Insulation between my wall and Neighbour's new wall

[MattSeeker]

Hi,

I live in a semi-detached property and My garage was converted into a Two-Storey extension 20 years ago. And now my neighbor is converting his garage into a two-storey extension.

They are in the process of their new wall, the gap between my wall and theirs is 50 MM. Their builder is suggesting keeping insulation in between the walls which will help to avoid damp issues in the future. He is suggesting the following polystyrene board( https://materialsmarket.com/products/50mm-eps70-polystyrene-insulation-board-2400mm-x-1200mm) as the material and will cover the gap in front/back by rendering. Also, block the gap at the top with some material.

My knowledge is limited in these and would like to know your opinion on the following question

1. Is insulation needed to avoid damp issues or this is optional?
2. if it is better to go with insulation, the above-mentioned material is the correct one or would u suggest a different one?

Thanks for your help in advance

 

[motorbiking]

It would be better to use Celotex. 50mm boards as it has a vapour barrier. its about twice the price though, but will this make your house a terrace now?

 

[MattSeeker]

The walls are independent . U mean whether adding the insulation would make it look like terraced ?

What would happen if I leave the gap between the walls ? I can see many house in the street with no insulation but little gap .

Is there any chance for weed to grow or rain water getting stagnant there

 

[Deluks]

If your walls are already cavity insulated and the neighbours wall will also be insulated, then there's no real point.
Just a simple cover strip at either end to hide the gap.

If whatever he uses to 'block the gap at the top with some material' gets compromised, then you will potentially have moisture stuck between the polystyrene and the brickwork, with nowhere to go.

Much better to leave it open. Weeds won't grow without light, leave a gap at the bottom of the 'cover strip' for ventilation, or use a grill.

Any guttering fitted at the top on your side?

 

[^woody^]

If you are saying that they are using your wall as part of theirs, then there should be a Party Wall Agreement in place to deal with this - it's not just the insulation that would be a concern.

If you are saying that they are building a separate external wall, and this gap you mention is just the gap between them, then you do not want anything, including insulation - be it a fire risk EPS or PIR or any other type between them. It should stay as gap. There will be no damp or other issues.

 

[MattSeeker]

Thx @Deluks and @^woody^.

@Deluks, No guttering fitted at my side of the wall as it is a flat roof at the top. But as you mentioned they are going to to block the top of the gap.

One more question/concern, In the front of the house, the gap between my wall and their new wall is blocked by the existing driveway separation brick wall( as seen in the attached picture. So if the rainwater goes in between these 2 walls, how the collected/stagnated water would go out? Do I need to speak with them to vent this water. ?

 

[^woody^]

It's not a waterproof tank, water does not collect or stagnate.

 

[MattSeeker]

It's not a waterproof tank, water does not collect or stagnate.

meant like 'If there is no actual out for water, would it not get into my house's wall over time'. Asking this with a bit of worry and limited knowledge of buildings

 

[^woody^]

meant like 'If there is no actual out for water, would it not get into my house's wall over time'. Asking this with a bit of worry and limited knowledge of buildings

What happened to your wall before the neighbour's wall was built in front of it? If it didn't have a damp problem then, it won't now.

 

[MattSeeker]

What happened to your wall before the neighbour's wall was built in front of it? If it didn't have a damp problem then, it won't now.

They had their garage and my wall was built very adjacent to that with no gap. Now, they are extending their garage with gap.