ASHRAE Journal - November 2020 - 69

COLUMN BUILDING SCIENCES

moisture can be absorbed
FIGURE 12 Wood Trim. All six surfaces of the trim
FIGURE 13 Wood Trim Over Air Space. Air space serves as a receptor
(the front, the back, the edges and the end cuts)
for capillary moisture.
by the bricks (brick is a
should be sealed.
Plywood/OSB
"reservoir cladding").
Sheathing
4
When solar radiation
6
Water Control
warms the brick veneer,
5
1
3
Layer
the moisture in the bricks
2
is driven inward out of
Spacer
the bricks into the air
Strip
space.
Coat Back and Front of
Wood or wood trim
Trim Pieces
pieces should be sealed
or coated or painted on
all six surfaces to control capillary absorpSeal All
End
Cuts
tion of water (Figure 12).
Additionally, these trim
pieces should be installed
FIGURE 14 Wood Shingles and Shakes. The wood shakes and shingles absorb deposited
over an air space (Figure 13).
rainwater due to capillary suction. The wood shakes and shingles dry to both directions.
Where wood shingles and shakes are installed
Wood Shakes/Shingles
over furring strips, or skip-sheathing on roof
Absorbed Moisture In the Shakes Can
Evaporate Inward As Well As Outward
assemblies (Figure 14), the same phenomenon
Rooﬁng Paper (Water Control Layer)
occurs. The wood shakes and shingles absorb
deposited rainwater due to capillary suction. The
wood shakes and shingles dry to both directions.
The function of the building paper/rooﬁng paper is
to facilitate drainage and air pressure moderation
across the roof assembly. The shakes/shingles are
Air Space
leaky, whereas the building/rooﬁng paper is tight.
Furring Strips (Skip
Where sheet goods are used for roof sheathing
Sheathing)
(such as plywood or oriented strand board/OSB)
Roof Rafters
with wood shakes and shingles, an air gap must be
added between the shakes and shingles to act as
FIGURE 15 Sheet Goods and Wood Shingles and Shakes. Where sheet goods are used for
a receptor for the capillary moisture. The air gap
roof sheathing such as plywood or oriented strand board (OSB) in conjunction with wood
is typically provided using a breathable drainage
shakes and shingles, an air gap must be added between the shakes and shingles to act as
a receptor for the capillary moisture. The air gap is typically provided using a breathable
mesh (Figure 15).
drainage mesh.
Life sometimes sucks. Sometimes you need to
Fully Adhered
Drainage Mat
give it some space.
Membrane

References

1. Straube, J. 2001. "The Physics of Building Science."
Presentation. Annual Westford Symposium on Building Science.
2. Straube, J.F., E.F.P. Burnett 2005. Building Science for
Building Enclosures. Westford, Mass.: Building Science Press.
3. Hutcheon, N.B., G.O.P. Handegord.
1983. Building Science for a Cold Climate.
Toronto: Wiley.
4. Lstiburek, J. 1990. "Insulation
induced exterior paint and siding failures." https://bit.ly/309ZpBE
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Journal of Thermal Insulation 13.

1.5 in. Overhang at
Eaves and Edges
Double Up First
Course
Roof Sheathing

N OVEM BER 2020

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