Types of retrofit insulation

If you are considering installing modern energy and heat generation in your property, e.g. solar panels, heat pumps etc. then you could be wasting a lot of money unless you also increase the efficiency of your property via insulation, double glazing, draft exclusion etc.

Types of insulation:

Rigid insulation (boards)

  • PIR – Polyisocyanurate  – (Celotex, Kingspan). This is a lightweight firm insulating board made of two aluminium facings with PIR insulation between. Various thickness are available.
  • PUR – Polyurethane. Similar type of board but less efficient. (Also available as spray on foam  – see below).
  • Expanded Polystyrene board.
  • Woodfibre. One of the oldest forms of manufactured insulation. This is coming back into fashion as an environmentally friendly and breathable material.


  • Fibreglass or mineral fibre insulation blanket. Readily available in rolls. Typically sold for topping up loft insulation
  • Recycled polyester blanket e.g. from plastic bottles. (Non-itch and Supasoft) more environmentally friendly and less itchy alternative to the above
  • Sheepswool. Usually a combination of actual sheeps wool and other materials such as recycled polyester.
  • Hemp wool. This is becoming more popular as it is made from natural materials.


  • Polystyrene beads used for blowing into wall cavities as a retrofit.
  • Multifoil (e.g. Superquilt). A very thin blanket made of foil layers which work only in combination with a sealed air gap on one or either side.
  • Aerogel (sold under the brand name Spacetherm). A highly efficient insulator developed by NASA. It is useful in very thin layers which can provide more insulation to other types of insulation. Available in blankets, or with a boarded backing.
  • Insulating plaster. Plaster mixed with polystyrene or cork. Better thermal resistance than normal (gypsum or lime) plaster but not hugely effective. (R-value .22 per 10mm)
  • Spray on foam (polyurethane). Now widely discouraged as it can seal in damp and mouldy timber work. Mortgage companies now sometimes refuse loans when this is used on a property.
  • Thermal wallpaper Available in various materials (e.g. expanded polystyrene, rubber) and thicknesses from 2mm – 10mm. This is an easy DIY retrofit for internal wall insulation however the actual benefits are small compared with the cost.

As you can see there is a lot of choice, and there are still, products we haven’t mentioned.

Comparing types of insulation

Understanding how insulation is measured can be a bit confusing as there are different types of measurements, all of which seem a bit baffling to non-scientists. Although you may often hear about U-value it is less useful when comparing products because it is mostly used when considering a whole structure, e.g. a wall, and is calculated from all the elements involved including masonry, cladding, stud work, render and insulation.

The typical values you will see on product specifications are Thermal Resistance (R-Value) and Thermal Conductivity (K-value or λ-value). The symbol λ is pronounced “lamb-da.”

If you are interested how these values relate to each other read on. Otherwise you can skip this bit go straight to the Comparison chart. The important thing to know is that for good insulation high R-values are best and low K-values and U-values are best.

  • R-value = thermal resistance (m2K/W) This means how good the material is at stopping the heat leaving your house, so higher numbers are good. The R-value depends on the thermal efficiency as well thickness of the material. For example 100mm insulation that has an R-value of 2.5 will have an R-value of 5 if it is doubled up to be 200mm. This is the main measurement we will be using to compare different materials as it is readily available from manufacturers and retailers.
  • K-value (or λ-value) = thermal conductivity (W/mK) The rate at which heat flows through something so low values are better. As this is a rate of flow it is not dependant on thickness of but is a good indicator of the overall thermal efficiency of a material.

If you know the thermal conductivity and thickness you can calculate the thermal resistance, and vice versa

  • R = metre thickness ÷ K
  • K = meter thickness ÷ R
  • U = 1 ÷ R . However, as mentioned, U-value is rarely used for single materials but relate to a whole structure, roof or floor so the R-value used is the sum of the different materials’ R-value

For a much more in-depth scientific explanation see Insulation Outlook

Convert Thermal resistance to thermal conductivity and vice versa Online calculator

Comparison chart

The tables below should be taken only as a rough guide. Materials can perform better or worse depending on the application and appropriateness of the installation. The prices quoted may be very approximate depending on the supplier, quantities ordered and  delivery. We have not included installation costs. Some products need extra materials and fixings. However as a very rough guide this can give you an idea of your options when considering a basic DIY retrofit insulation project such as insulating a loft floor.

Comparing insulation

*Airtec and superquilt are designed to be used in a system with sealed air cavities either side. The R-value given here assumes 25mm cavity either side. It makes little sense to provide a λ-value.

How much insulation to do a loft to building regulation standards?

The current building regulations for roofs require a minimum U-value of >0.11. In order to achieve this comfortably a realistic R value for the insulation alone is about 7. In real life it may be necessary to fit more than the required thickness depending on the availability of different thicknesses of the material itself. The last column shows the cost per metre required to achieve an R-value of 7.

Thickness to achieve recommended loft insulation

*Note that these equations do not work in regard to Superquilt or Airtec as it is a system that relies on different materials and air gap, so in order to achieve building regs there is a special triple layer system.

Some conclusions to achieve roof building regulations of U-value 0.18

  • Most types of batting = approx 270mm – but these are usually available in thickness of 100mm or 200mm only.
  • PIR =approx 150mm
  • Aerogel = approx 97mm. This is extremely expensive hence practical only where thin layers are necessary.
  • Sempatap = approx 335mm. This is 33.5 layers at 10mm so is neither practical nor economical at £729 m2

Cavity wall insulation (CWI)

The concept of building walls comprised of two skins of bricks with a cavity was introduced in some late Victorian buildings, but first became widespread in the 20s and 30s. The air gap did provide some insulation, but the idea of using an insulation material within the cavity was introduced in the 70s, however such energy saving measures did not become part of building regulations until the 90s.

Usually a PIR or expanded polystyrene board is used to fill approximately 50% of the cavity so the remaining empty 50% is available for airflow.

In the 1970s and 1980s grants became available to retrofit cavity insulation into older houses. In a retrofit it is not possible to fit boards (without demolishing a lot of the wall) so either insulation was injected via holes drilled into the outer skin. This was usually either polystyrene beads or urea-formaldehyde wool.

We are aware that some contractors may have done poor work, either not injecting enough or not sealing air bricks properly so that the insulation leaked out, either via an air brick in the outer skin or via the corresponding air brick or gap in the inside skin into the space below a suspended floor.

Cavity insulation

If you live in a property with cavity walls that were retrofitted with insulation, you can usually tell because the holes drilled into the walls would have been filled but often are still noticeable – see photo.

It’s worth noting that an EPC inspector will take note of such tell tale signs that insulation was carried out in the past and will assume it was done, however there is no guarantee it was done properly or still has its integrity because of this. EPC inspections do not do any kind of invasive survey that would reveal the actual state of the insulation.

Damp problems due to (incorrectly) retrofitted cavity insulation

As noted above some of the cavity insulation (especially when cowboy installers were attracted by the grant schemes) was poorly done. This can cause or exacerbate damp issues for various reasons:

  • Any cold spots caused by gaps in insulation can cause condensation
  • Properties in exposed areas (by the coast or top of hills) can be subject to a lot of wind-blown rain. Certain types of insulation would act like a sponge and retain this moisture in the cavity. See here for more info and map

More about cavity wall issues:

Case study: unblocking cavities

Loft insulation

Insulation can either be added immediately above the room ceilings (cold loft) or just below the roof tiles and battens, between the rafters (warm loft).

Warm loft

This means that the insulation is placed immediately below the roof battens so that all the loft area beneath can either be efficiently heated if required or else it will take heat from the rooms below. A warm loft is only really useful if you are using the loft space regularly enough as a room, either as storage, home office, playroom etc. Otherwise heat from the main part of the house still leaks into this area, and if the room is not used you would be much better off retain that heat within the main house.

Cold loft

This is much cheaper and easier as a DIY project (provided you have access to the loft) and probably more efficient than a warm loft for the house in general if you rarely or never use the loft. Insulation is laid between the joists that form the floor of the loft, ie immediately above the ceiling of the rooms below.

Again there were grants in the 80s/90s to have this done. Generally the joist were 4 inches (100mm) and so 100mm of insulation was laid between them. Modern building regs and EPC require at least 200mm [check] so is worth topping up older works by adding another layer. There is a limit to how much thickness is required for good insulation, topping up to more than 300mm is probably a waste of time in the UK climate.

Note that the joists in typical loft on an older house may only be about 100mm, and never intended for supporting the same load (furniture, fittings, multiple people etc.) as might be involved in a living space, yet can still be very useful for storage.  Be especially careful not to tread between the joists, ie directly onto the ceiling boards or lathes.

You can lay insulation between the joists and board over that. Fibreglass or mineral wool rolls can be used, however there are now more eco friendly products  such as Non-Itch or Supasoft, both of which are made from recycled plastic and (as the name implies) much nicer to work with than fibre glass. Ideally you may want it thicker than the joists so you could add extra timbers across the joists before boarding. [check – does this need building control just for storage space?]


Possibly a better solution to achieve good insulation without adding timber is to use something like Superquilt or Ecoquilt Expert multifoil. This is a much thinner but very efficient material. You would lay this across the joists and then recess between the joists and staple it so that it hangs about half way between the ceiling and the boards that you lay onto the joists. This system relies on a sealed airgap of about 50mm above and below. In simple terms, this creates a “radiant barrier” which reflects heat back downwards into your home and coldness back upwards.


Many lofts did have some insulation fitted back in the 80s/90s but is very much worthwhile topping up to 200 – 300mm. Even if you are boarding a small area you can do this by adding some extra timber to the existing joists create more space for the added insulation. Alternatively use Superquit as above but to create the air gap you would need to remove the old insulation.

Underfloor insulation

Some properties may have easy access to the space underneath a suspended floor via a crawlspace or by exposed floorboards. Unless the boards are tongue and groove, it is possible to inslutale by taking up only a few of the boards, e.g. every fifth or sixth board (see Insulation case studies DIY underfloor insulation)

Underfloor insulation is absolutely necessary when installing underfloor heating in order to ensure the heat is used for the room above, not the ground below.

Exterior wall cladding

This is usually not so common as a retrofit installation as it can adversely alter the appearance of a building that was not originally designed for it, as well as being very expensive. A common method is to add 50mm (2″) of foam board insulation and then cover this over with a cosmetic cladding such as timber or brick slips. On existing buildings you would need to pay special attention to the look of doors, windows and eaves. It is unlikely to get planning permission in conservation areas.

Various renders with insulating properties are available.

Interior wall cladding

This is generally recorded for solid walls. For cavity walls there can be problems due to condensation occurring if not correctly fitted. Again, foam board or polystyrene can be used, but to be effective needs a thickness of at least 25mm and probably more like 50mm which will have a serious impact on the size of the room. It will need decorating and skirtings/coving to be moved and refitted.

More breathable solutions include wood fibre board and insulating lime plaster such as Limco and Cornerstone.

There is a modern hi-tech alternative – Aerogel or Spacetherm – which has very high insulation properties and is only about 10mm thick. This is available as a blanket that can be used behind regular plasterboard but is now also available with a stiff backing and can be glued directly to a wall. This sounds like the ideal solution but there is always a catch – yes it is incredibly expensive at about £80 per square metre.

Theoretically it could replace existing plaster so have no impact on the room size. ideally it should also be fitted behind skirtings to avoid cold spots causing potential damp issues due to condensation.

Insulating plaster

Various renders and plasters with insulating properties are available. In particular lime plasters are very good for this purpose as lime is “breathable” as opposed to the more modern gypsum. If you need a room replasered in an older property, then it is worth thinking about using a lime plaster with insulating properties such as Cornerstone or Limco.

It’s worth noting that when original lime plaster has been skimmed with gypsum, this can cause problems both with the integrity of the plaster itself and also with dampness. Lime needs to be slightly moist anyway or it turns to dust and can break up behind the skim of gypsum. Alternatively the gypsum can seal in any moisture that accedes the normal small amount the lime needs.

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