We're doing a single extension with EWI. If we use aircrete blocks what thickness block should I assume?

What are the alternatives? Is it worth having look into hempblocks, etc.?

Yeah timber's an option, but blocks would have the added advantage of providing more thermal mass. Which could be important given that we're pretty much west facing, so overheating could be a problem.

Another advantage of timber: we are also doing a loft conversion in timber so using timber instead of block would get rid of a trade.

150mm medium density blocks?

From your earlier postings, I take it that this is a permitted-development single-storey rear extension on a terrace house - hence 3m deep max, and max 3m (normally less) high at the front, sloping up to 4m (normally less) max at the back (other shapes are possible of course)!.

I'm doing something similar now...

There are many things which impact the minimum thickness of a wall "panel" in this situation. Walls fail in two main ways 1. vertical overloading leading to crushing and cracking, 2. sideways actions (often wind) tending to push them over. For single story extensions, the wind tends to be the limiting factor. Given this, influences on thickness include:

. Local wind conditions (higher -> thicker blocks needed)
. Length (longer -> thicker needed)
. Height (taller -> thicker needed)
. Loading (including self-weight) (less lightly-loaded -> thicker)
. Strength of block (up to a point) (weaker blocks -> thicker)
. Strength of mortar (up to a point) - including mortar quality control proceedures (weaker blocks -> thicker)
. "Fixity" - how strongly each edge of the panel is fixed to the surrounding structure (less fixity -> thicker)
. Whether you have piers in the wall (no, or fewer / smaller piers -> thicker main part of the panel)
. Bed reinforcement (steel rods in mortar beds -> thinner blocks in some circumstances)

Under England+Wales building regulations, there are currently three acceptable ways to arrive at a wall thickness, these are:

. Use one of the simplified methods in Approved Document A (this will generally allow a 190mm single skin wall, or in some circumstances a 90mm single skin wall - worth looking at in case your's falls into that).
. Design each wall panel using BS5628 (still acceptable, but theoretically in the process of being phased out in favour of EC6) - this takes account of all of the factors above
. Design each wall panel using Eurocode 6 (similar to BS5628 but a bit more complex, and gives slightly different results, but usually the same to within a few %)

I'm planning on 140mm blocks for the side panels, and 215mm thick for the back (which has a big lift+slide door in it, so is basically reduced to a couple of 800mm piers with a 3500mm gap in the middle). We could even use down to 90mm for the side panels, but decided on 140mm (we had to increase fixity further at 90mm, or 100mm if I remember correctly).

I'm going to use a 7.3N/mm² light-weight 140mm blocks with recycled aggregate content (cheaper, and more stable than AAC, higher thermal mass, and slightly less embodied energy too). I'm planning on 2.9N/mm² AAC to reduce thermal bridging downwards into the ground.

As Tony said .
Single storey rear extension ,similar to Timsmall dimensions with vaulted ceiling
I build using 140mm 3.5N block (fibrolite) for all walls
no piers required, footing 450mm wide

Recycled medium dense agg. block would be better, harder work to lay though

probably lots more to it but I used
reg A1/2 2C6 " solid walls contructed of coursed brickwork or blockwork should be at least as thick as 1/16th of the storey height " to justify design, 2.4/16=150mm

We do have a structural engineer on it so I will find out from him. Just trying to get loads of detailing done this weekend and can't get hold of him.

So no thumbs up for hemp-based blocks or anything like that then?

The other reason I was thinking aircrete blocks was their thermal conductivity, but if I'm using blocks of thermal mass then of course it makes more sense to use dense blocks.

Posted By: borpinWith EWI? There is no approved method that I can find unless you put a cavity between the wood structure and the EWI.

Slightly off the wall suggestion - Light Steel Frame and EWI?

EWI stuck outbourd of timber frame sheathing OSB 9 or 11mm, very quick and easy to build, no wet trades needed, lots of pluses

You can render direct onto insulation when using timber or steel frame, but the NHBC, for example, make you add a 20 mm drainage cavity behind the insulation (with drain channels over tops of windows etc.)

Timber is an option but we really need thermal mass (particularly if we insulate over our structural slab).

Posted By: an02ew

Posted By: TimSmallYou can't directly render onto the EPS this way tho', need to use a render board, or some other sort of ventilated rain screen?

We use a polymer mesh render system from Parex, directly over stuck EPS, no problems.

Let me re-phrase that... NHBC, and my local building control say that you shouldn't do render-on-EPS-on-timber-frame without a drained cavity somewhere (and the only non-insane place for that cavity is outboard of the EPS of course).

My personal opinion is that it's OK if you pay very good attention to detail around things like window openings, but I do have some sympathy with their stance, because you are leaving subsequent building owners open to which-ever muppet with a Sky van comes along later and makes a big water-admitting hole in it.

Posted By: TimSmallSee pages 12 to 16:

http://www.nhbcfoundation.org/LinkClick.aspx?fileticket=%2ByHbjZ7R3QU%3D&tabid=339&mid=774&language=en-US" rel="nofollow" >http://www.nhbcfoundation.org/LinkClick.aspx?fileticket=%2ByHbjZ7R3QU%3D&tabid=339&mid=774&language=en-US

Internet Explorer is shutting down when I try this link ....

Posted By: an02ew

Posted By: TimSmallYou can't directly render onto the EPS this way tho', need to use a render board, or some other sort of ventilated rain screen?

We use a polymer mesh render system from Parex, directly over stuck EPS, no problems.

Our renderer is going to use Parex ... Are you using anything like the Wemico joint beads around openings?
Our openings will be set in about 100mm and the overhang as shown is about 250mm above all opening tops ...
Are we needing a drip channel or bell top do you think???

Posted By: an02ew

Posted By: borpinWith EWI? There is no approved method that I can find unless you put a cavity between the wood structure and the EWI.

Slightly off the wall suggestion - Light Steel Frame and EWI?

EWI stuck outbourd of timber frame sheathing OSB 9 or 11mm, very quick and easy to build, no wet trades needed, lots of pluses

The negative from my point of view is that BC insist on a cavity somewhere and finding a company that I trust to do the render properly has proved impossible so reluctantly I have gone for a second block skin and a traditional wet dash (to be painted). More expensive I suspect (double width foundations for instance) but the inability to find someone I could trust forced me in this direction.

(edit) BTW if you can show me a manufacturer's BBA certificate that shows EWI fixed directly onto a TF OSB outer board with no cavity I'd love to see it. I could not find one or any ADs to satisfy BCO which is why I went for Light Steel.

Posted By: an02ew as for a good EPS renderer thats such a shame that somone in your area isnt prepare to take a little time and effeort to learn a new skill set, seems like an opening for someone. none of these skill are that different from there daily grind as a plasterer

Yes, I'm getting the sense the suppliers/manifacturers are getting ready for a big surge in demand for EWI due to potential funding via the Green deal.
Let hope it doesn't result in unsuitable poor installs that can be common place in these type of 'gold rushs'

anO2ew wrote:

"100mm set back seems good and the frame seal bead has 100mm wing of mesh already attached which works well with your setback ...
... One of the great advantages of EPS is its ability to be shaped and planted on to create the look that you require so if you can find a suitable way of incorporating a bellcast above the windows that fits with your design then i think its a must. at very least try to encourage water to run/drip away from the windows"

The beads I have are the tear-off type for neat flexible joints ... no mesh ... product code 3712, as opposed to 3721, which is the one you're describing I think?
Don't remember seeing these when I bought mine a few months back. Are they new?

EPS definitely good for sculpting ... see pic of "thirties" corners.
The plan is to put a 1 in 30 slope on both the upper and lower horizontal sections of the "reveal/rebate/set-back".
A bell on top wasn't in the design but I guess it's worth modelling ... maybe a straight edged one to echo the overall shape of the double hip roof?

Thanks for the feedback