ASHRAE Journal - November 2020 - 68

COLUMN BUILDING SCIENCES

PHOTO 6 Retrofit "Wedgie." Siding can be gapped to provide a capillary break.
Siding wedges have been used in the past to address peeling paint problems
resulting from capillary rise between overlapping siding.

FIGURE 11 Brick Veneer. Air space serving as a capillary break as well as serving

as a receptor for capillary moisture.

Brick Veneer
Absorbed Moisture Can Evaporate Into Air Space
Air Space (Acts as a Capillary Break
And Receptor of Capillary Moisture)
Building Paper
(Water Control Layer)
Sheathing
Cavity Insulation in
Wood Frame Wall

pores on the surface of the wood. All six surfaces of the
siding (the front, the back, the edges and the end cuts)
should be sealed. Additionally, the siding should be
installed over an air space behind the siding to act as a
receptor for capillary moisture (Figure 10).
Siding can be gapped to provide a capillary break.
Siding wedges have been used in the past to address

peeling paint problems resulting from capillary rise
between overlapping siding (Photo 6). See "Peeling Paint
Problems" sidebar.
An additional example of an air space serving as a capillary break as well as serving as a receptor for capillary
moisture is where a brick veneer is constructed over an
air space (Figure 11). Signiﬁcant amounts of deposited

Peeling Paint Problems
In the mid-1980s I was looking at peeling paint problems on wood sidSidebar Figure 1 Oval Headed Nail. Blind nailed oval
headed nail creating a capillary break.
ing. I linked the problems to capillary rise between the overlaps in the
siding and a reduction in drying potential due to an increase in thermal insulation in wall cavities. I wrote it up for the CIB-W40 Meeting,
Oval Headed Nail Opens Up
Laps of Siding, Breaking
Victoria, BC., September, 1989, based on a paper presented at the
Capillarity, Providing Drainage,
th
Energy Efﬁcient Building Association (EEBA) 5 Annual Conference
And/Or Ventilation
and Exposition, November, 1987. Republished in the Journal of Thermal
Insulation, January 1990.4
Siding
Building Paper
Check out Sidebar Figure 1, Sidebar Figure 2, Sidebar Figure 3 and Sidebar Figure
Sheathing
4-all from that paper. Not very complicated to avoid the issues-we just
Insulation in
Wall Cavity
don't do it. Sucks, doesn't it? The capillarity.
Sidebar Figure 2 Oval Headed Tack. Spacer on the
back of siding creating a capillary break.

Plastic "Wedge" Opens
Up Laps of Siding,
Breaking Capillarity,
Providing Drainage,
And/Or Ventilation

Oval Headed "Tack"
Opens Up Laps of Siding,
Breaking Capillarity,
Providing Drainage,
And/Or Ventilation

Siding
Building Paper
Sheathing
Insulation in
Wall Cavity

ASHRAE JOURNAL

Sidebar Figure 3 "Wedge." Plastic wedge between
the laps of siding creating a capillary break.

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N OVEM BER 2020

Sidebar Figure 4 Furring Strips. Air space created by
furring strips provides a "receptor" for capillary moisture.
Installing Siding On Furring Strips
Creates An Air Space That Acts
As a "Receptor" for Moisture In
The Siding (And in the Case of
Permeable Sheathing, a "Receptor"
For Interior Wall Moisture)
Siding
Furring Strips
Sheathing
Insulation Wall
Cavity

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ASHRAE Journal - November 2020

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