I've read various articial and threads on the benifits of high thermal mass but I'm still unsure of the answer to the above question , I realise its not straight forward question with one answer
but,

Here are 2 wall construction I've the option to use
which would be considered the best and why?

1. dense concrete block ( 1.28 w/m2)
100mm mineral wall ( 0.32w/m2)
dense concrete block

2. aerated block (supablock 0.11 w/m2)
100mm mineral wall ( 0.32w/m2)
aerated block

I presume its option 2, but this block has a quarter the density of the DCB

finishes are external render, internal 2 coat plaster

thanks

Jim


PS. is this a good compromise

3. aerated block (external)
100mm mineral wall
dense concrete block (internal)

tony
it for a customer,

I'm trying to tweek an existing design for a extension to improve the u-value , drawings are passed etc.
so the changes are only ones I could make on site with the building controls officer on product selection
it will pass with the DCB or aerated.

I guess its a bit of a theorecticial question, in this limited situation is it better to stick with DCB
for there superior structual qualities , rather than go for the aerated and there insulation benifits

I was interested in the general opinions

i think the 3rd option is probably the best as the inner skins the structual one

or should we stay clear of aerated block altogether ?

does the superior insulative benifit ever out way the problems of poor key for finish, higher embodied energy, allumium use and
potential structual weakness
a lot of people out there think there the green option

cheers Jim

medium always a good middle ground

what problems might be caused by having different block types in the same wall ?

I like the solid wall with external insulation myself , but it a bit late to move to that
lintel changes etc ,
, i could always speak to the BC officer get his thoughts, though any major changes make them nevious

So it looks best to stay away from aerated blocks then

Why are you bothering to build a cavity wall when it is rendered. If a wall is waterproof on the outside cavity construction is pointless. You should go for solid lightweight blocks (Celcon Solar or suchlike) and put all the insulation on the inside. Forget all about thermal mass and insulation on the outside. In this day and age when people go out to work all day and the house is empty you need buildings that are thermally responsive. All the most heat efficient buildings have low thermal mass (straw bale, timber frame etc). A solid wall of lightweight blocks with a waterproof facing has lots of advantages. Ignore those who say lightweight blocks have no strength, blocks over 200mm give superb robust walls.

Some of the advantages of solid block and internal insulation are:

You can build the whole wall from the inside - no scaffold required until you reach roof level.
Put windows and doors at any depth you like - no messing with thermal breaks and dpm.
You can use standard cavity wall lintels - just groove the up stand into the block.
Airtight construction - no sealing round joists - you can bed them in the wall.
The insulation goes down to the floor slab insulation - no cold bridge at ground floor level.
Best of all real workmen on the site can actually do it and make a good job.

I have just built an extension with the following construction:
Outside face - 18mm timber
Breather membrane
265mm Celcon Solar
50mm Kingspan
50 x 25 batten to give 25mm certified low E air space and somewhere for services
12.5mm plaster board dry lined

No problems with building control and sailed though heat regs.

The Kingspan is fully taped with aluminium tape. It extends from the kingspan at rafter level to the Kingspan at floor slab level. This gives a continuous insulation and airtight layer over the entire construction. All the window and door openings have the blocks rebated on the inside so that the Kingspan meets the frames to avoid cold bridges.

I think solid walls are a very neglected form of construction. They are totally permitted in the regs as long as you have a waterproof external coating. My external coating is actually 18mm external ply routed to look like Georgian block rendering. It was then treated with a texture coating to look like rendering and a resin coat to make it waterproof. Nobody has actually noticed yet that it is not real rendering.

One point about solid construction is that you can use a solar or turbo block and actually render it. You cannot use a solar or turbo block as the outer skin of a cavity wall if it is rendered, unless your outer skin is 200mm plus compared to the usual 100mm. So you get even more insulation for the same total thickness of blocks. If you use thin joint bonding the insulation gets even better as you lose all those thick mortar joints.

Solid construction is difficult if you have face bricks or stonework which are obviously not waterproof. Interestingly my BCO told me that they (building control) had been looking at the possibility of allowing the fully taped Kingspan layer to be regarded as a DPM thereby allowing solid construction to be used with face bricks. Face bricks bonded to insulating blocks as backing.

Hi, Up here where I am its usually built solid as follows:

External render
Dense block laid flat = 225mm thick (= 140 course ht)
25mm batten horizontally across @ top/middle/bottom
Kingspan (or similar) 75mm (ish) top to bottom on the battens
50mm batten vertically to give a stud wall against the insulation and also gives 50mm available for services
Plaster board.

Services run up/down inboard of the insulation to avoid breeching that layer. With good use of tape/sealant and attention to detail it should be reasonably air tight. Around reveals or any penetration to the outside the 25mm battens are used around the perimter and likewise on top of the insulation and sealed well.

Cheers, Mike up north

JamesA
I used ply because I thought it was a greener option and also because it gives an even better U value than render. Also when you render the render obviously is attached to all the joints in the block work effectively giving a lot of cold bridges through the block work. If you do not account for the joints when you calculate your wall U value your value is grossly optimistic.

Jamesingram
Somewhere on the Celcon site is a table of what you can and cannot render. I have just been looking for it, but was unable to locate it. If you could render a 100mm Solar block you would never see a Standard block on a site. The outer skins of cavity walls are always in Standard blocks for the reason you cannot render a 100mm Solar block.
You should fire your labourers if they are chucking the Kingspan about. It is much too expensive to ruin.
My preference is for low thermal mass on the inside so that the thermal response is fast. Quick to warm up and easy to control . I know there are others who prefer the opposite, but that can of worms is elsewhere on the forum!