Smoke testing a cavity wall

[SairHeed]

During driving rain water permeates the outer Fyfestone blockwork of a gable cavity wall, it then runs down the inside of the outside wall, tracks along an RSJ and drips into a downstairs window. I've stripped out the plasterboard from around an upstairs window and, with an endoscope camera into the cavity, can see water running down the inside of the external wall.
Although the Fyfestone pointing appears very sound, a few vertical joints were found to have little mortar behind the outside mortar skin after experimentally grinding them out. Following repointing these joints the water ingress has improved but not completely. I believe a few remaining poor vertical joints are still letting water in, but finding these dodgy vertical pointing joints has been very hit and miss.
I am reluctant to do a complete repointing job for a handful of poor joints, which look perfectly sound until you cut into them. I am now exploring carrying out a smoke test into the cavity to see if I can pin point areas of water ingress.
I would be most grateful if forum members could advise of any experience they have had of cavity wall smoke testing? I understand that a smoke test will seek and display cracks in pointing but would it show up porous pointing?
Any advice would be most greatly appreciated.

 

[SairHeed]

CORRECTION
To avoid confusion, the RSJ referred to above is actually a steel lintel.

 

[^woody^]

Smoke testing is a waste of time.

A masonry wall is permeable and water will always run down the rear face of the outer leaf once the masonry is saturated.

 

[SairHeed]

Thanks for your input, Woody.
The water appears to be coming pretty early, long before saturation. I was hoping to make sure the pointing was tight before applying Antel StoneTone to reduce permeability

 

[stuart45]

Interesting idea. I've never heard of anyone smoke testing a cavity, but I'd guess it would find any weak spots. It's generally the cross joints where penetrating damp gets through first. BRE did some tests on cavity walls, and found that badly built ones with half filled cross joints allowed water through after 20 minutes of the sprinkler on them. Clay bricks are relatively good at resisting moisture for longer, as once the front gets saturated they get better at keeping it out. However, in continual driving rain, they do eventually get soaked. By that time though a lot has already got into the wall through the cross joints.

 

[SairHeed]

Thanks for your input Stuart45.
In my case the verticals seem to be the problem. I put this down to the shiny end faces of the manmade Fyfestone blocks not being roughed or scratched before laying in place. The cross joints would appear to have the benefit of gravity pushing down on the mortar during the setting period. One vertical joint near a window, which was found to be letting in after hose wetting, looked perfect on the outside but after grinding away the surface, the rear of the mortar joint came out in a slab. Several other verticals have been found to have little mortar behind the outside 1" of mortar.

 

[stuart45]

The vertical joints are the cross joints. The horizontal ones are the bed joints. Cross joints are are called the perps.

 

[SairHeed]

Many thanks for the clarification Stuart45; I erroneously thought that cross joints were the horizontal ones as they go across the way, d'oh! That also makes your post all the more pertinent to my problem now that I understand the lingo.

 

[stuart45]

Many thanks for the clarification Stuart45; I erroneously thought that cross joints were the horizontal ones as they go across the way, d'oh! That also makes your post all the more pertinent to my problem now that I understand the lingo.

You are correct in your thinking about the compression of the bed joints, as they are normally quite full as well. The cross joints, on the other hand, are often only half filled. Also, as the walls expand and contract small cracks go into the joints. Add to that that when the bricklayers get to the centre of the wall when running in along the line, if the last joint is a bit too wide they shuffle some of the other bricks along a bit to close up the last joint a bit, so that affects the bond of the joints a bit.