Good Old Delroy, he gets all the best Jobs :)

[Mike Johnson]

The cavity is there to provide a gap and prevent damp from penetrating to the inside skin of the building, wall/brick ties have drips on them to allow the water condensate to reach the cavity tray at the bottom of the wall and drain through the open perps to the outside, its not there for any electricians to run cables in and compromise the drips on the ties.

Do a google search and you will find that cavity's are no longer filled with insulation, it was always a bad idea and many house's have been ruined by the practice.

 

[goasis]

The cavity is there to provide a gap and prevent damp from penetrating to the inside skin of the building, wall/brick ties have drips on them to allow the water condensate to reach the cavity tray at the bottom of the wall and drain through the open perps to the outside, its not there for any electricians to run cables in and compromise the drips on the ties.

Do a google search and you will find that cavity's are no longer filled with insulation, it was always a bad idea and many house's have been ruined by the practice.

Cavity's definitely are still filled with insulation! Regardless of what google may say!!

 

[Pretty Mouth]

The cavity is there to provide a gap and prevent damp from penetrating to the inside skin of the building, wall/brick ties have drips on them to allow the water condensate to reach the cavity tray at the bottom of the wall and drain through the open perps to the outside, its not there for any electricians to run cables in and compromise the drips on the ties.

Do a google search and you will find that cavity's are no longer filled with insulation, it was always a bad idea and many house's have been ruined by the practice.

That's the theory. Real world, even a 9" solid wall doesn't allow significant damp to pass through to the inside, unless it's very exposed to the prevailing wind, or the rainwater goods have failed and are pouring water all down the wall. The odds of significant moisture making its way to the inner leaf of a cavity wall via cables run in the cavity are very low indeed.

I have never heard of cavity trays and weep holes being installed at the base of cavity walls, or of any houses being damaged by cavity insulation. What are your sources for this? Genuinely interested to hear more about this.

 

[pirate]

I think it's fair to say that there is a fairly strong view that cavity wall insulation is not the blessing it was thought to be at first. In particular, poorly installed insulation can have a detrimental effect on some structures, notably timber-frame building, especially if the method used is to pump polystyrene at pressure without proper supervision. I have personally witnessed damage to a few houses caused by such a method.
The polystyrene balls, being preformed and thus not "foaming" into the cavity, do not seem to cause such problems.
However, a cavity wall is there for a purpose, as Mike Johnson says above, with ties to carry moisture away from the inner leaf (as well as tying the 2 leaves together).
It may be rare for water to penetrate from the outer wall to the inner, but I have personal experience of this. Failure of the external pointing and mastic led to penetration of exactly this type in my building which has polystyrene balls installed. The result was considerable moisture affecting the internal plasterwork in some of the flats, black mould and flaking plaster, and the clean-up measures after rectification of the cause were considerable and expensive.
The surveyor appointed to oversee the remedial work was quite clear that the damage would have been far less had the cavity been left clear of insulation, as the insulation allowed the water to track across the cavity, and held it in.

 

[Mike Johnson]

I have never heard of cavity trays and weep holes being installed at the base of cavity walls, or of any houses being damaged by cavity insulation. What are your sources for this? Genuinely interested to hear more about this.

Cavity trays and weep holes are the standard method of draining a cavity, and damage to houses by full fill cavity insulation has been well publicised by the RICS and the Institute of Civil Engineering, you will note below the insulation is only part fill any penetration of this by cables will cause a thermal bridge.

 

[mainline]

That's the theory. Real world, even a 9" solid wall doesn't allow significant damp to pass through to the inside, unless it's very exposed to the prevailing wind, or the rainwater goods have failed and are pouring water all down the wall. The odds of significant moisture making its way to the inner leaf of a cavity wall via cables run in the cavity are very low indeed.

I have never heard of cavity trays and weep holes being installed at the base of cavity walls, or of any houses being damaged by cavity insulation. What are your sources for this? Genuinely interested to hear more about this.

I do a lot of work on barn type conversions with no cavity at all, lots of those walls are around 3ft thick some are coated with a tar like stuff because of water ingression.

 

[Pretty Mouth]

Cavity trays and weep holes are the standard method of draining a cavity, and damage to houses by full fill cavity insulation has been well publicised by the RICS and the Institute of Civil Engineering, you will note below the insulation is only part fill any penetration of this by cables will cause a thermal bridge.

View attachment 96424

Mike this diagram is for radon protection rather than damp, see here: Radon Protection in Above-Ground Environments - https://www.safeguardeurope.com/applications/radon-above-ground

 

[Mike Johnson]

The purpose of the diagram was to show the cavity tray which someone said they had never seen, this is all standard building works you guys need to get out more.

 

[7029 dave]

During my apprentice years we done loads of council houses using the wall cavity as a wire way as more often than not it was quicker and cleaner than chasing (we didn't have twin disc wall chasers back then)

We did to, but they were all fully occupied rewires the council tenants were still living in them back in the good old days, so you had to keep disruption and mess to a minimum cant think of a better way to use the cavity pull through old to new lovely job.

 

[Pretty Mouth]

The purpose of the diagram was to show the cavity tray which someone said they had never seen, this is all standard building works you guys need to get out more.

I trained and worked as a bricklayer for a few years in the early 2000s, so I know a little bit about this.

Cavity trays and weep holes were installed above openings, and occasionally where insulation didn't run right the way up the cavity wall (eg. it stopped at loft-level). I never once saw anyone install one at ground level, right around the perimeter of the building, as per your diagram, so it certainly wasn't standard practice back then.

Both leaves of the wall had a DPC, typically 150mm above ground, and a weak mix filled the cavity up to ground level, to prevent the earth pushing the walls in. No cavity tray, what would be the point? You think the cavity just fills up with water until it comes out the top?

Perhaps things have changed since then, and they're now mandatory at ground level, but I'd like you to show me a more relevant image or document before I accept your word for it.

 

[Pretty Mouth]

Back then, 'cavity batts' as they were known, were slabs of rockwool type insulation that were placed in the cavity and typically bridged the whole cavity. Supposedly they were manufactured so that the fibres were oriented so that any moisture would travel straight down, rather than wicking across.

 

[philectrical]

Poor lad on that video is probably working for free.

 

[Mike Johnson]

Will this. do, note the perps not filled to act as weep holes to stop the cavity filling up with water and coming out the top.

 

[Pretty Mouth]

Will this. do, note the perps not filled to act as weep holes to stop the cavity filling up with water and coming out the top.

View attachment 96464

No Mike, this doesn't tell us anything.

Approved document C is here:

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/431943/BR_PDF_AD_C_2013.pdf

In fact, there is a relevant paragraph in there, 5.5c:

If the wall is an external cavity wall, (see Diagram 9a) the cavity should be taken down at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray should be provided so as to prevent precipitation passing into the inner leaf (see Diagram 9b), with weep holes every 900mm to assist in the transfer of moisture through the external leaf.

Bear in mind that the DPC is at least 150mm above GL, taking the cavity down 225mm is easily done, much easier than running a cavity tray round the whole building. So hardly standard practice.

On the matter of full fill insulation:

CAVITY INSULATION
5.15
A full or partial fill insulating material may be placed in the cavity between the outer leaf and an inner leaf of masonry subject to the following conditions:

....

then later...

When the cavity of an existing house is being filled, special attention should be given to the condition of the external leaf of the wall, e.g. its state of repair and type of pointing. Guidance is given in BS 8208-1:1985111. Some materials that are used to fill existing cavity walls may have a low risk of moisture being carried over to the internal leaf of the wall.

I infer from this last paragraph that the likelihood of moisture tracking across the cavity via insulation is low, is less likely to happen to a wall in good state of repair, and is even less likely to be a problem with walls that were insulated during construction

 

[Pretty Mouth]

So. No cavity trays at ground level required, full fill insulation is allowed (and common place IME), and risk of moisture crossing the cavity is low.

 

[Mike Johnson]

The original issue was and is the use of the cavity to run cables.

Cavity trays are used when solid concrete floors are cast supported by the ground or beam and block floors supported on lightweight fill, the construction of suspended timber floors is very rare in this age of quick build, cavity trays where used in all the housing estates I was involved with during my working life in the South of England due to the possibility of marsh gas from the clay sub-soil.

On Insulation the latest perceived wisdom is partial fill of cavities with a minimum of 50mm gap between insulation and the outer skin.

 

[Pretty Mouth]

The original issue was and is the use of the cavity to run cables.

Cavity trays are used when solid concrete floors are cast supported by the ground or beam and block floors supported on lightweight fill, the construction of suspended timber floors is very rare in this age of quick build, cavity trays where used in all the housing estates I was involved with during my working life in the South of England due to the possibility of marsh gas from the clay sub-soil.

On Insulation the latest perceived wisdom is partial fill of cavities with a minimum of 50mm gap between insulation and the outer skin.

Mike, can you please define 'strawmanning', so the forum can understand where you're coming from

 

[mainline]

That's the theory. Real world, even a 9" solid wall doesn't allow significant damp to pass through to the inside, unless it's very exposed to the prevailing wind, or the rainwater goods have failed and are pouring water all down the wall. The odds of significant moisture making its way to the inner leaf of a cavity wall via cables run in the cavity are very low indeed.

I have never heard of cavity trays and weep holes being installed at the base of cavity walls, or of any houses being damaged by cavity insulation. What are your sources for this? Genuinely interested to hear more about this.

There are lots of references to the use of cavity trays at the base of buildings, so it's strange that with all your building knowledge you hadn't heard of it before, It should make you happy that you have learnt something from Mike because as you have said in the past you come on here to learn.

 

[mainline]

His one endearing quality is that he has no airs and graces and is not afraid to say that he doesn't know what's up.
Obviously unscripted, completely as it happens.
So many other channels present the activities as a polished perfected series of events where everything seems to go flawlessly.
While my reaction usually ranges from "hmmm, that's a bit iffy" to "WHAT??!?!?!" I still usually find it an entertaining watch.

I agree it's entertaining, I would place it in the category of comedy.

 

[Petej999]

Mike, can you please define 'strawmanning', so the forum can why understand where you're coming from,That's why T don't comment that much

Think he means STRAMETT BOARD

The obvious one that springs to mind was him taking down a wall and then jointing cables in a wago box and ----ing it up into the ceiling and calling it MF. The materials and method he used did not constitute a MF junction box. I politely noted it on his comments and he removed the comment ?

He doesn't like to be criticised that much does he?

 

[mainline]

Think he means STRAMETT BOARD

He doesn't like to be criticised that much does he?

His son does all the web stuff, and I'm guessing he would find it difficult to tell his father that even after 40 years experience in his job that he's still blissfully ignorant.

 

[mainline]

ah, those beads that get pumped into cavities sometimes, fair enough.

I only asked because in the video the room has a short wall all the way round, presumably the cavity is open at the top, and i guess with rockwool or celotex in the cavity, so running cables through it would seem a reasonable way to do it in this instance, same as you see in alot of conservatories. Delroy was 'teaching' the new lad and said you weren't allowed to do that but he didn't know why......

I agree in this instance, using the cavity would be reasonable.

 

[Mike Johnson]

I'm sorry, but using the cavity to run cables is not reasonable or acceptable in any circumstance, possible bridging of the cavity tie which has a drip on it to allow moisture to drip to the bottom of the cavity, but any penetration of the skin/insulation will introduce a thermal bridge, in a conservatory the correct way would be to run conduit in the floor, if the inside skin is fair faced brick then the back of the brick on the inside face is chased out to provide a passage for the conduit.

 

[mainline]

I'm sorry, but using the cavity to run cables is not reasonable or acceptable in any circumstance, possible bridging of the cavity tie which has a drip on it to allow moisture to drip to the bottom of the cavity, but any penetration of the skin/insulation will introduce a thermal bridge, in a conservatory the correct way would be to run conduit in the floor, if the inside skin is fair faced brick then the back of the brick on the inside face is chased out to provide a passage for the conduit.

My point would be that in this instance he can ensure that no breach would exist as it's open and low, giving easy access.

I have no idea of the layout but assume the demand isn't going to be very high, so depending on the layout would think about a radial/s rather than the ring final.

 

[Mike Johnson]

Still the point of introducing a thermal bridge unless the cable can be placed on the inside face of the cavity insulation.

 

[mainline]

Still the point of introducing a thermal bridge unless the cable can be placed on the inside face of the cavity insulation.

The cable would have to be on the inside face for the socket feeds, and I can't think of any reason for any of the cables to be bridged across the cavity.

 

[DPG]

The cable would have to be on the inside face for the socket feeds, and I can't think of any reason for any of the cables to be bridged across the cavity.

If cables are dropped into a cavity then they will contact the inner and outer walls several times, causing bridges.

 

[mainline]

If cables are dropped into a cavity then they will contact the inner and outer walls several times, causing bridges.

How would a 2.5 drop down the inner skin of a cavity cause a bridge several times the wall is only about 3 foot high and the sockets would be above 450 mm off the floor not much of a drop'

 

[Mike Johnson]

The cable would have to be on the inside face for the socket feeds, and I can't think of any reason for any of the cables to be bridged across the cavity.

If the insulation is already in place i.e. positioned whilst building the brickwork, it would be too easy to just drop the cable down the void, then penetrating the insulation to access the pattress box, this would cause a thermal bridge between the temperature of the cavity and the inside of the room.

 

[DPG]

How would a 2.5 drop down the inner skin of a cavity cause a bridge several times the wall is only about 3 foot high and the sockets would be above 450 mm off the floor not much of a drop'

I assumed he would take them horizontally across the cavity as well.

 

[mainline]

If the insulation is already in place i.e. positioned whilst building the brickwork, it would be too easy to just drop the cable down the void, then penetrating the insulation to access the pattress box, this would cause a thermal bridge between the temperature of the cavity and the inside of the room.

That's right it would be too easy to drop it in the void but that wall is so short a drop it would hardly be any work to avoid a bridge, he may even have enough room to get behind the dot and dab.
Like I said in this instance, i think it would be reasonable.

 

[mainline]

I assumed he would take them horizontally across the cavity as well.

The cable would obviously have to travel horizontally.

 

[DPG]

The cable would obviously have to travel horizontally.

I know. But you said there would only be a short drop.

 

[mainline]

I know. But you said there would only be a short drop.

Yes if the run was across the top of the wall and dropped to each socket.

 

[DPG]

Yes if the run was across the top of the wall and dropped to each socket.

Well, true. The point remains though, it is bad practice to run wiring in the cavity.

 

[Mike Johnson]

That's right it would be too easy to drop it in the void but that wall is so short a drop it would hardly be any work to avoid a bridge, he may even have enough room to get behind the dot and dab.
Like I said in this instance, i think it would be reasonable.

You are now introducing a totally different scenario, if the construction is dot and dab on the inside skin why is dropping the cable down the cavity even being considered.

 

[DPG]

That's right it would be too easy to drop it in the void but that wall is so short a drop it would hardly be any work to avoid a bridge, he may even have enough room to get behind the dot and dab.
Like I said in this instance, i think it would be reasonable.

You are now saying a bridge should be avoided. We are in agreement.

 

[mainline]

Well, true. The point remains though, it is bad practice to run wiring in the cavity.

This is where Mr spark gives a quote of many £100s of pounds more to dig up his floors or hack his walls to bits and Mr bob the DI gets the work.

 

[Mike Johnson]

This is where Mr spark gives a quote of many £100s of pounds more to dig up his floors or hack his walls to bits and Mr bob the DI gets the work.

I did not know that Detective Inspectors did electrical work.

 

[mainline]

You are now introducing a totally different scenario, if the construction is dot and dab on the inside skin why is dropping the cable down the cavity even being considered.

I'm not introducing a scenario at all, we were discussing the Delroy instance, if you watched the video you would know that he said it was dot and dab.

 

[Mike Johnson]

I stopped watching Dell boys videos some time ago, the question I answered was why you can't drop a cable down a cavity, I gave an answer to that question and many other vagaries that came about because of it.

 

[mainline]

I stopped watching Dell boys videos some time ago, the question I answered was why you can't drop a cable down a cavity, I gave an answer to that question and many other vagaries that came about because of it.

The real point is that you can if certain criteria is being met.
In Dels case I think it would easily have been achievable, but as usual Del gets it wrong by thinking it's against the Regs.

 

[mainline]

You are now saying a bridge should be avoided. We are in agreement.

My point would be that in this instance he can ensure that no breach would exist as it's open and low, giving easy access.

I have no idea of the layout but assume the demand isn't going to be very high, so depending on the layout would think about a radial/s rather than the ring final.

 

[timhoward]

@mainline your avatar is negatively affecting my monitor ;-)

 

[DPG]

Must admit I like the animated avatar!

 

[Mike Johnson]

The real point is that you can if certain criteria is being met.

OK tell me how you would drop a cable down a cavity and meet the criteria.

 

[mainline]

OK tell me how you would drop a cable down a cavity and meet the criteria.

In Dels instance you come along the top and drop into each socket, i don't know if insulation is there or not but could easily cut a channel for the drops.

 

[Mike Johnson]

The real point is that you can if certain criteria is being met.
In Dels case I think it would easily have been achievable, but as usual Del gets it wrong by thinking it's against the Regs.

No No No the difference between the Del boy situation is the full stop after your first sentence, where you say dropping a cable down a cavity is acceptable "if certain criteria are met" now try to answer the question of how you would meet this criteria in other circumstance's, your answers so far could be very miss-leading to others reading these post who may take your statements as factual whereas the criteria can not be met in a retrospective installation.

 

[mainline]

No No No the difference between the Del boy situation is the full stop after your first sentence, where you say dropping a cable down a cavity is acceptable "if certain criteria are met" now try to answer the question of how you would meet this criteria in other circumstance's, your answers so far could be very miss-leading to others reading these post who may take your statements as factual whereas the criteria can not be met in a retrospective installation.

It is acceptable to run cable in a cavity if the criteria is being met, how can that be seen as miss leading ?.

If you grab a load of cable and stuff it down the cavity causing a bridge or are using cable not suitable for the conditions, then that would obviously wouldn't meet the criteria.
But to say you can't do it would be wrong.

 

[DPG]

It is acceptable to run cable in a cavity if the criteria is being met, how can that be seen as miss leading ?.

If you grab a load of cable and stuff it down the cavity causing a bridge or are using cable not suitable for the conditions, then that would obviously wouldn't meet the criteria.
But to say you can't do it would be wrong.

I'm pretty sure that was the original point. You pretty much can't do it on an existing property.

Obviously in a new build you could do it and avoid Bridges etc, but let's face it in a new build you wouldn't need to use the cavity!!