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Structural thermal break?

chm@

I am trying to improve a detail from my school portfolio. The building involves concrete construction and has a raised floor which rests on ground beams.I have produced some rough sketches and would like your opinion.

I want to incorporate a thermal break block within the footing. Example of this product: http://www.marmox.co.uk/products/thermoblock

Option 1

There is a concrete column resting on the footing, so I want to connect the rebar of the concrete footing with that of the column. If a thermal block is to be incorporated, how can the rebar run through? I am not familiar with strength of thermal blocks, so I am concerned that this can also cause structural failure. I have seen examples of reinforced mansonry where rebar runs through, but I don't think that could be achieved with a thermal block.


However, I have found this product which is a structural thermal break and I was wondering if anyone has used it before:

http://www.contechaccessories.com/our-solutions/thermal-breaks/sch%C3%B6ck-isokorb-structural-thermal-break

Option 2

Option 2 would be to use an insulated double wall system for the footing (a sandwich of two precast walls with a rebar cage in between them, filled with sprayed insulation instead of poured concrete), as shown in the image above. I am also skeptical of the loadbearing capacity of a twin wall, as also in this case loads from the column above would still need to be transferred safely to the foundation. 


Please let me know what you think.


 
Aug 8, 19 7:36 pm

1 Featured Comment

All 7 Comments

Non Sequitur

instead of relying on one particular manufacturer’s proprietary product, just design the wall with continuous exterior insulation. No need for fancy pants foam blocks interrupting a more important building feature: structure. 



Aug 8, 19 8:21 pm
chm@

Continuous insulation on the exterior of the footing, around and beneath the ground beam?

Non Sequitur

why do you need CI under the ground beam?

chm@

I 've seen it in pad foundation details. Would you use a thin thermal break (like Neoprene) between the footing and ground beam instead?

chm@

*sorry raft foundation, not pad.

Non Sequitur

No. Depending on soil cover, you can normally deal with this by extending rigid insulation horizontally beyond the exterior face of foundation wall. You probably want to design some sort of thermal gap as a bond-break between the slab and foundation wall although hard to tell since the rest of the detail's condition are unknown.

chm@

How would you resolve the thermal bridge between foundation wall and concrete column resting on top of it though?

Non Sequitur

You don’t need to. The integrity of the structure takes precedence over a minimal thermal leak.

chm@

I posted a revised detail. Let me know what you think.

mightyaa

NS is right. You do the continuous from the wall above, and continue it down the foundation perimeter to below the frost line. So if you have a shallow foundation like you’ve shown, you dig and bury rigid at a 45-degree angle until the frost line below your foundation. The theory is the earth temp is stable once you get below the frost depth (45-50 degrees F). So the cold/hot condition and major heat loss is from the frost depth up at the separation from exterior to heated spaces. The foundation is easy. That proprietary product is more for a balcony; which is a pain to detail because you’ll have to support it structurally and bridge the thermal boundary.

Non Sequitur

Cheers Mighty.

Koww

are there no professors at your school?

Aug 8, 19 8:48 pm
chm@

Currently on a summer break.

curtkram

HALFEN. it always gets VEed out, but it's awesome and they keep trying to sell me on it.

Aug 8, 19 9:14 pm
chm@

Good detail, but there is still a thermal bridge between foundation wall and concrete column (see my 1st image).

chm@

Option 3

There is still a thermal bridge between foundation wall and column.

Do you think that the rigid insulation block on the inner face of the foundation wall could be omitted?

Aug 9, 19 7:56 am
Non Sequitur

there is no longer a TB since you've provided continuous insulation. Also worth noting that this is a conc column, not a several m long wall. The TB is negligible and I don't think any of the interior u/s insulation is required. This is so complicated a detail that you would get laughed off the construction site. If you're really worried about it, increase the depth of exterior insulation and make it equal to your wall's U value.

chm@

I put a thin layer of insulation below the screed as there is an underfloor heating pipe system. The thickness of this insulation layer is not enough, so I provided a 300mm insulation layer underneath.

In regards to the vertical insulation block in the inner face of the foundation wall, I was concerned that significant thermal bridging can occur from the footing into the ground beam and underside of slab, since that is quite a large mass of concrete.


Non Sequitur

I think you're massively over-thinking this. We have very strict thermal performance codes in my area (-40c to +40c temp delta) and even they recognize that TB will occur thus you're able to have a 200mm vertical TB at the slab/top of foundation without affecting your conformance to the energy codes.

chm@

Another sketch explaining why I think there is need for the interior insulation. The underside of the slab is unprotected and the insulation provided below the screed is not enough to provide the U value required. Cold bridges originating from the ground beams through the foundation walls and to the underside of slab are reduced this way.

It is quite complex though.

Aug 9, 19 9:37 am
Non Sequitur

it's only complex because you've made it complex. It's a simple detail actually, just brush-up on your building construction fundamentals.

SneakyPete

Non, be nicer. This is a great discussion.

Non Sequitur

I've been nice up until now. Shesss, tough crowd.  I'v even solved his issue in an earlier comment.

chm@

Hi Non, thanks for your input. I believe you are right I have been overthinking this. It's probably better to keep the detail simple for constructibility purposes rather than overdesign for the benefit of minimal thermal gains.

Featured Comment
bowling_ball

chm@ don't get discouraged. I work in a very cold area of the world and it's not always easy designing details that are reasonable to build, AND provide a good thermal break. Compared to where I practice, Non Sequitur works in a relatively warmer climate so it's easy for him to brush off this question, but at the same time, he's not wrong. There are plenty of starting points that exist for you to reference - passivhaus for example.

Non Sequitur

I get your point bowling, but I still have -40c temps to deal with too. 8-)

t a z

Trace your thermal line.  Also, don't forget the WP and subgrade drainage.

https://www.buildingscience.com/documents/information-sheets/basement-insulation

PS Your structural thermal break product is for CIP concrete balconies.

Aug 9, 19 9:49 am

This is good advice on all accounts.

If you’re in a cold enough climate where that type of thermal bridge is going to be an issue, your footing isn’t deep enough. Get that sucker below the frost line.

Aug 9, 19 11:10 am
chm@

I have a deep footing actually, the last sketch was not showing the entire structure. My apologies.

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