Cavity Wall Insulation

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If you have cavities in your walls which are not already filled with insulation then you have a great opportunity to save quite a lot of money. Most household heat is lost through the walls and anything to slow this down helps. The story is similar to loft insulation, you can reduce your energy bill for heating by up to 25%, but again more realistically 20%. The table below calculates the effect of only insulating your cavities considering several main household fuels and estimates a payback time for the investment. For a larger house cavity wall insulation will cost £ 500 and for the smaller house about £ 350.

  Approx % energy saving Approx energy saving per year
Electric  (Std) Economy 7 LPG                      
Fuel   Oil        
Savings - 3 bed house 20% 4000 kWh £ 192 £ 616 £ 300 £ 232 £ 172
Payback time with cavity wall insulation costing £ 400 (3 bed house) 2.1 years 0.6 years 1.3 years 1.7 years 2.3 years
Savings - 5 bed house 20% 6000 kWh £ 288 £ 924 £ 450 £ 348 £ 258
Payback time with cavity wall insulation costing £ 550 (5 bed house) 1.9 years 0.6 years 1.2 years 1.6 years 2.1 years
Calculations assume the energy required to heat an un-insulated 3 bed house is 20000 kWh and a 5 bed house is 30000kWh.

Is it worth doing?

Yes.  This measure is on a par with insulating the loft as one of the most effective thing you can do to your house to save money and protect the environment.  And like loft insulation there are grants available to many of us to completely pay for the insulation, check for grants from the warm front scheme (this is now gone), energy companies and local authorities and check with your local energy advice centre.

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0 # U-values?LL 2013-01-30 20:53
Average U-value for cavity wall insulation? and what would you say is the average payback time for houses in general then?
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0 # Cavity Wall InsulationJane 2012-11-27 10:54
While I would agree generally that cavity wall insulation is a good thing please point out that it isn't appropriate for all properties. Houses that are exposed to wind driven rain, such as those on the west coast of the UK, may end up with significant problems from having cavity wall insulation. The rain is driven into the cavity and soaks the filling this can cause damp and condensation problems internally as well as then freezing and causing problems with external rendering etc.

So generally a good thing but not always. It is a shame that it is pushed so hard without consideration being given to the site and exposure to extreme weather that a property may have.
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0 # Cavity wall insulationjeff 2012-05-19 14:07
I had originally just used your 4000 Kwh as an average houshold should save. This caused great excitement as local gas prices are 12p/Kwh which meant we would save £480PA.
This seemed too good to be true and of course it was. Our annual bill is £1500 but taking out the standing charges and estimated gas usage for hot water we were left with £822 as a good estimate of what portion of the bill is actually heating. This equates to a possible £164 PA @ 20% saving due to cavity insulation which I think is more realistic. This is still a substantial saving and awaiting quotations to calculate possible payback period.
I was quite suprised to see that we use on average less than half the energy consumption for a 3 bed house than your estimator is using.
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0 # insulationRichard 2012-05-19 23:27
Hi Jeff
It is amazing the variations, last month we had someone write in to say they were using 33,000 kWh per year to heat their three bed bungalow. You are using less than 9000kWh for heating and yes 20% of this corresponds to about £200 at your charge rate.

In the article we state this:-

*Calc ulations assume the energy required to heat an un-insulated 3 bed house is 20000 kWh
(assumptio ns are here: ...

Uninsulated is the key word here. If you already have loft insulation and double glazing the proportional effect of adding cavity wall insulation will be lessened, the heat flow through the wall remaining static and being governed by thermal gradient and wall materials.

F rom you email, do you live in Jersey? 12p per kWh for gas, wow, is it really that much! Is it LPG?

the 20000kWh number comes from UK averages, and includes all climates by the way.

would you consider putting you data on our survey? ...


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0 # Applying for insulation grantsJustin Elliott 2010-07-28 09:43
Hi - you can quickly and easily apply for insulation grants by going to As your article rightly says, this can save you hundreds of pounds off your energy bill and help to create a more energy efficient home.
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0 # Applying for insulation grantsRichard 2010-08-13 12:34
Thanks justin, looks useful.
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