External solid wall insulation vs cavity wall insulation


External solid wall insulation vs a retro-fit cavity

Why exter­nal solid wall insu­la­tion is the answer – and a cav­ity wall insulation isn’t

Prop­er­ties built before the 1930s are almost always built with a sin­gle skin of brick, which leads to rapid heat loss, expen­sive heat­ing bills and a cold house. These prop­er­ties are now being retro-fit­ted with 100mm of exter­nal wall insu­la­tion, which brings down the U-Value, or heat trans­fer, to bet­ter than build­ing reg­u­la­tions. This means that the retro-fit­ted walls are becom­ing more energy effi­cient than new build prop­er­ties. How­ever, after the 1930s, build­ings started to be con­structed with a gap in between two skins of brick. This was designed to slow down the heat trans­fer from inside to out­side, and increase the effi­ciency of the prop­erty. The prob­lem is that these cav­ity walls were con­structed with­out any insu­la­tion, mean­ing that the heat trans­fer is still fairly rapid. There­fore mil­lions of peo­ple have now had these retro-fit­ted with insu­la­tion pumped into the cav­ity. How­ever, there is one prob­lem, it’s sim­ply not good enough.

Why do cav­ity walls have lim­i­ta­tions

A typ­i­cal cav­ity wall built prior to the 80’s has a gap of only 40mm. So you can fill this with as much insu­la­tion as you can squeeze in, but at the end of the day, when com­par­ing it to 100mm of exter­nal wall insu­la­tion you wont get any­where near the same effi­ciency. This explains why we get lots of peo­ple say­ing that their prop­erty is still cold after inject­ing their cav­i­ties with insu­la­tion. Although it’s a much cheaper method, it doesn’t get you towards cur­rent build­ing reg­u­la­tions.

Solid wall insu­la­tion – per­fect with heat pumps

One impor­tant fac­tor when look­ing at insu­la­tion cav­ity walls, and com­par­ing them to exter­nally insu­lated prop­er­ties is how they work with renew­able heat pumps. Retro-fit­ted cav­ity walls still leak heat, due to them only hav­ing 40mm of insu­la­tion fit­ted within, this means that heat pumps, which required incred­i­bly well insu­la­tion prop­er­ties, do not reach the level of effi­ciency that they are capa­ble of. This often means that run­ning costs rocket and you end up pay­ing more than you bar­gained for. Apply­ing exter­nal solid wall insu­la­tion reduces the U-Value to such an extent that heat pumps not only work incred­i­bly effi­ciently, but they often only need to be switched on in the deep mid-win­ter.

Exter­nal solid wall insu­la­tion solves damp

A fur­ther neg­a­tive for retro-fit­ting cav­ity wall prop­er­ties is the issue of damp. Thou­sands of prop­er­ties up and down the coun­try, who had their pre­vi­ously unfilled cav­i­ties injected, have found that the insu­la­tion has caused a build up damp and con­den­sa­tion that has started the for­ma­tion of mould within the prop­erty. The ben­e­fits of exter­nal solid wall insu­la­tion go fur­ther than pure effi­ciency. It pro­vides a com­plete weath­er­proof solu­tion that not only pre­vents water ingress into the cav­ity, but also pro­tects the brick­work, which main­tains prop­erty strength.

Don’t just do cav­ity wall insulation, exter­nally insu­late as well

So if you’ve  do your cav­ity wall insulation, don’t think that that is all you can do to improve the effi­ciency of your prop­erty. Not only can it boost the appear­ance and solve damp issue, but adding 100mm of insu­la­tion can dras­ti­cally cut those heat­ing bills.


    • Sharna Tunney
    • 24.04.2017
    • Reply

    I have a cavity wall but it is narrow cavity and cannot be filled. Is EWI the best solution for me or should I look at internal insulation?

    Very best,

      • Alan Bouquet
      • 21.06.2017
      • Reply

      Hi Sharna,
      EWI works well on narrow cavity properties. Internal insulation will have the same pros and cons as you get with a solid brick property, so we would always suggest EWI in most cases. Feel free to call us for a quote!

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