Best way to insulate the roof of this french property

[gimiq]

hello, we are in the process of converting an old longere, we want to convert the attic space and top floor of the attached barn into habitable area. for a family room in the barn upstairs and bedrooms in the attic. the house is about 16m long and around 6m deep.

the roof has no membrane. in the winter we hear very light droplets of rain getting through. no actual drips just tiny droplets landing on the 1st floor ceiling below the attic roof.

if we have to take the tiles off to lay a membrane were wondering if we should insulate on top of the rafters while we are at it. or what other solutions would be advisable?

the roofs here have volige under the tiles. the old tiles are overs and unders. the volige is in good condition. I will attach photos below. Whatever system we choose we are looking to:

maintain breathability of the roof
make the attic habitable.
use natural materials where advisable.

All the threads and systems i have seen do not seem to have volige boards on top of the rafters like we do, so not sure what the best approach would be. We hire the work out but want to ensure the specification is done with respect to the existing materials in the correct manner for a period property. Also if anyone has ideas of prices to expect for such work. Even uk prices will help us benchmark this. the house is detached sits on 5 acres with no neighbours or adjoining property, plenty of access all around

thanks in advance

 

[gimiq]

we also want to re use existing tiles and reclaimed tiles to preserve the aesthetic.

 

[Cubist]

When I see properties like this I am always torn between the conflicting demands of how best to make it a liveable space whilst keeping the character intact and as pleasing as it already is. Like it or no the tiles will have to come off to allow installation of a waterproof membrane of some kind to prevent future rainwater ingress. But that need could be translated into an opportunity to save that glorious interior when considering how to insulate the place. Most insulation systems, even the most expensive varieties are damnably ugly when viewed from the interior, as are the means used to keep them in place. As a consequence these tend to be panelled in out of sight and the newly panelled surface plastered and painted. Depending on the depth of the rafter beams, they look quite chunky, you could use something like celotex panels to insulate the roof and trap these behind say a plaster board and plaster finish between the rafters to leave them exposed.

However, as I said earlier, there is an alternative. Rather than insulating the roof on the inside why not do it on the outside. This could be done by fitting lengths of timber directly over and following the lines of the existing rafters above the plank (volige). Insulation panels can them be installed between these faux rafters, a waterproof membrane installed and the tiles reinstalled - thinking about it horizontal battens may be needed between the faux rafters to allow fixing for the tiles.

I've no idea how much either option may but hopefully others here can offer some further thoughts and or pricing guidance.

Oh, and one other thing comes to mind if you want to explore my above roof suggestion a bot further you may need to but some additional tiles because in going up even by say a few inches could translate in a need for another row of tiles being needed to match the current coverage.

 

[a twig]

Scratching at the back of my head here but doesn’t Scottish building regs require sarking boards / sheets underneath the felt / tiles, so a pretty similar arrangement to what you have. Perhaps there are some ideas you could take from that?

 

[a twig]

For e.g. you might be able to strip the roof, put wood fibre or some other breathable insulation on top of your sarking / volige, then breathable membrane, counterbatten and tiles back on?

 

[gimiq]

yes, if we are going to have to strip the tiles it seems to make sense to insulate on the outside. Minimise thermal bridging whilst also maintaining maximum visibility and space within the interior. Probably the most expensive option but since we have to strip the tiles to make it liveable anyway it seems to be the most logical. Our rough estimate is around 20k for the job. But we will gain approx 100m2 of space that could be made habitable. the closest systems i have seen to what we need involve 2 membranes and and thick wood fibre boards about 160mm deep, 1 type of membrane below the wood fibre boards and another type above with counter battens and battens on top of this. then tiles placed on top. we have about a half a cubic meter of the original tiles in reserve that came with the house and the neighbour has a few piles of them as well. Which are an original match for ours. So we will try and use these where possible. Since we have the volige It would be nice not to have to pay for the sarking boards as well.

this is the product i am looking at as a possible suitable option for the insulation https://www.isonat.com/produits/isolation-exterieur/multisol-140-isolant-en-fibre-de-bois-rigide "isonat multisol 140"

the truss in the second photo needs a bit of attention, a local artisan thinks the join is separating could be fixed with iron bracing.

if we were to do the above. I wonder what we would need to do the interior to ensure any condensation from a warm space to a cold boundary formed in a way that does not cause a problem to the old wood work

 

[Cubist]

One of the reasons much of that roof structure has survived is the free air flow around the timbers. You don't mention the nature/construction of the floors but when that roof is done the floor will become perhaps the largest source of moisture inside the building. As its safe to assume you will also be installing some kind of heating system in I would suggest you need to consider the siting and distribution of the same as this will have a significant impact through pushing moisture laden air up into the roof space which may then condense out or otherwise be absorbed into the surface of the timbers. I would suggest you look into how that moisture laden air could be rapidly expelled maybe through vents in both gables?