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Can you have both cavity wall insulation and external or internal wall insulation?

Posted by Tasha Kosviner on 7 May 2015 at 9:35 am

Got a cavity wall? Fill it. Got a solid wall? Then add internal or external wall insulation.

This is the standard advice for people looking make their houses snugger and more energy efficient. But there are those who go one step further.

Having filled the cavities in his mid-century detached house with polystyrene bead, Ian Mawditt then opted to install external wall insulation (EWI) as well.

Why do it?

“I bought the house as a project, and had the intention of cutting our heating bills by 80 per cent,” Ian explains. “When we moved in it was freezing and draughty in the winter and far too hot in the summer. Installing optimal insulation meant that we would be able to achieve a steady pleasant temperature year-round.”

Cavity wall insulation (CWI) effectively reduces air movement in the cavity – air that could carry heat out through the top of the cavity. It was very important to make sure the cavity was completely filled – any air pockets could have a detrimental effect on the efficacy of the insulation.

Once that was done, the external wall insulation was then applied, effectively wrapping the house in one, snug blanket.

“The effect was almost immediate,” Ian reports. “Wondering around our house it just felt much, much warmer.”

Double insulation and damp

Another great benefit of having both cavity and external wall insulation is that the EWI reduces the chance that the CWI will cause condensation and damp.

Insulation and home build guru Tim Pullen says: “Putting EWI on a cavity-filled wall actually reduces the likelihood of damp because it both provides a waterproof barrier on the outer surface and allows the inner surface to reach room ambient temperature. This reduces the potential for both damp penetration through the outer skin and condensation caused by the temperature differential between the outer and inner surfaces."

Ian’s experience

So did Ian achieve his 80 per cent saving? You bet he did!

“In the first year we were here we used 21,700 kWh in gas to heat our home,” he reports. “Fast forward two years, and our total gas use for heating has actually been reduced by more than 90 per cent. “

In monetary terms this means Ian’s annual heating bill has dropped from £1000 to closer to £100. With the total cost of the insulation sitting at £13,500 Ian is looking at fairly long term financial payback time of 15 years.

“But the immediate payback is a warm, cosy house,” Ian points out, “and you can’t put a price on that!”

Ian's SuperHome opens as part of SuperHome Open Days.

More information about Insulation on YouGen

Find an insulation installer.

Related blogs

Airbricks, Flooding and Cavity Wall Insulation

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4 comments - read them below or add one


KBabsComment left on: 16 October 2018 at 2:13 pm

Interesting read. 

What size was your cavity and how thick was the EWI?

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EnergySavingGrants.orgComment left on: 19 May 2017 at 9:00 am

We would definitely recommend having a 'breathable' render such as silicone render installed, as opposed to a non-breathable render such as acrylic. A breathable render will allow water vapour to pass through the walls, thus reducing the chances of condensation build up within the house.

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CarlBristolComment left on: 8 May 2015 at 12:56 pm

This is such an excellent solution for saving energy in the home and there really should be no problems with insurance; after all you are making your home more secure by both methods. Working in Consulting engineering I can say that your estimated savings are also spot on. Excellent and informative article

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pechisComment left on: 7 May 2015 at 2:16 pm

I'd be interested whether there was any issues with the insurance company or mortgage company. Does the NHBC have an opinion? 

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