Insulation Tricks for a Midwestern Climate

This modern Victorian belonging to the Pusack family was designed by Janice Sweet and Associates, architects, who modified plans originally from Home Planners, Incorporated.

Construction on this home took nine full months, and during the first three-quarters of that time, there was at least one change to the design each day.

Construction workers roll out a black polyethylene vapor barrier over 2-inch expanded polystyrene insulation. (Note the location of the pier footings, which are not covered.)

Insulation at work is visible in this winter photo of frost covering the vapor barrier. The pier footings, which weren't insulated with the polystyrene insulation, are the only spots where warmth from the earth has kept the frost away.

Two techniques, used around the entire envelope, have proved too convenient and effective not to mention.

Instead of using either expanding or minimally expanding foam to seal window and door jambs to the rough opening, I use foam backer rod. It comes in various diameters and fits easily and tightly. It can be placed and repositioned precisely without ever having to be trimmed, and there's nothing to dispose of or clean up afterward.

Be aware that this material now becomes a vapor barrier, so it's important to follow the rule of placing the vapor barrier on the warm side of the wall. In heating climates, place it as close to the inside as practical. In cooling climates, especially with high humidity, place it out near the nailing flange.

This principle was born out of vivid personal experience. I had to replace four second-story casement windows in a home that my wife and I had built 15 years earlier. The lower two-thirds of the window frames were rotted, and the lower on the frame, the farther outside the decay reached, so that ultimately, the bottom hinges fell free. In installing the windows, I had scrupulously caulked the brick mold to the foam sheathing and the siding, but I stuffed fiberglass only along the frame and the extension jambs. Water vapor diffused into the rough-opening space, condensed on the jambs, and had no way to escape.

For our climate, I have developed a higher-to-lower priority for what side of the wall and what level of the house to be most concerned about and most careful to seal. The rule is inside-out and top-down.

The second technique, shared with me by a conscientious builder in Arizona, is consistent with my "inside-out" priority: joint-tape the drywall to the subfloor. It doesn't have to be pretty. When the crew is taping the drywall joints, ask the workers to run one strip along the bottom edge and onto the subfloor. It helps to seal out air infiltration and eliminates one more path for bugs to come in. It never costs more than $250 for the whole house, and if you're on good terms with the crew, your own gesture of thanks might get you by with a lot less.

The rest of the details center on the basement and focus especially on the footing-floor-wall area.

When excess water is used to make poured concrete flow more easily, the concrete ends up porous; it can soak up water like a sponge. Even with the theoretically correct water-to-cement ratio, concrete is not waterproof. Pouring the footings on a 4-ft wide strip of 6-mil polyethylene is one good way to keep moisture out.

Even though it represents only a small area of a building's envelope, there's a lot going on at the footing-floor-wall joint. Since concrete is a good conductor, heat escapes through the footing. And soil gases, including water vapor, easily pass through the small shrinkage gap where the floor and wall meet. I can confirm that from my own experience too. One winter, when I was finishing our neighbor's basement, I picked up a fiberglass batt left on the floor the night before, next to an outside concrete wall. On the wall, coming out right at the floor joint, was a feathery damp patch where water vapor was condensing. Fifteen minutes later the wall had warmed and the damp spot dried up. The fiberglass probably didn't keep water vapor from leaking into the basement, but had it not been there to cool the wall, I wouldn't have seen the condensation.

To inhibit the capillary wicking of ground moisture through the shrinkage gap (usually 1/16 inch or less), I mop water-emulsified dampproofing on the footing and to within 1 inch of the floor line on the wall.

To reduce conductive heat loss and to keep the floor from making contact with the footing and the wall, I press foam sill sealer into the angle. Usually the dampproofing has just dried by the time I reach my starting point, and is tacky enough to hold the sill sealer without any other fasteners.

A month or so after the basement floor is poured, I seal the shrinkage gap with urethane caulk to keep out soil gases. Iowa leads the nation in EPA action levels of radon over 4 pico-Curies per liter of air (pCi/l). I have never had a basement reading over 2.5 pCi/l.

In addition to the basement walls, one area I think it really pays to insulate is under the entire basement floor--both for energy efficiency and for comfort. $475 will buy enough 2-inch thick expanded polystyrene to insulate a 1,500-ft² basement. And yes, I cover it with poly vapor barrier.

Incidentally, during the construction of a Victorian home a couple of years ago, weather conditions provided a visual glimpse of the effect of basement insulation. We poured the floor in October, when frosty nights follow warm days. Preparing for the pour the next day, the flatwork crew laid the washed rock, insulation board, poly vapor barrier and reinforcing steel, while I tended to my floor-wall details. That night, a light frost formed on the black poly vapor barrier everywhere except--and distinctly so--over the pier footings, which had no insulation over them (see photos this page).

Sure, the insulation kept the ground heat from radiating into the basement that night, but under finish conditions, the owner will keep the basement about 20°F warmer than the constant ground temperature, and the floor will feel warmer.

We might extrapolate the effect of basement floor insulation we saw here, to visualize how frost-protected shallow foundations work. With this technique, insulation is used around a shallow footing to trap escaping ground heat and keep frost from reaching underneath to heave the footing.

One pleasant endorsement from the owner of that Victorian home came late last winter, after a week of windy weather when the temperature climbed as high as 5°F. He called and said, "Don, I didn't realize it until I came up from the basement. It's warm down there!"

Don Otto runs DPO Construction in Iowa City, Iowa.

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