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Insulating A Sloping Ceiling With
Rigid Foam Insulation
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In
This Article:
Rigid foam insulation is fastened to an
angled ceiling and held in place with 1x3 furring strips. |
Related Articles:
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| Skill Level:
2-3 (Basic to Intermediate) |
Time Taken:
About 8 Hours |
By
Bruce W. Maki,
Editor
Start:
In the second floor of my own 1½-story house, I decided
to knock out the knee walls and rebuild them farther back.
Even though the new space had low headroom, it gave us
more area for storage and possibly a large home office.
| The upstairs
room after relocating the knee walls. Before
installing new drywall, I wanted to improve the
energy efficiency of the sloped ceiling. The
original insulation was 3½" thick R-11
fiberglass. The insulation I added between the
2x6 rafters (above the new section) was R-13
fiberglass. These are not good R-values
for a ceiling in a house in Northern Michigan. |
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Installing foam insulation on my ceiling was
simple. There was only one obstacle... this
chimney chase. |
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In my situation, the simplest and
cheapest way to increase the R-value was to apply a layer of
rigid foam insulation over (below, actually) the
rafters before installing new drywall. I chose
polyisocyanate foam insulation, which has the greatest
R-value per inch of commonly available insulation materials.
I used 3/4" thick foam because it was readily available at
my local Home Depot. This foam is available in greater
thicknesses... I've seen 6-inch sheets of
polyisocyanate foam used on the roofs of commercial
buildings.
| I installed
the sheets of foam so the long dimension was
running perpendicular to the rafters, just
like drywall is normally hung. On this sheet I
had to cut the length short by a couple of
inches, because the right-hand edge did not land
on a rafter. (I had installed a 2x4 nailer
before installing the fiberglass insulation.)
I also had to cut a large notch to go around
the chimney chase. |
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Cutting Rigid Foam:
Cutting rigid foam insulation is
simple... just use a long sharp knife and a goof
straight-edge. I prefer to use one of those knives with
the snap-off blades, because they can be extended very
long (if the blade segments haven't been broken off). A wide-bladed knife is
better than a skinny blade. I
mark the cut line with a permanent marker and a long
ruler. A drywall T-square (which is 4 feet long) is the
best tool for laying out cuts that are perpendicular to
the panel edges. It's common for the ruler to slip while
cutting foam, so marking the cut first tells me when the
ruler has slipped. I placed a
scrap of plywood beneath the cut so I wouldn't cut into
the existing hardwood floor. When cutting on a plywood
subfloor, this cutting board prevents the knife from
hitting nails or screws in the subfloor, which will dull
the tip. Keeping the knife tip sharp is very important,
otherwise chunks get torn from the foam. |
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The next piece I installed was a full sheet.
To hold the foam in place, I screwed a couple of
short 1x3 blocks to the rafter, using 2½" deck
screws. My goal was to get about one inch of
screw penetration into the framing. |
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These blocks of 1x3 were carefully
placed along the centerline of the sheet (i.e. 24" from the
upper edge). I did this to avoid interference with the 1x3
furring strips that will be installed later.
| After the
first row of foam was installed, I hung the
upper row. These sheets needed to be cut to a
narrower width. I found that two 8-inch long
blocks of 1x3 were adequate for temporarily
holding the foam up, though some of the edges
were not tight to the rafters.
Then I applied housewrap tape to the
seams between the panels. This is important to
prevent air leaks. |
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The "Normal" Method Of
Fastening Foam Insulation:
These plastic cap ring-shank
nails are the fastener most commonly used for installing
foam insulation.
The plastic cap prevents the
nail head from pulling through the foam. Since these are
ring-shank nails, they hold very well and are quite
difficult to remove.
Note that the container says
"Round Plastic Cap Roofing Nails". Cap nails were
originally used to fasten roofing felt (tar paper)
during shingle installation, though I can't imagine why
anybody would need 2-inch nails for that job...
especially ring-shank nails. |
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I fastened 1x3 furring strips over the foam,
using 2½" deck screws. I made sure the boards
"broke" over a rafter, so the ends could be
fastened securely. Pre-drilling the holes at the
ends is necessary to prevent splitting the wood. |
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Installation Tips:
To make installation easier, I held the
boards against the ceiling and marked the rafter
locations on the board. Then I pre-drilled the
holes in the middle of the boards, (to make sure
the screw heads didn't stick out) and started a
deck screw in each hole.
I just held the board in place and drove in
one screw to hold the board up. Then I made sure
the board was positioned properly (i.e. along
the 16" on-center lines I had marked earlier)
and I drove in the remaining screws. |
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Using a few simple tricks, I was
able to install these boards without a helper.
Pre-Drilling?: If I didn't pre-drill
the holes, I discovered that the foam would get crushed before the
screw head became sunk into the wood.
Furring Strip Materials:
Instead of buying 1x3's for
furring strips, I bought some good-quality 1x6's from
Home Depot and ripped them in half on my table
saw, so the final width was just under 2-3/4".
I could have bought 1x2 or 1x3 furring strips, but those
boards are often very low quality, and they are often
much thinner than normal 1x material. I have seen some
big variations in thickness in furring strips at Home
Depot and their ilk, so I prefer to spend a little more
money on the Swedish-made 1x boards that Home Depot has
carried for several years now. That wood is excellent...
exactly 3/4" thick and exactly the industry standard
widths (3½", 5½", 7¼", etc.). The knots are almost
always very small, so when the boards are ripped
narrower the wood doesn't fall apart because some big
knot dominates the board. I rue the day when Home Depot
discontinues those Swedish boards. |
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It took me about an hour to install 7 rows of
furring strips, spaced at 16 inches on center.
The room is just under 17 feet long, so I had to
use 10-foot boards and 8-foot boards to span the
entire length. |
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| After
installing the horizontal furring strips, I
installed some small pieces of 1x3 between the
horizontal boards. These extra blocks were
placed so they would provide backing for the
edges of the drywall panels. I did this because
it's easy to bump the ceiling, and pushing on a
drywall seam might make it crack. These extra
boards are not necessary, but they also should
help slow the spread of flames in the event of a
fire. |
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Horizontal Or Vertical
Furring?
One reason I chose to install
foam and horizontal furring strips was to allow me to
hang the ceiling drywall vertically instead of
horizontally. This way all of the ceiling drywall will
be just less than 8 feet long. If installed
horizontally, I would need some 8-foot sheets of drywall
and some others that would be almost 9 feet long. It was
difficult enough to carry 8-foot panels up the stairs
and around the corner... it might not be possible to get
longer sheets up the stairs and over the pony wall
around the stair opening. It's
not necessary to install the furring strips horizontally
like I did... it all depends on the desired orientation
of the wallboard. I could've just installed furring
strips directly over the rafters if I wanted to hang the
drywall horizontally. |
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More Info:
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- Basic
Carpentry Tools
- Cordless
Drill/Driver With Countersink Drill Bit
- Cordless Impact Driver
- Drywall T-Square
- 4-Foot Level or Straight-edge
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Materials Used:
- Polyisocyanate Rigid Foam Insulation
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1x6's, Ripped In Half
- Short Pieces Of
1x3
- Deck Screws, 2½" Long
-
Housewrap Seam Tape
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