Thermalite/celcon

[Pocster]

Hey all,

 

My architect has specified Thermalite shield block for the exterior walls. My building supplier doesn't stock them and suggests

Celcon standard block is equivalent.

I can't find any evidence to support this. Also after googling I get the impression people don't seem too keen on Thermalite blocks for external walls...... (the finished exterior will be rendered).

Any advice welcome!

 

Cheers

Edited March 14, 2017 by pocster

[Declan52]

I built my inside skin using a lightweight block . Outside skin is standard block. Why would you do the outside skin with them??

This type of block has a tendancy to crack badly ESP on large walls like gable ends.

 

[Pocster]

Yeah. I'm just following the architects spec!. I'm guessing because they have a better thermal value than bog standard concrete blocks.? and easier to work with......

[Russell griffiths]

I wouldn't use an aircrete type block if rendering, they suck the moisture like crazy and can be a pig to work with. 

Ok inside as plaster takes to them so much better. 

[Declan52]

Problems arise when plasterers don't saturate the wall before starting which means the scratch coat dries out before it goes off and ends up cracking which cracks the top coat and looks very bad.

[Pocster]

I forgot to mention that the outside wall sits on lintels which in turn sit on metal brackets bolted to a solid surface. So the external wall is sitting on brackets! - I'm guessing the weight of concrete blocks may be an issue compared to aircerte blocks.....

[PeterW]

13 minutes ago, pocster said:

I forgot to mention that the outside wall sits on lintels which in turn sit on metal brackets bolted to a solid surface. So the external wall is sitting on brackets! - I'm guessing the weight of concrete blocks may be an issue compared to aircerte blocks.....

 

That is a very odd construction ..! What does the solid surface sit on ..?

 

and have you got a cross section drawing you can share ..?

[Mr Punter]

I have had Thermolites rendered.  They applied Rendaid first.  I think you can also just make them very wet.

[AliG]

Other than the weight issue you have mentioned and any structural issues I might not be aware of-

 

If you run the Celotex U-value calculator and change between standard blocks and thermalise blocks for the outer skin it only suggests a 0.01 U-value difference.

 

I have the same issue, I am using Porotherm blocks. In reality I probably could have saved and used normal blocks for the external skin as it adds little insulation value.

[Fallingditch]

I built with block and block, using 215 inner and 100 outer with 200 polystyrene in the void. We used Ytong blocks because Celcon couldn't provide 215mm blocks to the same specification. (Nor could Thermalite).

 

Plan is to clad it rather than render it however (planners).

 

There were two reasons for using lightweight autoclaved blockwork:

• our Structural Engineer assured us we could go with a lighter reinforced concrete slab (we did not want to have to pile)
• as other threads have pointed out, airtighness is critical. With lightweight autoclaved concrete using a thin bond mortar, its much easier to avoid  gaps (especially if it is parged on the inside)

I did suggest using 100mm normal blocks for the outer skin, but the issue (I was told) is that if you are using thin bed adhesive for the inner, and standard mortar for the outer, you cant keep the courses in step. (Somebody who's actaully done this may come back and say "oh yes you can" but for me the argument was good enough).

[Declan52]

You would get away with it if it was only 1-2 course but after that the tie irons would be badly slopping towards the inside which is what you definitely don't want. 

[Pocster]

Yeah, sorry. My build is 'unconventional' so sometimes I just ask a question without giving my different context.

 

Hope the attached explains it. Think I will go with thermalite or equivalent..

 

As always appreciate all the advice and help. You guys are stars!

 

Cheers

DD401B2C-B95E-41AF-95FC-AE9C40607AA7.pdf

[Declan52]

Instead of blockwork could you use a wood fibre board system like @ProDave used. it would improve the u value and should weigh less and can still be rendered. He could give you a rough idea of cost and compare the two.

[Pocster]

1 hour ago, Declan52 said:

Instead of blockwork could you use a wood fibre board system like @ProDave used. it would improve the u value and should weigh less and can still be rendered. He could give you a rough idea of cost and compare the two.

 

I guess that's possible. Don't want building inspector querying any alterations. I'll do thermalite blocks though. Looks more manly if I've "built it" 

[Brickie]

Control of suction is critical to avoid cracking,as has been said. If it were me I'd look into using a lime/cement/sand mix both for the build & the render & be sure about your positioning of expansion joints. Hiding one behind a downpipe is always good,if possible. 

[Pocster]

How long a run can I do before I place an expansion joint ??

[Brickie]

Check with manufacturers but I believe it's 9m for block work. You want ties every course either side of the expansion & de-bonding ties with sleeves every 450mm vertically. Sometimes an exp joint can work to your advantage in terms of breaking up the run into more manageable sections. Can whack a profile up it & away you go. 

[Pocster]

Thanks 

my max run is only just over 9m so no need I assume :-)