Design Ideas – Structural Walls – Revised

3.2 Structural Walls

This was written before the hurricane came through the Keys. We still intend to build and we think the only way to build is with the ICF blocks, described below.

Where do you start! If you are scared of hurricanes as we are, we decided on a concrete structure. What type of concrete structure? Poured in place, tilt up panels, concrete block? I was first concerned by the cost. After speaking to contractors and architects I am concerned about what processes are available in the area we are building. Poured in place walls require forms to be used. Who is using forms in the area? Who is familiar with the process? It may not be available in the area we are building. Some contractors know what they know, and nothing else. In a remote area there may be limited choices to what you can get.

I have not had any luck with contractors who use tilt-up concrete walls, pre-fabricated walls, or poured-in-place concrete walls either.

The typical construction method is to use concrete block, with rebar. Some methods I have heard of call for only the cells with rebar to be filled with concrete. I don’t know if this method is still used or have the building codes been changed to disallow this. The safest method is to use concrete block, rebar and fill all cells with concrete.

This makes the structure stronger and heavier. All parts of the structure need to be designed for the additional weight. That makes the price go up. What finish should we use on the outside? I think I prefer a stucco finish. Hopefully waterproof and easily repaired. How do you clean a raw concrete block wall? Do you waterproof the concrete block? What process is used for applying stucco? What color, what texture?

I am getting closer to making ICF blocks our choice. They are rigid foam on two sides (similar to Legos) with concrete and rebar added between the walls after the forms are set.  The concern I have is that, within a hurricane zone, how do you finish the outside wall? The ICF blocks are lightweight, easy to cut and set up, and have built in insulation. But, the outside facing wall is still foam! How do you finish a foam outside wall? There are finishes that can be used but do they protect the house? If damaged can it be repaired?

How do you compare the “R-value” (insulation value) of the different manufacturers’ ICF forms? The “effective R values” aren’t consistently derived from one manufacturer to the next, and are climate-, site-, and project-specific. For comparison purposes between not only ICFs but all wall systems, steady-state R-values should be the prize number, and mass effect considered as a subsequent part of the equation. I have tried to find this number across a few of the major ICF companies online, without success.

Concrete block is another choice. It is easy to work with, familiar to most locals, and the labor force does not have to be highly experienced. But, not all concrete blocks are made the same. What PSI strength are they created to meet? In a hurricane area is a higher PSI block necessary, affordable or available?  What size block do you use 4” deep, 8” deep? Are all concrete blocks filled with concrete/mortar? ICF forms require it. What type is used? What is the strength? What size of aggregate do you use or is it more of a mortar mix? How do you make sure all cells are filled and worked around the rebar?

In an area of high salt content the use of rebar is problematic. Does it sit out in the rain and salt air, and begin to rust? Do you get zinc coated rebar? Zinc will take the corrosion before the steel will. Other coatings? Is it cost effective? Do you need any coating for rebar within concrete block? Talk to the engineer/architect I guess. For the concrete fill, what PSI do you use? Is using anti-spall additives worthwhile? I read that it is best to treat the steel rebar directly with corroseal, it adds 50+ years for spalling prevention.

I hope to install a concrete depression on the edge of the concrete slab so blocks are a little lower than the floors. This is to help prevent wind driven rain from pushing into the interior of the house.