What’s the best insulation material to use in eco renovation?

This article helps you find the best insulation material for your eco renovation project by comparing materials on thermal ability, cost, eco-friendliness and breathability.

We’re going to look at the best insulation material:

  • for each task
  • from a point of view of tackling climate change
  • for breathable construction
  • if you are short of space to fit it in
  • from a cost perspective
  • for thermal performance

We’ll start by looking at the basic questions of why you should insulate and how much insulation you need. If you want to jump to the answer to a specific question, just use to the index here at the top.



Why should I insulate and where?
Which form of insulation material is best to use where?
How much insulation do I need?
What is the best insulation for health and climate?
Which is the best insulation for cost by volume?
What is the best insulation material for thermal performance?
Find out more – visit a refurbished home


Why should I insulate and where?

First, let’s see where, on average, the most heat leaves the house, what the target insulation value should be and what measures are appropriate. This is found in Table 1.

Building feature Heat loss (%) Target U-value (EPC Band B) Possible solutions
Table 1: heat loss through building elements, target insulation levels and insulation solutions
Walls 35 % 0.15 Cavity, internal or external wall insulation
Windows and doors 15 % 1.6 Double/triple/secondary glazing / shutters and curtains
Roof 25 % 0.10 Pitched, warm deck or cold deck roof insulation
Floor 15 % 0.15 Floor insulation
Gaps, cracks, draughts 10 % N/A Draughtproofing >> ventilation with heat recovery

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Which form of insulation material is best to use where?

Material which is loose and which comes in rolls, like cellulose and glass wool, is used in lofts and flat areas predominantly. Batts and slabs can be used there but also used vertically on the inside or outside. Rockwool is commonly used on the outside, but so are woodfibre batts. Generally a render over the top of the insulation material protects it from the weather.

The preferred render from an environmental point of view is lime, since it is breathable, and my personal favourite system for this is called Steico Protect, which is easy for any plasterer to apply and is available, like many of my favourite products from Ecomerchant, a trading style of Burdens Ltd.

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How much insulation do I need?

The short answer to this is: the more the better. The level of insulation is ideally determined by the U-value of the overall building element when the work is completed.That’s how the Building Regs and Energy Performance Certificates quantify it.

The precise amount depends upon not just the insulation you use, but other materials present in the wall, floor, ceiling, door etc, such as timber, brickwork, concrete, metal and plastic.

It also depends on how much space you have, and your budget.

So this is where we need to get a bit technical, and look at the relationship between the thermal conductivity of any material (the k-value) and its heat transfer (the U-value) properties.

Thermal conductivity (k)

Thermal conductivity, k (also known as psi or Λ), tells us how well a material conducts heat. It is:

k = Q/T times 1/A times x/T

or, the quantity of heat, Q, transmitted over time t through thickness x, in a direction perpendicular to a surface of area A, due to a temperature difference T. The units used are W/mK, or watts per square metre Kelvin.

Each insulation material is ranked in Table 5 below by its K-value. To find out the U-value of your actual installation you have to multiply it by the depth of insulation you can or want to fit in.


The U-value is the ratio of the temperature difference across an insulant and the heat flow per unit area through it. The lower the number the better the insulant. To compare two insulants with different thickness and thermal conductivity, it is necessary to calculate the U-value for each.

U-value is described in watts per square metre Kelvin [ W/(m2K) ], or the amount of energy lost in watts per square metre of material for a given temperature difference of 1°C or 1°K from one side of the material to the other.

Another way of understanding it is to see it as thermal conductivity divided by the depth of insulation, or U = k/d where k is the thermal conductivity of a material, d the material’s depth.

Doubling the thickness of an insulating layer doubles its thermal resistance.
Building Regulations provide minimum standards of thermal insulation, typically expressed as a U-value for a given building element like a wall.

This is found by adding the k-values for the different materials times the depth and area used for each within the element. In each case, measurements are taken on-site and then reference is made to information tables for the purpose of the calculation.

As a guide, Table 2 shows the depth of insulation required to reach a U-value of 0.15W/m2K for some common or sustainable materials.

If space is limited and the depth of insulation is a consideration for you, you can use this table as a guide. In general, expanded polyurethane, XPS and some other materials derived from fossil fuels take up less space.

Material Depth
Table 2: depth of insulation required to reach a U-value of 0.15W/m2K
Expanded polyurethane 130 mm
Unfaced polyurethane 160 mm
Rockwool (60 – 100kg/m3) 195 mm
Glassfibre slab 205 mm
Expanded polystyrene 215 mm
Mineral wool 225 mm
Flax 230 mm
Expanded Corkboard (110kg/m3) 240 mm
Glass fibre quilt 240 mm
Cork slab (160kg/m3) 250 mm
Woodwool board 250 mm
Cellular sheet glass 280 mm
Foam glass (140kg/m3) 305 mm
Cork slab (140kg/m3) 325 mm
Foam glass (130kg/m3) 330 mm

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What is the best insulation for health and climate?

Just because it’s saving energy doesn’t make it necessarily sustainable! Chiefly this article is considering the climate impact of different materials.

Some, however, also have health aspects. In general, materials which give off gases that harm the ozone layer are now not available but it’s just as well to check; polyurethane foams and sprays for instance may contain HCFCs.

Materials which contain glue might contain formaldehyde which can off-gas and cause indoor air pollution with attendant potential health problems.

The net climatic effect of building insulation is the sum of the greenhouse gas emissions associated with the energy used in manufacturing (its embodied energy) plus the leakage into the atmosphere during use of any (halocarbon: significant, or pentane, less so) expanding agents that have a greenhouse effect, minus the emissions saved due to energy saved as a result of the insulation (which is zero if renewable energy is used for heating/cooling that would not have been used elsewhere).

Although insulation saves carbon emissions in the future, those emissions associated with its manufacture are already present in the atmosphere and causing harm.

Given that there is an overriding need to fight climate change urgently by immediately slowing down the release of global warming gases, shouldn’t we try to avoid using these insulation materials, and instead use materials which lock up carbon in the fabric of the building?

Table 3 lists materials in order of descending climate friendliness on this basis. The most friendly ones are made from cellulose and other natural materials, since they have captured carbon from the atmosphere while they were growing.

You are sequestering atmospheric carbon in the structure of your building for decades as a result of using them. Consequently, the top scoring materials have negative carbon emissions associated with them.

Although sometimes they are more expensive, this is not always the case, as Table 4 shows, depending on where you source them.

Using them creates a market for this type of carbon storage and discourages the market for more polluting fossil-based insulants.

In general, these materials also enhance the breathability of your structure and hence its ability to withstand fluctuations in internal humidity which can cause damp and mould.

Determined by the embodied carbon (kgCO2e) emitted during manufacture, minus any sequestered carbon per cubic metre of material. All of the materials at the top with a negative figure are made from natural materials which have absorbed carbon from the atmosphere while they were growing
Material Embodied carbon (kgCO2e)
Table 3: Most climate-friendly insulation materials – best first.
Cork slab (300kg/m3) -155
Cork slab (160kg/m3) -70
Cork board -65
Woodwool board -35
Flax -5
Recycled loose cellulose -1.9
Glassfibre quilt 3
Rockwool 7
Glassfibre slab 8
Expanded Corkboard 9
Rockwool (60kg/m3) 13
Expanded polystyrene 15
Rockwool (100kg/m3) 20
Cellular sheet glass 28
Foam glass (140kg/m3) 30
Foam glass (130kg/m3) 31
Mineral wool 38
Expanded polyurethane 160
Unfaced polyurethane 175

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Which is the best insulation material for cost by volume?

For the purpose of writing this article I did a quick market survey, attempting to calculate for common materials the equivalent price per cubic metre. The results are in Table 4 below.

It’s nice to know that the most environmentally sound is by far the cheapest, Warmcel, and I can’t understand why everybody doesn’t use it in their lofts. It’s so easy to buy and apply.

It’s nice to know that woodwool and sheep’s wool are relatively cheap as are Rockwool and other mineral wool.

Material/product Price £
Table 4: Approx. cost per cubic metre from cheapest upwards.
Recycled loose cellulose (Warmcel) 11.67
Expanded Polystyrene Board (Jabfloor 70) 13.56
Woodwool (NOVOLIT) 33.33
Black Mountain sheep’s wool 46.66
Rockwool quilt 53.81
Mineral wool slabs 56.91
Woodwool (HERAKLITH) 59.33
EPS Jablite Polystyrene Sheet 75.52
Expanded Corkboard 160
Hemp Steico Canaflex 81.28
Woodfibre batts (Steico Flex) 106.89
PIR (Celotex XR4000) 117.93
Woodfibre batts (NaturePro) 127.74
PUR (Kingspan Thermawall TW50) 151.54
Woodfibre Board (Steico Therm) 176.66
Hemp batts (Black Mt) 317.98
Woodwool KOMBIVOL 330

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What is the best insulation material for thermal performance?

Finally, Table 5 below summarises the properties of different insulation materials.

There are categorised by their source with the organic, natural, more sustainable ones first, followed by other relatively environmentally-friendly ones made from natural materials, and lastly the category of materials derived from fossil fuels.

In each case the ones with the lowest k-value, i.e. the most insulating, are listed first.

Within each category, the best-insulating materials are at the top and the worst-performers nearer the bottom (based on K-value).
Image (click to zoom) Material K-value (W/mK) Notes
Table 5: Summary comparison of different insulation materials
Organic sources
These have absorbed carbon from the atmosphere and so are more climate-friendly
Sheep’s wool batts and rolls 0.038 – 0.043 Can absorb some moisture whilst remaining efficient
Wood fibre batts 0.038 – 0.043 Good for most walls, ceilings, roofs, timber joisted floors.
Cotton-based batts and rolls 0.038 – 0.043 Best for horizontal surfaces.
Cellulose (loose, batt or board) (e.g. Warmcel, Homatherm) 0.038 – 0.040 Recyclable, renewable, made from finely shredded newspaper, easy to install, best for horizontal services.
Flax batts, slabs and rolls approximately 0.042 Hard to obtain and expensive.
Hemp batts 0.043 Relatively expensive.
 expanded-corkboard Expanded Corkboard (e.g. Amorim, Korktherm, Westco) 0.040 – 0.050 Commonly used as underlay under hardwood and ceramic floors.
Wood fibre board (eg. Pavatex) 0.039-0.46 Good for wall and pitched roof construction
Hempcrete (e.g. Hemcrete, Canobiote, Canosmose, and Isochanvre) 0.12 – 0.13 Made of hemp shiv with a lime matrix. High elasticity and vapour permeability. Used for external wall insulation. Typical compressive strength 20 times lower than low grade concrete. Density: 15 per cent of traditional concrete.
Naturally occurring minerals
Usually environmentally ok but some have high embodied energy – see Table 3
Aerogel (e.g. Spacetherm) 0.013 Flexible sheets and laminates, a type of glass and composite materials including plasterboard and sandwiched within PVC panels. Expensive but useful where width is limited as performance is so good. Not breathable.
Fibreglass mineral wool batts and rolls (BSI kitemarked available) (e.g. British-Gypsum Isover, Knauf, Superglass) or Fibreglass board (e.g. Isowool, Dritherm) 0.033 – 0.040 Made from molten glass, sometimes with 20 to
30 per cent recycled content. The most common residential insulant. Usually applied as batts, pressed between studs. Most include a formaldehyde-based binder – exceptions are beginning to appear.
Mineral (rock & slag) wool batts and rolls (BSI kitemarked available) (e.g. Rockwool) 0.033 – 0.040 Used for loft and cavity wall insulation.
Foamed glass slab (e.g. Foamglas) 0.042 High, durable compressive strength, non-permeable. Needs bitumen or synthetic adhesives to install.
Perlite 0.045 – 0.05 Naturally occurring volcanic glass that greatly expands and becomes porous when heated sufficiently. Must be installed in sealed spaces.
Exfoliated vermiculite 0.063 Clay-based, otherwise like perlite
Multi-foil insulation (or ‘Radiant barriers’) disputed Thinness makes it ideal for places where little width is available. Made from non-renewable petrochemicals and aluminium. Can have poor airtightness. Expensive, vulnerable to being punctured, which will render it useless.
Fossil fuels
These have emitted carbon to the atmosphere during manufacture. Avoid unless you don’t have the space or budget for natural products. All manufactured at high temperatures, derived from fossil fuels. Extremely high embodied energy. Non-breathable, so may cause damp problems.
Phenolic foam board (e.g. Kingspan Kooltherm) 0.020 – 0.25 For roofing, cavity board, external wall board, plaster board dry linings systems, floor insulation and as sarking board.
Expanded polystyrene board and beads (EPS) 0.032 – 0.040 Beads are used primarily in masonry cavities.
Extruded Polystyrene board (XPS) (e.g. Kingspan Styrozone) 0.028 – 0.036 Very high compressive strength.
Polyurethane/polyisocyanurate board and foam
(e.g. Kingspan Therma)
0.02 – 0.033 Foam or rigid board. Foam is sprayed in at high temperatures; within seconds it will expand by over 30 times giving a seamless rigid covering. Good for plugging gaps or leaks. High compressive strength.
Eco-wool (e.g. non-itch) – batts 0.039 – 0.042 Alternative to glass wool, made from 85 per cent recycled plastic. Comes in rolls or slabs. Suitable for loft and stud walls.
Structural Insulated Panels (SIPs) variable approximately 0.040 A building method using pre-cut expanded polystyrene (EPS) or extruded polystyrene foam (XPS) to erect an airtight structure quickly that eliminates thermal bridging.

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The bottom line then is:

  1. Use as much insulation as you can afford and you have space for
  2. Use insulation from natural materials wherever possible, in preference to polystyrene and other fossil-derived insulants
  3. Avoid the use of foams wherever possible (improperly used these can expand and cause structural damage)
  4. For lofts, use Warmcel recycled cellulose
  5. For exterior insulation use wood fibreboard with wood fibre tongue and groove cladding and lime render; or Woodwool/Rockwool with lime render.

Your effort will be repaid in a warmer, cheaper-to-run home, and you will have that warm feeling that comes from doing your bit to help protect the climate.

© David Thorpe, Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

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Find out more – visit a refurbished home

You can find out more about external and internal wall insulation at green open house events in September. Speak to real homeowners as they share their personal experience of refurbishing their homes as part of SuperHome Open Days. SuperHomes are older homes refurbished by their owners for greater comfort, lower bills and far fewer carbon emissions – at least 60% less! Entry is free. Book now.

Also see:
Boarding a loft over insulation
Cavity wall insulation work
Cork insulation
Insulate a floor
Insulating a solid wall

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