Hi All,

I have largely finished the internal insulation of my coach house renovation with 60mm woodfibre followed by warmcote and a topcoat.

One area remains. It has a brick wall at the bottom and then it is timber frame with feather edge cladding. I previously had t&g internal cladding and plasterboard in some areas. I will not be removing the external cladding for the time being. When the time comes to do this i will use a membrane underneath and possibly some external insulation if the building features allow.

For now my plan is to use a membrane over the studs and try and do my best to seal this. Then i will insulate in between the studs and finish with OSB then plasterboard/wood wool then plaster/lime plaster. The alternative would be to use the membrane after fitting the insulation but my hope is that doing it the other way would offer a little extra protection to the insulation from insects.

I could do another thin layer of insulation over the osb but funds are now running very low and i think without it it will probably have similar performance to the rest of the masonry/woodfibre walls, albeit without the thermal mass.

I can fit 140mm batts between the timbers. I now need to decide between woodfibre (most expensive), thermafleece (still expensive) or mineral wool (half the price of the others). I think they probably all have similar performance so i guess this will come down to my preference on environmental impact.

For the masonry wall i may stud this out in some of the challenging areas and use cork or woodfibre elsewhere.

Does anyone have any advice or thoughts? Thanks!

I would be cautious about closing up the back of the feather-edge boards and indeed the studs with a membrane and insulation. Presently the cladding is able to dry out to both front and back and benefits from free ventilation on both sides. I would expect that closing off the back will cause more dampness and increase the rate of decay.

I'd be tempted to build a separate insulated stud wall inside the cladding envelope.

When evaluating the types of insulation do consider their response to humidity as well as their insulation value.

Although probably no benefit to putting the membrane after the insulation if I put OSB straight on top of that!

Posted By: Tomasz_Peventually I will remove the cladding

How long do you think eventually will be?

I repurposed an old garden room left by our previous house owners and made it into a garden office. I insulated it directly behind the cladding with 100mm of Thermafleece. There are gaps in the cladding in places, a large one where a board had slipped due to an old nail falling out where I saw birds pulling the fibres out to make their nests one spring. I have osb to the inside and no breather membrane.

This was originally a temporary solution for max 18 months but has been, partly due to COVID etc. a more permanent solution used for over 3 years now.

Both the timber frame, cladding and sheepswool are functioning fine still. No issue with insects or damp.

It's obviously not without risk and certainly not recommended, but it can do the job. The good thing I've noticed about the sheepswool is that if the outside layer gets wet, the water stays beaded on the surface, then it tends to run off without necessarily soaking into the inner layers.

I reckon mine will be in use for another 12 - 18 months before I demolish the existing structure and then reuse the materials to build a new renovated garden office. It gets pelted by the rain on some days, so the claddings has definitely seen better days.

Anyway, hope that helps.

Another quick question on this. Seeing as I will most likely be lime plastering. Could I do away with the OSB and just use wood wool boards instead? Should I go for the 15 or 25mm? What are they like for fixing picture hooks etc to?

Posted By: Tomasz_Pjust use wood wool boards instead? Should I go for the 15 or 25mm?

Unfortunately it may not be as simple as that. I think that maximum stud spacing for the 25mm board would be 500mm so this would depend on your existing studs and perhaps as little as 250mm on ceilings. As djh says, I don't think they're rated for racking at all. If you look at recommended lime plaster buildup, they also seem to require a reinforcement mesh within the plaster on timber frame construction.

You may be better off sheathing with osb internally, then battening out to leave a service void and then using wood wool boards as plaster substrate.