My definition of an unvented attic is an attic
where there are no vents and where the attic insulation follows
the slope of the roof sheathing thereby including the attic space
within the conditioned building enclosure.
The rationale for venting attics in the South is to "flush"
heat. The dominant heat transfer mechanism in an attic is radiation.
Venting attics will not "flush" radiation. The air change
in a perfectly built and vented attic (code 1:300 ratio) results
in an average air change rate of 3 to 6 ach. At this attic air
change rate there is approximately a 2 to 3 percent reduction
in heat transfer to the conditioned space through the vented attic
as compared to an unvented attic insulated to the same level.
This assumes an airtight ceiling and no ductwork in the attic
and certainly not leaky ductwork in the attic. The moment ductwork
(assumed airtight in this instance and insulated at R-6) is installed
in a vented attic, the balance changes. There is approximately
a 5 to 7 percent increase in heat transfer to the conditioned
space as compared to my version of an unvented attic. This is
due to conductive heat gains through the surface of the ductwork
and air handler now located in a "hostile" location
(a hot, vented attic), rather than inside a 75°F conditioned
space (the "house"). The moment leaky ductwork is installed
in a vented attic there is approximately a 25 percent increase
in heat transfer to the conditioned space. Of course this does
not happen if you have airtight ducts and an airtight ceiling
(then the penalty for venting the attic is only 5 to 7 percent
as previously noted).
Now, if you locate the ducts within the conditioned space and
also build an airtight ceiling, this is approximately 2 to 3 percent
more efficient than my version of an unvented attic. I never said
that this wasn't the most energy efficient way to do it. Of course
when is the last time you saw ductwork below an attic ceiling
coupled with an airtight attic ceiling? Builders put things in
attics because they don't leave any room in the house for the
ductwork and air handler. If they continue to do this, then venting
attics is a dumb idea.
So much for the energy concerns. Now lets talk moisture. What?
Are you all crazy? The air outside is hot, humid and disgusting.
And you want to bring this into an attic where it can diffuse
through the vapor barrier-less attic insulation and get to the
cold, air conditioned ceiling? What were we thinking! Before it gets there it will
see those cold R-6 insulated ducts, fittings, etc. and drip all
over. Give me a break. Venting attics in the South was dreamed
up by some disgruntled Yankee pissed about the Civil War and wanting
to get even. Be sure when insulating at the roofline in humid climates to follow
moisture control principles as you would with any insulated wall so that the
roof assembly is self-drying (Unvented
Lets now talk about durability of shingles and shingle temperature.
Venting or non-venting a roof has about a 5 percent impact on
shingle temperature and roof sheathing temperature and even less
on shingle durability. The color of the shingle is more important
than venting or non-venting. And temperature is less important
than the shingle getting a sunburn. The biggest impact on shingle
durability is ultra-violet light. UV is more critical than temperature.
The best roof for hot, humid climates for all applications (including unvented
attics) is a concrete or clay tile
Crawl spaces are real simple to understand and deal with. When
you vent crawl spaces you bring in hot, humid air and cause moisture
and mold problems. The ground surface is typically colder than
the dew point temperature of the exterior air. The underside of
crawl space floor insulation is radiation coupled to the ground
surface and is very close to the same temperature of the ground.
Moisture droplets can be seen all over the top surface of typical
polyethylene ground covers as well as hanging from the bottom
surface of the crawl space floor insulation. Gee, I wonder how
all the water got through the poly ground cover? It must have
leaked through the walls. Give me another break. Now, when the
moisture is in the insulation where do you think it wants to go?
Where is the air conditioning? Moisture moves to the cold surface.
Venting crawl spaces made sense only when you had no air conditioning
and no insulation and no crawl space walls.
Interested in unvented design strategies? It will take a while to change
common practice as builders and contractors learn to adopt new approaches. Be
sure to consult local experts and code officials before attempting unvented
attics or crawl spaces on your own. Many design details that cannot be covered
here are important to achieve best performance. See
Designs That Work
for more information.
This is a no brainer. Negative air pressures in buildings in
hot, humid climates induce infiltration of hot, humid air. Period.
Leaky ducts in vented attics and vented crawl spaces lead to negative
You can never clean them. When they are wet
and dirty they grow bad stuff. They get wet and dirty. When they
are wet and dirty and when they grow bad stuff the only thing
you can do is throw them out. At least you can clean and decontaminate
the metal ducts.
You don't need lined ducts for acoustical reasons. The acoustical
argument to justify lined ducts is only used by engineers and
others who don't know how to design acoustically. Put them inside
and you don't need to insulate them much, especially if you control
the interior humidity. Of course you need to know how to do that.
When was the last time you ever saw an engineer figure out
how to handle the latent load? I'll let you in on a little secret.
You can't do it when you mix ventilation with sensible cooling.
Engineers love to complicate things. Lets get this one really
neat system to do everything. Get out of town. Can't do it anymore
with one integrated system. You need two systems. One to handle
the ventilation and its associated preconditioning requirements.
The other to handle the space temperature. When you handle the
ventilation, you handle the latent load. The rule is always
deliver neutral temperature, dry ventilation air. And not worry
about other moisture loads. If you keep the rain out, there won't
be any. Now keeping the rain out may be a problem (see # 5).
This is a vapor barrier on the wrong side of the wall. It should
be put on the outside of buildings, not on the inside in the South.
If a carpet is dry, at the same temperature
as the occupied space, and cleaned it is great. Especially if
it has a vapor permeable backing. Now, it allows the concrete
floor slab to "breathe" into the occupied space (vinyl
floor coverings don't breathe, just like vinyl wall coverings).
I like carpets that are dry, have no (or a "very small")
temperature gradient across them and are cleaned. In schools which
are slab on grade, forget it. The carpet is regularly colder than
the air, especially when the a/c is turned off and you get the
thermal lag. Now you have a higher relative humidity in the carpet
than in the air. Now you get mold and dust mites. When was the
last time you saw a school with a decent housekeeping budget?
They can't afford to pay the teachers! Of course there is always
money to pay the 16 levels of administrators. Then carpet is maintained
and in a building which is operated correctly (HVAC system-wise),
it is one of the best floor coverings around. But, if you aren't
going to use carpet properly, you are better off not using it
I love brick. I also hate brick. Let me tell
you when I hate it. I hate it when it does not have a drainage
plane behind it that is also an exterior vapor barrier. Wet brick
exposed to the sun is like a moisture capacitor which discharges
to the cold side. The cold side is the interior air conditioned
space. I love brick when there is a vapor barrier acting as a
drainage plane between the brick and the rest of the wall assembly.
Most brick in the South is installed without functioning drainage
planes and effective vapor barriers. If you are not going to use
it right don't use it. When it is used right, it is the best exterior
cladding system around (just don't paint it).
I love stucco. I
also hate stucco. Sound familiar, see Brick above. Stucco over moisture
sensitive materials such as wood framing and steel studs needs a drainage layer
and a drainage gap. With traditional stucco this requires two layers behind the
stucco rendering - a "bond break" and a "drainage plane". With "synthetic
stucco" this requires a "water managed" system that drains.
They can't handle part load conditions. Most systems that use
them are inherently destined to be negative
pressure systems (meaning the building conditioned space operates
negative). I like them when they only do cooling and supply no
Dilution is not the solution to indoor pollution
in the south. A bunch of cowardly Yankees refused to target the
real issue of source control in many buildings due to the related
issue of material off-gassing. Too many manufacturers of building
products would get pissed off. What? You want me to actually tell
you what I put in my product? And then tell me I can't put it
in? No way! Hey, why don't we increase air change and flush out
the nasties? Everyone wins! You get to sell more energy. You get
to sell more and bigger equipment. Every existing system is now
obsolete and now you have to make it bigger. Engineers get to
charge more. Contractors really win. And you can continue to put
the bad stuff in the products and sell them. Of course, no true
Southerners were in that meeting. In the South there is more bad
stuff in the outside air than there is bad stuff in the inside
air. It's the charm of the South.