Thermalite blocks for external skin?

[RedHerring2]

Quick question.
Specification for our extension to bungalow is render on 100mm blocks, 50mm partial fill (25mm Kingspan K8 ), 100mm Besblock Cellular Insulite blocks for internal skin with 25mm (K17) insulation and 12.5mm pb on internal.

I've substituted 25mm Kingspan K8 for 30mm Xtratherm and I was intending substituting Besblock Cellular Insulite for 3.6N Thermalite Shield blocks for internal skin.

1. Can I use same Thermalite blocks for outer skin?

2. Bearing in mind the speed I work, it'll be next winter before I render. Is it advisable to leave Thermalite blocks open to the elements for that long?

 

[^woody^]

Aerated blocks are more prone to cracking and give less adhesion for render than concrete blocks. Also they are more susceptible to showing the bond/block pattern through render.

They can be used but need more care, so follow the makers guides precisely

The problem with leaving aerated blocks exposed is that they can change rapidly from hot to cold and wet to dry, and so experience severe vertical thermal cracking if changes are not adequately controlled

 

[RedHerring2]

Cheers Woody, It's mainly the south facing side so it gets the full sun and seems to suffer the worst of the rain, so it looks like a no, then.
Thanks again.

 

[RonnyRaygun]

Celcon have plenty of info on their website.

They say it's fine to use their blocks for the external skin (but they would, wouldn't they?!)

There is a factsheet which you will find here,
http://www.hhcelcon.co.uk/c/document_library/get_file?folderId=54610&name=DLFE-14123.pdf , which lays out the recommended method for rendering onto their blocks, might be helpful...
Exposure conditions have a big bearing; and their solar blocks are more difficult to render onto, although still possible as long as you follow their guidance.

My extension was built three years ago from aerated blockwork inner and outer, and fingers crossed, no issues yet

 

[Mac73]

Hi Ronny,
I read your above post with great interest (I am currently in the process of buying a house (2014 build) which apparently has Thermalite aircrete blocks on the outside walls. The original builders used plain old sand and cement and the render is mostly all blown now. Back wall has been redone but NHBC refusing to cover the re-rendering of the side and front walls - basically because the render is not quite literally falling off yet! Anyway, I'm now in a renegotiation with the seller because i will have to pay for the re-rendering once we've completed, and whilst looking for a couple of quotes one plasterer has told me that the thermalite blocks shoudn't even be used on the outer walls. This led me to Goodle and i found your above post. I see that your extension was built in around 2008 - can i ask you these 14 years later if there have been any problems with it please?

And does anyone have any advice generally? Should i avoid this house? The builders were obviously a bit suspect (using the wrong materials for the render) and i'm worried i'll buy it and the blocks will all end up cracking and i'll be left with a wreck of a house!

Thanks very much in advance for any help...
Simon

 

[RonnyRaygun]

I don't live at the same property any more, but when I left in 2016 it was still fine. In the end I also used EML fixed to the blocks to help with the bonding.

It's not common to use aircrete for external walls - there might be other methods of rendering onto them with alternatives to sand and cement. But as my previous post, Celcon do (or did) provide guidance for doing just that. Might be some plasterers who can offer more advice.

The blocks themselves shouldn't be an issue, and if you have to strip back the render to bare block, you may have other options rather than direct rendering - hanging tiles, timber cladding, renderboard and thin-coat render for example...

 

[Mac73]

Thanks very much for getting back so quick, that's really helpful.

Yeah, apparently there is specific manufacturers' guidance about the materials and process for rendering on aircrete and there are some plasterers out there who can do it, but everyone I've approached has not been interested - as far as I can work out either because (a) it's a bit of a pig of a job; and/or (b) they don't have the experience.

What worries me more is I've heard talk of the aircrete blocks cracking after use on outer walls - what happens if they crack?? Does that not compromise the structure of the house?

thanks for the tips about other finishes - i might look into renderboard or thin-coat render - the latter sounds like a no-brainer, are there any disadvantages?....

Thanks again for any advice!
cheers

 

[RonnyRaygun]

See Woody's post above - there is more risk of cracking due to the thermal effects.

However, it's a cosmetic issue, not a structural one - all of the load is taken by the inner leaf. The inclusion of EML and/or other fabric mesh will help to prevent cracking showing through into the render even if the blocks themselves crack.

 

[Mac73]

Thanks again Ronny.

If you don't mind I have another question....

I'm now trying to get a renderer to quote for the works (completely replacing the render on the side and front walls). Knowing that it is essential that the block manufacturer's guidelines are followed precisely, I'm trying to work out exactly which blocks were used. The renderers who re-did the back wall render said they were "aircrete thermal blocks". But in the attached pictures ("rear elevation..." is a shot of the back wall during the replacement of the render and "rear corner" is during replacement of some rusty beading later on) I can't see any "scratch marks" on the blocks.

On the Forterra website (who I understand make Thermalite aircrete blocks) there are scratch marks in all their blocks - see here: https://www.forterra.co.uk/wp-content/uploads/2021/10/Thermalite-brochure-web.pdf)

The house was built in 2014, so perhaps the blocks didn't have scratch marks back then? (seems unlikely but I don't know).

Otherwise are they perhaps Celcon blocks? Or something else? How do I find out?

Thank you....
Simon

 

[RonnyRaygun]

Thermalite have scratch marks and Celcon have a coloured band on the ends. There are others too such as Durox and Y-tong.

 

[bsr]

Stab the block with a screwdriver. If it goes in and crumbles you have aircrete.