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Is insulation a good idea?

Heat loss

Heat loss

Insulation is the key weapon in the fight against heat loss. By insulating walls, roof and floors, you will use less energy to keep warm. The benefits will vary. Some people will have lower heating bills as a result. However, if, like many people, you under-heat your house, you will be able to keep warm and comfortable for the same level of expenditure on heating bills.

Loft insulation

mineral wool insulation

mineral wool insulation

Loft insulation is generally the easiest type of insulation to deal with (unless your loft is full of a lifetime’s accumulated junk and you can’t quite face dealing with it). It is also one of the measures that makes most difference in terms of warmth, comfort, saving on your energy bills, and reducing carbon emissions.

How much insulation is enough?
Building Regulations call for the equivalent of 270mm of mineral wool insulation (nearly 11 inches). Many people would argue that by today’s standards that is not enough. A house built to Passivhaus principles will have 450mm mineral wool insulation in the loft. In a retrofit aiming at something between 300mm and 450mm would be a good target.

What does it cost to install loft insulation?
An average 3-bedroom semi will cost anything up to £300 to install 270mm of mineral wool insulation. As a result you will save around £175 a year in lower heating bills.

Prices are much the same to top up your insulation if you’ve just got 100mm of insulation. The savings are much lower – about £25 a year on average according to the Energy Saving Trust. Surprisingly, DIY installation isn’t much cheaper – in fact, if you’re eligible for any of the free offers, it works out considerably more expensive.

The Green Deal is a finance mechanism to enable people to make improvements to the energy efficiency of their house such as installing insulation without having to bear the up-front cost. For more information, see below, or see our Green Deal Page

Which insulation materials are best?
The choice of insulating material will be dictated by a few factors; space, access and, of course, price. Mineral wool is one common choice. 

Blown-fibre insulation is also an option. This can be mineral wool or cellulose (usually shredded paper), and should be installed by a professional using specialist equipment. It is quick and easy to install and is usually the cheaper option for lofts that are large, multi-storey or difficult to access. If your loft is draughty, not all products are suitable. At the time of writing Rockwool Rockprime is the only material certified by the BBA as for not being displaced by draughts.

If you haven’t got much space, rigid foam insulation (the PUR or PIR Kingspan or Celotex types) are about twice as good insulators as wool types (sheep wool, mineral wool, etc.) and so take half the space. Where we have discussed 300mm as a good target, getting the same level of insulation from PUR or PIR will take only 150mm.

Other options include sheep's wool, hemp and cellulose which some people find more pleasant to handle. In tight, confined, airless lofts moving rolls of mineral wool while wearing gloves, goggles and a mask can be irritating – literally and metaphorically. Natural insulation needs no such protective measures and is more fun to play with.  However, it is significantly more expensive, and is still only as good an insulator as mineral wool.

Multifoil insulation can be an answer, but only a partial answer. The Local Authority Building Controls suggest that multifoils should only be used in conjunction with another form of insulation (basically because they are not as good as the manufacturers say they are). But they are good draught excluders and easy to fit.

How do I lay loft insulation?
Generally insulation is laid horizontally on the ceiling joists and there are good technical reasons for this. Lofts are usually ventilated spaces – the ventilation is necessary to keep the roof timbers in good order. Insulating at the rafters rather than the ceiling joist could mean that the ventilation will increase the heat lost from the house and put the roof timbers in danger of rot.  Insulating at the rafters level can be done but it needs to be done carefully and in full knowledge of all that is going on in the loft.

Find a local insulation installer

Cavity wall insulation

Cavity wall insulation (credit: NIA)

Cavity wall insulation (credit: NIA)

The typical cavity is 50mm (2 inches) wide and 50mm of foam insulation will reduce the heat loss through the wall by around 75%. As 35 per cent of the total heat lost from a house is through the walls, that gives an overall saving of 28 per cent.

What does cavity wall insulation cost?
Installation costs range up to about £350 for blown mineral wool. Other materials are likely to be more. Annual savings on heating bills for a gas-heated, 3-bedroom semi are around £135 per year according to the Energy Saving Trust.

The Green Deal is a finance mechanism to enable people to make improvements to the energy efficiency of their house such as installing insulation without having to bear the up-front cost. For more information, see below, or see our Green Deal Page.  

Is cavity wall insulation suitable for my home?
If your house is less than 10 years old, the cavities are probably insulated already. If you’re not sure whether or not your cavities are filled, or whether they are suitable, you can get them inspected. A reputable installer will do this for you, or ask a drain cleaning company to drill a few 15mm holes in the wall, and stick a camera in to see what the cavity is like.

Cavity wall insulation is an all or nothing thing: if some of your walls are difficult to access, make sure the installer can get to them before going ahead. 50% of the heat lost from a building can be attributed to gaps in insulation amounting to 5% of the surface area according to research by NASA in 2005. They liken the gaps in insulation to a pin prick in a balloon – the temperature difference inside to outside means that heat escapes more quickly through a small hole than it does through a big hole, but ultimately the same amount of heat escapes.

What cavity wall insulation involve?
It is a quick job, taking around 2 hours to fill an average house. The installer will drill a series of holes in the walls of your house, and blow insulation into the cavity through a hose. Once done, they will fill the holes again – make sure they colour match the mortar. You may be asked to take down pictures etc that are hanging on inside walls just for safety.

When NOT to insulate
Some properties, typically those built before 1940, have no vertical damp proof course (VDPC) around the windows. They rely on the ventilation in the cavity to stop rainwater penetrating to the inside wall. In this case you will need to either insert a VDPC or not fill the cavity.

In any house, cavity wall insulation will reduce ventilation, preventing natural moisture build-up from being removed. So if you have cavity wall insulation make sure you also have trickle vents or extractor fans.

It is important to check that your cavity is clean before filling it with insulation, otherwise it may cause damp.

What NOT to use
Most cavity wall installions use mineral wool because it’s cheap and it’s very quick to blow in. However, YouGen Energy Expert Tim Pullen warns of potential problems: "Gangs of installers tend to do lots of houses every day, and often do them poorly. This leads to problems with voids in the insulation, where they have missed bits out, and slumping, where the mineral wool was not packed in tight enough to hold in position. Both these will lead to cold spots, reducing efficiency and increasing the potential for condensation in the cavity."

His advice about cavity wall insulation is do it, but make sure you do it right. "Mineral wool is the worst option for this work and I would always prefer cellulose beads. If I can’t afford that I would go for polystyrene beads or injected foam."

Hard to treat cavity walls

There will be financial support for people with 'hard to treat' cavity walls, under the Energy Company Obligation (ECO). 

There is not a clear definition yet of hard to treat for ECO, but this is how BEIS defines it for statistical purposes:

  • Narrow cavity—Masonry cavities that are less than 50 mm wide.
  • Concrete construction—Prefabricated concrete constructions systems with cavities.
  • Metal frame construction—Metal frame construction systems wih cavities.
  • Random stone cavity—Uneven cavities formed in walls constructed of natural stone outer leaf and block/brick inner leaf.
  • Timber frame uninsulated studwork cavity (also has a masonry cavity, which must not be filled).
  • Too high, more than four storeys tall.
  • Exposed to severe wind driven rain.
  • Wall fault in its outer leaf which would need to be remediated before filling.

Find a local cavity wall insulation installer

photo by Elsie esq.

Solid wall insulation

External wall insulation (credit: NIA)

External wall insulation (credit: NIA)

About a third of UK homes have solid walls according to the National Insulation Association. It estimates that the heat loss through the walls of these homes is 45% (compared to 35% from buildings with cavity walls). Unlike cavity wall insulation, which is quick and cheap to install, solid wall insulation is expensive and disruptive. It costs between £5,500 and £13,000 for a typical gas-heated, 3-bedroom semi, according to the Energy Saving Trust. Insulating internal walls tends to be cheaper than external insulation, but is more disruptive. Typical savings on heating bills are £445 - £475 per year.

Internal wall insulation

This is the cheaper of the two options, but also the most disruptive. You will have to move radiators, electric sockets and skirting boards, and it will reduce the size of your room. To insulate effectively you will need around 100mm thickness of insulation (about 4 inches) on all the outside walls. Once installed, you will need to redecorate.

The Energy Saving Trust estimates costs at £5,500 to £8,500 for internal wall insulation. This is for the whole job to be done professionally. The good thing about internal wall insulation is that it is easy spread the cost by doing it a room at a time. Doing the decoration yourself will also reduce the cost.

External wall insulation

This involves fixing an insulating layer to the outside of the house, and putting a render or cladding over the top. Render is generally the cheaper option. Cladding comes in a variety of forms including brick, tiles, timber or shingles.  

You may need planning permission for external wall insulation, so make sure you check with your planning department before going ahead. It may not be suitable for the front of some period houses, but you may be able to insulate the back.

Help with the cost of installation

The Green Deal is designed to help people improve home efficiency through measures such as insulation without having to bear the upfront cost. See below for more information about the Green Deal. 

Stone walls

A typical stone wall will be constructed of two skins of stone with the gap between filled with rubble. Thinking used to be that the rubble fill was just a cheap form of construction, but the reality is that it is designed to work in a particular way. Rainwater will penetrate the outer skin and some of that water will find its way to the rubble-fill. Because the rubble is relatively loose the water drops through it to the ground and away, preventing the inner skin becoming damp.

The wall is also kept dry by air movement – wind acting on the outside skin and air moving through the wall. Interrupt those flows and the wall can become damp.

The way to insulate stone walls is with a breathable insulation, preferably set off from the inner skin to leave a 25mm air gap. Sheep wool, hemp or cellulose are all good insulators for this application and 100mm will reduce the U-value to about 0.35W/m2.

Bear in mind that walls are designed, they don’t just happen. And they are designed to work in a specific way. Interrupting the way they work with non-breathable materials like PUR insulation, gypsum plaster, damp-proof course, will stop them working properly.

Brick Walls

Rainwater will penetrate less far into brick than stone. Wind on the brick tends to effectively dry the wall. The critical issue is the dew-point. When warm air meets cold air it condenses and moisture is released. In an uninsulated brick wall that point will be towards the outer surface of the wall. Insulating internally tends to draw the dew-point into the wall. What needs to be avoided is drawing it in to the inner surface of the wall.

There are 3 ways of doing this:
1.    External insulation
This works particularly well if your house is rendered as they lend themselves to render finish.
2.    Thin inner insulation
Options include Therma-coat or Sempatap. These are, respectively, an insulating paint and a neoprene-like foam.
3.    An insulated stud wall set 25mm off the internal surface.
All of these will work but what is best in any given situation will depend on the situation.

Find a local solid wall insulation installer

How do I insulate the floor?

underfloor insulation

underfloor insulation

The heat lost through floors is smaller than through walls or roofs, so these should be the priority. However, insulating the floor can improve comfort, and the Energy SavingTrust (EST) estimates that it will save about £60 a year on energy costs for a suspended floor, plus a further £25 if you fill all the gaps between the floor and skirting board.

It is quite cheap - if you do it yourself - but as the following blog post illustrates, it's quite a big job: click the link for a DIY guide to insulating under the floor boards. A professional job will cost in the region of £770 for a 3 bedroom semi according to EST.

Newer properties are more likely to have solid floors. Insulating them will save about the same amount, but costs can vary significantly. It is much cheaper to insulate when you are doing other work or renovation. 

How to insulate under a suspended floor

This is where floorboards are laid on joists. A good way to tell whether you have got these is by looking to see if you have airbricks outside. It's fairly simple (though quite time consuming) to do it yourself. You lift the floorboards and put insulation material between the joists. This is usually mineral wool or loose fill insulation hung in netting. You will need to seal any gaps between boards, and between the floor and the wall.

If you have a cellar, you can avoid lifting the floor boards by insulating from below. To meet fire regulations you will need to fit plasterboard to the cellar ceiling (under the joists) and give it a skim of plaster.

English Heritage has a detailed guide for older properties. Click here to download it.

Upstairs rooms do not have to be insulated if they are above a heated room.

How to insulate a solid floor

You can insulate a solid concrete floor underneath or above the concrete. If you are insulating on top, you will lose a bit of height in the room, and will have to cut the bottom off any doors. Insulating below makes most sense when the floor needs replacing (for example if you were installing underfloor heating). Building regulations state that new floors must achieve 0.25W/m2K.

In older buildings, the installation modern non-breathable products may lead to damp problems if not done carefully. English Heritage has a guide which can be downloaded free. Click here to get your copy.

Find a local recommended insulation installer

Choosing an insulation installer

Make sure that your installer is a member of one of the following bodies: National Insulation Association, Cavity Insulation Guarantee Agency (CIGA) or the British Board of Agrement. They all have the facility to search for a local installer on their websites. They should offer a 25 year guarantee backed by CIGA, and be signed up to a code of professional practice. We also recommend that you take up references from local customers who have had their insulation installed at least a year.

Green Deal

The Green Deal is a finance mechanism to enable people to make improvements to the energy efficiency of their house without having to bear the up-front cost. Cavity wall insulation, roof insulation, room in roof insulation, loft insulation, and underfloor insulation are all items on the list of eligible improvements

At the heart of the green deal is the 'golden rule': as long as the expected financial savings in lower energy bills are equal to or greater than the costs of the energy efficiency measures installed, you are eligible.

The loan to cover installation costs will be in the form of a charge on the property. It will be repaid over a period of 10 - 25 years (depending on what measures are installed) through the property's electricity bill.

An accredited green deal adviser will assess the energy efficiency of the building, using a system that is based on an improved energy performance certificate (EPC). They will draw up a list of measures that can be taken to improve the energy efficiency and indicate which ones meet the golden rule. 

The Energy Saving advice line was set up to provide a central point of contact for inquiries about energy saving and questions about the Green Deal. You can phone the Energy Saving Advice Line for the best deals in your area on 0300 123 1234. 

Please see our Green Deal page for more information.


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