As a builder in the Energy-Crafted Homes (ECH) program in
Massachusetts, I’ve been putting together
energy-efficient houses for years. Homes in the program have to
meet stringent standards for total air leakage and total
heating load. In typical houses, roofs are a big source of heat
loss and air leakage. So we put a lot of effort into building
airtight roofs with plenty of insulation. Cathedral ceilings
are a particular challenge: When you have only the depth of the
rafter to work with, achieving a high R-value, a good air and
vapor barrier, and code-compliant venting takes a bit of
If I’m building a big house but only a small section
has a cathedral ceiling, I don’t go overboard. It’s
the whole house, not each little part, that has to meet the
standard, so putting a lot of work into improving the
insulation of that little piece doesn’t pay off.
I’ll usually settle for R-40 or so in a small section of
On big expanses of cathedral, however, I want close to R-50,
and a near-perfect air and vapor barrier. I try to be well
within the ECH standards, not barely inside the line.
I’ve tried a lot of different techniques over the
years, and I’m still experimenting. In this article,
I’ll discuss several ways to get a high-performance
cathedral roof and give you a close look at our latest
This method seemed like a simple idea when I tried it. On a
cathedral roof that needed only a 10-inch-deep I-joist for
structural reasons, we went with a 16-inch I-joist, installed
vent channel under the sheathing, stapled a reinforced plastic
vapor barrier to the rafter faces, strapped over the plastic
with 3/4-inch strapping, and blew the cavities full of dense
cellulose (see Figure 1).
1. Sixteen-inch-deep wood I-joists insulated with
dense-blown cellulose create a high-R cathedral ceiling.
However, the high cost of the I-joists and the complicated
ridge and eaves details associated with I-joists make this the
author’s least favorite system.
Subtracting 3/4 inches for the vent channel, we had a
151/4-inch space filled with R-3.6 cellulose, for a total
R-value of at least R-55 (more if you count the air space
created by the strapping).
I-joists have the advantage of being straight and true, so
this technique gives you a nice flat ceiling. Even so, I would
not do a ceiling this way again. For one thing, I-joists are
just too expensive. But more important is the labor cost of
working with them. Attaching the joists at the plate and ridge
is complicated (especially if there’s a hip or valley in
the plan), but the worst thing is the eaves and rake details.
Attaching soffit and fascia to I-joists requires all kinds of
packing out; compared with trimming out sawn rafter tails,
it’s way too time-consuming.
Building Down With Gussets
This technique has worked well for me, and I still like it for
some situations. We increase the rafter depth downward after
the roof is framed, using plywood gussets to hang the rafter
build-down from the main rafter (Figure 2).
2. Hanging 2x3s below the rafters using plywood gussets,
the author creates as wide a space as he wants for blown
cellulose. The insulation also fills the space between the main
rafter and the build-down, blocking heat conduction through the
As usual, we staple vent channel to the underside of the
sheathing. After installing a reinforced poly vapor barrier and
strapping across the ceiling, we fill the cavity with
On a big section of roof, the build-down goes quickly.
Getting the ceiling plane flat is a problem, though. Using
string lines often isn’t practical — for example,
when the roofline is cut up by valleys or hips. And when
efficiency is a factor, the last thing you want is for your
framers to start messing around with strings.
The fastest way for setting the build-downs is to cut the
gussets a uniform length equal to the insulation depth
we’re after, and nail the gussets onto the build-down
pieces first, making all the pieces identical. Then the framers
can just hold the pieces up so the gusset ends butt against the
underside of the sheathing, and nail the gussets into the sides
of the main rafters. Since the gussets are all uniform, they
transmit the roof sheathing plane through to the ceiling
framing. We try to use reasonably straight 2x3s for the
build-down, and we compensate by eye for any excessive rafter
crown. Most of the time this gives us a good result quickly. We
use a light nail gun for fastening the gussets to the rafters,
so the work’s not too hard.
The big advantage of this system is that you can get any
depth of insulation you want — it’s limited only by
the size of the gusset. And it also saves on lumber: If you
only need a 2x8 rafter for strength, you use a 2x8 rafter. But
you can still get a foot or more of insulation into the
This technique also creates a thermal break between the
ceiling and the main rafter, which cuts thermal bridging. That
makes a difference to system performance: A foot of insulation
with a 3-inch thermal break performs a good bit better than the
same amount of insulation with thermal bridging at every
When you’re blowing dense cellulose, you need to be
aware of the force that it exerts. The material goes in under
extreme pressure — it will belly out poly vapor barriers
and can crush some types of vent channel. When blowing
cellulose into ceilings, my insulation contractor uses Sturdy
Vent (Edwill Manufacturing, (10223 Timber Ridge Dr., Ashland,
VA 23005; 800/476-4295), a strong extruded polyurethane vent
channel with a center rib that resists crushing. For the vapor
barrier, we use a cross-woven three-ply reinforced
polyethylene, and we always strap over the poly at 16 inches
on-center before blowing in the insulation.
Sometimes, I don’t have room for a big build-down because
the area below the ceiling is taken up by high windows, light
sconces, or something like that. And sometimes I’m
dealing with a small area of ceiling and I want to get it done
quickly. In such cases, I occasionally use the simple method of
stuffing the rafter cavities with fiberglass batts, and
installing foil-faced sheets of rigid foam over the rafter
faces, taping the seams to create the air and vapor barrier
3. If space is limited, the author applies foil-faced
foam sheets to the bottom faces of the rafters for added
R-value and a thermal break. Taping the seams where sheets meet
creates a good air and vapor barrier.
I strap across the foam to provide a good screw base for the
drywall (the dead air space also adds a little R-value).
With R-30 high-density fiberglass batts and R-10
(11/2-inch-thick) sheets of foam, you can pack a touch over
R-40 (including the air space) into 103/4 inches. The work is
simple and quick.
However, you don’t get a lot of bang for the buck with
this method. The high-density batts go for 70¢ per square
foot installed, and the R-10 foam is 77¢ per foot —
a total of $1.47 per square foot for the R-40. When we use the
build-down method, my insulator charges me $1.12 per square
foot for a 12-inch depth of dense cellulose in the ceiling,
good for an R-43 or more — and the unit price goes down
as the depth increases. Even including labor and materials for
the build-down, I come out ahead by building down and blowing
I also prefer cellulose to batts for other reasons, like the
fact that it’s recycled and offers more resistance to air
movement. And when you install batts, you often get voids that
cut their effective R-value. On the other hand, batts perform
reasonably well in this application, particularly with the
added protection of the foam below. And in some cases,
cellulose has its own limitations. For instance, in a roof that
had metal roofing over strapping (with no plywood sheathing), I
used high-density fiberglass batts because I figured that blown
cellulose might find its way behind the strapping into the vent
channel and clog it up.