About 20 years ago, I watched a video about a house being
built with structural insulated panels, or SIPs. It was the
first time I'd seen the process: Instead of framing one stick
at a time, the carpenters were installing entire sections of
wall, which had arrived on site sheathed on both sides and
It impressed me as a faster and better way to build, so I tried
SIPs on my very next house. That first one was difficult
because I had no one around to explain the technical details.
But we stuck with it, and now my company builds only projects
that include SIPs.
What Are SIPs?
SIPs are made by bonding a sheet material — OSB, plywood,
steel, or fiber-cement — onto both sides of an expanded
polystyrene (EPS) or polyurethane foam core. By themselves,
these materials are not strong enough to support loads, but
once they're made into panels they can be used for structural
elements like walls, roofs, and floors. The most common panels
consist of OSB over EPS (see Figure 1).
Figure 1. The most
common type of structural insulated panel is produced by
sandwiching EPS foam between two sheets of OSB. The face
material can also be plywood, steel, or fiber-cement, and the
core can be polyurethane.
Raw panels are produced in factories and then cut to size in
fabrication plants, or sometimes on site.
Size and thickness. OSB-faced panels come in sizes up to 8 feet
by 24 feet. Foam cores are sized in thickness to match the
width of standard framing lumber; that way, you can reinforce a
panel or provide nailing by inserting a piece of framing stock.
For example, a 6-inch panel is actually 6 1/2 inches thick,
made with a 5 1/2-inch-thick piece of foam sandwiched between
two sheets of 1/2-inch OSB.
Walls are typically made from 4- or 6-inch panels. Floors and
roofs might be made from 6-, 8-, 10-, or even 12-inch
Why Use SIPs?
We use SIPs because it takes less time — fewer labor
hours — and less skill to assemble precut panels than it
does to stick-frame. The parts of the building made from panels
are straight and true, and won't shrink or warp. Plus, they are
exceptionally well insulated and sealed against air
Our clients want their homes to be "green," and SIP buildings
qualify because they're energy-efficient and make good use of
natural resources. The OSB skin is made from fast-growing trees
that are plantation-grown specifically for OSB.
Also, there's very little job-site waste with SIPs; the panels
are cut by a fabricator, who can easily recycle cutoffs or use
them when smaller panels are called for.
The R-values associated with various building materials are
misleading because they don't reflect how and where the
material is installed. For example, 5 1/2-inch fiberglass batts
are rated R-19, but a wall insulated with these batts is not
R-19, because there will be thermal breaks at every stud,
plate, and header.
Whole-wall R-value. A more realistic way to look at insulation
is to consider "whole-wall R-value," a method developed at Oak
Ridge National Laboratory (ORNL), in Oak Ridge, Tenn., for
estimating the R-value of various assemblies. The whole-wall
R-value includes the insulation plus everything else that's in
According to ORNL, a 2x6 wall framed 24 inches on-center with
plywood sheathing, drywall, and 5 1/2-inch batts has a
whole-wall R-value of 13.7. The same wall built with 6-inch OSB
SIPs has a whole-wall R-value of 21.6. Why the difference? The
foam in the SIPs has a higher R-value than the batts, and the
SIP assembly contains fewer thermal breaks.
It's possible to buy raw panels and cut them to size on site,
but it's better to pay a fabricator to do the cutting. Many
fabricators have computer-controlled equipment that cuts panels
far more accurately than we ever could.
Design. Like any building, a SIP structure starts out as a set
of plans. Just about any stick-framed plan can be converted to
SIPs (Figure 2), although it's easier when the initial design
is done with panels in mind.
Figure 2. Panels can be
used for any part of the building that isn't curved. The walls
and roof of this traditional-style house are made from
Either way, the first step in any SIP project is to produce a
detailed set of shop drawings that show door and window
openings, corners, edges, and wiring chases, as well as how the
pieces will be joined on site.
Once the drawings are approved, delivery of the panels takes
six to eight weeks. The process is a lot like ordering trusses,
except in our case we produce the shop drawings in-house.
The fabricator could draw them, but we prefer to do it
ourselves because we gain more control over how the panels will
Handling. SIPs arrive at the site on one or more semitrailers.
Small panels are light — a 4x8 6-inch panel, for example,
weighs about 115 pounds.
Larger panels are heavy, so we rent an all-terrain forklift to
SIPs can be installed over any type of floor system. In our
area of Northern California, most homes have wood-framed floors
on stem-wall foundations with crawlspaces below.
Structurally, there's no reason we couldn't build the floor
with SIPs. Doing so would be much faster than stick framing,
and the insulation value would be very high.
But on most projects we still use conventional floor framing;
even with the labor savings, SIP floors aren't always
cost-effective in a mild climate like ours.
In colder areas, of course, where insulating the floor is a
major concern, building a floor with SIPs might make more
Sound transmission. Even if they did
cost less, we wouldn't use SIPs for upper floors.
The panels are good at preventing airborne noise from entering
through the walls and roof, but walking on them creates a
drumming effect that's annoying to the people below.
Our panels arrive on the job cut to size with door and window
openings, but without solid lumber inserted.
The foam is recessed along the edges, so there's room to make
insertions: bottom plates to fasten panels to the floor;
splines to join them edge-to-edge; and top plates to stiffen
the top of the wall and provide nailing for the roof or floor
We install these lumber members over beads of sealant (provided
by the panel manufacturer), then nail them in place through the
face of the panel.
For an extra charge, some manufacturers will install the
nailers for you.
Plates. With SIPs, wall plates are nailed, screwed, or bolted
to the floor and then the panels are slipped over them.
If the wall lands on a stem wall or slab, the plate and panel
must be isolated from the concrete. To do this, we install a
strip of pressure-treated plywood — sealed to the
concrete with foam sill seal — and then install the
plates over a bead of sealant.
Before installing the wall panel, we run sealant along the top
and both edges of the wall plate, then stand the panel over it
Figure 3.Plates are
installed first. Here, a 3x6 has been screwed to the deck over
a continuous bead of sealant. In preparation for standing the
walls, a carpenter runs sealant along the face and edges of the
plate (A). The crew then stands the panel over the plate (B),
braces it plumb, and nails it to the sides of the plate (C).
When walls land on concrete, the plate is installed over a
wider strip of pressure-treated plywood, which is also sealed
to the concrete (D).
After bracing the panel plumb, we nail it to the plate through
the OSB skin.
Hold-downs. In many regions, this nailed connection is all
that's needed to hold panels to the floor or foundation. But we
build in a seismically active area, so some of the panels are
designated as shear walls and must be tied to the foundation
The old way to do this was to connect threaded rods to the
foundation and run them all the way up through the
An easier method is to put double studs in the edge of the
shear panel, cut a hole in the OSB, remove some of the foam,
and install a conventional hold-down inside (Figure 4).
Figure 4. The OSB and
foam were cut from the corner of this SIP shear wall so that a
hold-down could be installed. Later the crew will foam in
around it and replace the missing OSB.
The hold-down is then bolted to the foundation and the double
Another method is to run a strap up from the foundation and
screw it to the outside of the panel at a double stud.
We edge-join the panels with splines that fit into slots in
adjoining edges and work like gussets. They're installed over
beads of sealant and nailed in place through the skin of the
We use three types of splines: solid pieces of lumber; surface
splines, which are 4-inch rips of OSB; and block splines, which
are basically a smaller SIP that fits inside the edges of
adjoining panels (Figure 5). We prefer the foam block or
surface splines because they don't produce thermal
Figure 5.Panels are
connected edge-to-edge with splines. Here, a carpenter prepares
to install a block spline over continuous beads of sealant (top
left). The spline functions as a gusset and is held in place
with nails driven first into the loose panel (top right) and
then into the adjoining panel (bottom).
We use solid lumber splines only where we need a doubled stud
to carry a point load.