ASHRAE Journal - June 2020 - 76

BUILDING SCIENCES
COLUMN

Joseph W. Lstiburek

Conditioned Unvented
Attics & Unconditioned
Unvented Attics
But With Fiberglass and Mineral Wool
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

Yes, we have done unvented conditioned attics before but mostly with spray foam and
rigid foam insulations. But we do not have to use spray foam and rigid foam insulations. You can use almost anything as long as you follow a few rules. OK, not so much
rules, but experience based on measurements and field performance. Real-world
stuff.*
Attics can be insulated on the top of ceilings or attic
floors. Additionally, attics or roofs can be insulated on
the top of a roof deck, on the bottom of a roof deck and
both on the top and bottom of a roof deck.
Attics can be designed and constructed to be either
vented or unvented in any climate zone. The choice
of venting or not venting is a design and construction
choice-not a requirement determined by the building
code. The model codes allow both vented and unvented
attic assemblies.
Unvented attics are also often referred to as conditioned attics-when they are part of the house that is
heated and cooled-and when the insulation is at the
roof deck (directly under it, directly on top of it, or a
*Not modeling-you use real world stuff to "tune" models. You do
not believe models until you build the "modeled thingy" and measure how it works in the real world. Then, you believe the model.
Most models ought to be quarantined with their modelers.

76

ASHRAE JOURNAL

ashrae.org

J U N E 2020

combination of both. Note that unvented attics do not
always have to be conditioned. This occurs when the
attic is unvented and the insulation is at the ceiling; they
are not part of the house that is heated and cooled. The
model codes allow both conditioned and unconditioned
attics. More specifically, the model codes allow conditioned unvented attics with fiberglass and mineral wool
(Figure 1) and unconditioned unvented attics with fiberglass and mineral wool (Figure 2). But there are pretty
specific requirements as we will point out.
First, some background. Conditioned unvented attics
have significant advantages over unconditioned vented
attics. The building enclosure can be made significantly
more airtight much easier by constructing an unvented
attic, thereby making the building much more energy
efficient. Ductwork and air handlers typically leak,
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.


http://www.buildingscience.com https://www.ashrae.org/

ASHRAE Journal - June 2020

Table of Contents for the Digital Edition of ASHRAE Journal - June 2020

Contents
ASHRAE Journal - June 2020 - Intro
ASHRAE Journal - June 2020 - Cover1
ASHRAE Journal - June 2020 - Cover2
ASHRAE Journal - June 2020 - 1
ASHRAE Journal - June 2020 - Contents
ASHRAE Journal - June 2020 - 3
ASHRAE Journal - June 2020 - 4
ASHRAE Journal - June 2020 - 5
ASHRAE Journal - June 2020 - 6
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ASHRAE Journal - June 2020 - Cover3
ASHRAE Journal - June 2020 - Cover4
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