Superinsulation pays off in net savings if you figure the job carefully; Boston economist expects fuel costs to fall to one-fifth

With the stock market plunging these days, one economist predicts that today's blue-chip investment opportunity may be to superinsulate your drafty old house.

Steven Kropper points to his own rehabilitated triple-decker house in Boston to prove his unorthodox investment strategy.

''I borrowed money at 11 percent and got a 20 percent tax-free payback in fuel savings by investing in superinsulation,'' reports the youthful economist, who operates his own energy-consulting firm, Rowell Energy Group.

''You might say that the bank and I split the profit,'' he adds

As a result of the project, heating bills for the three-apartment building are expected to drop from $2,600 to $570 or less a year.

Superinsulation is a technique that is slowly catching on in new-home construction. However, according to William Shurcliff, author of ''Super-Insulation and Double Envelope Houses,'' Kropper's is one of only a handful of older homes to be superinsulated.

The method involves doubling or tripling the amount of insulation in a home, radically decreasing the amount of heat required. Superinsulation doesn't cost more than conventional construction because the additional framing and insulating costs are canceled out by the savings from using a significantly smaller heating system.

Two years ago Kropper bought a dilapidated 70-year-old triple-decker house in Boston's Dorchester neighborhood. For his $18,000 he got cracked plaster, rotting windows, and staggering heating costs.

Mulling over the possibilities, including active and passive solar, Kropper decided that the best return on his investment would instead come through superinsulation.

Thus, he planned to rip out the plaster and build a second frame of 2x4s on 24-inch centers (16-inch centers are normal) next to the house's existing frame. This ''staggered stud'' construction would allow the use of two layers of 31/2 -inch fiberglass in the wall, the usual method of framing new superinsulated houses.

However, contemplating the tons of plaster to be removed and trucked away, Kropper opted instead for a method that is fast, clean, and yet produces almost the same insulating value. It should be feasible for anyone with basic home-improvement skills.

The first winter he had cellulose insulation blown into the walls, plus 10 -inch (R-40) in the attic. Kropper estimates he saved about one-sixth of the cost, or $100, by buying the insulation in bulk. He also tuned up the furnaces and caulked all the windows - improvements with quick paybacks.

This year Kropper began the second phase of the superinsulation job by nailing strapping to the existing walls and attaching 2-inch-thick panels of polyisocyanurate sheathing (R-16) to them. When the sheets were tightly joined with duct tape they also became an efficient vapor barrier.

To complete the job he nailed furring strips to the sheathing and attached new sheetrock to them (half-inch fire-code sheetrock is a must because of the fumes that could be given off if the sheathing burned).

The result is an astonishing R-37 total for the walls, compared with the usual R-11.

Overall, each wall is about 4 inches thicker than before, but that doesn't result in any noticeable reduction in room size.

In addition, Kropper replaced the rotting double-hung windows with snug-fitting double-pane glass, using fixed panes where possible so as to cut heat loss even more. Then he insulated the water pipes and tank as well as the underside of the first floor.

One other necessary improvement is still to come. Because the house now is so tight, an air-to-air heat exchanger will be needed to insure that there are enough air changes in the house per hour to rid the air of cooking gases and other air pollutants.

Kropper estimates the total cost of the project at about $6 a square foot, or under $5,000 per unit.

As a result of all these measures, the energy expert estimates that no heat at all will be needed until the outside air temperature dips below 45 degrees F. Overall, the house should require a maximum of 46,000 Btus on a subzero day, compared to 175,000 Btus previously.

This reduction will allow him to use only one of the three furnaces in the house to supply all three units.

Beyond the energy savings, the renovations also will reduce the noise from the jets which pass overhead - and the new walls are much more attractive. Kropper says that even though he has raised the rent, the tenants are pleased because their total costs from rent, plus heating, will be lower than before.

With a $15,000 ceiling on the renovations, Kropper did have to make some compromises. He would have preferred triple-pane windows; and thermal shutters will have to wait.

Joseph S. Fitzpatrick, former Massachusetts energy secretary, says Kropper's work has proved that ''investing in a triple-decker delivers as handsome a return as a Texas wildcatter.''