Without Continuous Air Barrier System, a Project Might be Full of Holes

[Guest guest]

This article is interesting, if for anything, to find out

that an organization called the Air Barrier Association of America

exists. I didn't know of it. The article also cites a NIST

publication that might be of interest. Usual disclaimers

apply. If you have issues, talk to my lawyer. Matt

Consulting-Specifying Engineer

Without Continuous Air Barrier System, a Project Might be Full of

Holes

By Tom , Product Manager, BASF Polyurethane Foam Enterprises

LLC -- 6/5/2006 10:22:00 AM

It can be quite literally full of holes. The building draws unconditioned

air in, then leaks conditioned air back out through holes, cracks, gaps

and other leaks of varying size. It’s called “uncontrolled” air leakage,

and it costs a lot of money. The U.S. Dept. of Energy estimates

uncontrolled air leakage can account for 30% or more of a building’s

heating and cooling costs, and contribute to problems with

moisture.

Most industrial, commercial and institutional buildings in the United

States leak air. The good news is it’s not the HVAC engineer’s fault. You

can design the most efficient HVAC system imaginable, and the building

will still draw in air. However, a simple understanding of the

interaction between the HVAC and the building envelope can help you

prevent future buildings from drawing air in and leaking air out with the

inclusion of a continuous air barrier system. Here’s how it

works:

Air leakage occurs through cracks, gaps, holes, pores in materials and

other openings in the building envelope. Airflow is the result of

pressure differences. When air leaks, it takes with it heat, water vapor,

smoke, pollutants, dust, odors, allergens and anything else it can find

and carry. Energy moves from regions of high to regions of lesser

potential: hot and cold, high pressure to low, and so on.

There are three major sources of pressure that cause air to leak: wind

pressure, stack pressure and HVAC fan pressure. Of the three, wind is

usually the greatest. When averaged out over the course of a year, it is

about 10-15 mph (0.2-0.3 psf or 10-14 Pa) in most locations in North

America. If it hits the building straight on, air enters the envelope on

the windward side and exits on the other three sides and at the top,

through the roof. If the wind hits at an angle, air exits the building on

the two leeward sides and the roof.

Stack effect, also sometimes referred to as chimney effect, is caused by

buoyancy or the simple physics lesson that hot air rises. The weight of

the column of conditioned air inside the building compared with that

outside creates a pressure difference across the building envelope. The

taller the building is, the greater the stack pressure will be. Warm,

conditioned air escapes through holes at the top of the building and at

the roof. The resulting lower pressure at the bottom of the building

draws in air from the surrounding environment.

The third pressure comes from the mechanical system itself. Mechanical

engineers and on-site managers often choose to bring in makeup air to

increase pressure and overcome the infiltration at the base of the

building. Unfortunately, this increases pressure at the top, causing more

exfiltration problems in that area.

How does an air barrier system increase energy efficiency?

When uncontrolled air leakage occurs, the HVAC system has to work

harder to maintain the indoor environment. An effective air barrier

system, quite simply, controls air movement into and out of the building.

This allows the HVAC system to do its job uncompromised by having to make

up for a disproportionately large amount of the air it is conditioning

leaving the building.

And of course, increasing the operating efficiency of the HVAC system

reduces energy consumption and, therefore, operating costs. In fact, the

inclusion of an effective air barrier system may allow the HVAC system to

be downsized at the design stage – in some cases by a substantial

amount.

According to a National Institute of Standards and Technology (NIST)

report, Investigation of the Impact of Commercial Building Envelope

Airtightness on HVAC Energy Use, the inclusion of an air barrier

system in four sampled types and sizes of building can reduce air leakage

by up to 83%. This represents a large reduction in current and future

energy consumption and operating costs: potential gas savings of greater

than 40%, and electrical savings of greater than 25%.

The study evaluated the energy savings of an effective air barrier

requirement for non-residential buildings in five cities representing

different climate zones. The methodology included blended national

average heating and cooling energy prices and cost-effectiveness

calculations matching the scalar ratio employed by ASHRAE 90.1. Energy

simulations were performed using TRNSYS (Klein 2000). Simulations of

annual energy use were run using TMY2 files (n and Urban

1995).

The research team selected whole building airtightness levels that were

judged to be readily achievable and used these as the whole building

target used in the energy modeling. The baseline buildings used in the

comparison were modeled with leakage levels based on a database of

commercial building leakage measurements.

Let’s look at one of the building models used in the study: the office

building.

The model was a two story office building with a total floor area of

24,200 sq. ft. and a window-to-wall ratio of 0.2 with a floor-to-floor

height of 12 ft., broken up between a 9-ft. occupied floor and a 3-ft.

plenum per floor. The building also included a single elevator

shaft.

The internal gains for the occupied spaces were divided into three parts:

lighting, receptacle loads and occupants. The thermostats operated on a

set point with setback/setup basis. The heating set point was 70 °F with

a setback temperature of 55 °F and the cooling set point was 75 °F with a

setup temperature of 90 °F.

The HVAC system included water-source heat pumps (WSHPs) with a cooling

tower and a boiler serving the common loop. Each zone had its own WSHP

rejecting/extracting heat from the common loop. The outdoor air for each

zone was supplied to each individual heat pump, and the heat pump blower

was on at all times when the zone was occupied. When the location of the

building required an economizer, the outdoor air controls were applied to

the individual heat pump’s airflow. With this approach, different heat

pumps could have a different percentage of outdoor air at the same time

depending on the loads. Three of the modeled locations included

economizers and two did not. Return airflow was specified to equal 95% of

supply airflow.

The results showed that reducing the air leakage rate to the target level

by including a continuous air barrier system resulted in an average

reduction in infiltration of 83%. The economic impact is shown in Table 1

below:

Table 1 Energy cost savings for office building

City

Gas

Savings

Electrical

Savings

Total Savings

Bismarck $1,854

(42%)

$1,340

(26%)

$3,195

Minneapolis $1,872

(43%)

$1,811

(33%)

$3,683

St. Louis $1,460

(57%)

$1,555

(28%)

$3,016

Phoenix

$124

(77%)

$620

(9%)

$745

Miami

$0

(0%)

$769

(10%)

$769

Note: The full report, Investigation of the Impact of Commercial

Building Envelope Airtightness on HVAC Energy Use, is available for

download from the NIST website at

www.nist.gov

What makes an air barrier?

Air barrier systems must be constructed of materials with an air

permeance rating of less than 0.004 cfm/sq. ft.when tested at their

intended-use thickness in accordance with ASTM E 2178. They must be

continuous throughout the building envelope with interconnected, flexible

joints. The air barrier must be able to withstand positive and negative

air pressures without displacement and must be durable enough to last the

life of the building.

Of course, all penetrations in the air barrier must be sealed or the

assembly itself becomes leaky, which defeats the purpose of installing

the system in the first place.

The Air Barrier Association of America (ABAA) has published Master

Specifications for several different air barrier materials and systems

that meet the performance requirements of state and model Energy Codes on

its website: www.airbarrier.org.

One of the most frequently specified air barrier materials is closed-cell

spray-applied polyurethane foam. This is because in addition to providing

an air permeance rating of less than 0.001 L/s/m2 at an

application thickness of 1.5 in., the material also offers an effective

insulation R-value of over 6 per in. and in many states also qualifies as

a vapor barrier. Spray-applied polyurethane foam is a two-component

product manufactured on site but engineered in the molecular level to

meet required performance criteria for every code and climate.

Spray-applied and seamless, it conforms to any shape, fully-adheres to

the wall system and requires no fasteners, thereby eliminating thermal

bridging, convection loss behind insulation boards, and condensing

surfaces, while also increasing installation speed and reducing labor

costs. It can also improve structural strength, according to testing

conducted by the National Association of Home Builders (NAHB)

ResearchCenter.

For the future

Although the idea of mandating air barrier systems for new commercial

construction is a relatively new phenomenon in the United States, Canada

has included air barriers in its National Building Code for over two

decades. In recent years, Massachusetts, Wisconsin and Michigan have

begun mandating air barrier systems as part of their Commercial Energy

Codes. Although air barrier systems are now required by the American

Society of Heating, Refrigerating and Air-Conditioning Engineers

(ASHRAE’s) Advanced Energy Design Guide: Small Office Buildings, and the

New Building Institute’s Benchmark for Advanced Buildings, for the first

time, continuous air barrier systems may become a requirement by the

ASHRAE under Addendum z to Standard 90.1-2004, Energy Standard for

Buildings Except Low-Rise Residential Buildings; at the time of

writing, this Addendum was in public review.

Professional Engineers designing and specifying mechanical systems in

non-residential buildings can improve performance by understanding the

impact of uncontrolled air leakage and the role of the air barrier system

in optimizing building energy efficiency and durability, as well as

occupant comfort, health and safety. To this end, the Air Barrier

Association of America provides training programs for architects,

specifiers and engineers, as well as a certified installer program with

third-party quality control inspections to ensure correct installation of

the systems.

Additional Resources:

The National Institute of Standards and Technology (NIST)

www.nist.gov

The Air Barrier Association of America

www.airbarrier.org

Oak Ridge National Laboratory

www.ornl.gov

The United States Department of Energy

www.energy.gov

American Society of Heating, Refrigerating and Air-Conditioning Engineers

(ASHRAE) www.ashrae.org

The National Institute of Building Sciences (NIBS), Whole Building Design

Guide www.wbdg.org

National Association of Home Builders (NAHB)

www.nahb.org

Canada Mortgage and Housing Corporation (CHMC)

www.cmhc

-schl.gc.ca

National Research Council of Canada

www.nrc-cnrc.gc.ca