Dense blocks or aircrete?

[Ben100]

I'm laying the blockwork for my extension which consists of a large steel frame with timber in-fill. Most of it is single story with part double story plus roof/loftspace. The timber frame company have recommended 140mm blocks for the 145mm sole plate to sit on, which is fine. I've laid the first course using 140mm 7.3N high density concrete blocks directly onto the foundation and laid a second external skin as a plinth to DPC. I was about to start laying the 2nd and 3rd course when someone said that I was going way overkill using 7.3N high density concrete blocks and I should be using something lighter. So my question is, should I be using aircrete blocks instead of high density ones? This is taking into account that this is a masonry seat for the TF structure, and behind an external masonry skin plinth.

 

cheers!

[Russell griffiths]

Don’t use aircrete they are proper shit, use a concrete block but light weight, they are sometimes called ag-lights, look very similar to dense concrete but have a proportion of lighter materials incorporated. 

Go to your merchants and see what is available. 

[Ben100]

Something like this? https://www.buildbase.co.uk/lightweight-armstart-block-73n-100010511-2803076

[Russell griffiths]

Are you not using 100mm for outer skin or have I misread something. 

[Oz07]

Got to be careful here. He's below dpc I gather. Agglite type can only be used below dpc under certain circumstances. Airated may actually be the better bet they take to freeze thaw better don't they? Otherwise it's dense 7n

[Russell griffiths]

10 hours ago, Oz07 said:

Got to be careful here. He's below dpc I gather. Agglite type can only be used below dpc under certain circumstances. Airated may actually be the better bet they take to freeze thaw better don't they? Otherwise it's dense 7n

I thought he said he was up to damp, when his friend said why you using dense blocks. 

Will re read. 

[Ben100]

15 hours ago, Russell griffiths said:

Are you not using 100mm for outer skin or have I misread something. 

 

I'm using 100mm dense 7n on the external leaf below ground, then moving to brick just below ground level and up to DPC where the external leaf will finish. The example blocks I mentioned were for the inner leaf for the TF structure to sit on.

[Ben100]

Thanks for all the information guys. I guess I have a few question.

 

1. Below ground should I be using dense 7n blocks on both external (100mm) and internal (140mm) leaf?

2. As my structure is timber/steel frame the blockwork is finishing 1 block above DCP and moving to timber and steel, so almost all of the blockwork is finishing 1 block above DPC. Does this give me any chance to use lighter blocks or should I stick with dense blocks in this case? The only reason I'm really looking to move from the dense blocks is that they are heavy and hard to work with.

3. Another question I just want confirmation on. Is it ok to switch to red-bricks on the external leaf just below ground level for the plinth? Just for a better look.

4. And the last one (for now) What should I be using for the in-fill between the blocks below ground level? I guess a concrete mix of some sort?

 

Cheers all!

Edited December 24, 2019 by Ben100

[Russell griffiths]

1. Yes

2. You can change to whatever you like above dpc, but is it worth it for 1 course. 

3. Below dpc bricks need to be frost proof, so maybe a class b engineering brick in red then swap to face brick above dpc. 

4. You really need a section drawing of your wall build up as you don’t want a cold bridge at floor level or a cold sole plate, so it needs to be designed to have an insulation layer in the cavity to prevent this, also a tray dpc to prevent water inside the cavity reaching the inner frame. 

You need to get this design right before you lay to many blocks. 

[Oz07]

Treat internal and external leaf below dpc the same. Unless you have a fancy dpm detail it's a safe bet. I'd stick with 7n in your case you can get lighter ones. Otherwise I've used airated below dpc houses of mine before all good

[Simplysimon]

3 hours ago, Russell griffiths said:

so it needs to be designed to have an insulation layer in the cavity to prevent this,

as well as a layer of insulation under the kit wall

[Brickie]

On 24/12/2019 at 09:39, Ben100 said:

. As my structure is timber/steel frame the blockwork is finishing 1 block above DCP and moving to timber and steel, so almost all of the blockwork is finishing 1 block above DPC.

How come?

[Mr Punter]

You can get 7N aerated blocks as well.  If you really want decent insulation for the soleplate you can use a course of Perinsul Foamglas or similar, but they are pricey.

[dpmiller]

Our TF sits on a course of 7N Quinn lightweight thermal blocks.

[Ben100]

On 26/12/2019 at 07:40, Brickie said:

How come?

 

As I need to fix the TF soleplate to the blockwork with bolts it would pierce the DPC. It might not be that big of a deal, but I'm playing it safe and running the DPC a block below where the TF soleplate and blockwork meet.

[Ben100]

On 24/12/2019 at 13:52, Simplysimon said:

as well as a layer of insulation under the kit wall

 

4. I have a design for the wall build up (I'll try to upload the drawing later). Basically it's tanked in the cavity then filled with insulating/water resistant foam. The internal leaf on the inside also has a DPM running up it and into the blockwork above DMC.

 

Another question I've been thinking about is is there a need for thermal insulation of the inner leaf to address the cold bridging from the slab/foundation. For this I'm considering changing from 7n dense blocks to a 7n thermal insulating block as I come out of the ground.

[Ben100]

On 26/12/2019 at 18:55, dpmiller said:

Our TF sits on a course of 7N Quinn lightweight thermal blocks.

 

Interesting. I've been looking at these to address the cold bridge from the slab too.

[Simplysimon]

15 hours ago, Ben100 said:

Another question I've been thinking about is is there a need for thermal insulation of the inner leaf to address the cold bridging from the slab/foundation. For this I'm considering changing from 7n dense blocks to a 7n thermal insulating block as I come out of the ground.

i ran a course of marmox under kit leaf. http://www.marmox.co.uk/products/thermoblock and armatherm under the steels https://www.armatherm.co.uk/applications/armatherm-column-base-thermal-bridging-solution/

[Oz07]

Ok if aircrete I think you may be as well with 3.5n underground. Unless you need 7n for other reason 

[Ben100]

After some research I’m planning to go with the ‘140mm 3.6n Thermalite Shield Blocks’ as I come out of the ground below and above DPC. They seem like a good fit for what I need, thermal insulation, water resistant, lighter than the dense blocks, etc.

[dpmiller]

strong enough for expanding anchors and shot firing?

[Ben100]

2 hours ago, dpmiller said:

strong enough for expanding anchors and shot firing?

should be, but I’ve emailed the manufacturer to make sure.