ASHRAE Journal - February 2021 - 31

COLUMN BUILDING SCIENCES

FIGURE 2 Faced Fiberglass or Mineral Wool Insulation. The insulation is mechanically attached to the underside of the structural slab and extended outward past
the building edge. Rigid extruded polystyrene insulation (XPS) over a capillary
break is installed on the top side of the plaza deck.

Cladding
Continuous Exterior Insulation
Water Control Layer/Air Control Layer
Sheathing
Capillary Break
Cellular PVC Protection Board
Extruded Polystyrene
Extruded Polystyrene Rigid
Insulation Installed Over Drainage
Mat Projects Horizontally (2 ft to
4 ft) R-10 Minimum

2 × 6 Metal Stud
Fiberglass, Cellulose or Mineral
Wool Cavity Insulation
Gypsum Board
Floor Finish

FIGURE 3 Closed-Cell Spray Polyurethane Foam. The spray foam requires a sprayapplied cementitious or fiber fireproofing layer installed over it.

Cladding
Continuous Exterior Insulation
Water Control Layer/Air Control Layer
Sheathing
Capillary Break
Cellular PVC Protection Board
Extruded Polystyrene

2 × 6 Metal Stud
Fiberglass, Cellulose or Mineral
Wool Cavity Insulation
Gypsum Board
Floor Finish

Extruded Polystyrene Rigid
Insulation Installed Over Drainage
Mat Projects Horizontally
(2 ft to 4 ft) R-10 Minimum

" Chairs "

Stone Paver
Trafﬁc Surface
Plaza Deck
Waterprooﬁng
Faced Fiberglass or Mineral
Wool Batt Insulation

Podium Slab

Column Supports
Podium Slab

So how does this work vapor barrier-wise? First of all,
note that the concrete slab is the air control layer ( " air
barrier " ). Second, note that the concrete slab is also the
vapor control layer ( " vapor barrier " ). Montreal is a cold
climate. In fact Montreal is in a very cold climate. If
there were an International Energy Conservation Code
(IECC) rating for Montreal, it would be IECC Climate
Zone 7 (A) Moist. It is heating dominated. They play
hockey outside. The heat ﬂow is from the inside out.
There is no risk of interior airﬂow passing through the
concrete. Did I mention that the concrete slab is the air
control layer? I should have mentioned it is as close to
being a perfect air barrier as you can get.
What about moisture in the concrete wanting to " dry
out " -which means both upward and downward for
elevated slabs? You do not want to trap the moisture
of construction in the ﬁberglass or mineral wool batt
insulation. The facing on this insulation layer should be
vapor semi-permeable. In " Joe speak " it should be on
the " high side " of vapor semi-permeable (5 to 10 perms).
This typically means an aluminum or plastic facer that is
pin perforated.
Why not make the facing vapor permeable (greater
than 10 perms)? Well, they do have summer in Montreal.
And it does get hot and humid in July-particularly

Stone Paver
Trafﬁc Surface
Plaza Deck
Waterprooﬁng
Closed-Cell Spray
Polyurethane Foam
Spray-Applied Fireprooﬁng
(Cementitious or Fiber)

Podium Slab

Column Supports
Podium Slab

PHOTO 2 Closed-Cell Spray Polyurethane Foam. This approach works in all climate zones.

around their Jazz Festival. You do not want uncontrolled
vapor drive from the interior of the garage, which is
ﬁlled with lots of outdoor air that has moisture in it,
migrating upward through your vapor-open ﬁberglass
or mineral wool insulation and accumulating at the
underside of the concrete slab. So, you want your facing on the vapor-open ﬁberglass or mineral wool batt
insulation to be a " vapor retarder, " not a " vapor barrier. "
Note that the vapor drive upward from the garage is not
airﬂow dominated, but vapor ﬂow dominated. More
about that later. Having said that, you still want to seal or
tape the joints of the facing.
Do I have any other insulation options? Yes. You can
used closed-cell spray polyurethane foam (Figure 3
and Photo 2). The closed-cell spray polyurethane foam
needs to also have a spray-applied cementitious or ﬁber
FEBRUARY 2021

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ASHRAE Journal - February 2021

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