Energy & Moisture Matters
We ask a panel of building scientists and builders
— all keen, experienced observers of wood-frame
performance issues — to answer some of the questions
that never seem to go away
Do vapor retarders do any
Joe Lstiburek, PE, a
principal of Building Science Consulting in Westford, Mass.,
responds: The answers are "sometimes" and "it depends."
The primary purpose of a vapor retarder is to reduce the amount
of moisture that gets into an assembly. Unfortunately, the same
vapor retarder can also reduce the amount of moisture that can
get out of an assembly. So what we really want is a way to
prevent moisture from getting into an assembly and a way to
allow any moisture that does get in to get out. So far, this is
pretty logical and easy to understand.
But it gets more complicated. Moisture flows from "more to
less" (flow follows a concentration gradient) and from "warm to
cold" (it also follows a thermal gradient). Homes in cold
climates that are heated in the winter have a higher
concentration of moisture inside than out, and —
obviously — a higher temperature inside than out. Thus
moisture flow is from the inside to the outside during
But the same home may be air conditioned in the summer, and
then the flow is reversed. So where do you locate a vapor
retarder? If you put one on the inside, it may cause problems
during air conditioning. If you put one on the outside, it may
cause problems during heating.
So here is the final word on the subject (until I change my
mind again): In cold climates in non-air-conditioned buildings,
install vapor retarders on the inside. In hot, humid climates
in air-conditioned buildings, install vapor retarders on the
outside. Everywhere else, you're better off without them.
The 2009 IBC will provide detailed guidance on this subject,
including climate maps and assemblies — the whole nine
Is installing painted wood
siding and trim over an air space — so-called
rain-screen siding — really worth the effort?
Paul Eldrenkamp, owner of
Byggmeister, a custom remodeling firm in Newton, Mass.,
responds: The first project we did with rain-screen siding
was in 1989. It entailed stripping existing shingles, applying
rigid foam insulation over the old sheathing, screwing
horizontal 1x3 battens through the foam into the sheathing, and
then installing cedar shingles that had been prestained on all
edges over the strapping.
We have been back to the house several times since, including
a couple of months ago, and the whole installation has held up
superbly: The shingles lie flat, the stain is holding up well
(the house has been restained once in the intervening years),
and the strapping has consistently been bone dry when we've
tested it. This is on a house with minimal overhangs along the
eaves, no overhangs on the gable ends, and a lot of shading
from large trees, so water exposure is significant and drying
opportunities are limited.
We also used the rain-screen approach at my own house in 1997.
I have clapboards above first-floor window-sill height, but up
to the sill height we installed tongue-and-groove fir beadboard
for a wainscot effect. All the wood was preprimed and
prepainted on all sides prior to installation and installed
over strapping, which in turn was installed over rigid foam. I
have small overhangs — 6 inches on the gable ends, 12
inches at the eaves.
The 10-year-old paint job shows no sign of failure whatsoever.
There are a few mildew spots, which wash off easily with a mild
detergent solution when I bother to do it. On several occasions
after an extended rainy period I've tested the wood siding for
moisture content, and I've found elevated readings (18 percent
or so is not unusual), but never any sign of paint failure,
which to me indicates a resilient system.
Additionally, we once had occasion to do work on a stucco
house built in the 1940s. Felt had been applied to the
sheathing, then vertical wood strapping, then metal lath, then
three-coat stucco. The lath was a little corroded in places,
but overall the stucco was in good condition, and the strapping
and felt appeared to have another half-century of life left in
These are not the only jobs we've done (or seen done) this
way; there have been dozens over the years. They all perform
So rain-screen siding clearly works. For me, the question has
been, "Is it overkill?" In other words, is the extra benefit
really worth the extra work? Actually, I'm so confident it's
worth the effort, we don't install siding any other way. But
I'm also keenly aware of what I perceive to be still-unanswered
questions, such as "If I'm using wood siding, do I need to both
prefinish on all sides and create the air space, or can I do
one or the other?" And "How big does the air space need to
With wood siding (especially clapboards), back-priming seems
to be more important than the air space. We have done some jobs
with no air space but with wood siding prefinished on all
sides, and the paint's held up very well (10 years without
failure, in at least one case). These jobs do seem to have more
mildew and cedar bleed, but I don't know if there's a
connection (or even if that observation would really hold up if
I tried to quantify it).
Some researchers have suggested that a clear preservative on
the back of the claps would be preferable to primer or paint,
creating the best balance of antiabsorption and drying
properties. I think that this is over-thinking the problem, and
that it fails to acknowledge the realities of the job site.
Ordering the claps prefinished on all sides is much easier and
faster — and, in my experience, yields an entirely
effective end result. Plus, should any problems arise, just
imagine your conversation with the paint-manufacturer
representative when you tell him you have a different finish on
each face of the clapboard.
So if there is absolutely no way to create an air space
— a job, for example, where there's no latitude to
thicken the wall even by a fraction — at the very
least order your clapboards prefinished on all sides. Our
contracts often include a drop-dead date by which the homeowner
has to select an exterior paint color so we have time to have
the prefinishing done.
How big an air space to use is a harder question. Some
researchers seem to think that only drainage is important
— that the depth of the space needs to be just enough
to allow water to flow down behind the siding, something on the
order of 1/8 inch or even less.
Others seem to think that ventilation is important, too
— that there should be clear continuous channels not
only for top-to-bottom drainage but also for bottom-to-top
airflow. This is the thinking behind Benjamin Obdyke's Home
Slicker and the old-fashioned 1x3 (or plywood strip) battens
that we use. A few researchers seem to think that the air space
itself is what's important — to allow for even drying
of the cladding material when it does get wet.
My observation of our projects in our Boston-area climate is
that you will have a durable, trouble-free exterior regardless
of what products you use and of whether you actually achieve
continuous top-to-bottom drainage, as long as the following
conditions are met:
•• The flashings and building paper guide the
water away from the sheathing and to the outside with 100
•• You have an air space of at least 1/2 inch
behind the siding material.
•• All wood siding and trim is finished on all
sides before installation.
Every new exterior job we do gives us an opportunity to test
the durability of rain-screen siding. To complete the
experiment, we need to observe its outcome over an extended
period. There is no substitute for going back to past projects
in a systematic way and seeing first-hand how they've held
Ever since we began building
tighter walls and ceilings, it seems we've been getting more
moisture and mold problems in houses. Isn't it better to leave
our houses a little bit leaky than to make them too
Martin Holladay, editor of
Energy Design Update, responds: A hundred years ago, most
houses had uninsulated walls and numerous air leaks. Now that
tighter building practices are standard, any incidental water
that gets into a wall dries very slowly, and problems with wall
rot and mold have increased. But building a new home to be "a
little bit leaky" is more apt to increase than decrease the
likelihood of moisture problems.
Filling framing bays with insulation — rather than
leaving them empty — makes walls and ceilings less
forgiving of moisture intrusion, for three reasons: Insulation
can act like a sponge, absorbing water that might otherwise
have drained out; it reduces airflow, slowing the rate of
drying; and it makes the exterior sheathing colder, introducing
a potential condensing surface.
Since building uninsulated houses is no longer an option,
builders must learn how to assemble walls and roofs in ways
that minimize water intrusion. To keep out exterior water
— wind-driven rain — a house needs careful
flashing at windows and other penetrations, and the flashing
must be properly integrated with a water-resistant barrier.
Ideally, a wall should include a free-draining air gap (rain
screen) between the siding and the sheathing.
Interior moisture is usually less of a problem for walls and
ceilings than exterior moisture. Since recent research has
shown that interior polyethylene can do more harm than good in
many U.S. climates, knowledgeable builders in all but the
coldest areas often omit interior poly. The most common way
that interior moisture enters walls and ceilings is by hitching
a ride with exfiltrating air; that's why it's important for
most homes to have a very good air barrier.
What's wrong with leaving a house "a little bit leaky"? In
theory, a certain amount of air exchange is a good thing:
Introducing fresh air is good for a home's occupants, and air
movement through walls and ceilings can, in some circumstances,
help dry out moisture that would otherwise be trapped.
In practice, however, this approach doesn't accomplish either
task very well. If interior air enters a wall through one of
the "little leaks," moisture in the air can condense on the
back of the wall sheathing. In other words, even though air
movement through a wall assembly can help dry out moisture in
some circumstances, it can deposit moisture in others.
Moreover, infiltration levels vary with the weather. In cold
weather, the stack effect increases airflow through a house; in
mild weather, infiltration and exfiltration are lower.
Similarly, wind increases the rate of air exchange in most
homes. But people need a relatively constant supply of fresh
air, whether the weather is hot or cold, windy or still.
If you want a house with few mold and moisture problems, you
have two choices. The first — to build a house without
any insulation at all — is illegal in most locations.
The second and more logical choice is to build a tight building
envelope — designed to handle incidental moisture
— and equip the house with some type of mechanical
Is spray-foam insulation
worth the extra expense?
Paul Eldrenkamp, owner of
Byggmeister, a custom remodeling firm in Newton, Mass.,
responds: Often, but not always. For an effective
insulation job, you need both good R-value and good
air-sealing. Spray foam is an expensive way to get R-value but
a relatively cheap way to get good air-sealing —
especially in retrofits.
Spray foam is probably not worth the extra expense in the
following types of projects:
•• Closed-cavity retrofits, like the walls of
an older home with no insulation. Here, use cellulose, and try
to get it installed to a high density — 3.5 pounds of
material per cubic foot of volume. You may have a hard time
finding a cellulose insulation contractor who knows how to do
this (or has even heard of it), but it's worth trying.
•• New construction or large-scale additions
where you can cost-effectively wrap the structure (both walls
and roof) with a layer of rigid foam before applying the
exterior finish. This minimizes thermal bridging and provides
good air-sealing (as long as you tape or gasket the joints in
the foam boards). And it means that almost anything you use for
framing cavity insulation — including fiberglass batts
— will be effective.
•• New construction or large-scale additions
where you can get a quote for cellulose (blown-in/mesh system
or damp-spray system) that beats the quote you get for
spray-foam. A good cellulose installation is often less costly
and just as effective as a spray-foam installation.
•• Attics with a simple geometry in which you
choose to insulate the floor rather than the rafters. You do
need to make sure to seal all of the penetrations in the
ceiling plane before you blow in the cellulose, and you should
avoid putting mechanical equipment in the attic above the
•• Cast-concrete basements or crawlspaces
where the walls and floors are even enough that you can install
sheets of rigid foam easily and tightly.
Spray-foam is usually worth the extra expense in these types
•• Relatively small jobs, like small additions
or partial guts, where there's a logistical and scheduling
advantage to having just one subcontractor and one material to
•• Houses where the attic is wholly or partly
finished. Once the attic starts becoming living space, it's
almost always most effective to insulate and air-seal the
outermost plane of the roof structure (rafters rather than knee
walls, for instance). This is where spray foam shines. Install
it from the top plate of the wall up to the ridge all around
the attic, like putting a cap on the house. Don't worry about
venting the roof. Most researchers I've spoken with (Joe
Lstiburek, William Rose, Terry Brennan) advocate applying one
or two coats of latex paint to open-cell foam insulation to
minimize vapor diffusion. There's uncertainty as to whether
this is a necessary precaution, but it's cheap and easy enough
to do. Closed-cell foams have a low enough perm rating that
they do not need the vapor diffusion retarder.
•• Any house where it's difficult to define a
simple, continuous boundary between tempered and untempered
space. Houses with lots of angles, plane changes, split levels,
dormers, bays, and so on are often going to be easier to
insulate and air-seal with foam than with other methods.
•• Old, uneven basement walls and floors. We
use a closed-cell foam on old basement walls and then spray it
with shotcrete to get the flame-spread rating required by code.
We've even sprayed the higher-density closed-cell foam on
basement floors, then poured lightweight concrete over the foam
to create a level, insulated floor slab.
Ultimately you need to figure out for yourself when one
material or technique is more appropriate than another. A lot
depends on the relative skills of your crew, on which
insulation subcontractors in your area are most reliable and
knowledgeable, and on what types of houses you work on. One
unavoidable fact, though, is that you will never know for sure
which materials and techniques are working best unless you
regularly test your jobs with a blower door and infrared
camera; otherwise, you'll just be guessing.
My insulation contractor
installed dense-pack cellulose in the walls of a 100-year-old
house. Two years later, the exterior paint began to peel, even
though the paint job was only four years old. Did the
insulation cause the paint to fail?
Martin Holladay, editor of
Energy Design Update, responds: Indeed, adding insulation
to the walls of an older home can shorten the life of the
exterior paint. The phenomenon has been observed for decades;
when conducting research on vapor retarders, William Rose, a
building researcher at the University of Illinois at
Urbana-Champaign, unearthed evidence of failing-paint disputes
between insulation contractors and exterior painters dating
back to the 1940s.
The failing paint has often been blamed on the fact that the
walls of most old houses lack interior polyethylene. With
little to slow vapor diffusion, interior moisture is said to
travel through the walls until it reaches the sheathing, the
siding, or the back of the paint film, causing the exterior
paint to fail.
As it turns out, this explanation is incomplete and
misleading. Although vapor diffusion can occur through exterior
walls, the effect of vapor diffusion on exterior paint
performance has been greatly exaggerated. Moreover, since most
old houses have several layers of interior paint, the walls in
question usually do include a vapor retarder; even two coats of
paint have a relatively low permeance (1.5 to 5 perms), and
each additional layer of paint will improve the paint's
performance as a vapor retarder.
In fact, adding insulation to a wall does make the siding more
humid, but not because of the lack of a vapor retarder. Adding
a thermal barrier between the siding and the warm interior
makes the siding colder; under the same conditions of vapor
pressure, colder materials are wetter than warmer materials. In
other words, before the insulation was installed, the
relatively warm stud bays helped keep the siding dry.
Of course, damp siding doesn't hold paint as well as dry
siding. The source of the moisture absorbed by cold siding
under these circumstances — called "regained moisture"
by building scientists — is the exterior environment,
not the interior.
On a new home, several measures can lengthen the life of a
paint job, among them specifying siding that has been factory
primed on all sides and installing the siding over rain-screen
strapping. On an existing house, such measures are not usually
possible; in many cases, homeowners who choose to insulate the
exterior walls of an older home may have to get used to more
frequent exterior-paint jobs.
Despite evidence that it's harder to keep paint on an
insulated wall than an uninsulated wall, a good painting
contractor should not hesitate to stand behind an exterior
paint job on an old, recently insulated house. Exterior paint
will last longer when the siding is carefully prepped; ideally,
this work should include the complete removal of all the old
paint, down to bare wood. If quality paint is specified and the
paint is applied in good weather, the paint job should last for
Mold has been around forever;
many of us grew up in houses that had the occasional mildew
spots in the corner of the ceiling, or mold-stained lumber in
the basement or attic. But in recent years it seems like mold
has become a really big deal. Is this much ado about not very
much, or a justifiable concern? Put another way, is mold a
worse health problem today or are there just more
Terry Brennan, a principal
of Camroden Associates in Westmoreland, N.Y., who served as a
moisture consultant on the Institute of Medicine's Committee on
Damp Indoor Spaces, responds: Good question! Mold has
indeed been here much longer than we have and will no doubt
still be here in the distant future. And yes, there are
certainly more lawyers now than there were when I was a country
boy in upstate New York. Ben Wattenberg, in the PBS production
The First Measured Century, reports that the number of lawyers
per thousand people has increased from 1.3 to 3.5 in the last
While I don't have those kinds of statistics for mold, I do
believe the amount of mold in buildings has increased as
changes in construction have occurred. Some construction
changes made walls stay wet longer — filling the
cavities with porous insulation, replacing diagonal board
sheathing with sheets of plywood and OSB, and adding poly vapor
retarders (or unintentional vapor barriers like vinyl
wallpaper) to the inside of walls.
We also gradually replaced relatively mold-resistant materials
— such as brick, plaster, and old-growth heartwood
— with materials containing sugars and starches that
many molds can use as food, like paper-faced gypsum board,
wood-based composites, and wood species with little resistance
to mold growth.
During the same period, we also began air conditioning more
buildings, which cools indoor walls, ceilings, and floors below
the outdoor air temperature. When the outdoor dewpoint is
higher than the indoor air temperature, the ventilating air no
longer dries out the house, but wets it. Any surfaces in the
air-conditioned house that are colder than the room temperature
— for example, the supply ducts, the supply diffusers,
or anything the supply air blows on — will be the
first to collect moisture and grow mold.
So in general, there's more humidity in the house, better
food, and, yes, more mold. But is it harmful?
The Institute of Medicine of the National Academy of Sciences
convened the Committee on Damp Indoor Spaces to examine the
medical literature for evidence of health effects linked to
occupying damp buildings. In 2004, the committee published its
findings in a book, Damp Indoor Spaces and Health, which
reported evidence of an association between living in damp
spaces and upper-respiratory (nose and throat) symptoms,
wheezing, asthma symptoms in sensitized asthmatics, coughing,
and hypersensitivity in susceptible persons, as well as limited
evidence of an association between living in damp spaces and
lower-respiratory illness in otherwise healthy children.
What's the best way to
insulate a basement foundation?
Paul Fisette, director of
Building Materials and Wood Technology at the University of
Massachusetts Amherst and a JLC contributing editor,
responds: The answer depends on the budget, the R-value
you're trying to achieve, how the space will be used, and
whether you're simply housing mechanicals or creating a living
space in the basement.
Assuming you don't need a tempered basement space, the best
and most economical approach is to insulate the floor that
separates the living space from the basement. This will
minimize the volume of the home's thermal envelope and the
amount of energy required to condition the living space. Also,
it's easier to install insulation with higher R-values in the
basement ceiling. This is the design that will have the
greatest payback in reduced energy costs.
The cheapest way to insulate the basement ceiling is to
install unfaced fiberglass insulation. A better method is to
cover the ceiling with drywall, then blow in cellulose
— a comparatively inexpensive upgrade, considering the
benefits. Not only does this method provide superior R-value,
but — if detailed well — it stops air leakage
at one of the most critical places in the house. (Most of the
inward air leakage caused by the wintertime stack effect
happens at the bottom of a house.)
If the goal is to provide basement living space, then of
course you'll have to insulate the basement walls. Wrapping the
outside of the foundation with rigid polystyrene is a common
approach; the materials aren't expensive, and the foam board
needs protection only above grade.
However, the space between the concrete foundation and the
back of the foam board can become a termite highway —
a hidden path connecting the soil directly to the framing
— unless you add the cost of a carefully installed
termite shield. Also, consider that a 1-inch layer of
polystyrene provides a meager R-5 of thermal protection; if you
increase the thickness to 2 inches or more, the project gets
even pricier — plus it's tricky to integrate the foam
with the frame wall above.
A simpler, better approach is to insulate the inside of
foundation. First, though, make sure the basement doesn't leak
and is protected by a good drainage system. In a retrofit
situation, applying a layer of damp-proofing on the interior
surface of the foundation wall is cheap insurance. I've had
good luck using Sto Watertight Coat, a two-component
cementitious compound with a low perm rating.
On top of the waterproofing, I attach rigid foam insulation
directly to the inside surface of the foundation walls with
construction adhesive, then caulk or tape the seams so that
warm interior air can't reach the cold foundation. Finally, I
build an uninsulated wood frame, spaced away from the
foundation to make room for plumbing and wiring. The foam
insulation keeps the surface of the wall above dewpoint
temperature, reducing the likelihood that condensation will
form in the wall.
Even though I installed R-38
fiberglass batts on the attic floor, a house I recently built
has suffered from roof leaks caused by ice dams. What's the
Martin Holladay, editor of
Energy Design Update, responds: If the attic floor has
adequate insulation, the most likely cause of an ice-dam
problem is that warm interior air is leaking into the attic
through cracks in the ceiling.
Ice dams begin when warm attic temperatures melt the lowest
layer of snow on a roof. The water flows downhill and refreezes
when it reaches the colder roofing at the eaves, gradually
thickening until an ice dam is formed. Such dams can become
thick enough to trap upslope meltwater; in some cases, the
water can be forced under the roof shingles and can dampen the
In sunny, cold weather, icicles can appear on any roof. As
long as the ice doesn't lead to a wet ceiling, it isn't really
a problem. Heavy ice at a building's eaves, however —
with or without wet ceilings — is usually a sign that
too much heat is escaping through the ceiling.
To some extent, ice-dam problems can be reduced by the
installation of a rubberized roof membrane like Grace Ice
& Water Shield. While a 6-foot-wide band of rubberized
membrane at the eaves is always a good idea, rubberizing the
entire roof is a crude defense against roof ice. If the roof
sheathing is warm enough to melt snow, the solution is not to
install wider and wider bands of rubber, but to prevent the
heat from escaping the house.
Many builders try to solve ice-dam problems by increasing the
size of soffit and ridge vents, but this strategy rarely works.
In fact, since air leakage is a more common cause of ice-dam
problems than insufficient insulation, increasing attic
ventilation can actually make things worse. A larger ridge vent
tends to increase the flow of air into the attic; the source of
that air might be the soffit vents, but if the ceiling is
peppered with holes and cracks, it might also be the home's
interior. In other words, a better ridge vent can actually
increase the flow of heated air into the attic.
The first step in any ice-dam investigation should be to crawl
up in the attic and look for ceiling air leaks. (If the house
has a cathedral ceiling, air leaks should be sealed from the
interior.) Common leak areas include attic access hatches,
recessed can lights, plumbing and electrical penetrations,
chimneys, bathroom exhaust fans, poorly sealed bathroom and
kitchen soffits, and cracks between drywall and partition top
Once the ceiling is relatively airtight, the next step is to
verify that the attic insulation is thick enough, especially
near the eaves, and that its R-value is not being degraded by
The use of fiberglass batts can increase the likelihood of
ice-dam problems. Since fiberglass insulation does little to
slow airflow, it can't stop warm air from escaping through a
ceiling leak. Moreover, its effectiveness can be degraded by
the flow of cold air entering the soffit vents. Fiberglass
batts also have a lower R-value per inch than rigid foam and
sprayed urethane foam.
In some houses, the space between the perimeter wall plates
and the roof sheathing is too cramped for adequate levels of
fiberglass insulation, especially if a vent channel is
required. If the roof is framed with raised-heel trusses,
fiberglass batts may work well, as long as the ceiling is
relatively airtight and as long as the builder includes a
wind-washing dam above the top plate. In many attics, however,
the problematic area under the eaves is best insulated with
sprayed polyurethane foam or several layers of rigid
Once the air barrier is in place, the next line of defense is
an uninterrupted layer of thick insulation.
Compared with a ceiling air barrier and thick insulation,
attic ventilation is a relatively minor concern.
What exactly is the
difference between an air barrier and a vapor retarder?
Joe Lstiburek, PE, a
principal of Building Science Consulting in Westford, Mass.,
responds: Air barriers control airflow, and vapor
retarders control vapor flow. Vapor retarders are not typically
intended to retard the migration of air; that's the function of
Confusion between the two arises because air often holds a
great deal of moisture in the vapor form. When this air moves
from location to location due to an air-pressure difference,
the vapor moves with it. In the strictest sense, air barriers
are also vapor barriers when they control the transport of
Part of the problem is that we struggle with names and terms:
vapor retarders, vapor barriers, vapor permeable, vapor
impermeable. In an attempt to clear up some of the confusion,
here are the definitions that I use.
Vapor Retarder: An element designed and installed in an
assembly to retard the movement of water by vapor diffusion.
The unit of measurement typically used in characterizing the
water-vapor permeance of materials is "perm." There are several
classes of vapor retarders:
Class I vapor retarder: 0.1 perm or less
Class II vapor retarder: 1.0 perm or less, and greater than
Class III vapor retarder: 10 perms or less, and greater than
Vapor barrier: a Class I vapor retarder
Materials can also be separated into four general classes
based on their permeance:
Vapor impermeable: 0.1 perm or less
Vapor semi-impermeable: 1.0 perm or less, and greater than 0.1
Vapor semi-permeable: 10 perms or less, and greater than 1.0
Vapor permeable: greater than 10 perms
Air barrier: A system of materials designed
and constructed to control airflow between a conditioned space
and an unconditioned space. The air-barrier system is the
primary air-enclosure boundary that separates indoor
(conditioned) air from outdoor (unconditioned) air.
Air barriers also typically define the building's pressure
boundary. In multiunit construction, the air-barrier system
also acts as the fire barrier and smoke barrier between units.
In such assemblies, the air barrier has to meet the specific
fire-resistance rating requirement for the given
Air-barrier systems consist of individual materials
incorporated into assemblies that are interconnected to create
enclosures. Each of these three elements has measurable
resistance to airflow (in liters per second per square meter at
75 Pascal pressure). The minimum resistance, or air permeance,
for each is:
Material: 0.02 l/(s-m2)@ 75 Pa
Assembly: 0.20 l/(s-m2)@ 75 Pa
Enclosure: 2.00 l/(s-m2)@ 75 Pa
For more information on air barriers and vapor retarders,