Installing EWI over pebbledash

[JP_]

Is this OK? An installer has quoted what seems a reasonable price for add external insulation - about £80 a metre, so about £4500 in total. He said that he would fit insulation straight onto the pebbledash, render over, and silicon/silica, which I assume is to help waterproof?

Now, I have heard here that no everybody supports EWI, and it may lead to more problems. What sort of problems?

For the moment, heat is not really the issue for me - it's staying dry and mould free.

There is another cheeky, sneaky reason to do it though - it will make my house about 20cm wider, so if I wait until it is done before putting in my rear extension plans, I get an extra 20cm, which is significant for my plans (e.g. a 2.7m kitchen diner and 3.9m lounge can become a 2.8m kitchen and 4m lounge).

So, what are the problems? Is it the wall being less breathable? If so, then I assume some trickle vents will solve the problem, in the same way they do when doing a loft conversion? Or is it something else?

 

[Doggit]

What's he going to fix the Panels to the wall with, and how prominent is the pebble dash.

It wouldn't take too long to take the pebble dash off, and then apply a brick stabiliser, but then you'd have a flatish surface to work from. He should then use adhesive, and then pins, but if the pebble dash is left on then you may find the the panels aren't level, and that'll then lead to other issues.

The top surface isn't a waterproofer, but the walls aren't really breathable any longer, but the silicone is designed to throw off water and dirt, not to waterproof the walls Trickle vents are a good idea whichever way you go.

 

[JP_]

I think he said they would be screwed on ... but I might be wrong. Chatted on the phone, he's coming over on Satuday.

 

[tomfe]

I would want to add adhesive to the backs to take out the void, however this is not necessary as long as the substrate is suitable and the correct length fixing used.
Unevenness of the boards is taken out during rasping.

Your top coat is the waterproof level, but with silicone and eps they are breathable.
Make sure everything is siliconed, around reveals before basecoat and then after topcoat, and that you extend the roof and not use verge trim unless it is flashed extremely well. Please don't use phenolic unless you are really sheltered.

You might need trickle vents, EWI will make your house more airtight.

 

[Doggit]

They don't get screwed on, they should be pins that get hammered in, but there is a growing trend to use adhesive and fixings as well. If the pebbledash is solid, then you could use adhesive, but if it's uneven, then you can't guarantee the depth that the pins will go in each time.

 

[tomfe]

but there is a growing trend to use adhesive and fixings as well.

If the substrate is not suitable adhesive is a waste of time, a pull off test should be carried out if you are unsure.
But really you can just use fixings it'll just be 5 or 6 fixings per board.
You can also use screw fixings if they are suitable.