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DOUBLEWALL VAV TERMINAL UNIT ISSUES _______________________________

Doublewall VAV Terminal Unit Issues
Page:
1 of 5




Krueger Corporate Headquarters
1401 N. Plano Rd. Richardson, TX 75081
Phone: (972) 680-9136 | Fax: (972) 497-0450 | E-Mail:
kruegerinfo@krueger-hvac.com

www.krueger-hvac.com




DOUBLEWALL VAV
TERMINAL UNIT ISSUES
_______________________________

Dan Int-Hout
Chief Engineer


Doublewall VAV Terminal Unit Issues
Page:
2 of 5




Krueger Corporate Headquarters
1401 N. Plano Rd. Richardson, TX 75081
Phone: (972) 680-9136 | Fax: (972) 497-0450 | E-Mail:
kruegerinfo@krueger-hvac.com

www.krueger-hvac.com

OVERVIEW
We are seeing specifications on a number of projects that require double wall construction for VAV boxes.
We are not sure of the logic behind the specifications, but want to point out some of the disadvantages of
this type of construction, and offer a logical alternative.

Doublewall construction is often specified for air handlers, where moisture is often present in the areas
near the cooling coils, surface temperatures may be near to wet bulb air temperatures, moist outside air
may be present and there is a requirement to enter the unit for cleaning. None of these is true for a VAV
box. In a VAV box there are no cooling coils, the air temperature will have risen at least a couple of
degrees above the lowest building dew point temperatures, there should be no outside air anywhere near
the unit, and even the largest fan powered box is too small to physically enter the unit for cleaning. So why
is doublewall construction specified? One obvious answer is that someone makes one and has been able
to get an engineer to have it specified.

If fiberglass erosion is a concern, facings are available which can reduce this potential. Foil facings are
available, and have been in use for a number of years. Plastic films such as Mylar and Tedlar were often
specified until someone noted that these facings, when exposed, were not only flammable to the point of
being explosive, but one of them gave off cyanide gas when burned!

It is true that fiberglass is listed as a hazardous material in some codes, but it has never been proven to be
an asbestos-like material. Fiberglass will be harder to clean, and should it get wet, it will be a potential
microbe breeding ground. Of course, double wall construction is not watertight, and should condensation
(still cant figure out how that happens at this point in a system!) or other moisture get into the dark,
fiberglass filled spaces, microbe contamination is just as likely!

The real problem is that fiberglass is the best sound absorbing material available. And VAV boxes are at
the end of a duct run, near or over occupied spaces. There is little opportunity to attenuate sound
produced by VAV terminals, particularly fan powered VAV boxes, and especially if they are over occupied
spaces. If double wall construction is specified for the boxes, one must assume that duct lining is also
prohibited. (If not then we have other pointed questions to ask!).

Lining selection in parallel fan boxes (where the fan is usually fairly small, doesnt operate all the time, and
is acoustically isolated from the supply damper by a back draft damper) and single duct boxes is not sound
critical, when lined duct and flexible duct are involved in the design. Indeed, a non-heat single duct is little
affected by the lining selection. If duct lining after the unit is not allowed, add several NC to the predicted
space sound level, however.

Series fan powered boxes, on the other hand, are extremely sensitive to lining selections. With these units,
primary air is discharged directly into a cavity that is open to the plenum. The following are the available
linings for Krueger VAV boxes (not all linings available with all units check the price pages and catalogs).
Some VAV box selection software accounts for lining options in preparing sound performance data.

The following are lining descriptions, and the acoustic effects on our quiet (KQFS) Series Fan Box:
1/2 1.5lb Density (Std): Dual density fiberglass insulation with a tough outer layer. This is the base
line, ARI certified data. 1 3lb Density (drops 1 NC): A thicker version of the above. Note: Does not really add measurable
sound attenuation or condensate protection. Steriliner (Duct Board - Foil inside) w/Steel Flanges (adds 7-10 NC): High density ductboard with a
heavy foil facing on the exposed surface. Steel strips seal all edges.


Doublewall VAV Terminal Unit Issues
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3 of 5




Krueger Corporate Headquarters
1401 N. Plano Rd. Richardson, TX 75081
Phone: (972) 680-9136 | Fax: (972) 497-0450 | E-Mail:
kruegerinfo@krueger-hvac.com

www.krueger-hvac.com

Sterilwall (Double Wall), Solid (adds 12 NC) and Perforated (adds 8 NC), 5 Insulation Options: The
perforated is a 23% free area perforated sheet. Foil Encapsulated liner (0.5 and 1) (Adds 7-10 NC): This is a pillow of light density fiberglass with a
foil/scrim/kraft facing (duct liner) installed with the foil on the side exposed to the air stream. Edges are
taped with foil tape and tucked under. Cellular (Adds 3-5 NC): Closed cell plastic-type foam, meeting NFPA-90a and UL-181 characteristics.
Note; this is not Armaflex insulation. No liner (Adds 12 NC): Unit must be externally wrapped to avoid condensation.

Note: the above acoustical effects are for the KQFS series fan box. The effects are less pronounced on
the lower cost QFC series fan box as it doesnt have as much acoustical attenuation.

So what to use? When looking at costs and acoustics, if fiberglass is not to be tolerated (for whatever
reason), the next choice is Cellular. This is a closed cell plastic material meeting the requirements of NFPA
90A for appliances, UL181 air erosion, flame spread, smoke generation and mold growth requirements,
and is easily cleanable. It will not soak up liquid water. It is even available white in color to make soiling
more obvious. It is in use by almost all major VAV box manufacturers (so it is not a proprietary product).
And it only adds a few dB to the sound generation into the space.

Double wall on the other hand, can add 200% to the first cost of a series fan powered box. It can add 10 or
more NC to the space (in effect requiring a greater number of smaller units to handle the load), and
actually provides no real IAQ or other performance attributes to a project. We need to understand these
issues, explain them to the engineering community, and get the double wall requirement relegated to air
handlers where it makes sense.

The specification sheet for the cellular insulation is attached!


























Doublewall VAV Terminal Unit Issues
Page:
4 of 5




Krueger Corporate Headquarters
1401 N. Plano Rd. Richardson, TX 75081
Phone: (972) 680-9136 | Fax: (972) 497-0450 | E-Mail:
kruegerinfo@krueger-hvac.com

www.krueger-hvac.com






Doublewall VAV Terminal Unit Issues
Page:
5 of 5




Krueger Corporate Headquarters
1401 N. Plano Rd. Richardson, TX 75081
Phone: (972) 680-9136 | Fax: (972) 497-0450 | E-Mail:
kruegerinfo@krueger-hvac.com

www.krueger-hvac.com