Decisions start off sounding easy but then when you look into the details...

So single skin blockwork wall + EWI has been decided. - easy.

I understand 190mm thick external blockwork is required as a minimum. - regs - easy

To avoid unnecessary delays/costs the following choices present themselves:

go for a 390x190x190 block - if they can be found
or
go for a 440 x 215 x 190 block
or
go for 440 x 215 x 215 block which appears to be more common and easier to purchase (as are lintels for this thickness).

Is it better to go for Dense Concrete, light aggrgate or aircrete - dense gives me thermal mass, aircrete gives insulation value. I also understand that a wet plaster on aircrete is not the preferred combination for plasterers.

A full size dense block is 35+ Kg - is this an efficient block to lay in terms of manpower? If not - is it any better to lay blocks on their side?

Should I consider thin joint - it's a relatively small project - there may be no pre-existing experience available to me here - at present I suspect the benefits of thin joint would not overcome the dis-benefits.

If I know the block system I can set the openings to coordinate and save on block cutting and general faffing about. I want to make the building fit the materials - it seems sensible to do this but only if the materials I will need when the time comes will be readily available at a sensible price.

Words of wisdom/experience would be appreciated.

look at fibolite 190mm or 140mm ( ive done single storey work with 140mm)
https://www.plasmor.co.uk/products/fibolite-fl
good key, not to heavy
some where between a medium dense and an aerated.
also these in order of preference are worth a look ,
Fenlite (some recycled aggregates)
Hemelite

aerated are to be avoided, as others say. plastering them is hard work, they need a really good sealing primer which adds time, and then 6 months later the blocks will crack along with your plaster. This is why the volume house builders always dot and dab - it's cheap, fast and hides all the cracks in the blockwork.

I've used medium density aggregate blocks on my extension. There are loads of choices, from memory I think the medium density stuff is typically in the range of 1350 - 1550 kg/m3. Consider also the additional thermal mass of medium density blocks, in most cases I think this would be desirable, especially with EWI. Your brickies will thank you for going with a standard format block 440 x 215 x 100, these are on the heavy side but perfectly manageable. Also your local builder's merchant will likely hold stock which can be a factor on a big job as you can get deliveries quickly, you dont want to get held up waiting for a follow on delivery of exotic sized blocks.

lastly, have a chat with your brickie and ask what he would use. My brickie was much happier when I told him it was medium density and not the aerated on the architect's drawings. I think aerated are a pest to lay in warm weather as they suck all the moisture out of the mortar.

If I did it like that it would be four courses normal, wall ties on the second course, then two blocks laid flat, four more and then one laid flat under joists

Sorry to resurrect this thread during the Easter break - but what disadvantage would a 215mm wall built entirely with blocks on their side with a stretcher bond have over Tony's method above. My thinking is that a simple stretcher bond wall would be easy to inspect and difficult to get wrong.

Would you have the mortar pointed flush on the eps/ewi side and on the wet plaster side? presumably flush pointing would remove some of the convection paths behind the eps.

I have walls that are 11.5m long and 15m long - I believe these will need expansion joints. My architect has suggested placing these up one side of the doors on each of the walls. Although they would be less obvious here is there any advantage in placing the expansion joint mid panel - perhaps where an internal wall is located. Is it rare to have some sort of expansion beading in the wet plaster on the inside of the building - I don't recall ever having seen one?

Thanks Tony I see - more important to get the bed joints correct than the vertical joint between the two 'skins' - In fact should I be concerned if the brickies left mini cavity's, 4 blocks high, 10mm thick with your scheme (provided they used ties).

I've Just had some good news - I've chatted with an structural engineer and he said he can design all my structural walls with a 140mm solid block-work wall with a standard stretcher bond (however one wall may need a wind post).

Unfortunately he has confirmed piles, ring beam and beam and block floor as being the most cost effective solution for us.

As we have a simple rectangular building he suggests supporting the b&b beams on a steel angle bolted to the inside of the walls to save on cutting blocks between the beams. It means the blocks can be laid to screed level in one lift, then the steel angles then B&B, polystyrene, dpc, uvf and screed.

Instead of movement joints in the long walls he suggested bed joint reinforcement every two courses of block to control cracks for the plasterwork and the render.

As I intend using medium density blocks for the internal partition walls - is there any problem building them on top of the screed (B&B design permitting) - or do they need to penetrate the floor insulation and be directly supported by the B&B?

any other comments welcome.

Posted By: barneyAgain, matter of taste, but I'd put the insulation under the beam and block (fill the void from the over-site right to the underside of the beam

I don't think it's really a matter of taste. The only way to seriously reduce the thermal bridge is insulation both under and above the floor as I understand it. It also depends on why a beam and block floor is specified, of course. If it's because of risk of heave then there's no way I know of to fill the void. If it's radon risk then you wouldn't want to fill it.

OK - I will look again at the steel angle support of the B&B floor.

A suspended floor is required because of heave.

I am assuming that insulation systems that sit below the B&B are expensive compared to above the B&B - in addition finding them with 200mm or more of platinum eps below is even more difficult.

I presume that stud partitions usually sit on top of the insulation and screed and was hoping to do the same with my thermal mass blockwork - but if this is not the done thing I will have too look at using stud walls instead and forgo the thermal mass.

Isn't it an EWI system - so the whole lot inside FT's "tea cosy"

The only cold bridge is the long path through the wall to the founds ? - which exists in any case

If the B&B combats heave then loose fill the void with EPS beads and have the B&B mass inside the thermal envelope

You can still have masonry internal walls - just design the B&B for it

As I said, I'd not be hanging a B&B floor off bolted shelf angles

Regards

Barney

BCO will insist on no material in the void (other than air). In any case EPS beads would exert a significant up force on the slab if the clay heaves, and if the clay shrinks the beads will not be against the B&B and as ventilation will be required ...

If I insulate below the B&B it would have to be something like beamshield - I'll give them a call tomorrow to get prices but I don't expect they will compete with EPS over B&B.

Cellcore plus is also an option and I believe does not require ventilation. but again will be expensive.

There will be compromise - all I can do is make the best of it.

Posted By: barneyI was simply suggesting that it's credible the unventilated void can be filled with loose insulation (to combat heave) and retain the B&B within the thermal envelope - acting , as you suggest like cellcore would in a "heave" situation

Loose is not the same as compressible. Cellcore is an air-filled honeycomb and permanently deforms when heave forces crush it. Thus leaving a permanent air space if the ground subsequently drops. (some seasonal rainfall or tree removal/regrowth scenarios can cause repeated heave movements). Unless you can suggest the name of an engineer who will sign it off, it's a non-starter, IMHO.

Posted By: barneyI'm working on a project at the moment where it's failed spectacularly under a ground floor slab onto beams and piles

There is evidence that it's already collapsed whilst the floor slab was being poured (although the slab is about 750mm thick for various reasons so an awful lot of weight imposed on the cellcore)

Cellcore comes in several different grades - the grade should be chosen to fail at a load just above the dead weight of the slab - what grade of Cellcore was used under the 750mm slab? It should have been the blue colour coded Cellcore - safe to support 24KN/m2 will fail at 32KN/m2

Just reading this important thread properly, having prefered to minimise blockwork for many years, so not up to date.

Posted By: jamesingramlook at fibolite 190mm or 140mm ( ive done single storey work with 140mm)
https://www.plasmor.co.uk/products/fibolite-fl" rel="nofollow" >https://www.plasmor.co.uk/products/fibolite-fl
good key, not to heavy
some where between a medium dense and an aerated.
also these in order of preference are worth a look ,
Fenlite (some recycled aggregates)
Hemelite

New to me - I thought Leca blockwork was extinct. Their page
https://www.plasmor.co.uk/products-services/techniclay-aggregate/focus-on-expanded-clay
also a revelation - after years of banging on about Leca, had no idea there's a UK producer. The page is a model of good information. Plus other interesting, aware-looking services on
https://www.plasmor.co.uk/technical-services.

However, no tech info on the site at all - "coming soon" - are they a new or recently re-booted co?

Fenlite 'includes' blast furnace slag in 3.6N/mm2 but looks like much less of that, more of 'natural' aggregate, in 7.3N/mm2 'structural', such as goodevans will be using.
Blast furnace slag must be in ever-shorter UK supply, just like the power station ash discussed by Plasmor.
(The other hope, bottom ash from waste burning, has not taken off because toxic - has become a disposal problem rather than an asset).

Not clear what Hemelite is made of these days.

Funny - the link Quoted direct from James' post, where it does work.

Note, I've edited in a bit more discussion into my post.

Medium density for thermal mass - could just be that rare 'sweet spot' occupied by Hemcrete?