How can I build an exposed concrete wall, on 2 sides!
Gabriel_Ladouceur
| Posted in General Questions on
Hi to all!
I’m working on a new construction project in Vermont, and one intention of the client is to have an exposed concrete finish on both the inside and the outside of the exterior walls.
I already know about sandwich panels, but I was wondering if we could acheive a decent detail by doing a single concrete interior wall, then addind insulation (rigid for exemple) and then adding another layer of poured concrete on top? some sort of a non-prefab sandwich wall then?
As the shapes of the project might not be conventionnal, i’m trying to find the best option, while also staying realistic?
Thanks for your advice!
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Replies
So the plan is to have a poured wall, with insulation in the middle of the of the wall? I don't see a problem here, as long as at least one side of the wall is thick enough on it's own to handle the structural needs of the assembly. A sheet of XPS should be fine pinned in place in the middle of a form, although I'd probably try to pour both sides at the same time to avoid the rigid foam bowing too much.
Bill
Thanks for that answer!
And yes, the plan was to go as you understood. One of my big concern was to reach a significant R value with that insert of rigid insultation... But I guess as soon as the exterior wall is strong enough on it's own, as you said, I can add as many layers or thickness as needed between the two sides. And as for the pouring, I find your suggestion very clean. Alright.
Thanks so much again!
Gabriel_Ladouceur,
The trick is detailing the walls so that the exposed ends of the insulation are covered by the frames of the door and windows, while still providing support for those elements.
Leave an oversized hole in your concrete form, then build a sort of window buck into the wall, and fasten the window into that. Seems easy enough to do, and you'd have the same outie/innie question you'd have when working with a deep double stud wall.
Bill
Bill,
The tricky bit is that a big part of the aesthetic of modernist houses with exposed concrete inside and out is often having concrete returns on the windows and doors so the walls look continuous - just interrupted by a narrow window or door frame, as in this one by Peter Zumthor.
Ah, aesthetic needs getting in the way. Being an engineer, I'm all about function (form is for the architecture guys :D)
Maybe set a PVC trim board against the form over the insulation at the edge, with the PVC board slightly wider than the insulation? If you route a notch into the edge of the board, and set the wide part of the notch away from the concrete form, you'd end up with a PVC board embedded into the concrete and locked (by that notch) into place. The only downside would be the potential issues with fasteners holding in that PVC board. The board would provide a mounting surface for a door frame.
BTW, are those concrete-formed recessed can lights in your pic? That must have been pretty tricky to do if they are.
Bill
"Ah, aesthetic needs getting in the way"
What other reason can you think of to use that assembly? It's not very practical.
Zumthor is a starchitect. If he designs can-lights in the concrete ceiling, they put can-lights in the concrete ceiling. It's the opposite with me. Clients do whatever they feel like with the drawings I do. I sometimes think of putting a note on them "For entertainment purposes only".
Not sure if you're avoiding this type of wall (is this what you mean by a "sandwich panel"?)--but this is an off the shelf system for concrete-both-sides cast in place walls that minimize thermal conductivity through the connectors. I have seen it with 4 inches of rigid foam within the wall, but I believe they have even thicker options.
THERMOMASS SYSTEM CIP
https://thermomass.com/products/system-cip/
Some thoughts and challenges -
Climate zone 6. Your final result will be continuous insulation. So you will be needing R20. Assuming R5/in you'll be needing around 4" of foam.
A "sandwich" of a structural concrete slice, 4" of foam filling, and a non-structural concrete slice. We'll assume the outer slice is structural.
The slices will probably be different thicknesses. What parameters control these thicknesses?
From a constructibility standpoint, is is possible to make the inner slice as little as 4" thick and still get good concrete placement? I doubt it. I think 6" is more likely the minimum, especially if the wall is more that 8' high. You'll also be running electrical conduit in that slice, which means more stuff to block good concrete placement. You're going to want a strong form with a high slump mix and vibration. This isn't a job for your typical concrete contractor who mostly does foundations. Getting a decent finish on exposed cast in place concrete is a real art.
In seismic zones you'd need to have a good 8" wall thickness to get the double layer of shear reinforcing to contain a core of concrete material. I don't know what the situation is in VT though. Maybe you can get away with a single layer of reinforcement? You'll have the same issue with finishing. So again, 6" is probably the bare minimum, but maybe 8".
Is it practical to do an 8' high wall in a single pour? I don't have the answer for that. But if you do it in 2 lifts then the detailing of the joint between the 2 lifts is a critical aesthetic issue. Check out your local public parking garage to see how ugly it can end up looking.
I don't see why a wall like this couldn't be done. But it will take careful and skillful work. If it is done in a single pour with the insulation already in the form you'll need to fill both sides evenly and simultaneously. The insulation will have to be restrained in such a way that there is no possibility of it floating up as you pour.
Do it right and I'm sure it will look great. But there is a lot of opportunity for mistakes here.
Another interesting approach, but perhaps too experimental, would be to cast the sandwich panels flat and tilt them up into place. You might then be able to make them thinner and use considerably less concrete. But you'd probably have a lot of thermal bridges in the final product.
Don't get me started on the environmental downsides of this kind of profligate use of concrete.
Along those lines, what if the outer wall was done conventionally, then clad with foam and then a cast panel was installed on the inside?
Casting a thin panel vertically, in place, would pose the same problems I described earlier. But I should think a precast interior panel, cast flat on the ground and tilted up, could be quite thin.
We'd need to work out how it is attached and also provide space for utilities, electrical conduits, etc. All that sounds achievable but would certainly require some thought and advance planning. This could be an interesting exercise.