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Can I install 2"-3" of concrete in a 6" exterior wall?

The exterior walls on the house are 6" wood frame. I want to insulate the walls either with 2" or 3" of insulation and leave enough space for 2 1/2" to 3 1/2" of concrete. The concrete will only be poured to about 6' above the floor with the taller remaining space on the 9' walls to be 5 1/2" of blown insulation. Can I sandwich the cement between 1 1/2" sheets of extruded polystyrene? Will moisture weep on the inside surface of the cement in cold weather?

Wayne4

Asked by Wayne McConnell
Posted Wed, 02/05/2014 - 23:10

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7 Answers

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1.
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Wayne,
You don't explain why you want to install concrete between your studs. I'm guessing that you think there will be a benefit to having thermal mass in your walls, but perhaps you are worried that foreign agents will try to assassinate you by firing high-caliber weapons through your wall.

If you think that there will be a benefit to including thermal mass in your walls, I advise you to read this article: All About Thermal Mass. It's fair to say that you will get far more benefit from filling the space between your studs with insulation than from filling the space between your studs with concrete.

There are several problems with your plan, including the fact that fresh concrete bleeds water, and that the rigid foam forms will bulge unless they are properly braced. Concrete is dense.

The most common way to achieve the result you seek is to build your walls with insulated concrete forms (ICFs). If you really want concrete in your walls, I suggest that you plan to use ICFs.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/06/2014 - 06:52

2.
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OK, I have 6" wood frame exterior walls and wear an aluminum foil hat. Zombies with .30'.30's are a question mark with only vinyl siding, sheathing, and blown insulation on the outside walls of the house. Could one use a 2" cement interior layer in the exterior wall either between two layers of insulation or in front of/behind the insulation? With a moisture proof barrier facing against the cement on each side, humid air could not get to the cement to condense. I assume the concrete itself will bleed water diminishingly over time but don't know it that will be a problem in the short term. Hot humid interior air coming in contact with the colder cement panel in the wall could/would? be averted with 3" air/vapor/moisture insulation between the cement and moist interior air? Granted the R factor of the insulation would drop without the additional 2". Granted the cement would not be a source of thermal mass heating being on the outside of the insulation. But if one wanted to protect the house from rifle shots, could it work?

Answered by Wayne McConnell
Posted Thu, 02/06/2014 - 17:54

3.
Helpful? 1

Wayne, you could install steel plate cladding. This is sometimes done with weathering steel, for the bright rusty color. Weathering steel (used to be called Cor-Ten) is an alloy (A606) with a small amount of copper, which forms a tighter grain of oxidation so it doesn't erode too quickly. Sometimes people use ordinary carbon steel, relatively inexpensive and widely available. You'd need to do some research into ballistics to see what thickness is needed to stop a rifle bullet. Then figure out how you can attach that much weight to your walls.

Answered by TJ Elder
Posted Thu, 02/06/2014 - 18:06

4.
Helpful? 0

Wayne,
Needless to say, I'm surprised that my joke about stopping bullets turned out to be an accurate guess.

I don't think you need green building advice. Somewhere out there on the Internet, there must be a few web forums for people who are afraid that someone will fire bullets through their walls. This isn't one of them.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/07/2014 - 08:04
Edited Fri, 02/07/2014 - 08:05.

5.
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Actually I teach Sunday School, have served on our local school board for years, am a member of the Triple Nine Society, and do a lot of volunteer work. The question didn't address apocalypse probabilities but the feasibility of a practical application of cement in an exterior wall. Irregardless of the motive behind the question, the question remains: With 3 1/2" of either XPS, EPS, or blown cellulose and a sealed vapor barrier between the concrete and the insulation and interior wall, (given the loss of R value in the reduction of the thickness of the insulation and added time and material), would it work?

Answered by Wayne McConnell
Posted Fri, 02/07/2014 - 15:34

6.
Helpful? 0

Wayne,
The reason that I hesitate to answer your question is that you are looking for a way to stop .30-30 ammunition, and I'm not an expert in ballistics. So I can't tell you whether your plan will work.

Your main challenge will be the massive bracing needed to support the rigid foam when the wet concrete is introduced between the foam forms up to a height of 6 feet. Without proper planning, you will get bulging, blow-outs, and concrete all over your floor.

This plan will noticeably reduce the R-value of your walls, because you will be creating a thermal bridge. That said, I don't think you'll have condensation problems -- just high energy bills.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/07/2014 - 15:45
Edited Fri, 02/07/2014 - 15:47.

7.
Helpful? 0

Install a layer of EPS foam as a thermal break between the studs and the concrete. Pour reinforced wall sections in forms on the floor next to the walls, then stand them up and fasten them to the studs with heavy duty lag bolts. You'll have to figure out what thickness and density of concrete you need, and make provisions for the floor to hold the additional weight. A structural engineer can help you design this and to design the proper fasting scheduling.

Answered by Bob Irving
Posted Fri, 02/07/2014 - 17:20

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