Solid concrete and thermal block question

[pilsbury]

When building a blockwork cavity wall, I've seen loads of pics with the solid 100mm concrete block on the outer skin and thermal blocks on the inner skin. My assumption is the 7.3N concrete blocks are better load bearing and the lighter thermal block offer better insulation......

Is there any problem building both skins in the heavier duty concrete blocks? I ask as there will of course be 100mm of cavity insulation, but the solid blocks work out at £1/block and the thermal blocks £1.42/block. This is for a single storey extension.

 

[Doggit]

You're initial supposition is correct, and I've no idea if building regs would allow you to use concrete blocks on the inner leaf, as that would reduce the insulation properties of the extension and require additional heating to even get it warm, let alone keep it warm. It may cost you more to build it with thernal blocks, but you'll definitely get the costs back in reduced heating costs.

 

[noseall]

When building a blockwork cavity wall, I've seen loads of pics with the solid 100mm concrete block on the outer skin and thermal blocks on the inner skin.

Correct, so far.

My assumption is the 7.3N concrete blocks are better load bearing and the lighter thermal block offer better insulation.....

Your assumption is partially correct. It is the inner leaf of masonry which bears the greatest load.

Is there any problem building both skins in the heavier duty concrete blocks?

Yes, your bricklayer will want to kill you and if they are wet or there is wet weather forecast, be prepared for a s.l.o.w. build. As said, they are less thermally efficient as aerated blocks. You will need to account for this when calculating your heat loss values etc.

 

[tomfe]

You'd need to go with a wider cavity and more insulation so you would not save anything. Although you would have more thermal mass and stronger walls for hanging stuff on, so you might want to do it that way.

You could use a light block like a fibolite which has good thermal properties and are fairly strong to fix into. They also don't use aluminum or are autoclaved so are better for the environment. You can also get recycled aggregate blocks as well.

 

[garyo]

Noseall - which type of block would you use on the outer skin if you were rendering? It seems contentious and people say that 3.6's are too light and don't take the render well, and are too exposed. I like the idea of the ease of working with 3.6's on the outer skin but it feels like 7's is probably the belt-and-braces choice that I should make?

 

[Draughtsman]

I would keep any thermal blocks as dry as possible, if they get too wet they can expand and later on pull away from the mortar courses as they dry out and contract. Is the job/block storage under cover?

I think that thermal blocks are the way to go, great heat retention in comparison to conc.

 

[pilsbury]

Dain1 - I can keep the thermal blocks dry. Seems like the dense concrete blocks also need to be kept dry from Noseall's comments.

Keeping them stored under cover is fine, but what about when the wall is going up? sheet up? My build will be super slow as I'm DIY.

 

[^woody^]

You could put thermalite blocks in the oven at gas mark 8, in the middle of an El Nino summer drought and they would still end up shrinking and cracking. And the block outlines always tend to bleed through render.

Use a lightweight but concrete block for render.

 

[Draughtsman]

Woody, what about wet plastering? Thermalites cracking may not be helpful, how do you deal with that.

 

[garyo]

Use a lightweight but concrete block for render.

Can you give me an example... I'm confused by the way that a lot of aerated blocks - such as a Celcon, describe themselves as 'lightweight aerated concrete block'

 

[tomfe]

Can you give me an example... I'm confused by the way that a lot of aerated blocks - such as a Celcon, describe themselves as 'lightweight aerated concrete block'

Read my post.

 

[stuart45]

AAC is a form of concrete, but when most people use the term concrete blocks they don't mean AAC.
As stated above fibolites are a popular alternative to AAC.

 

[noseall]

Noseall - which type of block would you use on the outer skin if you were rendering? It seems contentious and people say that 3.6's are too light and don't take the render well, and are too exposed. I like the idea of the ease of working with 3.6's on the outer skin but it feels like 7's is probably the belt-and-braces choice that I should make?

We used 3.6's - i.e. fibolites or similar as they are grippy but forgiving regards suction etc. Deffo not aerated celcon type.
The guys doing the rendering wanted us to use solid conc' 7's. I said no chance. Soak the fibolites is what I said.

 

[^woody^]

Can you give me an example... I'm confused by the way that a lot of aerated blocks - such as a Celcon, describe themselves as 'lightweight aerated concrete block'

Just look for a concrete block, which will be proper concrete with bits of stone and not soft aerated like a crunchie.

 

[pilsbury]

So seems wise to use concrete for outside. If using aerated "crunchie" blocks internally, how well do they take the plasterboard adhesive? Suck all the moisture out and give a crap bond?