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  • Insulation - introduction

     Insulation representative image

    “All insulation is environmentally friendly. Some installation has better environmental credentials, but what matters is the energy it saves.” – Dick Strawbridge

    What is insulation?

    It’s any material that slows down the rate at which heat is lost through the fabric of your building (although it can also mean sound or fire insulation, or keeping heat out, we’re talking about keeping heat in). Air is actually the insulator, and the material is a way of trapping air. For example, you may sleep under a duvet containing eider (duck) feathers. The feathers trap air, which retains your body heat and keeps you warm (the feathers do exactly the same job on the duck too). Insulation can be in your loft; on interior walls, exterior walls or in the cavity between walls; under floors; or around pipes and hot water cylinders. It can be made from a wide range of materials, some natural (e.g. sheep’s wool or hemp) and some synthetic (e.g. polystyrene or fibreglass), that will come in different thicknesses, and with different methods of installation.

    Installing internal wall insulation.

    Houses had solid walls until the 1920s, when they started to be built with two walls with a cavity in between. This cavity started to be insulated in the 1970s, and it became compulsory for new buildings in the 1990s. It seems incredible that throughout most of the twentieth century, a thin layer of mineral wool insulation was added in the loft as an afterthought, or was absent altogether. It’s only recently, with rising fuel prices, that people have started to calculate the amount of insulation they need, as it’s now cheaper to insulate than to waste heat.

    Sheep’s wool insulation under a floor.

    k-values, R-values and U-values

    The insulation properties of a material can be expressed by its k-value. The k-value is the thermal conductivity of a material, and so obviously, for insulation, the lower the value the better. See here for the k-values of lots of common materials. The U-value is the important one for building regs – it’s a measure of heat flow per m², and so for an area of wall or roof, the U-value is a combination of the k-values of all the materials in it, and again, the lower the U-value the better for insulation. (NB: in the US they use R-value, which is based on the inverse of the k-value, and so a higher R-value is better as regards insulation properties). The k-value is a constant for any material (see list), but the U-value depends on the thickness of the wall / roof etc, as well as the materials involved. See the box below for how to calculate U-values.

    Using natural reed board to insulate a room.

    What are the benefits of insulation?

    It will reduce the amount of heat you need to generate, and therefore save energy, along with the associated pollutants and emissions.

    You can save even more energy in a well-insulated house by turning down thermostats. Temperatures will be more even, and so you’ll be more comfortable at lower temperatures than in an uninsulated house.

    It will also save money – and more so in future, as energy prices rise. However, if you spend a lot on wall insulation, say, then it might turn out to be quite a long payback time. But the benefits in terms of comfort will be immediate.

    Some building materials are themselves very insulating, such as hempcrete – here being used to insulate a roof; and straw bale walls have an exceptionally good U-value (0.13).

    Natural, local or recycled products have a lower embodied energy (the energy used to make them) than mineral wool, and don’t require the toxins and greenhouse gas emissions involved in the production of expanded foams. Mineral wool – the most common insulation – has a huge embodied energy, which reduces the total amount of energy saved. However, it’s not usually advisable to use unprocessed materials for insulation – for example, sheep’s wool insulation is manufactured with modern fire retardants and insect repellent to meet building regulations, otherwise it would be eaten by the larvae of clothes moths, and the lanolin would be a fire risk.

    Some building materials have good insulation properties in themselves, and therefore don’t require extra insulation – straw-bales and hempcrete, for example.

    insulation 5

    For k-value, see product spec or see here for the k-values (thermal conductivity) of lots of common materials.

    What can I do?

    You’ll have to consider issues around breathability, budget and the space available. It’s best to start by making sure you have adequate loft insulation, for the simple reason that heat rises. If wall insulation seems too expensive, don’t despair – you can achieve a lot with loft insulation, underfloor insulation if possible, and draughtproofing of windows, doors, letterbox, keyholes etc. If money is tight, you might consider this order: loft, draughtproofing, underfloor, walls. In your loft, insulate between the joists of the floor if the loft isn’t used, and between the roof rafters if it is. You can add batts (slabs) or rolls of synthetic or natural insulation, or you can pour in loose-fill material such as Warmcel between the joists – it comes in bags and is made from treated, recycled newspaper. Don’t remove any mineral wool insulation that’s there already – put extra insulation over the top of it. And don’t forget to attach some insulation to the loft hatch too.

    Warmcel recycled newspaper, loose-fill insulation poured into a loft.

    With walls, if you have a cavity, and it’s not insulated, that’s the first thing to do. You can insulate solid walls internally or externally, but remember that it’s only necessary to insulate perimeter walls (e.g. if your house is in the middle of a terrace, you only need to think about the front and back walls).

    Internal and external wall insulation both have their pros and cons. With internal insulation, there’s no external disruption, and it can be easier to do a DIY job – but the extra thickness will be inside your room, reducing personal space, and the insulation will be between sources of heat and the thermal mass of the walls (which can absorb heat and help maintain stable temperatures). External insulation means you keep the benefits of thermal mass, but you may have to extend eaves and move gutters, soffits and downpipes etc, which can be expensive. In conservation areas, you may not be able to change your frontage, and so you could insulate the front of your house internally and the back externally. External insulation can be breathable, but often isn’t. There are various types of wood-fibre boards available, and renders can be lime or hempcrete.

    Insulating a camper van with sheep’s wool; works for houses too.

    We wouldn’t advise the use of synthetic, non-breathable materials on a house made from natural, breathable materials such as timber, stone, earth etc. Breathable materials keep moisture moving, but synthetic materials trap water, which will then damage natural materials. It’s important to work out where the dew point is in your walls. The dew point is where water vapour will condense when there is a large temperature difference inside and outside your house. The dew point is often inside your walls. Adding insulation to the inside walls can move the dew point to the surface of the internal wall, which will cause natural insulation to get wet, removing its insulating properties and potentially causing it to rot. You may need an air gap between the wall and the insulation. Carry out research or ask a professional about the dew point in your building. However, hempcrete is a good internal insulation material that can deal with dew point moisture, as it doesn’t lose its insulating properties when moisture is present.

    If you’re doing insulation work yourself, you can check books or DIY guides online for instructions, and if you’re having your home retrofitted by a builder, especially with solid wall insulation, don’t assume that s/he will understand the issues outlined here. Do your own research, and contact product manufacturers for advice. It’s best to exceed building regs if you can (they’re not great by European standards).

    Cavity wall insulation: holes are drilled through the mortar all over the walls, fibreglass-based insulation is pumped in, and the holes are sealed with cement.

    Don’t forget to lag hot water cylinders and pipes properly too – you can get pipe and cylinder lagging from DIY shops. And thick curtains provide good insulation for windows (glass loses heat easily) – don’t forget to tuck them behind radiators.

    Grants: contact your local authority – they should be able to tell you if there are any local or national grants for insulation available. Also check the Energy Saving Trust and Government Grants websites.

    Thanks to Emma Winfield for information.


    Whilst you’re here, why not take a look at the other 25+ utilities topics available? And don’t forget to visit our main topics page to explore over 200 aspects of low-impact living and our homepage to learn more about why we do what we do.


    The specialist(s) below will respond to queries on this topic. Please comment in the box at the bottom of the page.

    David Thorpe of One Planet Life built his own passive solar timber frame studio and is the author of The Earthscan Expert Guide to Sustainable Home Refurbishment. He has built his own passive solar timber frame studio and retrofitted several homes. He has also written numerous ‘how to’ articles for the Superhomes website on insulation. He is an expert on the causes of damp and its relation to breathable structures and airtightness.


    The views expressed here are those of the author and not necessarily lowimpact.org's


    7 Comments

    • 1nae.bother August 17th, 2016

      I have just learned that my local council wants my building externally insulated (the flat is made from no fines concrete, built around 1955) The council want all homes in their area externally insulated by 2020. I don’t know the exact material they intend to use, but they are calling it cavity wall insulation. That is all the info I have at the moment, but I am concerned about :

      a) the health risks involved,
      jb) just where do these insulation materials come from (ie are there environmental risks)

      I may be wrong of course, and this project is just great for the folks who actually live in the buildings, and also for our planet, but I smell a rat.

      Can anyone enlighten me?
      Am I being paranoid because I believe that our councils are here to control us rather than serve us?
      Is the only benefit for the companies who produce the materials?

      Any comments would be gratefully received. Thanks for your time.

    • 2John Harrison August 18th, 2016

      Our council was externally insulating homes until they ran out of money. Very annoying to the chap who was half done when they told the gang doing the job who promptly decamped back to Poland. External insulation is effective for solid walled properties – although it doesn’t add up economically since it’s far more expensive than cavity wall insulation.
      Cavity wall insulation is generally cost-effective and a no-mess, no-problem undertaking except in properties subject to extreme weather – as many are in Wales with driving rain and wind – where the insulation can bridge the damp barrier causing awful problems inside with mould etc. Removing cavity wall insulation is far more expensive than installing it.

    • 3villa_bruce November 30th, 2017

      I’m looking to build a new house using a passive house design and was wondering about the possible use of sheep’s wool direct from the sheep as a cavity insulator, any thoughts ?

    • 4Dave Darby November 30th, 2017

      I’ll ask our specialist to answer this for you, but I believe that pure sheep’s wool needs to be treated so that a) it’s not eaten by critters and b) it’s not a fire risk.

    • 5David Thorpe November 30th, 2017

      Yes, Dave’s right. It should be washed and treated with fire and pest retardant. Its efficacy also relies on it not being matted, as it’s the air gaps that help with insulation.

    • 6homeminderuk December 20th, 2017

      Needing to insulate the sub-floor of a recently purchased property with a cellar & wanting to be planet-friendly, I opt-ed for sheep’s wool. More expensive than I could really afford (meaning I could only do half the floor), I was disappointed that the nearest supplier was in Wales, somewhat negating the eco-credentials but doubly disappointed on delivery at the amount of plastic it was wrapped in – huggins of the stuff! Needless to say, I feel that my ‘green’ aspirations had thoroughly wilted when I had to nip to Wickes’ to finish the project.

    • 7Dave Darby December 21st, 2017

      At least you’re thinking about sustainable alternatives – but shame about the plastic.

    • 8Hattie Duke September 30th, 2018

      Hi, we are building a round house with a reciprocal roof and sedum roof. We are looking at insulation options between the layers, any advice very welcome, thank you

    • 9David Thorpe October 1st, 2018

      With reference to both Hattie and HomeminderUK, the best insulant in terms of cost, ecological credentials and performance is recycled newsprint. This is available under the trade name Warmcel (https://www.warmcel.co.uk/). It is available from ecomerchant.co.uk. It is particularly ideal for horizontal surfaces/spaces such as underneath floors and in the lofts. It comes ready treated against vermin and for fire protection.

      In a flat space you can install it yourself. It comes in sacks quite tightly packed, you cut them open and spread it around frothing and fluffing it about as much as possible to ensure that there are air gaps. Putting it in a vertical space requires specialist assistance because it has to be applied damp so that it sticks to the surfaces and is therefore more expensive. I am also sceptical as to whether or not there is settlement over time, which would mean that there would be a space at the top where there will be no insulation, which renders the whole thing pointless.

      For vertical spaces I would recommend woodfibre batts. These insulants have the other benefit that they lock up atmospheric carbon in the fabric of your building. The only disadvantage really between these options and EPS, Polystyrene and other insulants that rely upon the fossil fuel industry and are non-breathable, Is that you need a greater depth of insulation in order to obtain the equivalent benefit.

      If you look at the website superhomes.org.uk I’ve written some articles which explain the amount of depth that Is needed for each type of insulation material to reach the required level of insulation. There is much more information in my book the Earthscan Expert Guide to Sustainable Home Refurbishment (Not a plug, it’s just true!).

      To comment on the opening paragraph of this article, insulation also helps to protect from overheating; As well as keeping heat it in, it will keep heat out in times of very hot weather.

      It’s also important to know the difference in effect between external and internal wall insulation; If you insulate the inside of the wall then only the air in your room will heat up; As soon as that leaves the room will cool down. If you insulate the outside of the house, then heating the inside will mean that the walls will heat up, so that even if you turn off the heating that heat will come out from the walls and keep the rooms warm for many hours afterwards.

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