9 inch Hollow Block with internal Dry-lining but cold house

leonlafrite

Hello,

I am looking for help, feedback, suggestion and other recommendation to the following problem we currently have:

Last November we moved in a new house, in 2008, the whole house was modernized from top to bottom and had a 2 storey extension added on side and back. All external walls have been dry-lined.
Extension Construction is typical for Dublin with 9 Inch cavity blocks and dash and dot plastering with insulated plasterboard (not the thickest but still about 8 cm in total)

Problem:
- All the extension rooms are always really cold (Kitchen, Kid bedroom, spare bedroom and TV room).
- They are quite quick to heat up (2 degree in 30 mins or from 15 to 17 with 7 degrees outside) but after about 4 hours we would have lost that heat.
- Temperature goes down 1 or 1.5 degree (19 to 17.5) overnight when no heating and an outside temp of 12 degrees (like last night)
- Checking the wall temperature with a heat gun thermometer - they are at 20 or so degree (so not cold really and similar to room temp)

My findings:
- We have a very strong draft in between the blocks and insulated plasterboard. We have seen this when removing the light switches and plugs plates. This is the same in all the extension rooms.
- Older external walls do not have this problem at all ( and we have no temperature issues either)
- Even when at temperature the rooms never feel warm hard to explain...
- We have upgraded the attic insulation and filled every gaps between the block and plaster from room below (this did not make any difference)

Am I correct in thinking this drop in temperature is caused by the draft (or is this normal and expected behavior)?
While we cannot afford it... Would doing external insulation help ? I don't think so since the draft is likely the source of the issue so this should be resolved before we undertake any other expensive work.

thank you very much in advance for your help.

I have a house similar to yours, in that it was built as a cavity wall, but we stuck a cavity block extension onto it.

Ours, also, is insulated with insulated plasterboard inside (no external insulation). Similar thickness to what you describe (probably less actually) and we rarely/never have heat loss issues. We do have, on external walls, some coldness if light switches etc are removed, but it's not 'drafty' as such.

Something to perhaps consider, would be to silicone around the edges of the fixtures that you feel are allowing cold drafts into the rooms. Also, do you have ventilation in the rooms in the form of wall vents or such? These are obviously going to allow cool air into the rooms. (personally, we didnt install any vents as I hate them, but our sliding door was fitted marginally improperly, leaving a small 2 inch x 1 inch gap at the bottom, which I didn't fix, and I allow that to be a form of ventilation).

I wonder if vents (if there are any) are the culprit?

RandomViewer

RandomViewer wrote: »

Is there insulation in the cavity?

It's generally not recommended to insulate a cavity block.

leonlafrite

Hi RandomViewer,

Do you mean in the Hollow Blocks themselves?
There is no cavity wall perse, there is the wall made of the hollow block + insulated platersboard glued directly on top of it using dot and dash. this leaves a gap in between the block wall and insulation of a few cm this is where the draft is see.

leonlafrite

Hi KKV,

Thank you, Like you we do not have any vents for the same reasons. I can even call the draft as Wind, it is that strong.
I fell this may be the source of our problem but I need to be sure as we need to focus our efforts (and euro) where is actually will matter

What is the outside of the build like? Is it finished (ie; plastered)? I'm not sure I can understand why it's so bad for you. Not that I'd recommend this as such, because it can be messy, but you could consider taking the affected switches etc off the walls, and spraying a bit of expanding foam in?

Any electrician that has to work on it in future will curse you, but you'll be warmer for it, I'm sure..?

leonlafrite

I did think about this but I have a feeling this will help a little but leave the bigger problem unresolved, ie why do I have so much draft in between the block and plaster.
the external wall are all finished and painted.

Am I right in thing this draft is the source of the heat retention issue?

Ak84

The draft is possibly coming from the ventilated attic above. The gap between insulation and block work possibly extends to the attic?

leonlafrite

Hi Ak84, this is what we suspected and so we had the perimeter sealed already when we did the attic but no luck...

RandomViewer

leonlafrite wrote: »

Hi RandomViewer,

Do you mean in the Hollow Blocks themselves?
There is no cavity wall perse, there is the wall made of the hollow block + insulated platersboard glued directly on top of it using dot and dash. this leaves a gap in between the block wall and insulation of a few cm this is where the draft is see.

Breeze blocks, didn't think they were used for houses, think you'll need some form of external insulation

leonlafrite

Hi RandomViewer, ok but how can I be sure this will actually help? Also I read having external and internal insulation is not a good idea, did you see that too , thanks.

Calahonda52

Ak84 wrote: »

The draft is possibly coming from the ventilated attic above. The gap between insulation and block work possibly extends to the attic?

generally not the case because of fire regs, the ceiling slab usually goes to the wall or else there is a batten.

OP: take off an internal wall vent and post a picture, my guess is the the wall duct is not sealed to the PB.
I remediated whole estates in Dublin when this issue arose.
Its easy to do

leonlafrite

HI Calahonda52,

There are no vents in the wall which is making this even more challenging l
Should there be a membrane or something in between the block and PB as there is none there, just the block dot of glue and a few cm of emptiness (I was able to check using a inspection camera) see pics

Calahonda52

So there is no insulation?
when you insulated the attic, was there a solid block on top of the hollow block wall as the last layer?

Penn

leonlafrite wrote: »

Hi RandomViewer, ok but how can I be sure this will actually help? Also I read having external and internal insulation is not a good idea, did you see that too , thanks.

You're correct. Having internal and external insulation isn't wise as it can cause condensation to form inside the structure with no way of getting out. If you were to go with external insulation, you could remove the drylining internally as it shouldn't be needed.

I would suggest however that air-tightness is likely your issue. A sand&cement scratch coat on the blockwork or a full airtightness membrane and airtightness tape throughout around all openings, joists built into wall etc would form an air barrier. Your drylining (properly taped at all joints and specified to achieve correct U-Value) could then be dot-and-dab fixed to the wall.

See standard Acceptable Construction Details for walls insulated internally
https://www.housing.gov.ie/sites/default/files/migrated-files/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad%2C18753%2Cen.pdf

Your own construction will vary slightly and obviously you won't be able to retrofit thermal blocks or similar, but at the very least it would reduce air permeability through the building fabric.

leonlafrite

I tried to check this but I don't think I am to see that if I recall properly ; I will go up and have a look again later today
Is this not something that can only be checked from outside under the roof-tiles ?

TheBoyConor

External isulation will do nothing to reduce the drafts. And in fact if you put on EWI and there is a draft blowing along within the wall, then the EWI will offer practically no benefit whatsoever in heat retention as the heat is being lost within the wall, not to the external face.

I would say the source of the draft is a combination of the wall pasterboard gap communicating with the attic and the window and door opes. Of course the gap should be closed off for fire safety reasons but in reality the builder probably took an out of sight out of mind approach and did whatever was fastest and most convenient for him. Unfortunately, in my experience, that is the way the vast majority of contractors are unless you watch them like a hawk and pulled em up on every little imperfection.

Also, are the cavity blocks ones with horizontal or vertical voids? If vertical, these might be left open and uncapped at the top which will allow are to freely circulate within the blocks and if any of the pointing between the blocks is iffy, air will blow right out of the mortar joints into the gap with the dry lining.
Again, in my experience vertical mortar joints are usually a hit or miss affair, often pointed up nice on the outside and half hollow behind it. It is my experience that if there is every an opportunity for a tradesman to pull a fast one or take a short cut and get away with it, they will do it. Over time I have developed zero trust in any of them.

leonlafrite

Hi Penn,

Thank you so I am in for a good bit of work then (external + plastering would be way too pricey house is 150 sq meter)

I would need to Remove all existing internal dry-lining, have membrane + tape fitted on blocks and then have properly installed insulated PB.
Would redoing the internal insulation in some rooms be sufficient or is it an all in or nothing scenario?

Any idea how much per Sq/M this will cost me ; I was quoted 2k euro to redo the dry-lining for a 20 sq-meter room - does this sound ok?

thank you

Penn

leonlafrite wrote: »

Hi Penn,

Thank you so I am in for a good bit of work then (external + plastering would be way too pricey house is 150 sq meter)

I would need to Remove all existing internal dry-lining, have membrane + tape fitted on blocks and then have properly installed insulated PB.
Would redoing the internal insulation in some rooms be sufficient or is it an all in or nothing scenario?

Any idea how much per Sq/M this will cost me ; I was quoted 2k euro to redo the dry-lining for a 20 sq-meter room - does this sound ok?

thank you

I would say that the air is coming in through the blocks as there's no scratch coat that I can see from your images (considering I can see what I believe are the mortar joints in the blocks). That air is then in the gap between the blocks and the plasterboard, and that's what's seeping through into your room.

Whether you applied airtightness measures to the back of the plasterboard or the wall, you'd have to remove the plasterboard, so the best measure is a sand&cement scratch coat on the wall (with airtightness tape if required, but may not be if the scratch coat is applied correctly). This would greatly reduce the air getting into the gap between wall and plasterboard.

I'm not great with costs so perhaps someone else could advise, but 2k would seem okay to me. It's not huge work in itself, but could be further complicated if there are kitchen units or stoves involved etc.

leonlafrite

Hi Penn,

You are correct the blocks have nothing on them so this looks like my main issue then ; They are probably not capped either then (need to confirm/check as per TheBoyConor suggestion)

Would adding sand&cement scratch coat + airtightness membrane and tape + insulted PB take care of the draft 90/100 % (even if the blocks are not capped?)

Should I cap them if not done or this will not make much difference and will need to have the wall Dry-lined properly

denismc

People are posting some fairly expensive fixes for your problem which may or not work.
My suggestion would be to have a heat loss survey done before you undertake any remedial work, whoever does the survey should be able to recommend appropriate measures that may not cost an arm and a leg.

I had an energy survey done a number of years ago as I was experiencing heat loss similar to yours, turns out airtightness was the issue.
The reccomendation was to treat each room individually from an airtightness perspective which involves sealing up any openings and cracks in the room.
While this is painstaking it is not very expensive and is being done over a number of years.

TheBoyConor

€2k, which sounds very cheap for pulling down drylining and redoing it. What does this include?
Does it include removal and full reinstatement of things like:
- removal and reinstatement of joinery like architraves & skirting,
- removal and reinstatement of electrical fittings, and isolating them to make them safe while the work is under way
- removal and reinstatement of heating and plumbing fixtures
- skimming the new plasterboard
- re-painting the walls.
- skip bag for disposal of all the old broken plasterboard and insulation.

I think that €2k cost is grossly underestimated and might be just covering the main plasterboard work and ignoring all the other stuff that has to happen to enable it. I'd say you could at least double it.

And at the end of the day you need to figure out whether the savings in heating costs will be enough to pay for the work over a few years. If the payback is any more than 5 years, I'd say it would hardly be worth the hassle. You might be better off making more modest improvements by capping off the open voids at the top of the blocks, and sealing up the other main gaps with a few tubes of silicone and plugging voids with firestop or rockwool.

imho, I think you'd be mad to go pulling down a whole extension of dry lining just to retrofit an air tightness barrier whose absence may or may not be the main factor contributing to the drafts. I think seal up any more obvious gaps and voids and air circulation paths at on a minimally invasive and minimal cost basis, and then if that does not give a decent enough improvement, then reconsider the more invasive work. However, I would not be inclined to go down the road of re-doing the whole drylining unless you can genuinely justify the large cost and hassle involved in doing so.

leonlafrite

Hi TheBoyConor,

The quote of 2K was just for one room, about 20 sq meters of walls.

The problem is the house is always cold and does not feels comfortable. We are not planning in moving again so we are looking to do it right to save in the long run (plus the cost of full will only increase over time)

Hi denismc, Fair point could you recommend the company you used? Having said that it seems that I have some obvious and quite major gaps in my house insulation....

TheBoyConor

I would say start with the easy cheaper fixes like plugging voids and sealing up visible gaps in the room and the attic.

If that fails to deliver an acceptable result then and only then, consider the bigger scale work and expense. Be aware than it doing so you are likely getting into the zone of diminishing returns, spending more and more to save less and less.

Also you seem to be focusing on this draft issue. You need to also take a look at the building and extension as a whole rather than just one aspect of it. For example, are you sure the attic is properly insulated? Have you got a large area of glazing? It is possible that the draft is only one part of a wider heat loss issue.

There might be other relatively inexpensive easy fixes that could mitigate the heat loss to an acceptable level and possible do away with the need to take more invasic drastic measures.

leonlafrite

Hi TheBoyConor,

Right but I am not sure where to start to be honest, I was thinking about injecting expansion foam around the wall perimeter of the room (started behind the floor board but this did not help)
I will need to drill many small holes but this should reduce the air movement significantly or is it a waste of time?

TheBoyConor

Start with non-invasive stuff. Go into the attic, identify any improper gaps or voids and seal or pack them up as appropriate. Then seal up any gaps and crevices around windows and doors. Window boards are a notorious leakage spot. Ensure the attic insulation is up to scratch. make sure all your windows and doors are closing properly.

Then if that is not delivering results maybe look at doing the drill and inject with foam thing you have there.

Also be aware that if there is a cooker hood, bathroom vent, dryer or stove or fireplace operating that they will drastically increase the pressure differential drawing any drafts through the house.

leonlafrite

HI TheBoyConor,

thank you so Attic has been done properly and all gaps at ceiling level underneath were sealed properly, no more gaps there.
Then I guess next step is for me to try seal any gaps I can find in the attic (and check if the blocks have been capped - will take a look shorty).
Also will using silicone be sufficient to fill gaps and crevices around windows and doors?

TheBoyConor

Normal silicone shrinks over time. I don't remember the name, but there are other mastic products are specifically intended for air tightness sealing. I think it is Siga or Sika is the manufacturer.

Another thing, are there floor joists build into the wall? Floor joist penetrations are notorious draft sources. They are not easy to seal up if they have been sloppily done from day one.

Penn

leonlafrite wrote: »

HI TheBoyConor,

thank you so Attic has been done properly and all gaps at ceiling level underneath were sealed properly, no more gaps there.
Then I guess next step is for me to try seal any gaps I can find in the attic (and check if the blocks have been capped - will take a look shorty).
Also will using silicone be sufficient to fill gaps and crevices around windows and doors?

I think as a previous poster suggested, your best bet might be to get a heat loss survey done, and recommendations based on same. The rest of us can only guess, and small bits of work you might undertake yourself might not have any effect.

When the extension was being built, was there an architect or other professional involved specifying and inspecting the works? Did you have photos taken during the build which might show some of the details used?

TheBoyConor

Penn wrote: »

I think as a previous poster suggested, your best bet might be to get a heat loss survey done, and recommendations based on same.

Are those surveys actually any good though? Or are they usually just the regular half arsed surveys with generic piss poor reports telling you the usual non specific yada yada of insulation and new windows etc.

I'd also be concerned of a conflict of interest with the people doing these types of heat loss surveys. They could very well be attached to people or companies offering retrofit services and as such it'll be in their interests to give you a report that recommends a lot of work - work that their company or connections offer.

Unless you can be confident of their competence, thoroughness and impartiality, I'd take all these surveys and surveyors with a pinch of salt.

Perhaps 12 years working in construction and civil engineering has just made me a cynic, sceptical of everyone, professionals included and from bitter experience I have come to expect that both work and services will be done to a shítty, half arsed standard unless you stand over them like a hawk. At this stage I'm genuinely surprised when consultants and contractors actually deliver what they originally promised rather than a joke of a job that ends up in disputes and arguments.

Penn

TheBoyConor wrote: »

Are those surveys actually any good though? Or are they usually just the regular half arsed surveys with generic piss poor reports telling you the usual non specific yada yada of insulation and new windows etc.

I'd also be concerned of a conflict of interest with the people doing these types of heat loss surveys. They could very well be attached to people or companies offering retrofit services and as such it'll be in their interests to give you a report that recommends a lot of work - work that their company or connections offer.

Unless you can be confident of their competence, thoroughness and impartiality, I'd take all these surveys and surveyors with a pinch of salt.

Perhaps 12 years working in construction and civil engineering has just made me a cynic, sceptical of everyone, professionals included and from bitter experience I have come to expect that both work and services will be done to a shítty, half arsed standard unless you stand over them like a hawk. At this stage I'm genuinely surprised when consultants and contractors actually deliver what they originally promised rather than a joke of a job that ends up in disputes and arguments.

For a heat loss survey, the first step would likely be using a thermographic camera to see where heat loss is happening, which areas of the wall are colder than others. That would show what the pattern of heat loss is and the recommendations would likely be based on what such pattern would typically signify.

However, regarding your general comment about surveys/surveyors, such surveys can only be carried out based on the evidence available to them. For example in this case, a heat loss survey with a thermographic camera would only show so much. If they were allowed to then open up the wall by removing some of the dry-lining, that would show more and might change their findings. But if the wall was opened up before the thermographic survey, that would affect the findings of it by throwing it off.

Any surveyor coming in with no prior knowledge can only base their findings on what they can reasonably observe within the parameters of the survey, and there will always be caveats to account for what can't be determined due to being outside those parameters. But standard forms of construction regularly have the same issues and whatever information the surveyor can garner from their survey will likely allow them to make recommendations based on their experience with similar previous cases/issues.

Regardless, your comment about conflicts of interest and surveyors recommending unneccessary works due to links to another company are ridiculous. I coincidentally also have 12 years experience working in a consultancy firm, and I've never experienced or heard of what you're suggesting. The vast, vast majority would never put their reputation and business on the line by doing that. There are pricks in every industry, but they are few and far between, and just because you're jaded by your experience doesn't mean you should go around badmouthing an entire profession.

If a client asks us for recommendations of people to carry out the works, we recommend a few we trust, and we still oversee the works as directed by the client and pull up the contractor on any issues. We receive no remuneration from the contractors, we recommend them because our duty of care is to the client and we trust the contractors to be competant to carry out the works correctly or to fix any issues we raise. If the client gets a different company to carry out the works, our job doesn't change from that.

TheBoyConor

Fair play to you. I admire your optimism.

I find the most common word I use in emails is "outstanding". As in, "it still isn't done after telling ye 5 times" rather than, "wow great job".

leonlafrite

Hi Guys -

I have just been in the attic and it looks like the blocks have been caped with other narrower blocks see pictures, can you confirm this the case?

Interestingly the temperature in that extended part of the attic is colder then the main house (pic old vs new) does this means something that can help me find confirm the issue?

thank you for the help so far.

Calahonda52

It does not help when they are sideways or upside down
In any event they tell me nothing, what I want to know is what is happening on the far side of the wall plate in image 3.
In addition some wider angle shots would give some perspective

the_pen_turner

doesn't look like the cavity is closed under the wall plate.
what is going on with the rafter in pic 2 . is the building off square . why is there a piece added on at an angle

blackbox

It sounds like there is too much air movement between the wall and the plasterboard.
As mentioned above, the top of the wall may be an issue, but have you carefully checked the external walls for gaps or openings? Underneath the window sills is a real possibility.

You can stop the draughts internally around the switches, but if outside air is circulating behind the plasterboard, your walls are only effectively the thickness of the plasterboard.

TheBoyConor

Those blocks on the flat close off the gable cavity blocks ok but it is the wall under the wall plate that is of most interest. It looks as if there is no top course of block on flat there. If the cavity blocks have the horizontal void then that is OK. If they have vertical voids then that is a problem and there should be a block of flat course to top them off. Without that, the insides of the walls running along the top

the pen turner, it is a cavity block wall, not a cavity wall.

You would expect the temperature in the attic to be colder than inside the house. You are above the insulated envelope when you're in the attic. If the new attic has more insulation than the old attic the you would also expect the new attic to be colder than the old one.

leonlafrite

Hi TheBoyConor, What can I show you to help identify exactly what you want to see, could you point on the new pic?

Also both the extension and the original house have the same attic depth but the temperature are different by a few degrees

I uploaded wide angle pics

TheBoyConor

Those pictures are showing the gable wall but that is not what we are looking for. We need to see the side wall, right down in the very corner of the roof near the eaves. You might need to get a borescope to see the top of the wall there, or look by removing a roof tile on the eaves, or putting your hand up through the soffit.

you'd also need to see the junction between the insulated plaster slab and the ceiling from the attic side. You'd have to lift the insulation and expose the joint, both at the gable wall (easy) and at the eaves wall (difficult).

A couple of degrees difference between each part of the house could be caused by multiple things. An extension probably has a higher surface to volume ratio than the main block of the house, and possibly thermal mass. As such even with the same overall U-Value of the building envelope, the temperature will drop quicker in the extension than the main house.

Penn

leonlafrite wrote: »

Hi Guys -

I have just been in the attic and it looks like the blocks have been caped with other narrower blocks see pictures, can you confirm this the case?

Interestingly the temperature in that extended part of the attic is colder then the main house (pic old vs new) does this means something that can help me find confirm the issue?

thank you for the help so far.

In image Cap3 showing the wallplate, are you able to get your hand to the other side of the timbers and check if the cavity has been closed? It doesn't look like they've used a solid block on its flat to close the cavity there, but they may have bedded in a slate across the cavity to close it. Or even if you can stick the camera over the timber you might be able to get a photo

the_pen_turner

are there 2 layers of insulation.
are they 90 degrees to each other. the insulation doesn't look like a well installed job . is there a walk way to that tank.
the tank is not insulated
. it wouldn't surprise me that the insulation is not installed far enough into the corners . are there black spots on the eiling below at the wall to ceiling joins

what kind of ventilation is there at the eaves level and higher up the roof.

pull back the insulation at the wallplate and post a pic of that area

leonlafrite

Ok so I looked at the area from attached pic named "attic issue" and found no caps on the block in the area in red but it looks like block are capped in the area in green (gable slope) see other pics.

Problem is that it is really hard to get access to the wall plate as the roof is very low but will try later today ...

I am thinking that having someone come in and check this from the roof will be the easiest way to go - so we could then fill all the gaps with some foam, does that make sense as a first step?

the_pen_turner

is it a tiled roof or slates. whats access like to the soffit area

leonlafrite

Hi the_pen_turner

The roof it tiled and It is easy to access as far I can see. See pics
On the pic call "back extention kitchen and kid bedroom.jpg" the small roof is with slates and has no insulation and will need improvement)

Thank you

Calahonda52

IMO, that sewer pipe vent should go up beyond the gutter.
the red zone pics are part of your problem

leonlafrite

Calahonda52 wrote: »

IMO, that sewer pipe vent should go up beyond the gutter.
the red zone pics are part of your problem

I have put some foam in there now red zone holes - I hope it will not just sink into the hole - I will check in the morning.
But even if this helps I think there is more to the problem than this?

Ignore the attic for a moment; earlier you posted these pics:


https://www.boards.ie/vbulletin/attachment.php?attachmentid=513443&d=1589877336

https://www.boards.ie/vbulletin/attachment.php?attachmentid=513445&d=1589877354


Maybe I'm looking at it wrong, but what's going on there? Why is there a big gap with nothing in it? Where did you take that photo from and is there any insulation on the back of the plasterboard? To me, that looks like you have no insulation at all?

Insulation is like a thick foam of sorts, that's stuck to the back side of the plasterboard sheet. This foam should be tight to the brick work, like in this picture:

https://thorhelical.com/wp-content/uploads/2015/10/fixing-insulated-plasterboard-laminates-banner.jpg


There, to my knowledge, should not be a gap between the insulation and the block. It should be fixed directly to it.



EDIT: I'm not a builder! Just a DIY'er of sorts, so I'm always open to correction.. just to cover myself

TheBoyConor

KKV you are dead right. On a plaster dab dry lined wall there should hardly be a 10mm gap between the block wall and the slab. You certainly should not be able to get a camera or phone in there.

If what we see in that picture is indeed the gap between the slab and wall then there is something very fishy going on that needs investigating. That would not be a normal set up at all.

Calahonda52

the presence or absence of insulation is a separate issue from the obvious porosity problem, not solving the latter first will negate the value of an additional insulation work.

leonlafrite

Thank you all.

There is insulation on the plasterboard and the space in between the wall and the back of the PB is may a couple in cm only
Is this even OK ?? I would agree with KKV that it should stuck to the wall (I used a borescope to check)

Silly question probably but if I need to fix the walls porosity problem, is there any way to do this without removing the existing dry-lining?

Based on all the feedback I think the house is just not air tight in so many ways .... The blocks could be only the tip of to iceberg here ...

leonlafrite

leonlafrite wrote: »

There is insulation on the plasterboard and the space in between the wall and the back of the PB is may a couple in cm only

How is there a gap, though? Are there wooden battens fixed to the wall, and the plasterboard fixed to them?

It would definitely give a reason as to why the otherside of the wall is drafty when you remove light sockets etc. as you effectively have a cold air 'shell' around your extension.

Perhaps this is a legitimate way of doing things, but I personally can't understand why you'd do it that way.

(theoretically, though.. you still do have insulation plasterboard, so, again, in theory, your house should still be warm inside..?).

A boroscope probably has an ultra-wide angle lens that makes the gap look bigger than it actually is.. Could you give an idea on how deep the gap is?

To be honest, it might be worth your while cutting a small square out of the wall, and having a proper look in there (you can always fill it back in, just use a stanley blade to cut out a square/circle and keep the piece you've cut out, then use a bit of filler to put it back on).

It'd take a lot of the guesswork out of it. I reckon you may need someone that's a bit more clued in to have a look, though, or ask the builder that did it what the reasoning was behind doing it that way.