Foundation details (MBC twinwall) - can I do better?

[Dunc]

Architect's current foundation detial.

Can I do better in terms of thermal bridging?

 

 

Although the actual product is not specified, Forterra's thermalite trench blocks are 0.24 W/m.k below ground (i.e. when wet). A dry aircrete block seems to be closer to 0.17-0.18 W/m.k but obviously needs to be kept dry.

I've trawled previous threads but haven't found a foundation detial for the MBC twinwall on a non-raft foundation. @JohnMo's ICF detail seems helpful so I'm wondering if something like this is possible and worth the effort?

@Kelvin I belive you used Mannok's high strength 7 - presumably you need to keep that dry? Were they the full 300mm width?

 

Would I be causing problems for lift& slide door threasholds later?

[IanR]

Has anyone calculated the psi values at thresholds, especially for large sliders or bi-folds?

 

22 minutes ago, Dunc said:

non-raft foundation.

 

Why are you discounting an insulated raft? It would perform "better", but by how much is unknown without the calcs for what is proposed.

[Dunc]

I would love to use a raft but can't find anyone willing or with experince to install one up here (NE Scotland). I'm not brave enough to DIY it!

[Russell griffiths]

I would also look into the hold down fixings, unless it’s on a different drawing, those spit fired nails will not be adequate into trench blocks. 
needs a hold down strap bolted down and secured to some of the studs. 

[saveasteading]

It's not a bad arrangement, thinking diminishing returns.

If thermalites are so poor when wet, can you keep them dry? Dpc, and bitumem paint? Black will look better too.

The thermalites might break along the fixings line, or locally. Can you get the fixings more central, and perhaps not used impact/expansion mechanism?

 

To my costings, insulated rafts are much more expensive. Other people like them.

[Kelvin]

Yes 300mm. I also rendered them using a specialist Weber product for insulated blocks. Be mindful that I had a nightmare trying to get them shipped to Scotland. Most of the block makers do something similar and none of them are stocked in Scotland, or at least weren’t 18 months ago. Therefore before you commit to that kind of foundation design look into how easy it will be to get them. 

Edited August 9 by Kelvin

[Dan F]

3 hours ago, Dunc said:

I would love to use a raft but can't find anyone willing or with experince to install one up here (NE Scotland). I'm not brave enough to DIY it!

Do MBC no longer offer this as part of package?

[Dunc]

Thanks for the thoughts!

 

@saveasteading keeping the blocks dry was exactly the point of trying to find an improved design. Mannok have a variety of examples (for traditional cavity wall block construction) on which I based my sketch.

@Dan F they do, but were very reluctant to send the groundworks team this far and this was reflected in the eyewatering quote. Odd since they've no problem doing the TF bit...

@Kelvin will look into the Weber render. Yes, I noted your issues in sourcing I'm looking in to that.

@Russell griffiths Duly noted. This is the architect's drawing; hasn't been through the Engineer's hands yet.

[JohnMo]

12 minutes ago, Dunc said:

keeping the blocks dry was exactly the point of trying to find an improved design

If the thermolite blocks are inside and the DPC goes under and up and joins the DPC they will be dry. So as drawn with the thermolite inside.

 

Sourcing in Scotland is not easy. I got mine via Insulation Hub, only place I could find at the time, that didn't want a small fortune for shipping

[Kelvin]

My issue was I couldn’t get anyone to deliver at all as the order was too small at 6 pallets. 

Edited August 9 by Kelvin

[saveasteading]

The bottom 2 blocks in standard concrete wouldn't lose any significantly  more heat. Or you can put 25mm of insulation down the inner face if desired. I am guessing that you can expect £5 a year maximum energy saving, at a cost of a few hundred.???

Personally I would do it all in standard block, for durability, a better fixing of the timber and longevity. Painting in bitumen* is cheap and easy. Only 100mm is visible and nobody is looking. It won't  go green with algae either and can be retouched in 10 years if you think it needs it.

I've done this probably  200 times, and in brick about 10. The black block looks the better after a few years.

 

* one coat. another over the mortar joints, then a second full coat.

[Thorfun]

Do you need 300mm of PIR above the slab? Seems excessive. Most folk (us included) were happy with 300mm EPS for our insulated slabs. Could save quite a bit of money. 

[Dunc]

On 09/08/2024 at 18:37, Thorfun said:

Do you need 300mm of PIR above the slab? Seems excessive. Most folk (us included) were happy with 300mm EPS for our insulated slabs. Could save quite a bit of money. 

I suspect this is a typo from the architect; it's already on my list of things to check, but thanks for pointing it out!

[Dave Jones]

here is marmox detail for shed build.

 

 

[Dunc]

thank you @Dave Jones. Not sure I can use Marmox thermoblock as these require the sole plate to be at lest the same width as the block (100mm seems to be the narrowest availalbe). The MBC frame has 89mm verticals.