Carpenter Jeff Wilson likes to stand in the house he's building right now and sight down a wall's long, straight line. Walls usually aren't like this in new houses today, he says. They tend to bow in or out.

"In the old houses, with plaster, you'd get that straight look," said Wilson, but today it's usually missing. "Our lumber isn't the way it used to be."

The reason for the ruler-straight walls in this house is a new building method called SIPs--structural insulated panels. Fans say the panels produce a very straight, very strong, very heat-tight house and do so at affordable prices.

Several thousand SIP houses have been built in Michigan, but the public has not seen them. They're one-of-a-kind houses designed by architects.

Now at Orchard View Estates near Fenton, Mich., people can finally see what a typical, moderately priced house built with SIPs is like. Base prices in the area will start at $140,000 to $170,000.

Orchard View houses look like conventional subdivision homes, out in a country location, but the building method is big news.

Of the 200 houses that will be built, home buyers have their choice of the conventional stick-built method or the new structural insulated panels. The houses will look the same from the outside. The big difference is inside the walls.

Viewed from the top or bottom, these wall panels look like a sandwich. The center of each is four inches of white foam--extruded polystyrene. It's enclosed between two sheets of heavy exterior-grade plywood.

The sheets are built from 27 thin layers of shaved wood bonded together with glue. It looks like the typical processed wood you might see on any house that doesn't yet have siding, but it's actually thicker and stronger. A single wall panel can be up to eight feet high and 24 feet long.

With SIP houses, much of the labor takes place in the factory, where each wall is custom-built for each house, and holes for windows and doors can be cut in.

When the walls reach the home site, they are glued and nailed together. Either at the factory or at the home site, tunnels are drilled through the foam plastic core to let electricians and plumbers install wiring and plumbing.

SIPs get high ratings from many fans--including the National Association of Home Builders and the energy-conscious magazine Environmental Building News, which last year devoted a cover story to them.

The consensus is that SIPs do a very good job of meeting a house's basic needs. They give:

* A higher level of insulation than the usual fiberglass batting.

* An air-leak seal with fewer seams than house wrap.

* A structure that's two times as sturdy as conventional 2-by-4 stick building.

They do this at an affordable cost. And SIP houses go together quickly, with less labor in the field, a boon in the face of today's shortage of skilled house framers.

SIPs are the main building material at the South Pole, says Jim Tracy of the Structural Insulated Panel Association. Down there, buildings must come together fast before the construction crew freezes, and structures must stand up to high winds and deadly cold.

Energy-conscious groups love SIPs. Even the thinnest SIP wall--4 1/2 inches thick--has a robust insulation value of R15. Better yet, its whole-wall insulation value is excellent, without all the weak spots found in a stick-built house with conventional insulation. For example, in a conventional wall, each wood stud creates a spot with no insulation every 16 inches.

In Lansing, the family of Leroy Harvey lives in a 1,300-square-foot house built 3 1/2 years ago from six-inch-thick structural insulated panels.

It was built for $99,000 as a demonstration project for the energy-efficiency group he heads, Urban Options. Harvey's heating bills have been less than $200 a year.

Finally, environmentalists like SIPs because they can be built with only recycled wood or young scrub trees. They help spare old-growth trees, which must be cut for larger-size building lumber.

"Being carpenters, we go through a forest in a year," Wilson said. "I like these things. I like to save as many trees as possible."

Stand inside one of these newly framed SIP houses and you get an unusual sensation, compared with standing in the frame of a stick-built house. Even with windows and doors missing, you feel the solidness, and the lack of give in the structure, and you hear an unusual quiet. You'll also see the straight lines Wilson loves.

"These are really rigid walls," he said.

Any new building method needs transition time. All local building codes now accept SIPs, says Tracy of the SIP association. But Crosswinds Communities, the Novi, Mich., builder of Orchard Estates, is still weighing the best ways to use the 8-by-24-foot panels, says Andrew Seidel, who's spearheading the project.

For example, is it better to use them horizontally or vertically? Is it better to cut window openings in the factory or on the job site?

When the first houses are built here, Crosswinds will measure their airtightness, Seidel says. It's expected that the houses will be very airtight. If they are too tight--all houses need to breathe a little--Crosswinds will add extra ventilation near the furnace.

At Orchard View Estates last week, Wilson gestured up at an unroofed SIP house. "We should be done by tomorrow," he said.

"We're getting pretty used to these houses. We're pretty impressed with them so far."

Any building method comes with issues a smart house shopper should think about. Most have been addressed by SIP makers. Some issues and possible solutions:

QAren't these houses too airtight?

AIt's much better to build a very airtight house, then add a little inexpensive ventilation, than to struggle with a leaky house.

Aren't we damaging the ozone layer when we manufacture that extruded white foam?

No, 95 percent of SIPs are made with extruded polystyrene (EPS), which does not have ozone-damaging byproducts. That includes the R-Control brand used at Orchard View Estates. If you encounter the rare SIP that says it's made with HCFCs, avoid it. Its manufacture does damage ozone.

Doesn't all that glue in the wood panels give off gases that can hurt chemically sensitive people?

It could if the glue is formaldehyde-based, so the consumer should check. Some SIP makers use formaldehyde glue on the exterior panel and a different glue inside. Some use only a water-based glue. Once the water dries, what's left is an inert plastic.

Doesn't the extruded foam make a good home for carpenter ants?

It would, so many SIP makers include an insecticide. R-Control infuses its foam with a relative of the boric acid many people sprinkle to kill ants. It is harmless to humans.