Insulating a Cathedral Roof - Mould and Condensation

[LeRouret]

Hello.
We bought a house in France and clearly have condensation problems in the lounge/roof.

The lounge is big - 7m X 6m - and has what the Frogs call a "Cathedral Ceiling".

There is no "loft" space in the lounge, just plasterboard, minimal insulation in places and then "roman" tiles.

Note - the way roofs are done here a somewhat different to the UK - our roman tiles/roof is very typical in the south of France. House built 1970.

Attached are various pictures and a graphic of how the tiles are put together on the roof - only the first row and last row of tiles are fixed...the remaining are just placed with no fixation.

The slope of the roof is fairly steep...see piccy with Christmas tree :-)

You can see the minimal insulation with the inspection camera image - this is by the windows where the condensation is worse - presumably because warm air meeting the cold plasterboard and this is where the minimal/none insulation is.

Our heating is reversible air-con so not sure how this helps with lounge humidity and condensation.

Some advice on how to remedy this ?

I can take all the tiles off for access - as stated, they are not fixed - except first and last row.

I want to keep the existing plasterboard.

And can't even lower the ceiling for more insulation as it will fail building regs.

Between tiles and plasterboard the gap is approx. 10 cm.

 

All I can do, I think, is add a vapour barrier than say 80 cm of insulation....something like this:

 

https://www.castorama.fr/panneau-d-isolation-thermique-polyurethane-2-5-x-1-2-82-mm/5411545052587_CAFR.prd

Thanks in advance.

 

[Mr Punter]

If you want to add a vapour control layer and insulation from above it looks like you would need to strip the triangular battens to get access.  You could add insulated plasterboard from underneath and still not be lower than the beams.

[LeRouret]

Hello Mr P and thanks..

 

I can probably remove a few of the triangular battons and slide the panels under.

Not sure how I am going to attach them..I can glue the vapour barrier to the plasterboard and then glue panels to vapour barrier…I just need to stop them moving when it’s windy…does not seem ideal…

I can barely lower the ceiling…the lowest point by the big windows only gives me about 30mm before I violate building regs

[Radian]

Nice big room. I bet it takes some heating though in the winter. Nobody can say what anyone's future heating demand might be so if you plan to live there a while it might be sensible to prepare for the worst and construct a proper  Warm Roof.

[Dave Jones]

taping anything isnt a vapour barrier and is a le bodge.

 

What you have is an unsealed warm roof, the cold is 'coming through' the insulation until it hits its dew point and water forms inside, it cant dry out easily and so gravity takes over and down it comes.

 

Pitting a VCL over the top wont help at all, the cold will still go through and a dew point will still form inside the insulation only now it has zero chance of getting out.

 

You need to make this a cold roof, massive ventilation. Take all the muck off the ridge and make sure its vented, make sure the eaves are all open and vented . you need as much aiorflow over the insulation as possible. put as much insulation in as you can and still have at least a 50mm gap between it and the tyvec membrane (which you need to fit).

 

 

Edited December 30, 2021 by Dave Jones

[ProDave]

How much work do you want to do?

 

the best thing you could do is strip the roof tiles completely, and turn it into a warm roof by fully insulating between the rafters and then insulating a layer above the rafters before putting the roof back on.  That will raise the roof by perhaps 100mm and you would have to be confident of re roofing it the same, or if allowed re roofing in a different way.  Do you want that much work?

[Radian]

49 minutes ago, ProDave said:

turn it into a warm roof by fully insulating between the rafters and then insulating a layer above the rafters before putting the roof back on

 

ProDave, I think that would make things more complicated than necessary for @LeRouret because the vapour control layer needs to be the first thing the air from inside meets. This would mean getting the VCL under the rafters which would mean removing the plasterboard ceiling.

 

If all of the insulation goes on top of OSB or ply sheeting on top of the rafters then the VCL can simply be put down first and everything below the rafters stays put.

[LeRouret]

Thanks all for comments....as you all know the roof/insulation is shambolic...this is France.

 

I am very reticent to get a "professional" (or "artisan" as they are called in France)  down here to have a look...if you think you have seen cowboys in the UK, think again..having sorted out electrics and plumbing I am not going to let an Artisan anywhere near this house..

 

I don't even need to ask but I think they will charge about 20k just to strip tiles and put insulation down...and will not be a good job...in other words I want to do it myself.

I think I will have to compromise on the amount of insulation but that is OK if I can avoid condensation.

 

@ProDave How much work ?

Of course you are right - effectively starting again is the ideal fix - so I want to cost and have some idea of the complexity for two solutions (i) The solution you have proposed and (ii) Cold roof within the limites of not starting over again

 

@Dave Jones Thanks - yes that looks like my "compromise"...looks like I can get Tyvec here as well...I guess I have to glue the PU panels to the plasterboard ceiling, I assume weight is not an issue ?

 

@RadianI am being a bit thick here...are you suggesting I add OSB ? Recall (from piccy below of how roof is constructed) all I have is plasterboard->100 mm space->Triangular battens->Tiles

[Radian]

2 hours ago, LeRouret said:

 

@RadianI am being a bit thick here...are you suggesting I add OSB ? Recall (from piccy below of how roof is constructed) all I have is plasterboard->100 mm space->Triangular battens->Tiles

 

To create a 'warm roof' the aim is to put the insulation as near to the outside as possible. In your case you seem to have 100mm rafters supporting triangular battens above and plasterboard below - is that correct? The 100mm space thus created might seem like a natural place to put insulation but the rafters themselves act as cold bridges thermally connecting outside to inside in a direct path at each rafter. This would be called a 'cold roof' and cerates lots of challenges with trying to keep moisture from condensing.

 

To make a warm roof you would put a continuous layer of rigid insulation on top of the rafters and then batten on top of that. Now the rafters, void and plasterboard are all at room temperature. Of course you need to support the insulation - so usually an OSB or plywood deck is put down on top of the rafters and to keep moisture from finding its way up to the cold side of the insulation and condensing, a vapour barrier is simply laid across the deck before laying the insulation. Construction is really, really simple. Your triangular battens might be OK screwed down on to the insulation. Otherwise another thin deck goes on first.

 

Another assumption I am making is that structurally, your exposed beams support the 100mm rafters - else the weight of the tiles would be too great.

[Radian]

 

I couldn't find a decent one earlier but here's a picture of what I'm describing:

 

I found this image from A flat roof company but it applies equally to a pitched roof. In your case, if you want to DIY it I think this is realistically your best bet as everything goes down in big pieces. While it may seem simple to cut rigid insulation and fit it between rafters, in practice it's horrible stuff to cut and you can forget sliding it along unless you leave big gaps. Nothing will be square and you would have to get an air tight seal from the plasterboard upwards otherwise vapour getting past the insulation will condense on the cold side and run back inside rotting everything in its path.

 

If the base of the typical triangular batten is broad enough it may be possible to screw through the insulation into the supporting deck below. This fixes everything into place so long as some screws go into the rafters to ensure security.

[LeRouret]

@Radian Thanks, very clear, understood.

I am going to lift some tiles off and see exactly what is going on.

As a matter of curiosity...for the scheme described, 100 m2, how much would you charge ?

And how long would it take two people ?

Thanks again.

[LeRouret]

11 hours ago, Radian said:

 

I couldn't find a decent one earlier but here's a picture of what I'm describing:

 

I found this image from A flat roof company but it applies equally to a pitched roof. In your case, if you want to DIY it I think this is realistically your best bet as everything goes down in big pieces. While it may seem simple to cut rigid insulation and fit it between rafters, in practice it's horrible stuff to cut and you can forget sliding it along unless you leave big gaps. Nothing will be square and you would have to get an air tight seal from the plasterboard upwards otherwise vapour getting past the insulation will condense on the cold side and run back inside rotting everything in its path.

 

If the base of the typical triangular batten is broad enough it may be possible to screw through the insulation into the supporting deck below. This fixes everything into place so long as some screws go into the rafters to ensure security.

@Radian One thing...weight. Considerations ??

[Radian]

1 hour ago, LeRouret said:

@Radian Thanks, very clear, understood.

I am going to lift some tiles off and see exactly what is going on.

As a matter of curiosity...for the scheme described, 100 m2, how much would you charge ?

And how long would it take two people ?

Thanks again.

 

I'm not in business myself and have no real feel for how much a trade would charge for this job. Here in the UK I would think the materials might amount to somewhere between £3K-£4K

I'm guessing a couple of people might be able to do it in 3 or 4 days but I've never seen an actual example of your kind of tiles so don't have a feel for how long they would take to strip/replace. If it were mine I'd definitely have a go myself.

 

1 hour ago, LeRouret said:

@Radian One thing...weight. Considerations ??

 

Yes, well that's what I can't gauge from your description - how the load is distributed and onto what. The rigid insulation is relatively lightweight but here in the UK we would start at 150mm thick minimum and maybe go up to 250mm the further North we are which makes it somewhat heavier. I wonder what would be considered an appropriate U-value in your location? You might make enquiries. In your case I would also look into EPS (Expanded Polystyrene) as this might be the most lightweight option available to you. Depending on rafter pitch, the decking might be as light as 9mm ply and might be possible to be kept as a single layer under the EPS if, as I previously suggested, you mount the triangular battens directly on top. In this case you can work off of boards to spread your weight on the battens.

 

The shallow pitch of your roof, the low height above ground and the lightweight nature of materials involved would definitely tempt me to DIY this and save several £K and get it done right.

 

[LeRouret]

On 31/12/2021 at 11:57, Radian said:

The shallow pitch of your roof, the low height above ground and the lightweight nature of materials involved would definitely tempt me to DIY this and save several £K and get it done right.

Absolutely...the Romain canal tiles are really childs-play..fix first and last row...remaining just placed with no fixation so relaying tiles for 100 m2 is probably half days work...

I think 4k is about right for materials for 150 mm...even if I assume 250 day labour per person....two people five days is 2.5k....so I can get to 7k in total but down here the wankers will charge at least double...

I am still considering a cold roof for simplicity and as a temporary measure...

One thing I have to deal with with the strategy you have described is how to deal with the ridge...I guess I would have to do the other side as well..

 

[gc100]

21 hours ago, LeRouret said:

wankers will charge at least double

as someone who has lived in Le Rouret for 20 years I did find this insulting to the locals. Like any trade there are complete cowboys and some really good people just like the Uk - it’s no different at all. Perhaps learn the local language and little better , immerse yourself with the locally born and get recommendations like you would in the Uk… 

 

[Radian]

13 hours ago, gc100 said:

as someone who has lived in Le Rouret for 20 years

 

Did you ever bump into Richard Wright?