Building With Structural Insulated Panels - Continued
Solid nailers. Any vertical edge that is not
joined to another edge with a spline must be filled with a
piece of solid lumber. This provides nailing where there
otherwise would be nothing to nail into.
Wall corners are made by butting the edge of one panel into the
face of another and then screwing back through into the nailer
(Figure 6). The exposed foam edge of the overlapping panel is
filled with lumber to provide nailing for the wall
Figure 6. At corners,
the crew installs nailers flush to the edge of the panels,
butts the panels together (left), and uses screws to fasten
through to the nailer beyond (right). These panels are 6 1/2
inches thick, requiring 8-inch-long screws.
Once the walls are up, we insert top plates. This stiffens the
walls and provides solid nailing for the second floor or
Sealing the Seams
There are a number of ways to seal the seams between panels. We
run beads of panel mastic on mating surfaces, but you can also
use polyurethane foam from a can.
As an added measure, some panel manufacturers require you to
surface-seal the interior joints by covering them with SIP
tape, a type of peel-and-stick membrane. This is primarily a
concern with SIP roofs in very cold, wet climates, because warm
interior air will carry moisture through the gaps and can cause
the outer layer of OSB to rot.
In some locales, the building code may require that you install
a continuous vapor barrier inside the building.
And to the extent that it reduces air leakage, a vapor barrier
can be an improvement.
But the real issue with SIPs is not moisture diffusion through
the panels — it's air leakage at the seams. In most
climates, if you properly seal the seams you should not have
problems, even without a vapor barrier.
Because SIP buildings are so tight, it is necessary to
mechanically ventilate them to remove excess humidity and
provide fresh air. The best way to do this is to install a
heat-recovery ventilator (HRV).
If the budget allows, a project might have a SIP roof. A truss
roof is cheaper and, if the roof is complicated, easier to
install. But a SIP roof is tighter and better insulated.
With a SIP roof, beams are required, except where the panels
span from wall to wall. There is typically a bearing ridge and
beams at hips and valleys. Roof panels are joined edge-to-edge
in the same manner as wall panels, then screwed to the beam or
Many of the photos in this article are from a house with a flat
— or, more accurately, very low-slope — SIP roof
surrounded by a short parapet (Figure 7). The panels are
supported by interior beams and ledgers screwed to the inside
faces of the walls. The ledgers are sloped to drain the rubber
membrane roof toward scuppers in the parapet; inside the house,
we dropped the ceilings to make them flat, leaving space for
ductwork and wiring above (Figure 8).
Figure 7. Roof panels
are lifted with an all-terrain forklift (A) and lowered onto
glulam beams and sloped ledgers screwed to the wall panels (B).
This carpenter fastens a panel by screwing through to the beam
below (C). The parapets terminate with a double top plate,
specified by the engineer (D).
Figure 8. Many of the
photos in this story are from a house with a flat SIP roof and
parapet walls. The roof panels are supported by ledgers, which
provide a slight slope toward drainage scuppers. Inside, the
ceiling was dropped to provide space for ductwork and recessed
Door and Window Openings
Door and window openings are often cut right through the panel.
Headers are not usually necessary unless the opening is more
than 5 feet wide and or very close to the top. If the opening's
large enough, you can save on material by piecing in around it.
In such a case, the edges of the flanking panels should contain
full-height studs plus jacks to support a panel or a header and
Cutting in the field. Occasionally the owner will want to add a
window or make slight design changes after the panels are
delivered. As long as the changes are minor, we can accommodate
them by cutting the panels on site (Figure 9).
Figure 9. Mistakes and
changes sometimes force the crew to alter panels in the field.
Here, a carpenter trims a panel to size (top), then uses an
electric hot knife (bottom) to neatly remove the foam so there
will be room for a block spline.
After cutting, we use a hot knife to remove foam from the edge
so there's room for a spline or nailer.
Because SIP buildings are engineered, we have to get changes
Effect on Subs
As with any alternative method, using SIPs affects the
Drywallers and finish carpenters love SIPs because they are
flat and straight and they don't shrink or bow. Also, finding
nailing is easy because the panels are continuously sheathed on
Roofing over SIPs is no different from roofing over any other
Mechanical trades. Since partition walls in SIP houses are
normally stick-framed, the hvac installer can easily run ducts
in them. The only time there's a problem is when there's no
attic and both the floor and roof are SIPs. Then we have to
The plumber is in the same boat as the hvac contractor —
most of the pipes go in partition walls. If the kitchen sink is
on an outside wall, we either run plumbing through the toe
space or bring it up through the bottom of the cabinet (Figure
Figure 10. To avoid
putting pipes in the wall, the author had the plumber install
the drain and supply lines for a sink just inside the panels at
the sink-cabinet location (top). If plumbing must go in an
exterior wall, the author creates a chase by cutting out the
panel and removing some of the foam (bottom). Once rough-in is
complete, the author's crew uses spray foam to fill in around
We typically build an interior chase for the vent pipe; when
necessary, we leave an open space between two panels for pipes,
then fill the space later with EPS and spray foam.
Electrical. The electrician faces the greatest
challenge because it's hard to avoid putting switches and
receptacles in exterior walls.
We order panels with one vertical and two horizontal wire
"chases" — 11/4-inch holes that run edge-to-edge through
the foam (Figure 11).
Figure 11. Wire chases
are provided by panel manufacturers. The electrician accesses
the chase by cutting a hole through the OSB and removing some
of the foam. He can then fish wires through the chases and
connect them to remodeling boxes in the panels.
The first horizontal chase is at outlet height, and the second
is at switch height. Since they're marked on the OSB, their
location is obvious.
The electrician accesses the chase by cutting a hole through
the face of the panel and digging out some of the foam. He is
then free to fish wires vertically and horizontally and install
remodeling boxes as needed.
When the wiring is done, we seal everything with spray
With a little planning, you can run most of the wire through
interior walls and minimize the amount that runs through
Panels cost more than conventional framing material, but they
require less labor.
In my business, building a house with SIPs costs somewhere
between 1 percent less to 5 percent more than stick-framing the
Because a SIP house is tighter and better insulated, we can
downsize the hvac system — but we have to install an
We don't have to hire an insulation contractor, and our dumping
fees are lower because there is much less waste.Gary Pugh owns Alternative Building
Concepts, a green building company in Santa Rosa, Calif.