item to put a value on not only because of the wide variety of sources but also the degree to which you allow for the sequestering of carbon in building timber.
(16)meaning long term contact with a damp surface

Just to put other materials into perspective, to match the insulation value of 30 cm of fiberglass insulation (roughly the equivalent amount you would need in a wall to achieve the forthcoming EPBD standards set for new houses in 2021) you would require a thickness of about:

  • 10 metres of common brickwork or sandstone or
  • 25 metres of granite or
  • 12 metres of solid concrete or
  • 3.5 metres of cob (see The thermal properties of cob buildings of Devon) or
  • 1.4 metres of plasterboard or
  • 1.2 metres of softwood or plywood

These figures indicate how traditional solid masonry walls are almost as good as useless in terms of insulation. Even a 50 mm cavity wall with no extra insulation is only equivalent to about 6 mm of mineral fibre insulation. Cavity walls were never intended to provide insulation. They were introduced to prevent driving rain from getting into the inside of the house. The moisture cannot cross the cavity (well not one that is constructed properly and there starts another story).

Embodied energy

Embodied energy matters because so many of the materials used in house building absorb a huge amount of energy in their manufacture and distribution and this energy is usually associated with global warming and pollution. The embodied energy of building materials depends not only on producing the materials themselves but also on the energy associated with conversion and distribution. Sawing down a local tree and converting it into usable timber might give a very low value whereas the value for cellulose insulation can vary greatly depending on the extent to which you include the manufacture and distribution of newsprint and where the original timber came from.

The plastic insulating materials like polystyrene and polyurethane have a high embodied energy but where moisture is an issue there is probably no alternative except foamed glass. Cellulose fibre, cork, hemp and wool have very low embodied energies. Mineral fibres are quite low but still about ten times that of natural materials. See a table of values here. It should be borne in mind that because insulation is mainly light in weight that its embodied energy in a building is a relatively minor factor compared with say masonry.

Column F in the table above is a little misleading because it does not give a correct picture of the actual values for embodied energy in a practical situation. This is because embodied energy is measured in MJ/Kg rather than MJ/cubic metre. In a practical situation what matters is the latter. In other words, for instance expanded polyurethane may have a very high value for its embodied energy but because it can be foamed up so much it actually uses very little material, whereas wood starts out with a low embodied energy but because it is so dense, it uses a large weight of material.

A useful guide to the relative merits of insulation is on the Superhomes web site.

The BRE Green Guide rates a wide range of insulations against their environmental impact


An integral part of obtaining Building Regulations approval is the calculation of all the insulation values that make up the various parts of the external envelope of a house. This includes walls, roofs, floors, windows and doors. This is normally done by whoever does your drawings. It gets entered into a large spread sheet along with other factors such as how much solar gain the building might enjoy, what type of heating boiler is used and whether solar collectors are to be used. This all becomes part of the SAP calculation which indicates whether you have enough insulation.

However, with the realization of the need for higher energy saving goals, more stringent standards are being used by many people and standards such as Passivhaus are being adopted. With Passivhaus there is a much more comprehensively designed spread sheet which takes into account a much wider range of insulation criteria.

Commercial software is available to calculate insulation values, thermal mass and risk of condensation. E.g. Build Desk


Apart from simply saving heat, insulation is important to prevent condensation. Condensation occurs when moist air comes in contact with a cool surface and when this happens it is said that it has reached its dew point. If the moisture is finding its way out from a warm room through an outside wall or roof and reaches a point near the outside surface (but still within the wall’s thickness) where it is cooler then it may condense out somewhere in the middle of the wall. This is called interstitial condensation and can be very damaging to the building’s fabric, especially if timber becomes damp or fibre insulation gets wet. Calculating this kind of thing is not simple.

Software for predicting condensation within the shell of the building (interstitial condensation) takes into account several parameters including –

  • Thermal conductivity
  • Vapour permeability
  • Density
  • Specific Heat Capacity
  • Porosity
  • Water sorption coefficient
  • Sorption Isotherm
  • Liquid water diffusivity
  • Emissivity

It also takes account of internal and external conditions including temperature and relative humidity. It then works out whether, on balance, over a period of a year, moisture is going to dissipate from the structure or continue to build up. When submitting plans to Building Control you may well be asked to provide calculations regarding vapour barriers. BSI BS 5250 Code of practice for control of condensation in buildings is the document which the Building Regulations refer back to.

It’s worth noting that this type of software is still being developed and refined because it is so complicated, especially when there are cavities in walls or other parts of the fabric or when the sandwich of different materials which make up the fabric is complicated.

To get an idea of the values of various construction types there is an on-line insulation calculator at Vilnis Vesma’s site. It also flags up when interstitial condensation might be a problem.

Decrement delay

Decrement delay is a fairly new subject which is about how insulation behaves in a dynamic situation (dynamic in the sense of fluctuating temperatures). It is quite a simple subject in theory but there are immense numbers of variables in terms of materials and their positioning and also with regard to climate. It still does not figure in the building regulations although it can have a very marked effect on how insulation behaves.

You can think of decrement delay as the time lag between when the external surface of a wall or roof reaches its maximum temperature (around mid day) and when the heat gets through to the internal surface and causes it to reach its maximum temperature (possibly many hours later).

Because it is a new and complex subject you will need expert advice if you are thinking about using the building’s fabric in this way.

The traditional way to evaluate insulation in a wall or roof is to assume a steady state where the temperature on one side of the insulation is fixed at one level and on the other side it is fixed at a different level and then see (or predict) how quickly heat flows through the wall. For instance a 225mm thick solid brick wall (which has very little insulation) with a winter air temperature on the outside of 0°c and a temperature on the inside of 20°c will have a certain number of watts per square metre escaping out through the wall (about 53 watts/sq.m.).

Similarly in summer the same wall might be at 40°c with the sun beating down on it and if it is 20°c inside then heat will transfer towards the inside.

It gets more interesting when you have a situation where there is the same summer temperature but you start with a cold wall (cold from the night before) rather than one which is already warm. The heat starts moving in through the wall but before it gets through it has to heat up the bricks, so it gets delayed and only seriously starts to show up on the inside after a good few hours.


Similarly, when it starts cooling down at night it takes considerable time for the hot bricks to cool down so much of their heat goes into the room before it starts to travel to the outside. In this way a time lag has been introduced. It’s a bit like the time lag with the earth’s seasons which causes the coldest months to be around January/February rather than in December and the hottest ones to be July/August rather than June when the sun is highest.

The trick is to design walls and roofs so that there is about a 12 hour time lag so that it is always exactly out of phase with the sun. In this way you can cancel out the effects of overheating and overcooling.

It is important to notice that this only works when there is an outside temperature range which swings above and below the indoor desired temperature. Ideally it swings equal amounts both directions. None of this works in a very long, even, hot or cold spell. Only insulation will work then because once the bricks have heated up (or cooled down) after (say) 12 hours they cannot soak up (or give out) any more heat.

The really interesting bit comes when you have a wall which includes mass and insulation either combined (such as insulating clay blocks) or in sandwich layers using dense materials such as woodfibre board. It gets more complicated if you vary the position and size of layers because you can play around with the decrement factor to create different decrement delays. The diagram below shows heat coming in quickly through the insulation (but much less of it) and then slowing down when it hits the thermal mass of the solid wall.

Later on during the night the trapped heat has trouble getting out through the insulation so it goes into the room.

The obvious application for this is in desert areas where it is regularly very hot during the day and very cold at night but in the UK climate there are also seasonal periods when this effect can be utilized, especially in the spring and autumn months.

There is an interesting study from the Netherlands which shows two identical houses, except one has a high decrement time and the other not. The graph on the right in this study has a blue trace showing the lower swing of internal temperature.

1 comment to Insulation properties

  • john holland

    i looked at sip and straw bale house, getting straw bale to code 6 is very hard, i cannot see how? I do think that house do need to get more better in terms of insulation, I do think some of the tick box on code 6 as just to much red tape, and needs to re looked at again.

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