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

Assuming that the question of designing in enough insulation has been dealt with the question is about what kind of materials to use. Here the issues are -

• 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?

Moisture

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.

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[for the purposes of part B of the Approved Documents] - A space enclosed by elements of a building (including a suspended ceiling[for the purposes of part B of the Approved Documents] - A part of a building which encloses and is exposed overhead in a room, protected shaft or circulation space. (The soffit of a rooflight is included as part of the surface of the ceiling, but not the frame. An upstand below a rooflight would be considered as a wall.)) or contained within an element, but not a room, cupboard, circulation space, protected shaft or space within a fluepipe to conduct gas, typically ventilation air or boiler exhaust. see Flue, chute, duct, pipe or conduit. 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 +/-»

Fire resistant and acoustic insulation

In many situations the thermal insulation may also serve as fire resistance[for the purposes of part B of the Approved Documents] - The ability of a component or construction of a building to satisfy, for a stated period of time, some or all of the appropriate criteria specified in the relevant part of BSBritish Standard 476. and acoustic insulation. This is often a complicated matter which is dictated by the building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see 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.

Sustainability

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 CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming pollution in their manufacture.

Wool may cause concern because of the damage sheep do to the countryside and may also be a problem from a vegetarian standpoint as their rearing is intimately bound up with the meat market.

There is a study of the LCALife Cycle Analysis (also sometimes called 'Cradle to Grave' or 'Cradle to Cradle' assessment) see Building materials of various natural insulation materials which uses mineral fibres as a benchmark. See the UK’s National Centre for Biorenewable Energy, Fuels and Materials

Health

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.

Insulation values

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 bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a thermal bridge. see more on 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 development.

Notes:

(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 most values are missing
(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.
(13)Polyisocyanurate
(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.

Just to put other materials into perspective, to match the insulation value of 30 cm of fibre glass insulation (roughly the equivalent amount you would need in a wall to achieve the forthcoming standards set for new houses in 2016) 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
• 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

The embodied energy of materials can vary enormously depending on how you measure it, where it is located etc. 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.

So a more useful way of comparing the in-use embodied energy is to take into account the K value, the density and the embodied energy. By multiplying them together you arrive at a figure, in column E below, which gives an index of comparison for the embodied energy of various materials which give a similar degree of insulation.

So cellulose (recycled newspaper such as Warmcell) wins hands down because it has good insulation properties, it is light because it fluffs up well and it has a low embodied energy to start with (although this relies on the production of newspaper in the first place which is relatively energy intensive).

Softwood comes out worst in this list because it is not a particularly good insulator and it is very heavy in its solid form. Interestingly however, if walls were made of solid softwood (such as traditional log cabins were), although the walls would have to be about 4 times thicker than with other insulations, there would be the advantage of sequestering large amounts of carbon which might otherwise turn into carbon dioxide or methane as timber rots. Furthermore the timber would replace brick, concrete etc. which have high embodied energy. On top of this is the factor mentioned above regarding the wide range in values for the embodied energy of timber which depends on how it was grown, how far it had to travel, whether it was kiln dried, treated etc. No, it’s not simple!

The BREBuilding Research Establishment. Green Guide rates a wide range of insulations against their environmental impact

Calculations

see also Vapour barriers

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 SAPStandard Assessment Procedure - the method used in the building regulations for calculating the energy use of a house. see Part L and 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 PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. 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 massthis is about how much heat something can absorb - so it involves its specific heat capacity and its volume. It can be useful for levelling out the peaks and troughs of temperature within a house. See the page on thermal mass and risk of condensation. E.g. Build Desk

Condensation

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 pointWarm air contains (invisible) water vapour. If you cool the air down you will reach a point where this vapour turns to liquid water. This is called the 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.

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 Controlthe local authority department which deals with the Building Regulations you may well be asked to provide calculations regarding vapour barriers. BSIBritish Standards Institute 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.

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.

A few suppliers are publishing information on decrement times such as Natural Building Technologies who  show decrement delays of up to 12 hours in their literature for pitched roofs.