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

[Dustydazzler]

 

[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.