After a lot of searching, I finally found a way/product that will thermally break a concrete slab balcony. Of course the solution was found by a foreign company. I couldn't find anything here in the USA that addressed the problem. The engineering was done by the German company Schock but here is the North American division of it:


Schoeck Canada

The Schöck Isokorb® type CM is a load bearing thermal insulation element for cantilever concrete slabs such as balconies. The element transfers bending moment stress and shear forces. The integrated hanging and perimeter tensile reinforcement, fitted as standard, saves the unnecessary and costly use of extra stirrups or hooped mat.

I can now get a good nights sleep. Next to the problem of Ash bore this has bothered me the most

Thanks Lbear! Looks like a good solution for cantilevered porches, and perhaps for brick ledges as well, both of which I am incorporating into my home, still in the planning stages. Though I haven't lost any sleep over it, I have spent time worrying about the thermal bridging in my current design. Therefore, I do agree this is an exciting find!

They do have a US sales office as well.
http://www.schock-us.com/

Are they using some special type of non-thermally conductive rebar with that thing?

Posted By arkie6 on 26 Jan 2013 07:28 PM
Are they using some special type of non-thermally conductive rebar with that thing?

I was trying to find out myself. Apparently the bar going through the form is of a different type of material; stainless steel (which has 70% less conductivity). They do state that the product provides a 90% reduction in thermal bridging so it is not 100% but it is viewed as a thermal break for the concrete.

Per the site:
"Compared to non-insulated connections, the Schöck Isokorb® element achieves a 90% reduction in thermal conductivity in the connection area for standard load-bearing scenarios."

"Due to thermal insulation and corrosion protection requirements, stainless steel is used around the gap. The stainless steel bar is extended by welding reinforced steel at both sides until the required length is achieved. This design is approved by several material test establishments and has been tried and tested for over 25 years. The flash butt welding method is used for the welded connection between stainless steel and reinforcement steel. The requirements for quality assurance of this black-and-white connection are very high and are met by internal quality control as well as third-party inspections."

On a different note, can one access the IBC for concrerte construction parameters online without paying? Trying to find out what the smallest dimension requirement is for a concrete beam. Along with other info.

I wonder if they have investigated using rock rebar (basalt) or fiberglass.  Both should conduct less than stainless steel.

Posted By Alton on 27 Jan 2013 03:20 PM
I wonder if they have investigated using rock rebar (basalt) or fiberglass.  Both should conduct less than stainless steel.

From what I have been told, it comes down to strength. In order to transfer shear loads, stainless steel bar welded to whatever length is required, is the only method that the engineers found to be able to withstand the required shear transfers. In non-load applications, the use of the other bar might be possible but when it comes to balconies, decks, etc., the only way to maintain and transfer those loads was by using SS bar.

Here is a detail of that step down thermal break design:




As you can see, this product provides the thermal break in the concrete balcony/deck all while allowing it to be poured and function as a monolithic slab design. It will transfer the shear loads from the inside to the outside balcony while stopping the thermal bridge.

And that's fine, unfortunately doesn't work with cantilevered slabs

Posted By Chris Johnson on 09 Mar 2013 06:49 PM
And that's fine, unfortunately doesn't work with cantilevered slabs

That is incorrect. They are designed to work with cantilevered slabs.

"The standard Schöck Isokorb® products can be used to connect the typical projecting components, such as cantilevered balconies, supported balconies, loggias, slab edges, and canopies. Balconies with live loads can be connected with a free cantilever length of up to 9.8424 ft (3 meters) with the standard products. Secondary components, such as canopies, can also be connected with significantly higher cantilever lengths. Our Schöck specialists would be happy to assist you."



Schöck Isokorb® type CM - for cantilever concrete slabs

Posted By Lbear on 09 Mar 2013 09:32 PM

Posted By Chris Johnson on 09 Mar 2013 06:49 PM
And that's fine, unfortunately doesn't work with cantilevered slabs

That is incorrect. They are designed to work with cantilevered slabs.

Schöck Isokorb® type CM - for cantilever concrete slabs

The Schöck Isokorb® type CM is a load bearing thermal insulation element for cantilever concrete slabs such as balconies. The element transfers bending moment stress and shear forces. The integrated hanging and perimeter tensile reinforcement, fitted as standard, saves the unnecessary and costly use of extra stirrups or hooped mat.

The CM version, yes, but not the other ones, which from what I see includes the step down one you just posted. I don't see how the step down version with that looped rebar can possible be rated for cantilever. Also, based on their illustration you really need to find out how far the cantilever can extend out. No problem for a balcony, but you're wanting to do a deck, right? The deck will have a whole more tension pulling on the rebars.

Are you familiar with bending moments, forces, torsion forces, etc.? If so do a simple calculation of how much the weight of the concrete in a cantilever beam form translates into tension on the rebars. I think you'll be shocked.

Lbear - with that much steel in it, I doubt that the conductivity will be much less then a straight concrete pour. Might be better to have an engineer design mini concrete beams and than pour the outside slab on that later. You would just have the X section of the beam transferring heat which should be significantly lower than have the whole slab exposed and connected.

Posted By dmaceld on 09 Mar 2013 11:19 PM

The CM version, yes, but not the other ones, which from what I see includes the step down one you just posted. I don't see how the step down version with that looped rebar can possible be rated for cantilever. Also, based on their illustration you really need to find out how far the cantilever can extend out. No problem for a balcony, but you're wanting to do a deck, right? The deck will have a whole more tension pulling on the rebars.

Are you familiar with bending moments, forces, torsion forces, etc.? If so do a simple calculation of how much the weight of the concrete in a cantilever beam form translates into tension on the rebars. I think you'll be shocked.

Sorry for the confusion. There are many different designs and the CM version is rated for a cantilevered deck (up to 9 feet). The other versions allow step downs and they even have versions for seismic conditions.

These products can thermally break a balcony slab and have been in use for over 30 years all over Europe. Until recently here in the U.S., nobody really thought about or cared about thermal breaks in balcony slabs.





For thermal breaks in cantilevered slabs (such as with balconies in high-rise residential buildings), Isokorb Kprovides an expanded polystyrene (EPS) layer of insulation--either 80 or 120 mm (3.1 or 4.7 in) thick--with carefully engineered steel re-bar passing through it to carry the loads. The Isokorb module includes both straight pieces of re-bar at the top that function in tension and specially bent re-bar that functions in compression to maintain the strength that is lost by substituting EPS for some of the concrete (see illustration showing loads and forces).

Special steel is used in Isokorb. Stainless steel 316TI is used both for its corrosion resistance and because it allows just a quarter as much heat flow as standard steel re-bar, according to Alexander Krenczik, the area manager at Schöck.

Here is a Building Science Article on it:

Building Science

Posted By FBBP on 09 Mar 2013 11:30 PM
Lbear - with that much steel in it, I doubt that the conductivity will be much less then a straight concrete pour. Might be better to have an engineer design mini concrete beams and than pour the outside slab on that later. You would just have the X section of the beam transferring heat which should be significantly lower than have the whole slab exposed and connected.

The stainless steel rebar in Schock reduces conductivity by 90% vs. standard rebar steel.

I've been researching case studies done in Europe and now in the USA and the product does work.

Cost wise, it is not that expensive for what it does. On a 6 foot wide balcony, you are looking at $450 for the product.

The stainless steel rebar in Schock reduces conductivity by 90% vs. standard rebar steel.

Thermal conductivity of steel: 43, Stainless Steel: 16

Of course the right alloy would also allow thinner sections, but that is even more true of other steel alloys. Maybe people should be using stainless steel nails and screws in the exterior of wood/foam houses.

Well seen as how they are somewhat local, I'm going to follow through see if the local engineer will approve it...and at what cost.

Cost wise, it is not that expensive for what it does. On a 6 foot wide balcony, you are looking at $450 for the product.

Just when you think everything has been invented....up pops another thing. As a commercial product, it will probably do well. It's too bad green residential building can't get traction like that.

The product thermally breaks the balcony slab and transfers the loads at the same time. While the product is just coming out to the US market, it has been in use for decades out in Europe.

It is a great solution for those who are using InsulDeck or other concrete balconies, both cantilevered and supported. The foam in the product thermally breaks the concrete slab from the outside/inside and insulates that transition area. The stainless steel rebar reduces any conductivity through the rebar.

I've searched for a solution on how to thermally break a balcony slab and nobody had a real solution. I called InsulDeck, LiteDeck and everyone in between and nobody had a viable solution. Schock just came into the US market back in 2011/2012 and they are starting to gain some ground. While mostly a commercial gain, it has its residential use also.

If they had used the product on that Chicago high rise as mentioned in the Building Science article, think about how much energy would be saved. You are talking hundreds of units losing heat through those balcony slabs over the next 100 years.

JonR, you can mock the product if you so wish but it's a green building solution. I'm just trying to get the word out so others can know about it.

I'm not mocking it, although the claims might be somewhat exaggerated. Thermal bridging is an issue and the typical cantilevered balcony IS like a fin on a heat sink.

I'd also look at a diagonal cable running from the outer edge of the balcony back to an anchor on the wall (ie, not a cantilever). Or use a corbel. Or cover the entire balcony with foam (perhaps with another layer of concrete over it).