Why 50mm of External Solid Wall Insulation is Just Not Enough?

Why 50mm of External Solid Wall Insulation is Just Not Enough

As you can imag­ine, we get a huge amount of inter­est in solid wall insu­la­tion, espe­cially since the launch of the Gov­ern­ment GDHIF scheme which pro­vides gen­er­ous grants to help cover the cost of the works (up to 67% of the total cost!).

What peo­ple are often sur­prised to find out when they get in touch with us is that we rec­om­mend installing 100mm of insu­la­tion to the wall. Many peo­ple assume that 50mm or less will be more than suf­fi­cient to lower their energy bills.

Well in a way they are cor­rect – it will lower their bills, how­ever we are about to show you why 50mm is never enough!

Why not just insulate with 50mm of insulation?

Using 50mm of EPS insu­la­tion does improve the U-Value a huge amount – as a reminder the U-value is sim­ply a mea­sure of heat loss through an object, the lower the value the bet­ter in terms of ther­mal effi­ciency.

Solid brick walls have a U-value of around 2.1-3W/m² and adding 50mm of expanded poly­styrene (EPS) will take the U-value down to 0.5W/m²k, which is a sub­stan­tial improve­ment, but 100mm takes it down to just 0.27W/m²k.

A new cav­ity wall built today must be built to a u-value of 0.3W/m²k which means if you ret­ro­spec­tively add just 50mm of EPS insu­la­tion it won’t be suf­fi­cient to adhere to build­ing reg­u­la­tions. If 25% of the exter­nal wall is being insu­lated you MUST adhere to build­ing reg­u­la­tions, there­fore you are legally obliged to go for this larger thick­ness. In addi­tion, if insu­la­tion is being installed under the GDHIF scheme, then it must adhere to build­ing regs, so if some­one has installed 50mm of insu­la­tion on your home it is worth get­ting in touch of them to make sure they come and do it cor­rectly!!

But aside from building regulations, why else would you go thicker?

Well obvi­ously the thicker the insu­la­tion the greater the sav­ings, so 100mm of insu­la­tion will pro­vide big­ger energy sav­ings than 50mm of insu­la­tion. With 50mm of insu­la­tion you should expect to see a reduc­tion in your heat­ing bills by around 30%, but with 100mm that per­cent­age will shoot to over 50%. Your heat­ing sys­tem won’t have to work as hard, and as a result won’t use as much gas – so more cash in your pocket! Especially great in the current energy crisis!

50mm vs 100mm – the cost

Obvi­ously that is a pretty sig­nif­i­cant improve­ment on the energy bills sav­ings, but many people’s counter argu­ment to this is that 100mm of insu­la­tion will be so much more expen­sive!

Well, while thin­ner insu­la­tion is cheaper to buy, if you break down the costs of the solid wall insu­la­tion works, you will soon see the dif­fer­ence is price is very mar­ginal. All EPS insu­la­tion gets attached to the wall in the same way – so both 50mm and 100mm thick EPS still get attached to the walls in the same way, they get the same rein­force­ment layer and the same ren­der fin­ish. You will still require the same scaf­fold­ing and both will use the same amount of labour!

On a £8,000 job you might only end up pay­ing £200 more to install 100mm over 50mm – so really based on the energy sav­ings, we really do rec­om­mend going for the thicker insu­la­tion!

50mm vs 100mm – the comfort

We can talk about the sav­ings and costs of the dif­fer­ent sys­tems until the cows come home, but a lot of poten­tial cus­tomers are purely inter­ested in the increased com­fort lev­els.

Would it be worth the extra width and cost for the added com­fort, or is the dif­fer­ence min­i­mal? To look at this you need to take the prop­erty as a whole. Pre­vi­ous to any exter­nal solid wall insu­la­tion, most prop­er­ties suf­fer from hot and cold spots through­out the prop­erty –  this is caused by the heat from the heat­ing sys­tem, rapidly leav­ing the prop­erty in cer­tain areas.

Apply­ing 50mm of exter­nal solid wall insu­la­tion helps reduce the like­li­hood of these hot and cold spots. How­ever, due to the thick­ness only being 50mm there is still a chance of cold bridg­ing, which is the main cause of cold spots within the home. Cold bridg­ing is caused by areas of a higher U-Value. With 100mm, these spots are almost always com­pletely cov­ered, mean­ing that you are always warm and the house remains a con­stant, com­fort­able tem­per­a­ture!

Should you always go for 100mm then?

In some cases, 100mm is just not going to be viable. If you have a path run­ning by the side of your house for exam­ple, 100mm may make it too thin to walk up and down or get a car up. In this case you will obvi­ously need to use less, but in these cir­cum­stances we rec­om­mend using a prod­uct that is more ther­mally effi­cient, so you are still achiev­ing the same u-val­ues, just with a thin­ner amount of mate­r­ial – for exam­ple XPS instead of EPS.

You might think hav­ing read this that we would be advo­cates of installing more than 100mm, well strangely enough we aren’t – or at least we aren’t in some cir­cum­stances. 100mm can be installed with­out hav­ing any real impact on the amount of light enter­ing the prop­erty, but as you start going above this then it does start becom­ing an issue. Obvi­ously 150mm will pro­duce larger energy sav­ings, and so for walls with few or no win­dows then it may well be worth con­sid­er­ing, but in many cases 100mm will more than suf­fice. In Europe 150mm is becom­ing the stan­dard, but they do have much harsher win­ters, and in the British cli­mate the sav­ings just aren’t that great for most prop­er­ties.

Insulating previously insulted properties

It has been inter­est­ing to see how solid wall insu­la­tion has evolved though. In East­ern Euro­pean coun­tries, that are obvi­ously treated to slightly harsher weather con­di­tions com­pared to us here in the UK, a new insu­la­tion indus­try is now boom­ing where homes that have pre­vi­ously been insu­lated with 50mm are being retro­fit­ted with a fur­ther 50-100mm of insu­la­tion. It seems sen­si­ble that we make the jump now and make 100mm the indus­try stan­dard so this doesn’t occur here in the UK in the future!

Read more about external wall insulation & energy bills, passive houses, and more on our partner’s blog!