Larger Homes with Less Impact

Efficiency
Written By Ted Caplow

Green building begins with a healthier, longer-lasting home

by Ted Caplow, PhD

A house over 40 in Miami is a dinosaur. When lots are redeveloped, many of these houses will be torn down and discarded. Why? Several reasons. First, the interior volumes of a former era don't always meet the expectations of today. Second, the materials have deteriorated: Drywall crumbles and stains, mold moves into the hidden void spaces in the walls and ceilings; toxins leak out of old insulation and synthetic finishes. Third, modern hurricane codes have stiffened, and windows and roofs need to be replaced frequently – and the underlying structure may not be strong enough in the right places to receive these upgrades. Fourth, the heart, brain, and lungs of the home – the plumbing, wiring, and air conditioning – are typically in need of replacement, but so embedded into the skeleton that it is impossible to do this work without tearing the house to pieces anyway.

The result is a home that lasts 40 or maybe 50 years.¹ Just by creating a home that can last 100 years (or more), we can halve the resources needed to build and operate that home, per year of residence.

Material intensity

It turns out that in making a home longer lasting, we also have the opportunity for material subtraction, because an entirely different approach is required to some fundamental building components. Consider CM2, the home that we designed for the Belle Meade neighborhood north of downtown Miami (a building permit has recently been issued for this home and construction will be complete in 2026). Compared with other houses being built down the block, CM2 uses less concrete, less steel, less insulation, and less wallboard, yet achieves a longer-lasting structure that is equally, if not more, thermally efficient.

The columns, beams, and floor slabs of CM2 are made of concrete reinforced with fiberglass bars in place of the conventional steel rebars, saving weight (for example, using fiberglass saves approximately 10 tons of steel in just the floor and roof slabs of CM2, with comparable savings in the column and beams throughout the house).² More importantly, the fiberglass rebar eliminates the risk of rust from within – a menace that can doom an aging concrete structure.

The exterior walls of CM2 are made of autoclaved aerated concrete (AAC) block, a type of masonry developed a hundred years ago that incorporates a multitude of tiny air bubbles chemically formed in wet concrete, which is then baked into blocks. Mortared together into walls, the AAC replaces the conventional concrete masonry units (CMUs) that make up almost every other new house in town. But a square foot of AAC block wall weighs only 25 lbs,³ where a square foot of CMU wall weighs 40 lbs.⁴ Both materials are made of essentially the same concrete, and both are 8" thick. There is another important difference: the AAC block provides five times the thermal insulation of the CMU block. Most modern houses have fiberglass insulation on the inside of CMU walls, and this fiberglass is held in place between metal studs that in turn support gypsum wallboard (called sheetrock, or “drywall”). The drywall, studs, and insulation diminish the longevity of the conventional structure. The drywall is vulnerable to mold; the insulation cannot tolerate water; the cavity between them harbors dust and pests; and when the plumbing breaks, the whole wall often must be torn apart and discarded – it cannot be restored once damaged. By contrast, the AAC block wall at CM2 is covered by a thin layer of plaster directly applied to the block, and there is nowhere for water, pests, allergens, or mold to hide.

The house is also built without suspended ceilings - when you look up from the living room or bedrooms, you see the polished concrete bottom of the slab above. No framing is required on this ceiling, and no drywall. Another 3 or 4 lbs per square foot is saved ⁵ (approximately 7 tons of material at CM2), along with the environmental impacts of these materials. Best of all, plumbing lines don't cross through void spaces in the walls nor creep along above the ceilings. Instead, they are arranged neatly in almost entirely vertical chases behind panels in the central atrium. The lines can be accessed, repaired, replaced, upgraded over the lifetime of the house, without destruction of any building materials. Leaks will be easily detected and mitigated before they lead to widespread damage and possible health problems.

Looking at the entire house as one object, the entire mass of CM2 has been reduced between 12 and 15% because it was designed with an approach that we call “hypostruction”, i.e. building with less.

More space

Consider a two-story, 5,000 sqft conventional home, constructed as a pair of connected squares, one measuring 30 ft x 30 ft, and one measuring 40 ft x 40 ft. Assume the floors are 12 feet apart. This geometry is close to the shape of CM2. In the conventional home, the exterior walls are 12" thick and there is an 18" cavity above the suspended sheetrock ceiling. In the hypostruction version of this exact same home, the exterior walls are 8" thick and there is no cavity above the ceiling. As a result, the interior volume of the hypostruction home is a whopping 20% larger, and there are almost 200 sqft of "recovered" floor area in this home compared to conventional construction. At today's four-digit prices per square foot in many upscale areas of Miami, this extra area is worth $200,000 or more. The higher ceilings create additional value.

Three times less material

Adding it all together, CM2 is 20% larger inside, lasts twice as long, and uses at least 12% less material than a conventional home on the exact same footprint. In an ecological sense, these figures translate into a 65% reduction in material impact for every year that this interior volume is occupied.

Ted Caplow
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