All About Frost

Construction season is underway, and road crews are out in force repairing frost heaves and potholes. Both of these driving hazards occur because ice is less dense than water (which is why ice cubes float), meaning water expands when it freezes. Frost is just another word for underground ice.

If you take a soil sample, you’ll find anywhere from 1% to 10% water content. But that’s not enough water to swell and damage a road or a building foundation when it freezes. The real problem occurs when there’s a source to replenish the water as it freezes, causing a buildup of ice. How might that happen?

Go deep enough underground and the soil isn’t affected by surface temperature swings, staying around 50 degrees Fahrenheit year round. Fine soils, like silts and clays, have pores that can draw water up from the water table. (This gravity-defying property is called capillary action, and it’s also how trees draw water from the ground up to their leaves) With a continuous supply of water freezing above the frost line, an ice-filled region forms underground in the shape of an oval or lens.

As an ice lens grows it displaces the soil around it, causing a frost heave. Then, in the spring, the ice melts and leaves a void. With nothing left to support it, any soil above collapses into the void… and there you have your pothole.


A pothole in a road. (flickr- creative commons)

A frost heave (or pothole) can really mess up a work of construction, so engineers and builders design to avoid them. One way, of course, is to prevent the ground from freezing. If your house is heated, then you could rely on heat escaping through the floor to keep your foundation above freezing… but that means you’re wasting energy, and it leaves you forever paranoid about what happens if you lose your heat source. A better solution is to surround your soil with insulation – in the floor and around the perimeter – so it neither draws heat from your house, nor freezes.


from Mark Wallace, “How to Prevent Frost Heave.”

Many buildings are built with a deep foundation wall extending below the depth of maximum frost penetration. As the map shows, that depth depends on your location. New York City and Long Island can expect less than 31 inches of frost, while the same state’s North Country designs for penetration up to 70 inches. (In Massachusetts most builders use 48 inches as a rule of thumb, though this depth is conservative for most of the state.) An alternative is to build a frost-protected shallow foundation extending out from the building on all sides.

Another important frost-protection technique is to prevent the capillary action that allows ice lenses to form. Coarse soils like sand and gravel have large voids between particles, too large to draw water. If your existing soil is finer, you can excavate and then fill below the structure with an appropriate soil.


Frost wall cross section from a recent PERCH project in Massachusetts.


Over at Colin’s house, the front porch proceeds at a steady clip. The porch’s ceiling, so recently a jumble of roof trusses and exposed cable, is now finished with a spread of white solid soffit. A narrow swath of vented soffit encloses the shed roof overhang and forms a sort of border.

Our support columns got wrapped in a neat product called a post sleeve. It’s basically PVC (low-maintenance all the way!) but manufactured in four 5½-inch-wide sections with flexible edges in between. In other words, it wraps around a 6×6 post cleanly and with very little effort. Terry installed the sleeves nice and snug, and he bent and cut aluminum to fit around the top beams’ as-yet exposed faces. Every bit of lumber above deck is now covered.

Meanwhile the screen porch begins to look less like a deck and more like a room. Colin built half-walls around the perimeter, added aluminum sills for the screens, and framed the doorway to the main porch. Still some work to do here, but it’s moving along quickly.

One seamless gutter. They surround the house and the Barn now.

One seamless gutter. They surround the house and the Barn now.

Of more practical concern: we have gutters! A team from the supplier came Thursday to furnish and install them all. With the ongoing drainage problems at Colin’s house (and the gloppy Champlain clay that continues to plague his yard), it’s a big relief to shed the water where we want it.

The big picture.

The big picture.

The Little Things

We must remember all kinds of details to finish this house properly. Early this week we brought a variety of subcontractors back to the site. Now that drywall is up, Chuck’s Heating and Air Conditioning can install ductwork and machinery in the mechanical room, and Bugbee can blow insulation in the attic. Tim from Pillsbury Excavation returned to improve our footer drains so the backyard doesn’t flood, and the stove contractor stopped by to install and connect the propane tank. It’s rarely easy getting contractors to come when you want them, so kudos to Colin and Terry for their persistence.

Joe installs a maze of ducts in the mechanical room.

Joe installs a maze of ducts in the mechanical room.

The propane tank, hidden along the south side.

The propane tank, hidden along the south side.

Outside, Arnie visited another day to install more decking. The cool detail here is the way the planks fit around the posts. Terry left room for perimeter planks to form a continuous border, and cut out square notches so they fit snugly. (Eventually we will clad the posts to match the farmhouse style.) It looks really slick.

Don't you agree?

Don’t you agree?

Inside, there’s lots of touch-up paint required. The window extensions got really beat up over the past few months, surrounded as they were by construction activities, so I sanded off the bumps and ridges and then Abby brushed on two coats of white paint. That’s a work in progress. Abby also touches up each ceiling after she finishes the walls, covering up the inevitable daubs of paint that miss their mark. It’s tedious work, but someone has to do it.

Library walls are all painted; the living room and first-floor hall have one coat to go. Nat comes to help as needed. Painting is now on the critical path, because we want the paint to dry before we install flooring… and yesterday the flooring was delivered!

Several wall and ceiling colors visible looking up the stairs. I love the crisp lines.

Several wall and ceiling colors visible looking up the stairs. I love the crisp lines.