Exterior insulation of cavity wall

Chickereeeee

I have a 1920's house with cavity walls (albeit a narrow cavity) for which I would like to improve the heat insulation. I am loath to install cavity insulation as-is due to the possibility of damp penetration. The layout of the  rooms (high, curved ceilings, picture  rails etc) makes the use of internal insulation difficult. I assume there is no point in applying exterior insulation while a ventilated cavity still exists. 

Would filling the cavity, but with either exterior cladding or insulation, overcome the damp worry? Would exterior insulation be required, or just some form of waterproof cladding/render?

Thanks

stuart45

How small is the cavity?

Chickereeeee

stuart45 said:

How small is the cavity?

About 20-25mm I would say.

Bendy_House

From what I understand about the potential risk of damp penetration due to cavity insulation breaching the two skins, it mainly/only occurs if the outer skin is in poor condition, and/or in locations where heavy driving rain is common. So, it strikes me as tho' also adding external insulation - which will have an outer finish which will, of course, be fully weatherproof, should completely eliminate that risk.

I would personally never ever fit ext insulation to a wall with an empty cavity, for the very reason you suggested. I would imagine, tho', that cavity PLUS ext insulation would be super- effective, and carry no risk.

stuart45

CWI companies won't fill a cavity of that size. It's not really a cavity wall, more a solid wall built in Stretcher bond. When you open up those type of
walls you can see they aren't proper cavities. 
A solid wall built in Stretcher bond will have a 10mm cavity when used for a garden wall, but on houses they were made a bit bigger to make it easier to lay the inner bricks. In theory the cavity (known as the wall joint) should be filled with mortar, but this tends to make the wall belly out as you lay it.

Chickereeeee

stuart45 said:

CWI companies won't fill a cavity of that size. It's not really a cavity wall, more a solid wall built in Stretcher bond. When you open up those type of
walls you can see they aren't proper cavities. 
A solid wall built in Stretcher bond will have a 10mm cavity when used for a garden wall, but on houses they were made a bit bigger to make it easier to lay the inner bricks. In theory the cavity (known as the wall joint) should be filled with mortar, but this tends to make the wall belly out as you lay it.

No, it's not stretcher bond. It has normal wall ties.

Maybe I am underestimating the gap, as it is not easily accessible to measure now. I just remember it was difficult to get your fingers down the gap. I will try and find some way to get a better measurement.

stuart45

A solid wall built in Stretcher bond will still have wall ties. It looks like a cavity wall from the outside because it's built with stretchers. It's the same when you build a 9inch garden wall in Stretcher bond, you use ties. 
The cavity here can only be 10mm because the wall needs to be the correct thickness for a brick on edge finish. With a house wall it could be increased to about 20mm to make the laying of the inside stretchers easier.
Bricklayers used to call it a finger cavity, because it was just enough room to get your fingers down when laying the bricks.
Although most solid walls were built in Flemish bond, stretcher bond was a bit cheaper as it used.less face bricks.
Normally CWI companies won't fill a narrow cavity.

Bendy_House

stuart45 said:

A solid wall built in Stretcher bond will still have wall ties. It looks like a cavity wall from the outside because it's built with stretchers. It's the same when you build a 9inch garden wall in Stretcher bond, you use ties. 
The cavity here can only be 10mm because the wall needs to be the correct thickness for a brick on edge finish. With a house wall it could be increased to about 20mm to make the laying of the inside stretchers easier.
Bricklayers used to call it a finger cavity, because it was just enough room to get your fingers down when laying the bricks.
Although most solid walls were built in Flemish bond, stretcher bond was a bit cheaper as it used.less face bricks.
Normally CWI companies won't fill a narrow cavity.

If adding ext insulation, Stuart, what would you ideally do with that small cavity?

Chickereeeee

stuart45 said:

A solid wall built in Stretcher bond will still have wall ties. It looks like a cavity wall from the outside because it's built with stretchers. It's the same when you build a 9inch garden wall in Stretcher bond, you use ties. 
The cavity here can only be 10mm because the wall needs to be the correct thickness for a brick on edge finish. With a house wall it could be increased to about 20mm to make the laying of the inside stretchers easier.
Bricklayers used to call it a finger cavity, because it was just enough room to get your fingers down when laying the bricks.
Although most solid walls were built in Flemish bond, stretcher bond was a bit cheaper as it used.less face bricks.
Normally CWI companies won't fill a narrow cavity.

OK, I (obviously) don't know ow much about brick laying.

What bond is a normal cavity wall, then?

 I an not sure what you mean by 'correct thickness for a brick on edge finish'. I assume this is the final layer along the top of a garde wall? If so, I don't think there is such a restriction here (top is open) but they would use the same approach?

stuart45

Bendy_House said:

stuart45 said:

A solid wall built in Stretcher bond will still have wall ties. It looks like a cavity wall from the outside because it's built with stretchers. It's the same when you build a 9inch garden wall in Stretcher bond, you use ties. 
The cavity here can only be 10mm because the wall needs to be the correct thickness for a brick on edge finish. With a house wall it could be increased to about 20mm to make the laying of the inside stretchers easier.
Bricklayers used to call it a finger cavity, because it was just enough room to get your fingers down when laying the bricks.
Although most solid walls were built in Flemish bond, stretcher bond was a bit cheaper as it used.less face bricks.
Normally CWI companies won't fill a narrow cavity.

If adding ext insulation, Stuart, what would you ideally do with that small cavity?

I don't think there's much you can do with a 20mm gap Bendy. You have to think of it as a solid wall. Even if the Bricklayers wanted to keep the cavity clear, it would be too small to get your trowel down properly.
Even a 2 inch cavity was a bit tricky, 2.5 or 3 was easier.
CWI companies wouldn't do anything less than 50mm in the past, but there are a few that will do 40mm now. Years ago some of them tried doing finger cavity walls not realising what they really were.
I remember years ago an old bricklayer telling me about working in the 20's, when there was a big push for builders to use cavity walls. Many of them were suspicious of them, questioning the strength of 2 skins with a big gap.
Some of them still used a 9inch wall/cavity/4inch wall, other did a solid wall in 2 skins of 4 inch stretcher bond. In theory the gap should have been solid mortar.
The reason they used stretchers instead of Flemish was that one of the attractions of the cavity wall was that it needed less expensive face bricks.
So some builders used that idea, but still built a solid wall.
There are still a lot of surveyors who believe that these solid walls built like this were intended to be cavity walls.

Chickereeeee

Bendy_House said:

stuart45 said:

A solid wall built in Stretcher bond will still have wall ties. It looks like a cavity wall from the outside because it's built with stretchers. It's the same when you build a 9inch garden wall in Stretcher bond, you use ties. 
The cavity here can only be 10mm because the wall needs to be the correct thickness for a brick on edge finish. With a house wall it could be increased to about 20mm to make the laying of the inside stretchers easier.
Bricklayers used to call it a finger cavity, because it was just enough room to get your fingers down when laying the bricks.
Although most solid walls were built in Flemish bond, stretcher bond was a bit cheaper as it used.less face bricks.
Normally CWI companies won't fill a narrow cavity.

If adding ext insulation, Stuart, what would you ideally do with that small cavity?

I am also wondering that.