and forth through the thermal envelope as the
with mastic.
seasons change. During summer months, when
Fiberglass tape and mastic can also be used to the air outside is warmer, it tends to move inward.
seal gashes in insulated flex ducts, but because
In colder months, heated indoor air migrates out-
such ducts “give” somewhat as pressure is
ward. As water vapor migrates outward along
applied to them, oriented polypropylene (OPP)
with the heated air, the moisture condenses as it
tape and foil-backed butyl tape are often used to
hits cold exterior surfaces. If that condensation is
patch flex ducts because both tapes stick aggres-
sustained and excessive, it can soak and, in time,
sively. Ironically, you should never use cloth-
rot the framing. (Warm, moist air escaping
backed duct tape to seal ducts because heat
through ceilings causes the picturesque but dam-
causes its adhesive to dry out and come undone.
aging ice dams of New England roofs.)
In response to winter condensation problems,
VApoR RETARdERS
builders began installing a polyethylene vapor
barrier on the living-space side of insulation to
Any discussion of air-sealing would be incom-
prevent moist air from migrating into wall cavi-
plete without mentioning vapor retarders, com-
ties and condensing there. In winter, vapor barri-
monly called vapor barriers. As essential as water
ers work well enough. But in warm months, when
is to life, excess moisture inside a house can lead
moist outside air migrates into wall cavities, it is
to big problems—especially as houses become
blocked by the vapor barrier and stays in the
more airtight. Thus, materials that are part of a
walls. Heating and cooling climates have oppo-
building’s thermal envelope are chosen, in part,
site problems when it comes to poly vapor
on how permeable or impermeable they are to
barriers. In hot, humid climates, where air
water vapor.
conditioning runs constantly in the summer, a
Permeability is measured in perms: The lower
polyethylene vapor barrier installed behind
the number, the more impermeable the material
the drywall becomes a condensing surface for
is and the more it will impede moisture’s move-
moisture.
ment. The higher the perm number, the more
These days, there’s an uneasy consensus that
easily moisture can pass through it. Accordingly,
vapor barriers should be used sparingly in
vapor retarders are classified:
regions of extreme cold, such as Climate Zones 6
class I (impermeable) vapor retarders
and higher (see the map on p. 407). That is, vapor
have permeance levels of 0.1 perm or less and
barriers should not be used in moderate or hot
are rightly called vapor barriers. The group
climates. Take note renovators: If a home has
includes polyethylene sheeting, rubber
polyethylene vapor barrier behind the drywall, do
membranes, sheet metal, aluminum foil, foil-
not compound the problem by retrofitting foil-
faced sheathing (insulated and not), and glass.
faced insulation on the exterior of the building.
class II (semi-impermeable) vapor
Moisture in the walls would have nowhere to
retarders have permeance levels between
go, and that’s what builders are increasingly
0.1 perm and 1 perm. Class II materials include
concerned about. There must be some drying
oil-based paint, kraft-paper facing on fiber-
potential for exterior walls, either to the outside
glass batts, unfaced extruded polystyrene
or the inside.
(XPS) panels thicker than 1 in., and traditional
Better understanding of the movement of air
stucco applied over #30 building paper.
and moisture has led to a wider use of class II
class III (semipermeable) vapor
and class III vapor retarders. Some builders use
retarders have permeance levels between
what’s called the Airtight Drywall Approach to
1 perm and 10 perms. The group includes
stop air leaks and the movement of moisture into
plywood, OSB, #30 building paper, most latex
walls. And a new generation of exterior latex
paints, unfaced expanded polystyrene (EPS)
paints are proving more durable than oil-based
panels, and unfaced polyiso (polyisocyanurate)
paints because latex, being semipermeable, is
foam panels.
less likely to be lifted off of a substrate by
trapped moisture.
Energy Conservation and Air Quality
397
Vented and unvented roofs
Vented attics with insulation on the floor are marvels of nature and
man. After you air-seal openings in the attic floor and cover it with an
appropriate amount of insulation, nature does the rest. Rising as it
warms, air flows up from the eaves and out of ridge or gable vents, car-
zzzzzz Insulating under the Roof
Ridge vent
rying off moisture that may have migrated from living spaces. In cold
climates, insulation and ventilation combine to keep the roof cold,
thus minimizing melting snow and ice dams. In hot climes, the same
combination rids attics of sultry air and moderates temperatures in the
floors below.
Codes require a net-free ventilation area (nFVA) of 1 sq. ft. of vents
for each 300 sq. ft. of attic space, equally divided between eave and
ridge vents. But building scientist Joe lstiburek takes issue with that,
opting instead for more ventilation at the eaves—say, a 60/40 split,
with the eaves getting the greater proportion. lstiburek reasons that
giving eaves more ventilation “will slightly pressurize the attic. A
Closed-cell foam
depressurized attic can suck conditioned air out of the living space,
or batts
and losing that conditioned air wastes money.”
As near perfect as vented attics are, though, they have weak points
Prefab vent
energy-wise. you can pile insulation as high as you like in the middle
baffle
of the floor to attain required R-values, but the tight spaces where roof
Soffit vent
slopes meet sidewalls are a problem. do your best to air-seal and pile
insulation over top plates—without blocking soffit vents and the like.
Installing rigid insulation over wall sheathing can reduce energy loss
Venting the roof deck. If you want to turn the attic into conditioned
in this vulnerable juncture, but it’s a prohibitively expensive fix unless
space or you’ve got a room with a cathedral (vaulted) ceiling, insulating and
you already need to strip siding for some other reason.
ventilating become more complex. The venting path is essential y the same—
air flows up from the eaves and out at the ridge—but creating a vent channel
under the roof deck can be chal enging. (The IRC requires 1 in. o
f airspace
under roof sheathing; Joe lstiburek cal s for 2 in. of airspace, minimum.)
Instal ing a prefabricated vent baffle (above) is certainly the fastest and
zzzzzz A Vented Soffit
perhaps the most cost-effective way to create a vent channel: Staple prefab
Roof deck
Rafter
baffles between rafters and use canned spray foam to air-seal baffle edges.
Vent-holes
drilled into blocking
Insulating under the roof is the conventional way to go and probably the
between rafters
most affordable if the roof is in good shape. your heating zone, your budget
and the depth of your rafters will decide what type of insulation to use.
Filling joist bays with closed-cell spray polyurethane foam will provide the
greatest R-value per inch, but it’s the most expensive option. If you have
2x10 or 2x12 rafters, you may be able to reach requisite R-values, with
Insulation
some combination of batts between the rafters and XpS rigid foam under
Ceiling joist
them. Furring out rafters to gain additional depth is another option, albeit
Fascia
Drywall baffle
a labor-intensive one.
Insulating over the roof may be a more attractive option if (a) your raf-
Blocking
ters aren’t deep enough, (b) you can’t afford to spray-foam insulation, or
Top plate
(c) your roof is worn out and you need to strip it anyway before reroofing.
The built-up roof shown on the facing page relies on an array of 2x4 purlins
Soffit vent
(horizontal pieces) that capture the foam panels installed over the existing
roof deck and provide nailing surfaces for materials installed over the foam,
as well as and an array of vertical 2x4 spacers that create vent channels
To keep air flowing from the soffit to the ridge, it’s
important to keep attic insulation from covering the
over the foam and under the new plywood sheathing (a second roof deck)
vent to the exterior. Typically, builders use rigid foam to
to which the new roofing will be nailed. A very complicated assembly,
weigh down loose insulation along the sidewalls or
which should be attempted only by seasoned builders.
install combination baffles/air chutes to ensure airflow.
398 Chapter 14
An unvented roof is often the only viable option when roof framing is
complicated, such as when there are hips, valleys, dormers, or skylights
that would prevent eave to ridge ventilation; when the house has no soffits
and hence no soffit vents; or when adding eave vents would clash with the
zzzzzz unvented Roof
house’s architectural style. unvented roofs also make a lot of sense in high-
wind or high-fire areas, where vents might admit drenching rains or embers.
Here again, the choice of what insulation to use and whether to install
it over or under the roof deck depends on the condition of the present roof,
the depth of rafters, the R-value you must attain, and cost. Cost aside, the
most straightforward route is spraying closed-cell polyurethane foam on to
Drywall
the underside of the roof deck. If you need additional R-value, install XpS
foam panels to the underside of rafters, and then cover the foam with
drywall. Correctly done, this assembly creates a premium air-and-thermal
barrier. Again, it’s a complex roof that needs to be impeccably detailed
by a pro.
Closed-cell foam
of course, there are caveats. (There are always caveats.) unvented roofs
in the snow belt may still need some type of venting to keep the roof
Foam air-seals
and insulates
cold and prevent ice dams. And some roofing manufacturers won’t honor
top plate
warranties if their shingles are installed over unvented roofs. For more on
this complex and constantly evolving topic, visit Joe lstiburek’s website:
www.buildingscience.com.
zzzzzz Insulating over the Roof
2x4 spacers at 2 ft. o.c.
Two layers of
rigid foam insulation
5/8 -in. plywood
(new roof deck)
Drywall
Vent strip
Rafter
Spray foam air-
seals and insulates
Doubled 2x4
top plate.
purlins at perimeter
Energy Conservation and Air Quality
399
soffit-to-ridge ventilation can prolong shingle life
Increasing Control ed
and make upper-floor rooms appreciably cooler.
Ventilation
Keeping vent channels open from soffit to
ridge is essential to keeping air flowing.
After air-sealing the house, you may need to add
Continuous soffit vents are typically screened to
controlled ventilation—some mixture of mechan-
keep animals and insects out of the house. Inside
ical ventilation (fans) and natural convection—to the attic, it’s equally important that as you air-
exhaust stale air, remove excess moisture, and
seal the perimeter and insulate the attic floor, you
avoid furnace backdrafting. To moderate temper-
prevent insulation from clogging vent holes
atures under the roof and get rid of moisture that (intakes) drilled in the blocking (see “A Vented
has escaped from living spaces, attic and roof
Soffit” p. 398) or drifting down into soffit vents.
vents work like a charm as long as uprising air is
The simplest way to keep insulation where it
allowed to flow freely. Better yet, this venting is
belongs is to install baffles where attic floor joists
free. Looking at mechanical solutions, whole-
meet the rafters.
house exhaust fans, and a few passive intake
Standard how-to advice is to construct baffles
vents don’t cost much.
on site by cutting up rigid-foam panels—without
SoFFIT-To-RIdGE VEnTIlATIon
saying exactly where or how you attach the
baffles in the tiny triangular spaces under the
If air-sealing and insulating attic floors are the
roof. Moreover, the spaces between rafters almost
first steps to reducing excessive moisture and
always vary, so precutting baffles outside the attic
heat in an attic, increasing ventilation is the
won’t work. In a 30-ft. by 40-ft. attic, you’ll need
second. Nothing exhausts moisture or cools the
roughly 80 lin. ft. of baffles, which could take
area under a roof as effectively as soffit-to-ridge
days to fabricate, fit, and seal.
ventilation, as discussed here and in chapters
AccuVent, a baffle fabricated out of recycled
5 and 7. (Gable-end vents help but are usually 1 ft. plastic, seems a good solution to some of the
to 2 ft. below the highest and hottest air.) As a
problems just mentioned. Unlike rigid-foam baf-
bonus, in winter, cool incoming air can prevent
fles available at many home centers, AccuVents
snowmelt and ice dams along eaves. In summer,
are flexible and less likely to fracture when fit
when unvented roofs can reach 150ºF to 160ºF,
into the cramped triangular spaces where sloping
roofs meet walls. Staple the flexible vent’s lower
end to the inside of a top plate, roll its upper end
Both an air chute and a baffle that
snug to the roof deck and staple it, and then seal
zzzzzz A prefab Baffle/Air Chute
restrains attic insulation, the
its edges with spray foam. Ridges in the back side
AccuVent seems a good solution if
of the vent keep it spaced about 1 in. from the
roofing nails sticking down
sheathing, so air can flow over it. It’s both a baffle
through the sheathing don’t
and an air chute, in other words.
impede its installation.
InSTAllInG An EXHAuST FAn
Baffle
The least expensive mechanical ventilation is a
whole-house exhaust fan—similar to a bath fan
Seal baffle edges with
but rated to run continuously. (This fan is in
expanding foam.
addition to a bath fan and a kitchen range hood.)
It’s hard to imagine an exhaust fan as small as
Batt or loose-fill
insulation
50 cfm ventilating a whole house, but that size
can vent up to 1,500 sq. ft. Moreover, Energy
Star–certified fans such as Panasonic’s FV series
do so cost-effectively, with a 15-watt fan running
8 hours a day costing less than $10 annually to
operate. To avoid depressurizing the house and
causing backdrafting, it’s wise to install two or
three passive intake vents as well. The cost of
fan and intakes is modest—only a few hundred
dollars—so spend a little more and have an
HVAC specialist size and locate components for
you and help you choose a 24-hour automatic
timer to control the fan.
400 Chapter 14
P R O T I P
Bath-Fan BEllS And WhIStlES
There are complex formulas
Removing moisture is the primary function of a bathroom fan—good to remember
for sizing bath fans, but a good
when considering all the extra features you could buy. First, get a quiet fan: 2 sones to
rule of thumb is 1 cfm (cubic
3 sones is tolerable, 1 sone is quiet and there are even 0.3-sone fans. Next, consider
foot per minute) per square foot
switches. Because fans usually need to continue venting after you leave the room, get
for bathrooms 100 sq. ft. and
smaller. For bathrooms larger
an electronic switch with an integral timer so the fan can keep running after the light
Renovation 4th Edition Page 93