Cantilevered element - cold bridging

[divorcingjack]

Has anyone successfully managed to engineer a cantilevered element into their timber frame build? 

 

We have a design feature of a cantilevered "pod" section to the house - about 750mm wide, which brings extra light into both floors with a full height window and provides a section of seating area (downstairs) and a bath location (upstairs). The reason it was originally cantilevered was that it sits next to a neighbours dug-down flat (about 2.4m below our current site level), and this allowed the foundation to avoid loading the neighbours basement wall. 

 

However, I've not been able to find a detail that allows for a cantilever (balcony/structural element) in a passive house. If we have to bring it to ground, it potentially introduces a world of problems with their neighbours basement. Our engineer (not MBC) is now not keen on the passive slab and wants standard foundations. Obviously, I don't!

 

Any suggestions? I have attached the plans FYI. The "pod" is on the east side of the building, adjoining the kitchen. 

PL004-910139-1.pdf

PL003-910138-1.pdf

[ProDave]

If I am understanding it correctly, all the cantilevered bit is contained within the walls of the building so within the insulated envelope of the building. the only bit needing any careful attention being the "floor" of the pod.

 

I think the cold bridging problems start when you are talking of cantilevering an external structure such as the floor of an open balcony.
 

[tonyshouse]

If it is a heated area then the insulation barrier should consistently run all round, over and under it.

[IanR]

I assume the steels are close to the underside of the floating element. If so, could you not drop the underside a little further and provide space for wrapping insulation underneath?

 

You may lose some of the "floating" aesthetic, but then one option you're considering it taking it down to floor.

[TerryE]

Yup, the floating element is purely structural to move your footings clear of the neighbour's.  And I've even question that -- more a dead area to collect weeds, crap and undesirable creatures!  Have a word with the architect.  There's no reason why you can't pack the void with EPS and take the skin (and the floor) down to G/L.  Just take lots of photographic evidence during construction.

 

As to the bridging, this is going to a matter of proper insulation of all of the exterior facing steel beam surfaces.  The conductivity of steel is frightening and in relative terms will be virtually a perfect conductor of internal heat out to the surface.  So what is critical here is that the exterior faces of the steel (that is on the outer sides) is wrapped with a decent insulation.  Just do the basic calcs or ask your architect to do them and post them here or email me and I can double check them.  If you have 100mm min insulation then you might get away with PUR but any less and I think that you will need aerogel or something of equivalent performance.

 

Also be very aware of the potential cold bridging of any vertical steel into the foundation.  The last thing that you want from a thermal perspective is to foot these in the foundation as this will suck the heat out of the house into the ground table.  The only steel that you should need in this build is the U section at first floor level carrying the cantilever.

 

There's nothing wrong in principle with a conventional foundation with beam floor -- you can just use the foam infill technology. @RandAbuildhas just done one of these and maybe he can comment.  But you will need something like a Perinsul Foamglass thermal break. 

Edited August 27, 2016 by TerryE

[PeterW]

Another option for you would be to suspend the lower section from the upper steels. 

 

Increasing the steel size would allow you to "hang" the lower portion using a UC section in tension. This is sometimes used for commercial buildings and needs the connection detail clearly designed by an engineer. Best suited to lightweight building methods but is an option if you are using timber frame and render. Still needs the insulation details done properly.  

[RandAbuild]

Given that it's a relatively short overhang, can't you use a cantilevered timber construction? It would reduce the likelihood of cold bridging and make insulating it a lot easier. Treat it like a balcony?

 

[TerryE]

I was thinking along the lines of Peter, and Richard does have a point. The cantilever is only 800mm or so and this should be doable with racked timber and Glulams or even ecoJoists / PosiJoists.  Does your architect have much experience of designing passive constructions?  Steel is a wonderful construction material, but also has a conductivity roughly 300 × construction grade timber.