We are about to embark on the next renovation project. A barn conversion of a modern agricultural building.
As a long time GBF member I would have liked to externally insulate, but this isn't going to happen for various reasons. So this leaves internal insulation.
We would like to achieve Passive House levels of insulation and airtightness.
The existing concrete floor will have to come up and be dug out, EPS in the floor should be straightforward.
But what to do with the walls and roof?
Considerations so far include I-beams for the walls with either EPS or rockwool between. Would we need to leave a cavity between this and the existing structure? Drained? OSB for racking, with a membrane facing the cavity?
We quite fancy OSB as the internal face, to avoid plasterboard and skimming. All services could be surface mounted for an industrial feel and for ease of alteration in future. I expect we may need a sprinkler system due to open plan kitchen, dining, lounge etc?
I have been playing about with U-values, which is easy enough, but I'm struggling with the finer points of construction details.

The GBF hive mind is powerful and varied, so any ideas or suggestions would be gratefully received.

Approved Elevations - North and East

You could build a 3x2 stud wall internally about 250mm away from the existing structure, line it with 9mm OSB and fill it with 300mm cellulose.

Just a few thoughts, that may of course be impractical with not much background...

- keep the internal conc slab, and build on top. Raise the external ground locally at the front, breaking out some of that concrete apron, to allow planting, softening the frontage visually, and allowing gentle ramps to bring you to the slightly higher internal FFL. The side entrance is stepped anyway, so a couple more steps may not matter. Add some raised beds for planting along the side elevations to disguise this level diff, and again visually soften. This may also help with issues with DPC's or lack of them around the structure. Your wet services are all around the perimeter, so should get drain falls in insulation depth, without cutting into exist slab too much. Big cost and carbon saving, keeping all that concrete.

- demolish the dwarf block wall, so it's no longer a constraint. You're over-cladding anyway, so it's not part of the visual conservation. Maybe leave the bottom course (or whatever height), to give you the underbuild structure to set the internal frame on, so it is then of use, rather than something to compromise your design. It then becomes more like a simple, typical timber frame construction. The internal frame can be of whatever build-up you prefer. You then have a choice, not forced down a route.

- I suspect the roof will be a trickier problem than the walls, assuming you keep he original structure, and build inside it, with a thermal layer (similar to the approach for the walls). Maybe a light weight metal stud to carry the insulation and internal surface finish? Are there solar panels planned? Additional load onto the existing roof? Could add a couple of posts, say hidden in the bedroom wall, to give additional roof ridge support if needed.

- OSB internally and timber cladding externally with adjacent buildings... remember fire spread regs. You'll likely need to treat all those surfaces with a fire retardant to meet the approp Euro Class. BSO's are super hot on anything fire related, (post Grenfell). With no supported floors, you don't have any "elements of structure", so that makes it a bit less arduous perhaps.

I'd start drawing out some sections at various junctions/interfaces, to clarify what you currently have. That makes the issues more obvious, and so will help formulate solutions.

So should end up something like this

For context, this is from the south. The Alder tree will provide summer shading.

Posted By: WillInAberdeenthe steel frame is a thermal bridge too.

which is why I wanted to externally insulate and carry it on down to include the foundations. Tea cosy.
We could have then had the steel frame exposed on the inside, and extended the concrete block walls upwards to provide thermal mass inside the heated envelope.

Thanks for the comment. Knock it down and rebuild is not an option in this case (due to planning and SWMBO), VAT should be reclaimable for self-build barn conversion so no worries there.

First off, great project. Look forward to seeing how it goes.

Posted By: Dominic CooneyWe quite fancy OSB as the internal face, to avoid plasterboard and skimming. All services could be surface mounted for an industrial feel and for ease of alteration in future.

Bear in mind surface spread of flame restrictions. We have Cross Laminated Timber frame which we wanted to expose internally on walls and ceilings. The Building Regs required that we coat it all with a flame retardant finish and I eventually found a nice translucent treatment (NT Deco by Nordtreat) which looks like limewash.

I actually prefer the finish to the bare timber now, but just remember to budget for the painting. That aspect isn't a saving on plasterboard (although you do of course save on the skimming etc.)

The other anecdotal advice is that whilst it does mean you can mount pictures and shelves on walls fairly easily and with confidence (unlike the paper thin plasterboard most developers use), you then can't fill the holes so easily if things move. There are wood fillers of course, but so much of the DIY market is geared to white plasterboard walls that it's quite eye opening when you don't have them!

Posted By: Dominic CooneyConsiderations so far include I-beams for the walls with either EPS or rockwool between. Would we need to leave a cavity between this and the existing structure? Drained? OSB for racking, with a membrane facing the cavity?

We used the timber I-beam method to overclad our frame, with OSB on the outside as racking and support for waterproof membrane. Comparatively it was v. cheap and reasonably straightforward. Note that the flange and web shape of the beams means there is a risk of voids at the inner and outer edges if you just use standard insulation batts, so we chose our thicknesses to match the different 'layers' of the beam e.g. our total insulation thickness was 400mm but that was a layer of 50mm between the outer flanges, 300mm between the webs and then another 50mm on the inner flanges. This meant each layer could compress to fill its relative width. Possibly overkill but it resulted in a very good flush fit.

You'll need EPS for at least the bottom 150mm 'below DPC', but comparatively this was a lot harder to fit tightly. We staggered joints etc. but you really want to foam glue the edges as you go too.

No problem.

I also just spotted this write-up of a passivhaus barn conversion project. Page 38 onwards. Sounds exactly like what you want to do:

https://passivehouseplus.co.uk/issuu/uk-edition

EDIT: edited to add that the issue I was referring to is number 44 (the hyperlink just takes you to the latest version)

Full version attached.

Posted By: WillInAberdeenYou can see daylight through that kind of wall cladding - treat it as 'decorative' but in no way weather resisting - it's designed to keep cows very much 'ventilated'!

In a balanced rainscreen it is the inner drainage plane that needs to be waterproof. Probably a membrane in this case. The outer screen intercepts some of the rain and slows the rest via pressure effects.

Posted By: WillInAberdeenno-maintenance waterproof layer on the outside of the 'inner box'.

I wonder if this could be tanking slurry on the inner face of the concrete block walls? increase their height up to eaves level so that the vertical timber boarding is just decorative. It could even have a roofing type membrane behind it to help shed driving rain that gets through the gaps.

"an insect mesh could cause more harm than good" ?

Edit: post deleted as this is going OT. Just treat the existing cladding as decorative and put a no-maintenance beastie-proof waterproof outside layer on the new building within the barn.

Posted By: Viking Housepumping the void with EPS beads

Would beads normally be cheaper than solid slabs of EPS for the same volume?