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Insulation is king - part 2

Posted by Tim Pullen on 3 April 2009 at 8:20 am

A question I am often asked at exhibition seminars is should cavity walls have cavity wall insulation. Almost invariably the answer is yes. 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.

An average three to four bedroom house, around 150m² floor area is likely to have a heating bill of about £700 per year (assuming a condensing gas boiler). Cavity wall insulation will save about £200 per year (and reduce carbon emissions by 2.5 tonnes per year).

Getting the cavities filled is likely to cost around £500, but remember the CERT scheme. This is Carbon Emissions Reduction Target, imposed on energy companies by the Government. All the major energy companies (British Gas, Npower, Scottish & Southern, EON and EDF) are required to provide support to householders to reduce the carbon emissions, which support comes in the form of grants.

If you are over 70 years old, on benefits or disabled you can get the walls and loft insulated free. Even if you are none of the above you can still get a grant of up to 50 per cent. Any of the energy companies will do a free survey and tell you what needs to be done, if cavity wall insulation will work and how much it will cost (or you can ring your local Energy Advice Centre on 0800 512 012 to see what grants are available in your area).

When NOT to insulate
Some properties, typically 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.

What NOT to use
Cavity wall installers’ favourite material is mineral wool because it’s cheap and it’s very quick to blow in. 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.

My 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.

photo by Elsie esq.

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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6 comments - read them below or add one


JTec Energy PerformanceComment left on: 13 July 2010 at 11:26 am

I remember being part of a piece of research that included a study on the impact of thermal mass on energy performance.  The research reports that we found showed an enormous variation - there was no clear conclusion that could be made, as to whether thermal mass improved, or worsened, energy performance. 

The one conclusion that could be reached was that this depended on how the building was being used.  If the building was heated 24 hours a day, then thermal mass reduced energy use.  It the building was heated intermittently, it increased energy use.  

So the problem with adding insulation in a way that increases the thermal mass of the home - say, by using external, not internal, insulation on a solid wall - is that it may help the current occupier who is at home all day and therefore heats it all day.  When she sells it, the next occupier might see higher than necessary fuel bills, because she doesn't use the home the same way.

Differences like this are very difficult to deal with in a rating system that rates the house and not the occupier, such as SAP and RdSAP, because the rating must be independent of the way the house is used.

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EnergyLink Ltd

EnergyLink LtdComment left on: 12 July 2010 at 4:33 pm

I just wanted to mention that if you wanted to go down the grant route for insulation CERT (carbon emissions reduction target) funding has been extended until December 2012.

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 23 March 2010 at 9:35 am

Absolutely agree with Steve. But it is a refinement and renovation projects do not always have the luxury of options. They just have to get insulation in where they can. Thermal mass can be helpful in smoothing the peaks and troughs of the heating cycle but essentially only make a contribution if the energy heating the mass is free, i.e. solar energy. The thermal tent, or tea-cosy effect, helps to reduce the rate at which the heat you are paying for escapes.

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Countrylines ArchitectsComment left on: 19 March 2010 at 10:43 am

I totally agree that insulation is the most effective investment when building - new or an extensions - however, its also important where the insulation is positioned in the construction. Some new builds especially linked to a heat pump could benefit  with a "thermal mass" construction while others would benefit with a "thermal tent" form. Its important to consider the way the home is used.


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Tim Pullen

Tim Pullen from Weather WorksComment left on: 25 September 2009 at 1:46 pm

The short answer is no. Cellulose, like all natural insulation materials, are treated in such a way as to prevent this.

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vimokshadakaComment left on: 14 September 2009 at 1:50 am

would celulose beads get damp and ineffective unless it's a breathing wall?

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