Lath and Plaster Ceiling Discovered - Roof Insulation Advice

[lauren b]

Hello 

 

I am currently converting a Chapel building and unfortunately discovered a lath and plaster ceiling in the roof in between the decorative tongue and groove wooden boards and the rafters ...I have some photos if needed. I had an insulation spec from my architect who I have not taken through to construction stage and I now cannot achieve this spec and the lath and plaster is obviously in the way.

 

The original wooden sarking boards on the inside I really do not want to take down (from a cost perspective) therefore I think the L&P has to remain in place, you could not take it out from above.  The building is not listed.

 

Building control do not have a solution as apparently it is not part of their sign off to get the insulation to the spec below.

 

Does anyone know how I would go about solving this problem to achieve the same U value in the original spec, there is maybe an option to raise the roof an inch and therefore create or create a warm roof instead (??) (currently spec is a cold roof design), there are parapet walls in the building so raising the roof would not be too hard.

 

But essentially, if I wanted a re-calculation of the U value to make sure any different insulation arrangement maintains the same U value 0.13 - who would I employ impartially to calculate this or are good roofers able to do this? Sadly my current roofer does not seem engaged in finding a solution which is a shame or potentially it is normal to be provided with an exact design / insulation build up. 

 

I have contacted Celotex technical department to see if they can help, but yes sadly this is my first roof so I am quite sad about it

 

Are lath and plaster ceilings frequently found, are there any work around suggestions?

 

The roof spec is as follows snippet from my main spec doc:

 

Install new vaulted warm roof to buildup as follows:

I. Existing t&g cladding (painted white)

II. 100mm Celotex GA4075 or equiv between purlins, sealed to form airtight barrier to interior with expanding foam/ GAPO tape & foil tape on all joints

III. 75mm Celotex GA4075 or equiv between 100mm rafters, allowing 25mm air gap

IV. 25x38mm treated battens to form 50mm air gap (continuous  ventilation zone to be created from eaves vents to proprietary tile vents placed at suitable

intervals near ridge)

V. breathable sarking eg Tyvek Supro or equivalent

VI. 25x38mm treated counter battens

VII. 600x300mm Cedral Thrutone Smooth, Blue-black artificial slate

tiles.

 

U=0.13W/m2K

 

thank you if you can help and thank you anyway - Lauren 

 

 

 

[jayc89]

If you weren't intending on removing the existing internal cladding boards, I'm not sure how the discovery of the plaster behind them introduces a problem. Perhaps some pics would be useful. 

[lauren b]

Hey Jay 

Thanks. Discovered the lathe plaster ceiling from above when we lifted some tiles from the roof off ..so if insulation goes in from above the ceiling is in the way? 

Thanks 

[Nickfromwales]

  On 09/02/2025 at 21:45, lauren b said:

so if insulation goes in from above the ceiling is in the way? 

Thanks 

Expand  

Can it not simply remain and be considered a dysfunctional part of the makeup, and you do as you suggest with the roof 'growing'?

 

175mm PIR is a decent spec. Is the rest of the building being upgraded to reflect this?

 

You say airtight barrier, is the entire conversion to adopt airtightness to a specific value?

[Nickfromwales]

Oh, and Hi, and welcome, by the way lol  

[lauren b]

Hello... apologies I hadn't done the introduction and hello section. Sorry!

[lauren b]

Hello Nick 

I'm just worried if I start from above the lath and plaster maybe cross batten to make sure there is a gap ..that the full space original sarking...L&P ...start of insulation is this massive gap that will get cold air stuck in it? 

The problem I have is I am not an expert so I don't really understand roof insulation or air tightness to a specific value ...yes the whole building is being upgraded, walls, floors and roof 

I'm really looking to confirm who I can go to ..to under write a new solution that insures I have a warm home 😁

[G and J]

Context is so helpful in these things. 
 

On the face of it simply ignoring the lath and plaster, which I imagine will be vapour permeable, and carrying on with your planned build up would seem a viable plan. 
 

However, if you give an overview of your plans then it would be easier for peeps to comment.  For example, if you are going to be at or near passive house standard with whole house heating by a single chunky candle it’s a very different prospect from you planning to retain the single glazed sash windows and have a 25kW gas combi running. 

If you are a renovation presumably you don’t have minimum insulation requirements?

 

So go on, share.  There’s phenomenal knowledge on here and most of us don’t bite lol

 

[ProDave]

Your architect is also clueless.  He talks of a warm roof make up, but then mentions a ventilation gap which would make it a cold roof.  Which do you want?

 

Some sketches or pictures showing the roof construction and timber sizes is needed to have a meaningful conversation.

 

If the current panelling is just fixed directly to the L&P ceiling I don't see it makes a difference.  Or are you saying there is some kind of gap between them?

[Redbeard]

  On 10/02/2025 at 10:41, ProDave said:

 

Some sketches or pictures showing the roof construction and timber sizes is needed to have a meaningful conversation.

Expand  

 

 

 

Yeah, this is head-scrambling! So the original plan was to take off the roof-covering, insulate with PIR leaving the T&G below the rafters and install further PIR below, yes? Insulation-to-insulation contact is, in my view, always best, but yes, if it was only the T&G there and you could get both layers 100% tight to the T&G there should be no risk of thermal by-pass (cold air in the 'layers of the sandwich'). However with the L&P there you are really going to struggle because of the 'snots'. HOWEVER... Since the L&P is 'under-boarded' and no longer needs the 'snots' to hold plaster to lath you could rake off and hoover out the snots and insulate tight to the lath. If it were me in these (less-than-ideal) circumstances I would primes the sides of the rafters and, after foam sealing any gaps, air-tightness-tape the PIR to the rafter edges.

 

It also took me a while to 'orientate' myself re the 25 x 38 battens but I now see they form the 50mm vent path as per BEIS (now DESNZ) best practice guidance.

 

As @ProDave says, this is not a Warm Roof. You will have a Warm Room below it but the roof structure is a(n insulated) Cold roof.

Edited February 10 by Redbeard

[lauren b]

Hi Everyone - thank you for all your responses... Some photos of the L&P attached and a word doc with the roof build up elevation  

 

He talks of a warm roof make up, but then mentions a ventilation gap which would make it a cold roof.  Which do you want? ; - i would like a solution and a warm home - ideally cost effective, and also he is no longer my architect.  

 

If the current panelling is just fixed directly to the L&P ceiling I don't see it makes a difference.  Or are you saying there is some kind of gap between them? - I do not know as I have only investigated from the top (outside in to L&P) - I could take some panelling off from the inside to see how tightly it is all affixed

 

thank you again, also trying to upload of video of the building, we are also insulating the floor, walls etc - and i have had building control approval and i have gone back to them to speak about this potential change

roof build up.docxFetching info...

[lauren b]

 

couldnt upload the video from inside. 

Edited February 10 by lauren b

[lauren b]

[Mr Punter]

The warm roof / cold roof is confusing on a pitched roof, where it often means to refer to the loft, not the rafters.  Almost no pitched roofs are truly warm, with all insulation above the rafters.

 

The architect has specified insulation between rafters (I assume fully filled), breather membrane above, counter battens and battens, then new tiles.  You won't need to touch the ceiling.