External wall insulation

[octonuddle]

Has anyone had this done and if so, has it made a noticeable difference to the warmth of your home? Also interested in cost…how much would you consider reasonable for a 4 bed detached house?

Did mine a few years ago and it's brilliant, about 4k for small mid terrace.

Regarding the heat, yes it keeps it cool up to about 30 degrees outside. In the extreme heat, I ran around opening doors and windows as soon as the temperature tipping point had been reached, like everyone else did!

That was everything but roof and pipes. He didn’t propose to take off our pebble dash, just to go right over it.

you don’t factor out windows in the calculation btw. Just measure the width of your house, and then the height should be around 5-6m (depending on the height of your ceilings). We’re an end of terrace, so we would have 7m wide by 5m high on the front and back, and 7x5 on the side as well (our house is a square). So all told 105m2, so we were looking at £14k-ish to get it done.

at that price I’m not really interested. It’s liable to reduce my heating bill considerably for sure- would bring it down by 60-70%, but frankly I could do the same internally with 50mm myself for a fraction of the cost (probably cost me around 2-3k all in to do it myself), which means for the total cost I could do internally + get my windows changed (they’re 20+ years old) and my heating bill is approx £2,500/year atm (damn energy prices). So saving myself £1,500/year sure, but I don’t have the money now and can do it more cheaply myself sacrificing only a minimal amount of internal space.

Internal wall insulation is cheaper but very much depends on house features and your skill level. Our house is 1912 and we have coving, picture rails, window surrounds and the original skirting boards ie all non standard. I am not skilled enough to do any of that eg the window surrounds would need removing and the lining boards extending to enable 50 mm of boarding fitted. All adds to the cost and disruption

Thermally It is easier to get 100% coverage with EWI. It’s 2-3 weeks of disruption and then done, and for us it has modernised the house with the smoother render rather than the dated and very very difficult to paint pebbledash.

@Summerhillsquare Is there a threshold beyond which you find that the external wall insulation makes a heatwave worse? In other words, is it the case that, if it is, say, >30C for 5 days straight, the house will eventually get hot anyway, and, once it does, the insulation makes it worse because it takes longer for the house to cool down?

May I ask how long ago you did it and how many heatwaves you have had after fitting it? I think the worst one was the summer of 2020, because, even if the peak temperatures were not as high as this week, it lasted longer and it was hotter at night, too. I remember 28 degrees at midnight, like in Spain!

What was it like during the extreme heat of this week? Are you in the SE where we saw 37ish? How long did the house take to cool down?

Thanks!

No, it doesn't make it worse. There is a point at which it ceases to be effective in keeping the internal temperature reasonably cool. I'm in the NE, and once it got above 30 outside it was 25 inside, which for me was just too hot. It topped out at 27 degrees inside. I'd still rather have it than not, has saved me a fortune and the house is super quiet. And a pretty colour cos you have the render any colour.

We are having this done in a few weeks. Our house js 5 bed 1930s breeze block single skin. At the back we have a full height modern extension and the difference between the 2 halves of the house is incredible. Plus we have mould on all the old walls. We are doing the front elevation and one side. The render has blown on the side and is being hacked off but they are going over the front existing render. Other 2 sides are just being re rendered (the other side has out garage and utility so not bothering with EWI there). Whole cost c 35k plus vat but we are having the thinner more expensive EWI as our window reveals can't cope with the thicker stuff.
New Windows are 11k
Then add on c 6k for electrics (moving and replacing lights and EV charger), plumbing (all taps, overflows etc need extending), guttering, scaffolding, alarm move, Sky dish move and roof cleaning.
It's not cheap but hoping it makes the front of our house liveable.
On the heat front I really can't see why it would be any worse than any other well installed new build. We are SE and so yes for those few days a yeat its really hot it will be hotter in the front but as the front is north facing it stays cooler anyway.

My advice it to get a really good firm for render and windows. The windows for mine are not standard because we need deeper sills on some sides and the windows pushing forwards by 5 mm. Its been a real headache to tie everyone up. We have got great trades.
The new render is colour through so no maintenance and guaranteed for 15 years.

EWI is the gold standard. It doesn’t make the house “cooler” or “warmer” what it does is prevent the external temperature affecting your wall temperatures so rapidly, keeping the inside more constant.

so solid brick walls have a u-value of around 3 IIRC, but if you put 120mm EWI properly fitted with no air movement inside the insulation etc you reduce it to less than 1 (I think something like 0.7 or 0.8).

u values are a measure of how fast a material transmits heat. The basic principle is that energy (heat) moves to entropy (cold) until the two equalise. There are other factors (air leaks etc) but u value is what EWI affects. The lower the value, the slower the transmission of energy. If you do not have EWI and it’s 40 degrees outside, you have that heat baking into the walls which absorb that energy and start transmitting it inside the house. Aside from uvalue, materials also have something called thermal mass, which refers to how much energy they can absorb. So essentially when it’s really hot, your walls heat up. they don’t heat up immediately because as the heat is being transferred it’s also warming up the walls. This is why even at night in the UK (when it’s very cool even in summer) the house is unbearably warm because the walls are still giving off residual heat from the day.

So EWI takes advantage of both of these factors- if you have a way of regulating the internal temperature, EWI will allow you to compete with the external temperature by making the thermal transfer from inside to outside very slow.

Internal wall insulation on the other hand isn’t as good- you can’t take advantage of the thermal mass of the walls to help regulate the temperature. You can however slow down the thermal transfer. Additionally I wouldn’t be putting 120mm of insulation inside my house because it would ruin my internal space. So the improvements will neither be as good or as efficacious. But as ever, it’s a balancing act.

coupling EWI with a properly insulating ceiling/loft, and then properly sealed floors from air leaks, your house will be substantially more stable. The problem is UK houses are designed to breathe through their air leaks, so just make sure you open windows for 10 minutes every few hours to allow the air to exchange.

£35k plus VAT! OMG <cries>

We have a big house.... but yes its expensive... be less if they could have gone over existing render but they can't on 2 walls.... taking it off is v labour intensive....

@Lottsbiffandsmudge are you able to benefit from the zero vat on energy efficiency measures?