Basement Wall with rigid insul in middle 1

[henryd]

I'm an Architect and I am questioning putting 2" of rigid insulation in the middle of a basement wall.

Is this a good detail for insulating a basement wall?
It seems to me that in areas where there is lateral pressure on the wall due to expansive soils this would not be good because it would make the wall weaker.

 

[Ltwine]

Two questions:

1. Is the wall reinforced?
2. What prevents you from putting foams against exterior wall face?

 

[dik]

Ltwine is correct. YOu would be better served by having the insulation on the outside face, bringing it above grade and parging the outside surface.

If you need it where shown, it will want to 'float' when you place concrete and you will require ties going through the insulation to connect the inner and outer concrete portions.

Dik

 

[JAE]

There is a system you can use with this detail where triangular composite rebar ties are used to link the two concrete sections together. The use of composite rebar reduces the thermal link through the insulation and the triangular shape create shear transfer between the two concrete sections to allow them to work together as a "thicker" wall than the sum of their parts.

However, this is not a typical system.

Some insulation is rigid enough (40 psi or 60 psi rated) such that it wouldn't be compressed by the lateral earth pressures, but without adequate shear ties, the wall is very weak in bending as you essentially have two, thin sections trying to do the work of one thick section.

 

[dicksewerrat]

There is a forming system that uses an insulation product for the forms. Insulates inside and outside.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[henryd]

Thanks for the replies...I always have put insulation on the face of the wall. But I am hearing of contractors using this detail and it is shown as an alternate system in basement insulation articles. So I was curious as to it's structural implications; thanks JAE you confirmed my thoughts:

"but without adequate shear ties, the wall is very weak in bending as you essentially have two, thin sections trying to do the work of one thick section"

So my next question would be if say this was a 10" thick wall with 2" of foam in the middle and adequate shear ties
will this wall function as a 10" wall without the middle insulation...

 

[Ltwine]

I think now you are likely having an eight inches wall the most, depending on how the shear tie works.

Now half of the wall is cold, half is warm, how to avoid coedensation?

 

[hokie66]

henryd,
The answer to your question is that if you can provide adequate webs across the cavity to make the two wythes act together, the wall would be almost as strong as a 10" solid wall. But achieving that linkage is not easy.

 

[JAE]

ltwine, that's why you have to use a composite tie vs. a steel tie. That helps minimize condensation issues.

I agree with hokie. - the composite ties we used on a previous project worked fine. But the system, as I said before, is not "typical" so you will struggle with contractors knowing how to construct it all properly, and inspectors that will have concerns since its not in the code.

 

[Ltwine]

Yes. I think I have difficulty to understand unless there is an example.

Puzzle #1 - Usually, bearing rarely controls in wall like this, however, just for the sake of argument, unless the foam (2") has the same bearing capacity as the wall material (8" total), how the sandwiched 10" wall has the same capacity as a solid 10" wall?

Puzzle #2 - Obviously the sandwiched wall relies heavily on the shear reinforcement with little, or no, contribution from the wall material, as general theories do not cover this type of application, we can only provide estimate/design through test results.

On top of difficulty in construction, any problem would be very difficult to correct. For this reason alone, I will stay away for now.

 

[JAE]

Puzzle #1 - I was thinking of a wall that has bearing AND lateral flexure due to earth or wind/seismic loads. So flexure enters in. Also, with taller walls, it isn't just bearing but rather axial compression coupled with lateral bending. With axial compression you have buckling to deal with if the wall gets fairly tall - thus the flexural stiffness of a solid 10" wall vs. the sandwich wall is similar and thus the axial compression capacity is similar.

Puzzle #2 - You are correct that a sandwich wall panel using some type of shear tie is not as common...but the concrete tilt panel industry uses these all the time. The mechanics of it aren't that difficult. What is the main challenge is understanding how the stiffness of the shear ties affects the flexural stiffness of the wall ties. Also knowing what that tie stiffness is can be tough to determine - requiring, as you suggest, testing.

 

[Ltwine]

JAE:

Thanks for the excellent explanations. But, I am still in doubt, and hesitate to place an equal sign between these two applications without further information as you have put into assumptions in making the comparison.

Of course, my doubt and hesitation are purely based on lack of understanding of the mechanics and its behaviors. And, I don't see it is a cost effective, or better, solution to conventional method.

But I do agree extensive testing can make up for the lack of understanding.

 

[ToadJones]

I would think having the insulation on the outside would also help mitigate water infiltration, somewhat.

 

[jike]

This does not seem like the right application for a below grade wall plus it will be more expensive than a conventional poured reinforced wall with insulation and waterproofing on the outside face. Above grade possibly; below grade no!

 

[structuresguy]

Everyone here seems to be assuming that this is a cast in place wall. The detail provided looks as if this is a tilt-up sandwich panel wall. If this is the case, this detail is very common in tilt-up construction. I can see no reason why it could not be used for a basement wall just as easily as an above grade wall.

 

[BAretired]

If the wall is cast in place, it would be difficult to prevent the insulation from being displaced by the pressure of the fluid concrete.

The detail has no obvious advantage over the normal practice of placing insulation on the exterior face of the wall after the concrete has cured for a while.

BA

 

[concretemasonry]

Two other choices are:

1. A conventional cast in place wall with drain tile around the footings and 2" extruded polystyrene on the exterior. This will give you a hard wall on the interior.

2. Insulating concrete forms (ICFs) with drain tile are also an alternate. For details you can go to the Reward site or any of the major sites that provide complete technical information and suggested details.

It all depends on the criteria, use and finish requirements.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.