Insulation: introduction

What is insulation?

It is 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, this factsheet is 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 of 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.

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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 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.

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 is 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 box below for how to calculate U-values.

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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).

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.

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.

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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.

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).

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

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.

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.

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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.

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 to contact product manufacturers for advice. It’s best to exceed building regs if you can (they’re not great by European standards).

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.


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.

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