ᐅ Waterproof concrete + flush-mounted electrical boxes in the basement – how can this be addressed?
Created on: 13 Jun 2017 02:22
D
DReffects
Good evening,
We have commissioned a single-family house with a basement from a developer. Three of the four basement rooms will be used as living space or offices.
The basement exterior walls are made of 24cm (9.5 inches) thick waterproof concrete. From the beginning, it was clear to me that in a new build, electrical and network outlets should naturally be installed flush-mounted. Surface-mounted trunking is absolutely unacceptable to me, especially in living areas.
We have discussed this several times with the developer and were always told, “No problem, there have never been any issues with this.” Now, however, the electrician has raised concerns, and the developer is now siding with him.
So, my question to the community: What is the actual risk here? I thought that either the concrete is waterproof or it isn’t.
Are there any guidelines on how deep a flush-mounted box should be installed or other practical experience you can share?
Surface-mounted installation is, unfortunately, a huge no-go for me — which is exactly why we chose a new build, so that my office rooms don’t have to show something like that.
There are also decentralized ventilation units in the basement rooms that require electrical connections embedded in the concrete.
What options are available for this?
Thank you very much!
DR
We have commissioned a single-family house with a basement from a developer. Three of the four basement rooms will be used as living space or offices.
The basement exterior walls are made of 24cm (9.5 inches) thick waterproof concrete. From the beginning, it was clear to me that in a new build, electrical and network outlets should naturally be installed flush-mounted. Surface-mounted trunking is absolutely unacceptable to me, especially in living areas.
We have discussed this several times with the developer and were always told, “No problem, there have never been any issues with this.” Now, however, the electrician has raised concerns, and the developer is now siding with him.
So, my question to the community: What is the actual risk here? I thought that either the concrete is waterproof or it isn’t.
Are there any guidelines on how deep a flush-mounted box should be installed or other practical experience you can share?
Surface-mounted installation is, unfortunately, a huge no-go for me — which is exactly why we chose a new build, so that my office rooms don’t have to show something like that.
There are also decentralized ventilation units in the basement rooms that require electrical connections embedded in the concrete.
What options are available for this?
Thank you very much!
DR
Diffusion-tight? Wow, I’ve already read several times that a waterproof concrete basement releases a significant amount of moisture inward. That’s exactly why we were required to install a decentralized ventilator in each room.
You mean just adding a layer of bricks in front? I think 6cm (2.4 inches) thickness would be good, as that would be ideal for 45mm (1.8 inches) flush-mounted boxes. But how do you connect that layer to the concrete wall at the back?
Alex85 schrieb:
It’s hard for air to “breathe” against the ground anyway.
I think it makes sense to build the interior walls with masonry. Then, in the worst case, you have two concrete walls in the room that cannot—or should not—be equipped with flush-mounted boxes without special solutions (such as surface-mounted systems).
You mean just adding a layer of bricks in front? I think 6cm (2.4 inches) thickness would be good, as that would be ideal for 45mm (1.8 inches) flush-mounted boxes. But how do you connect that layer to the concrete wall at the back?
C
Caspar202013 Jun 2017 22:0377.willo schrieb:
Waterproof concrete basements with this wall thickness are diffusion-tight.Yes, they are. However,
In waterproof concrete structures, diffusion slows down over time in the drying zone and practically comes to a halt after the release of internal moisture [5], [12]. Also, in [14], even under constant water pressure (e.g., basements below groundwater level), no cases of elevated air humidity were reported.
In other words, with cast-in-place concrete it takes some time for the wall to dry through to the core.
@DReffects
There is a fairly detailed technical datasheet on this topic called "Waterproof Concrete Structures"—try searching for it.
And that’s why even shallow floor drains are not a good idea. You quickly drop below the mandatory 200mm (8 inches). All it takes is forgetting to remove the centering drill before installing the sleeve...
DReffects schrieb:
I am already assuming that the basement walls are plastered in the usual wayIs that just an assumption or part of the construction specification?
DReffects schrieb:
Vapor-proof? Oh, I’ve read several times that a waterproof concrete basement actually releases a lot of moisture inward. That’s why we were required to install a decentralized ventilation unit in each room.
You mean just a layer of brick in front of it? I’d say 6cm (2.5 inches) thick would be good, that would be ideal for 45mm (1.75 inches) flush-mounted boxes. But how do you connect it from the back to the concrete wall?No, I mean that only the exterior basement walls are made of concrete (precast or cast-in-place, doesn’t matter), and the interior walls are built as masonry. Then you run conduits normally flush-mounted inside the interior walls and avoid the hassle with added wall claddings, etc.
Caspar2020 schrieb:
Yes, they are. However,
with cast-in-place concrete, it takes some time for the wall to dry all the way through to the core.
There’s quite a detailed information sheet on the topic of "Waterproof Concrete Structures"—just search for it.
That’s why even shallow flush-mounted boxes are not a good idea, since you quickly fall below the mandatory 200mm (8 inches) thickness. It only takes forgetting to remove the centering drill bit before inserting the box...
Is that an assumption or part of the scope of work? Part of the scope of work: "Concrete ceilings and walls in the basement will not be plastered (except basements 1, 2, and 3, which will also be plastered)."
I’m trying to understand the information sheet. I previously spoke with another contractor who told me that the waterproof concrete is completely sealed after about 3cm (1¼ inches), since no water should penetrate the inner layer, which is apparently the load-bearing part. So, from a water perspective, he didn’t see a problem. However, if the installation of the flush-mounted boxes isn’t done very professionally, there could be issues, because using an angle grinder or chisel can damage the “structure of the wall.”
To be honest, I’m completely confused now.
Alex85 schrieb:
No, I mean that only the exterior basement walls are made of concrete (whether precast or cast-in-place, doesn’t matter), and the interior walls are then masonry. Then you install the cables normally in flush-mounted boxes inside the interior walls, avoiding the hassle with drywall installations, etc. Oh, that’s how we did it too. It’s just quite unprofessional when you end up having cables running across the office in a new build—and so there remain several boxes that really should just be in the exterior walls...
A brief update on this matter.
I have now spoken with:
- the site manager
- another construction company I know
- the Homeowners’ Protection Association
Results:
Site manager:
Due to the reduced thickness of the reinforced concrete wall caused by the electrical boxes, the watertight concrete shell may be compromised.
So far, I have not had any negative experiences in this regard.
(This strikes him as a late realization – until now, he always said this would not cause any issues.)
Construction company I know:
A watertight concrete shell (“white tank”) is waterproof after about 3cm (1¼ inches) thickness – so I have nothing to worry about there. However, he believes the overall wall structure is affected by the cutouts. Normally, electrical boxes and conduits are cast into the concrete during pouring. (We explicitly did not do this, following the site manager’s advice; the electrician actually wanted to do it that way…)
Homeowners’ Protection Association:
“We strongly advise against installing electrical components behind the plaster on external walls made of waterproof concrete (WU concrete) after the fact. The reasons are the weakening of the concrete thickness of the external wall below the required level, and the risk of cracks developing in the concrete due to subsequent chipping and drilling work. Both factors can impair the watertightness of the concrete.”
“The possibility of using a secondary wall structure to create an installation space depends, among other things, on ground conditions and the type of load from external water pressure or soil moisture.”
“The installation space should, in any case, be ventilated. The same applies to connecting room ventilators as it does for electrical installations added later on external walls in general.”
Now, regardless of the electrical boxes, I face the problem that the decentralized fans have already been cast in, but their electrical connection has not. This *must* be done by chipping and drilling unless they rebuild the house.
What do you recommend? How can I ensure liability protection for myself here? The site manager has changed his position just one week before the electrical installation, due to the electrician’s skepticism (!). However, for the decentralized fans, he will have no other option than to chip into the waterproof concrete.
For the electrical outlets, I am reluctantly leaning towards installing a baseboard trunking system but am extremely frustrated, especially since this would not have been a problem with proper planning.
I have now spoken with:
- the site manager
- another construction company I know
- the Homeowners’ Protection Association
Results:
Site manager:
Due to the reduced thickness of the reinforced concrete wall caused by the electrical boxes, the watertight concrete shell may be compromised.
So far, I have not had any negative experiences in this regard.
(This strikes him as a late realization – until now, he always said this would not cause any issues.)
Construction company I know:
A watertight concrete shell (“white tank”) is waterproof after about 3cm (1¼ inches) thickness – so I have nothing to worry about there. However, he believes the overall wall structure is affected by the cutouts. Normally, electrical boxes and conduits are cast into the concrete during pouring. (We explicitly did not do this, following the site manager’s advice; the electrician actually wanted to do it that way…)
Homeowners’ Protection Association:
“We strongly advise against installing electrical components behind the plaster on external walls made of waterproof concrete (WU concrete) after the fact. The reasons are the weakening of the concrete thickness of the external wall below the required level, and the risk of cracks developing in the concrete due to subsequent chipping and drilling work. Both factors can impair the watertightness of the concrete.”
“The possibility of using a secondary wall structure to create an installation space depends, among other things, on ground conditions and the type of load from external water pressure or soil moisture.”
“The installation space should, in any case, be ventilated. The same applies to connecting room ventilators as it does for electrical installations added later on external walls in general.”
Now, regardless of the electrical boxes, I face the problem that the decentralized fans have already been cast in, but their electrical connection has not. This *must* be done by chipping and drilling unless they rebuild the house.
What do you recommend? How can I ensure liability protection for myself here? The site manager has changed his position just one week before the electrical installation, due to the electrician’s skepticism (!). However, for the decentralized fans, he will have no other option than to chip into the waterproof concrete.
For the electrical outlets, I am reluctantly leaning towards installing a baseboard trunking system but am extremely frustrated, especially since this would not have been a problem with proper planning.
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