ASHRAE Journal - September 2021 - 64

COLUMN BUILDING SCIENCES
The connection between the
gypsum board and the edge
of the exhaust grille unit is
not airtight. The duct that
extends to the shaft is not
airtight. So, the entire bathroom
dropped ceiling area is
negative. Photo 8 shows the
connection of the negative
pressure field of the bathroom
dropped ceiling to
bathroom/hotel room wall,
which is then subsequently
connected to the hotel room demising wall, which then
extends outward to the exterior. The infiltration of hot
humid air from the exterior through the hotel room
demising wall ultimately ending up in the bathroom
dropped ceiling leads to the mold shown in Photo 9.
So how do we make this interstitial negative pressure
field go away? Ah, easy: " bleed " the negative pressure by
installing " transfer grilles " connecting the hotel room
pressure field to the bathroom dropped ceiling pressure
field (Photo 10).
What if this does not work? Ah, the voice of experience
talking. Then you add interstitial air sealing by injecting
spray foam into the hotel room demising wall. Note
that you can't install a " transfer grille " in the hotel room
demising wall, as it is a fire rated assembly. Don't go there.
Is there more? Yes. Thirty 30 years ago I did not have
aerosol duct and building enclosure sealing available. I
have that now. Service raceways and exhaust shafts can
be sealed this way. You might end up having to do all
three: bleed the negative pressure with transfer grilles,
Gypsum Board Closure
Supply Air
Supply " Sleeve "
" Transfer " Grille to Balance/
Equalize Air Pressure Across
The Gypsum Board Closure
And the Room
Return
Grille
Return Grille " Sleeve "
Return Grill Filter
FIGURE 5 Not Being Negative. Install a ducted return sleeve between the return
grille, the AHU and a " transfer grille " to equalize or " bleed " pressures.
Supply Air Opening
Air Handler Unit
Return Air
Opening
PHOTO 7 Bathroom Exhaust. The
exhaust grille unit is not airtight. The
connection between the gypsum
board and the edge of the exhaust
grille unit is not airtight. The duct
that extends to the shaft is not
airtight. So the entire bathroom
dropped ceiling area is negative.
inject spray foam to seal demising walls and use aerosol
technology to deal with exhaust shafts.
Now, for the record, how do I feel about vinyl wall coverings?
I am good with them. I have no issues with them
on interior walls that are not air pressure-connected to
the exterior.
Also, for the record, I like air conditioning. So air conditioning
and vinyl wall coverings work if you control
the pressures. And, to be extra safe, don't install vinyl
wall coverings on the interior of exterior walls. Moldy
hotels sucked literally and figuratively. Get rid of the
interstitial negative pressures and don't have the vinyl
wallcovering acting as a vapor barrier on the wrong side
of the wall in the wrong climate zone, and we are good.
References
1. 2021 ASHRAE Handbook-Fundamentals, Chapter 26, Table 6.
2. Wallcoverings Association. Undated. Mold: Cause, Effect and
Response: A Study of Wallcovering Products. Chicago: Wallcoverings
Association.
PHOTO 8 (Left) Interstitial Negative Pressure Field. The negative pressure field of the bathroom dropped ceiling is coupled to the bathroom/hotel room wall, which is then
subsequently connected to the hotel room demising wall, which then extends outward to the exterior. PHOTO 9 (Center) Mold Damage on Interior Wall. The infiltration of hot
humid air from the exterior through the hotel room demising wall ultimately ending up in the bathroom dropped ceiling leads to the mold damage shown. PHOTO 10 (Right)
" Bleed " the Negative Pressure. Installing " transfer grilles " connecting the hotel room pressure field to the bathroom dropped ceiling pressure field controls the negative pressure
in the demising wall.
" Transfer " Grilles to Balance/
Equalize Air Pressure
64
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