I’ve noticed a lot of Styrofoam houses going up around here. From an engineer’s perspective what is the scoop on these?
Frank M.; Snohomish, WA
By Styrofoam, I assume you mean Insulated Concrete Form (ICF) homes. In a word, I like them very much.
Here’s the scoop. Most ICF’s resemble hollow Styrofoam (expanded polystyrene, actually) LEGO blocks. You build walls with them, put rebar in the hollow space in the middle, then pour full of concrete. The blocks stay in place permanently, providing excellent insulation. Most brands have plastic or sheet metal tabs embedded in the forms at regular intervals — typically eight-inches on-center — to receive sheetrock and siding nails or screws. These tabs or ‘webs’ extend through the hollow core also serving to hold rebar in place.
The finished product is essentially a reinforced concrete wall with an integral insulation cladding on both sides — very slick system.
I’ve engineered dozens of these, and in fact am building a portion of my own home using ICF's. Following are the advantages and disadvantages as I see them.
Reinforced concrete walls are incredibly strong, both in terms of lateral (wind and earthquake) resistance and gravity load resistance (holding the weight of floors, roofs, beams, etc.).
They are fast to construct. An experienced ICF contractor can have standing insulated walls ready for gyp and siding every bit as fast as a good wood framer, perhaps faster. There is no separate step for insulating nor for plywood/OSB sheathing.
They are soundproof. I like a quiet house, and you can’t get much quieter than solid concrete.
They are well insulated. Most ICF walls are R-30 or better, depending on the thickness.
They are fireproof. Concrete doesn’t burn and neither does polystyrene (it melts).
They are great for basements. Using ICF, you don’t have to frame an additional wood wall on the inside. ICF is waterproof (using manufacturer’s recommendations) and is suitable to receive sheetrock directly.
You get the advantages of concrete without the mess of forms. There are no heavy forms to lug around and none to scrape and oil. Even the slightest-built workers can whip this stuff around.
Rebar is held in the right place. The engineer in me particularly likes this feature. Sometimes rebar needs to be placed toward the tension face of the wall; sometimes in the center. In either case, ICF’s integral web holds the rebar positively where the engineer specifies it — as opposed to normal formed walls, where rebar flops all over the place, rarely winding up where the engineer specified.
(Note: although the following list seems long, many issues will go away or become minimal after becoming proficient with ICF)
In my neck of the woods, an ICF home is generally about 5% more expensive than a similar stick-framed house.
Plumbing and electrical are routed in a burned-out groove in the Styrofoam, which is relatively simple and easy to do. However, if you forget something and have to penetrate a wall later, coring it is an expensive hassle. Also, large diameter sewer pipes don’t fit in the Styrofoam, adding a complication.
You need to know what you're doing. Concrete is not very forgiving of mistakes. All builders will make a few mistakes with ICF at first, though some brands offer training to minimize this.
There can be blowouts because wet concrete is heavy, particularly when poured several feet high. Some ICF brands appear to be more prone to blowouts than others.
If you precisely follow the ICF manufacturer’s recommendations, you shouldn’t have problems keeping walls true and plumb. However, if you cut corners, watch out. Some brands even recommend proprietary shoring/bracing/scaffold systems.
If you follow manufacturers' recommendations to the tee, you should be fine with door and window penetrations. Most contractors I’ve spoken too, however, have problems on their first few.
On structural connection to horizontal diaphragms — because concrete walls are heavy, they are more affected by earthquakes than wood. As shear walls, ICF's are about five times stronger than wood, but in the out-of-plane direction, ICF walls depend on floors and roofs to be held from flapping during an earthquake. The connections between concrete walls and wood floors or roofs can be difficult and expensive.
Local building officials may not be familiar with ICF's. I’ve had problems educating wary building officials about them. Some have insisted on expensive and time-consuming special inspections.
If I were a builder, I would definitely get involved with ICF's. It is a terrific product with a very bright future.
Tim K. Garrison is a licensed professional engineer from Mount Vernon, WA, the author of “Basic Structural Concepts for the Non-Engineer," and is the president of ConstructionCalc.com. He can be reached by e-mail.
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