Dot Dab with IWI

[Gaf]

I have regretted the decision to go ahead with cavity insulation and additional IWI. In retrospect the advice here for 200mm cavity would have been so much more straightforward. I’ve sat with it knowing the decision was made for a reason at the time.

 

House is at the stage of internal plasterboards going on. Only done 15% of the house so far and stopped for Xmas.

 

Got a first look at it today and noticed they’re going dot and dab throughout. When I first met builder I had asked about how they’re attached and he said fit directly onto the wall, as in not dot dab. I was comfortable enough with this as it appeared to align with some online reading Id done and a video Id come across on IWI installation.

 

With the dab thickness, there’s now an approximate 2.8cm void between the IWI and the inner leaf block wall. 
 

Have searched the forum and haven’t just found an exact answer to my query. With all the best attempts at airtightness work, surely internal air will find a way in behind the IWI? Doesn’t this create the real chance of condensation and then mould growth behind the IWI? 

 

Even once I start fixing TV brackets, curtain poles, etc, Ill be creating a hole into this void. I’ve seen suggestions to use silicone or other similar materials to seal screw fixings, but I can’t help but think it will never be perfect and air will get in.

 

Any break in the airtightness for this IWI will also effectively make the IWI redundant surely?

 

Wall buildup is internal to external:

 

- Skim Coat

- IWI - 37.5mm (25mm IWI, 12.5mm plasterboard)

- Parge coat (sand cement)

- 100mm block

- 150mm cavity bead

- 100mm block

- Render (Parex or K-Render)

 

 

 

[joe90]

I am not a fan of dot and dab but some here are, from what I have read here the boards should have a perimeter band of adhesive all the way round then dabs in between @nod does this for  a living so will give you a better answer.

[lookseehear]

I think although dot and dab can be unpopular, in this case your risks are likely fairly low. You've only got 25mm PIR, so most of your insulation is within the cavity. This means that while you have a gap between IWI and blockwork, that inner skin of blockwork is on the warm side of the CWI and therefore shouldn't go below dew point and cause condensation ordinarily.

 

The parge coat should hopefully reduce risks further by making the blockwork somewhat airtight, meaning that air flow from living spaces into through small gaps into any void behind should be minimal.

[nod]

As Joe has already pointed out There should be a continuous band of drywall adhesive around the perimeter  and electrical boxes casings etc This is crucial No one bothers with Parge coat nowadays Except self builders 😁 You need to be sure that the builder isn’t assuming that because they have put Parge on the blockwork There’s no real need to bother sealing the boards You should be able to form a good seal with the boards and stop the air getting through to the block work in the fist place 

It’s also worth foaming around the bottoms (blue) prior to skirtings 

[Gaf]

15 hours ago, lookseehear said:

I think although dot and dab can be unpopular, in this case your risks are likely fairly low. You've only got 25mm PIR, so most of your insulation is within the cavity. This means that while you have a gap between IWI and blockwork, that inner skin of blockwork is on the warm side of the CWI and therefore shouldn't go below dew point and cause condensation ordinarily.

 

Cheers yeah that makes a lot of sense. I couldn’t see the obvious that sure many houses probably have 150mm cavity bead without IWI and don’t have condensation problems. We have MVHR so hope that also keeps humidity levels down.

 

14 hours ago, nod said:

As Joe has already pointed out There should be a continuous band of drywall adhesive around the perimeter  and electrical boxes casings etc This is crucial No one bothers with Parge coat nowadays Except self builders 😁 You need to be sure that the builder isn’t assuming that because they have put Parge on the blockwork There’s no real need to bother sealing the boards You should be able to form a good seal with the boards and stop the air getting through to the block work in the fist place 

It’s also worth foaming around the bottoms (blue) prior to skirtings 

 

Cheers for the clarity on this, good to get the advice from someone who does this. Is there any way for a lay person to know if this is all being done? I can always ask the builder, but at the risk of sounding paranoid, I could be told they’re doing it when they’re not. All the work done so far is continuous up against walls so I can’t get a look at what’s done at edges.

 

Would a heat camera show up where the dabs, continuous bands, etc are?

[nod]

A simple tap with your knuckle will tell you if the a continuous band 

I can’t emphasize enough how important this is and the same around sockets and especially window reveals where most of the heat is lost 

Our air test came back both times  as a 2 without any special measures 

probably worth mentioning it to the builder If only to keep him on his toes 

[Redbeard]

If you suspect, or know that it has been done 'old-school dot and dab', rather than 'Full Perimeter beads and cross-hatchings' (often shortened to - wait for it - Dot and Dab) then there is a remedial course of action. 10-12mm holes at 40-50mm ctrs and a second or two of air-tight foam in each should 'stitch it up'. I have done this on a number of wall D 'n' D'd in the 1980's, with some success.

[Gaf]

21 hours ago, nod said:

A simple tap with your knuckle will tell you if the a continuous band 

I can’t emphasize enough how important this is and the same around sockets and especially window reveals where most of the heat is lost 

Our air test came back both times  as a 2 without any special measures 

probably worth mentioning it to the builder If only to keep him on his toes 

 

The photo on the first post is looking down from where the window cill will go (bottom reveal - if that’s the right term). That would want to have a piece of the adhesive running the whole way across to seal it?

 

Forgive the dodgy attached drawing. Black pen is the outline for each plasterboard, with a socket, light switch, and window openings. Blue pen is where I’m currently understanding there should be an unbroken bead of adhesive to create a sealed pocket of air behind each piece of plasterboard. Green pen is rough positioning for the dot-dabs. Do I have that right?

[Mr Punter]

29 minutes ago, Gaf said:

 

The photo on the first post is looking down from where the window cill will go (bottom reveal - if that’s the right term). That would want to have a piece of the adhesive running the whole way across to seal it?

 

Forgive the dodgy attached drawing. Black pen is the outline for each plasterboard, with a socket, light switch, and window openings. Blue pen is where I’m currently understanding there should be an unbroken bead of adhesive to create a sealed pocket of air behind each piece of plasterboard. Green pen is rough positioning for the dot-dabs. Do I have that right?

 

I have never seen this happen on site and would not trust that it is done correctly.

[Gaf]

Just realised I never uploaded the photo for the original post.

[Iceverge]

On 26/12/2024 at 21:08, Gaf said:

Only done 15% of the house so far and stopped for Xmas

 

Pull the boards off the wall. 

 

Batten out the walls with 45*45 timber as a service cavity and use mineral wool infill. Add some noggins or ply where you need to hang stuff. You'll thank yourself later. Your sparky will love you too. 

 

Send back what boards you can and flog the used ones. 

 

The whole thing will still work out cheaper than insulated plasterboard which is very dear and half of it ends up in the skip as off cuts. 

 

Changing course isn't mentally easy but avoiding the most logical action because we don't have the energy to rewire a few neurons is highly illogical. 

 

 

 

 

 

 

[nod]

1 hour ago, Gaf said:

 

The photo on the first post is looking down from where the window cill will go (bottom reveal - if that’s the right term). That would want to have a piece of the adhesive running the whole way across to seal it?

 

Forgive the dodgy attached drawing. Black pen is the outline for each plasterboard, with a socket, light switch, and window openings. Blue pen is where I’m currently understanding there should be an unbroken bead of adhesive to create a sealed pocket of air behind each piece of plasterboard. Green pen is rough positioning for the dot-dabs. Do I have that right?

Correct 

[Redbeard]

1 hour ago, Gaf said:

Black pen is the outline for each plasterboard, with a socket, light switch, and window openings. Blue pen is where I’m currently understanding there should be an unbroken bead of adhesive to create a sealed pocket of air behind each piece of plasterboard. Green pen is rough positioning for the dot-dabs. Do I have that right?

 

...ish. 2 points though:

 

1. as per @Iceverge I would not start from where you've started from and 2. If you are going to go this way I would suggest what I referred to in my post above as 'cross-hatchings' - further horizontal bands of adhesive at 400 - 600 ctrs, the thinking being that if, due to wibbly walls, the perimeter bead does not completely 'close off' the air-space the leakage can at least be limited to smaller 'pockets'.

 

It's somewhere between extremely hard and completely impossible to check from the 'front' of a board whether the 'back' has been 'buttered' properly. 

[nod]

7 hours ago, Redbeard said:

 

...ish. 2 points though:

 

1. as per @Iceverge I would not start from where you've started from and 2. If you are going to go this way I would suggest what I referred to in my post above as 'cross-hatchings' - further horizontal bands of adhesive at 400 - 600 ctrs, the thinking being that if, due to wibbly walls, the perimeter bead does not completely 'close off' the air-space the leakage can at least be limited to smaller 'pockets'.

 

It's somewhere between extremely hard and completely impossible to check from the 'front' of a board whether the 'back' has been 'buttered' properly. 

Or simply look once it’s been skimmed As the boards dry All the dabs or continuous lines are clearly visible 

Same goes for when the paint goes on 

But it’s a bit late then 

[Gaf]

7 hours ago, Iceverge said:

Pull the boards off the wall. 

 

Batten out the walls with 45*45 timber as a service cavity and use mineral wool infill. Add some noggins or ply where you need to hang stuff. You'll thank yourself later. Your sparky will love you too. 

 

Send back what boards you can and flog the used ones. 

 

The whole thing will still work out cheaper than insulated plasterboard which is very dear and half of it ends up in the skip as off cuts. 

 

Changing course isn't mentally easy but avoiding the most logical action because we don't have the energy to rewire a few neurons is highly illogical. 

 

I 100% take your point. I won’t bore you to death on how we got to the decision to have CWI + IWI. Personally I have no affinity to this IWI approach, it was drawn up by our architects. Originally he suggested exclusively EWI in fairness to them, but with the IWI approach it was never mentioned at the time that we could do battens. At our current point, all electrical and plumbing chasing has been done, first fix electrical wiring and plumbing is all all done. Although Id do it differently now that I know the batten approach would have been the way to go, (though if I could go back in time I’d have done exclusively 200cm CWI), I’ve made my peace with this being how it’ll be done but I want it done properly. 

 

7 hours ago, Redbeard said:

 

...ish. 2 points though:

 

1. as per @Iceverge I would not start from where you've started from 

 

As best I can tell, we’re stuck with this approach unfortunately. Electrical and plumbing chasing and wiring / piping is all done. 

 

 

7 hours ago, nod said:

Correct 

 

7 hours ago, Redbeard said:

2. If you are going to go this way I would suggest what I referred to in my post above as 'cross-hatchings' - further horizontal bands of adhesive at 400 - 600 ctrs, the thinking being that if, due to wibbly walls, the perimeter bead does not completely 'close off' the air-space the leakage can at least be limited to smaller 'pockets'.

 

It's somewhere between extremely hard and completely impossible to check from the 'front' of a board whether the 'back' has been 'buttered' properly. 

 

Got a visit in. One section was exposed at an edge and got a good look behind the plasterboards. It’s exclusively just dots of adhesive. As far down a 7m long wall as I could see with a torch, there are just blobs of adhesive, so that entire 7m long wall is one open void. I tapped away at everything done and it’s all the same, all hollow. So if my understanding is correct, a single unsealed area (socket) will mean the entire 7m of insulation is doing nothing other than providing a void that will be heated by the internal air leaking into it.

 

Trying to keep it short so a few other bits:

- There is nothing between the floor and the plasterboard. I could run my finger along the entire base of the boards.

- There is no adhesive, nothing at all, around the plasterboard holes for the sockets, light fixings. I could see in around the sockets.

- There is nothing between the top of the plasterboard and the service void above.

- Where the service void is, the internal leaf of the external walls does not have a parge coat, its just bare blocks. Doesn’t this mean the ~200mm service void will be potentially allow in air? 

- There is no parge coat on the window revels.

- They have used uninsulated plasterboard for the window reveals.

- As best I can tell, the conduits used for the electrical wires have not been sealed. They go up into the service void and attic (fron what I can tell), so they’ll just allow cold air in through the sockets I gather.

 

Bah.

[Gaf]

[Redbeard]

The dot 'n' dab problem is worst when the masonry is not really well sealed, as you can get cold air in from the structure, where it can then freely circulate if it is done as you have described.

 

All in all your description of the issues does rather make the heart sink.

 

Think positively, though. If there is no insulation to the reveals you could glue up your own 9mm plasterboard/12mm PIR sandwich, or even plaster on XPS tile-backer board. Or, if you have lots of money, Spacetherm WL withg 10mm aerogel and 3mm magnesium oxide board.

[nod]

Every wall should look like this Real basic stuff 

[Gaf]

49 minutes ago, Redbeard said:

The dot 'n' dab problem is worst when the masonry is not really well sealed, as you can get cold air in from the structure, where it can then freely circulate if it is done as you have described.

 

All in all your description of the issues does rather make the heart sink.

 

Think positively, though. If there is no insulation to the reveals you could glue up your own 9mm plasterboard/12mm PIR sandwich, or even plaster on XPS tile-backer board. Or, if you have lots of money, Spacetherm WL withg 10mm aerogel and 3mm magnesium oxide board.

 

Cheers for the suggestion 👍

 

My plan is not to lift a finger on this and put it all back on the contractor.

 

I’ve rechecked our construction drawings. The specified IWI is exclusively mechanically fixed and is not to be adhesively fixed. The IWI should extend into the service void.

 

This work will have to be completely redone. 

Edited Saturday at 22:18 by Gaf

[lookseehear]

That at least should offer you some comfort - less of a discussion about how you think it should be done vs how the contractor does, but a simple failure to follow the agreed plans. I hope it doesn't set you back too much time-wise.