Mind the Gap

Jeff bought his home in the middle of summer several years ago. The first winter, a gap opened in the drywall between the gable-end wall and the vaulted ceiling. The gap seems to close every summer and open again every winter. Jeff hired PERCH to diagnose the problem and recommend solutions.

What on Earth was going on here? My initial investigation ruled out several possibilities. The gap is seasonal, not progressive (although Jeff does think it gets worse each year), so it doesn’t indicate a problem with the superstructure but something environmental. Frost heave seems unlikely, as Jeff has no uneven floors or major cracks elsewhere in the house.


The gap between gable-end wall and ceiling.

For a little while I was wooed by the idea of truss lift. Roof trusses are known for expanding and contracting in cold climates: in winter, the bottom chord stays warm and damp because of its exposure to inside air, while the top chord and webbing gets cold and dry right below the roof. The top chord and webbing contract and pull up the bottom chord. But Jeff with his vaulted ceiling clearly doesn’t have roof trusses. They must be rafters, no deeper than 2×6, for which any differential contraction would be barely visible.

The best-fitting explanation was not frost heave, but a different kind of heave. Certain soils (typically soils with a lot of clay) are known as expansive soils because they collect groundwater and expand during wet seasons, then lose the groundwater and contract during dry seasons. If the gable end wall was built on an expansive soil, it would tend to drop in the winter and rise in the summer.

But wait. Wouldn’t the rest of the house move, too? I found evidence to the contrary during my site visit. On the first floor, a wall adjacent to the offending wall had a few minor cracks in the drywall. Directly below, in the basement, a crack ran across the plaster covering the concrete foundation. Aha – the walk-out side of the basement is framed by a stud wall, which is much lighter than the concrete walls on the other three sides. The soil under the stud wall hasn’t compacted as much as in the other locations, so it’s more susceptible to subsidence when the soil contracts.


Cracking in the first-floor wall, about 6 feet from the corner.

Sometimes structural engineering is like solving a mystery. I search for clues and weigh possibilities against the evidence, and hitting upon the right answer is very rewarding.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

Sir Arthur Conan Doyle

Tiny Tuesday: Net Zero, Affordably

Dave Posluszny designed and built a net-zero house in Massachusetts from an existing foundation. No fossil fuels are used to power the house: HVAC and electric all come from solar panels on site. Posluszny aimed to make the house inexpensive (not commonly associated with net zero, at least up front) and easy to build, which led to some unusual details. Green Building Advisor shares this article he wrote about the design, and it’s worth a read to understand his decision-making process.


Vapor barrier, WRB, and thermal break at the top of the leveled foundation wall.

One detail actually generated a disclaimer on the article and much discussion in the comments. For his the water-resistant barrier (WRB) on the wall sheathing, Posluszny used Ice & Water Shield, which is also impermeable to vapor. This detail results in what some builders call a wrong-side vapor barrier, located outside the insulation instead of inside, which threatens to trap condensation in the walls and cause mold. Posluszny claims there won’t be any vapor to trap because the house is airtight and properly flashed, but a lot of builders feel uneasy.


The entire house is wrapped in Ice & Water Shield.

Posluszny describes several other unusual but logical decisions he made. Instead of making the insulation better (at a cost of square footage and budget), Posluszny opts to just install more solar panels instead. A roof vent above the scissor truss was accomplished with two layers of sheathing in order to place the WRB on the underside. The house has few windows, which is very smart from an insulation perspective; a semi-gloss white paint on ultra-smooth plaster ceilings brightens the interior.

I applied this thinking to all my decisions, and found that the least expensive way to be net-zero is not always the most energy-efficient way.
–Dave Posluszny


Open floor plan, showing one of the two lofts.

BONUS! Here is a glossary of the four barriers a house requires to maintain the temperature and humidity inside – in other words, to keep you comfortable.

Air barrier: The layer that prevents ambient air from entering the building, or conditioned air from leaving it. The sheathing usually serves as the air barrier, but builders need to be religious about filling in accidental holes (with spray foam, perhaps) in order to make a house completely airtight.

Thermal barrier, aka insulation: The layer that prevents heat from moving in or out of the building. The thermal barrier defines the building envelope. A conducting material that passes through the thermal barrier is called a thermal bridge, which wastes energy and should be avoided.

Vapor barrier: The layer that prevents water vapor in the air from entering the building. Exterior-grade plywood, plastic or aluminum sheeting, or Ice & Water Shield may be used as a vapor barrier. (If the house has a perfect air barrier with no holes, then the air barrier also serves as a vapor barrier.) Installed incorrectly, a “wrong way” vapor barrier may trap water vapor in the walls, leading to mold.

Water-resistant barrier (WRB), aka drainage plane: The layer that keeps rain and other liquid water from entering the building. Most houses use felt paper, also known as housewrap, but this house uses peel-and-stick Ice & Water Shield, which doubles as a vapor barrier as described above.

Tiny Tuesday: You Gotta Start Somewhere

Real estate agents often describe a small house as a “starter home” – implying, in the truest American tradition, that the owners will get a bigger one as soon as they can afford. Some people do trade up to accommodate kids or absorb a higher income. Others find that small suits their needs, and stay in the “starter home” for most of their adult lives.

And then there’s Luke Thill from Iowa, who built his own starter home but will probably never move into it. Luke lives with his parents and plans to keep that arrangement for a while longer. He’s 13.


Luke Thill and his “starter home.”

According to this Des Moines Register article, Luke bankrolled the whole project himself. He reclaimed 75% of his materials, spent $1500 he earned from lawnmowing and online fundraising, and bartered labor – for example, he cleaned a neighbor’s garage in exchange for the neighbor, an electrician, to help him wire the house.

The 89-square-foot groundbound house has a shed roof and a front deck. Inside is a kitchen with hot plate and refrigerator, a living room with couch and TV, and a sleeping loft. There’s no plumbing, which means the house cannot be a legal dwelling. Nevertheless, Luke uses the structure for homework and entertaining friends, and he sleeps there a couple nights a week. He plans to eventually sell it and use the proceeds to build a larger house on a trailer that he can bring to college. A starter home, indeed.

“Everyone had to have a big house, and now people have changed and realized it’s not practical. You can save money, travel the world and do what you want instead.”
Luke Thill