which mortar for Edwardian house with cavity walls & cavity insulation?

[Tangus]

Our Edwardian terraced house is booked in to be repointed at the end of the month and I think it currently has a lime mortar in as it has just disintegrated.

The walls are cavity walls and we are pretty sure it has those little grey cavity wall insulation balls in as we have found some upstairs.

At the front, there is just one air brick and there are damp patches to both sides internally (coming down from the stone cills although we can't quite work out yet where it is coming from).

I've read conflicting advice on what mortar to go for so I don't know.

-Lime would be breathable which is better if there is no cavity insulation
- cement mix if it has cavity insulation so it doesn't drawn in moisture/get wet.

Is this right??

BUT

What if the cavity insulation has not been done properly ie if it has gaps in and that's why we have the damp patches? because currently it's lime mortar.
What if we end up at some point getting it removed and it's had cement mix - so the house becomes unbreathable.

As it's costing nearly 2k to do the building, I want to make sure we do it right.

thank you

 

[malcolm]

Always lime either with or without insulation. The purpose of the lime is to allow water to evaporate from a wall. Cement is fine for modern walls which are designed to be wet and are isolated from the internal walls, but if the cavity is filled you'll want to keep the outer wall dry and use lime.

Make sure it is lime with no cement in it at all. Typically this will be NHL 3.5. Any cement at all will stop water from evaporating but typical builders often like to put a bit in. Sand ought to be sharp sans or a mix of sharp and builders sand.

 

[Tangus]

Ah brilliant, thank you so much for putting my mind at rest.

We will go with the lime then.

Another quick Q. At the bottom of the window (where the damp is) it has been painted over so could this be causing the damp issues? Ie rendering the lime mortar unbreathable?
I don't know what type of paint it is but it's under a bay window.


Thank you

 

[paulc]

Our Edwardian terraced house is booked in to be repointed at the end of the month

End of January up in the dark depths of Scotland ?
It is going to be ruddy cold, and really isn't a good time of year to be working outside and doing work with lime (or even cement).

 

[Tangus]

Scotland? Ha no, we are Lancs by the seaside and it's been quite mild here ;o)

 

[paulc]

Scotland? Ha no, we are Lancs by the seaside and it's been quite mild here ;o)

My bad - I had it in mind that FY postcodes were up in Scotland.
Even although it is mild down south, it was just 2°C this morning when I got up. Even that is too cold to be doing pointing outside.

 

[Tangus]

Yes I think there will be some leyway on timing to allow for any bad weather but we have had 10 degrees up here not too long ago then it dropped so

 

[alexander]

do not use lime mortar outside if the temperature is going to fall below 5 degrees for at least 72hrs