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Winter 2015-16 • Vol. 6, No. 4
Materials • Technology • Trends
COP 21’S IMPACT ON
CONSTRUCTION
p10
DESIGNING FOR
EXTERIOR CONTINUOUS
INSULATION
p16
BRUNER/COTT
REIMAGINES
STUDENT DINING
p26
MORTARS & ADMIXTURES
p36
MATERIALS | INSULATION
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Materials • Technolog y • Trends
DESIGNING FOR
EXTERIOR
CONTINUOUS
INSULATION
By Alejandra Nieto
Image Copyright of Paolo De Santis | 123rf.com
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MATERIALS | INSULATION
THE FACT THAT WE HAVE TO USE INSULATION IN OUR
BUILDING ENCLOSURES IS NOT NEW TO THE BUILDING
PROFESSIONS. Codes and standards have been prescribing specific
R-Value and U-Value requirements for the last few decades as a means to
increase energy efficiency and to improve overall building performance.
Good design shows that the best performance will be achieved when the
insulation is placed on the exterior of the building, as opposed to the interior
in between the structure. Most recently, codes and standards also have
figured it out, and are prescribing the use of continuous insulation.
There are several benefits with using continuous insulation, which include
increasing the durability of the assembly, reducing the risk of condensation, reducing thermal bridging, and increasing performance. However, due to vague
understanding of what is considered “continuous,” the benefits of using continuous insulation often are times significantly reduced.
What do the codes say?
Thermal resistance requirements will differ between codes. Enclosure code
compliance will depend on where the project is located, the climate zone, and
the type of building being constructed. Typically, codes will have two compliance methods: prescriptive and performance.
As it relates to thermal performance, the prescriptive method will indicate
the assembly specific R-Value requirements of the insulation to be used within
the wall assembly. Newer codes typically require both interior and continuous
insulation (i.e., insulation between the framing and exterior insulation).
The performance method will indicate maximum overall U-Value to be
reached or total effective R-Value. With this method, all layers of the assembly
are considered; reductions to the prescribed R-Value are assessed, depending
on the structural components within the assembly.
Designing with exterior insulation
For optimal energy performance, it is best to go the performance path
when designing the building enclosure. Since it takes into account all the factors
within the assembly, it encourages the use of exterior insulation. As it relates
to temperature and heat loss, you want to wrap the building in a warm layer
as opposed to stuffing it. Along with reducing
thermal bridging, exterior insulation significantly
reduces the risk of condensation within the assemblies, resulting in a more durable and resilient building enclosure.
Different types of building enclosures will
have different critical layers. For example, in a
typical steel frame wall assembly, the exterior
sheathing is the critical layer within the wall assembly. This is so because it is the first “cold”
interface surface within the wall assembly,
which translates to being the plane for potential condensation to occur. The design strategy
to prevent condensation at this plane is to use exterior insulation to keep the
sheathing “warm,” and ideally, above the dew point temperature. For example,
as noted in the figure on page 19, when using R8 of continuous insulation, the
temperature of the sheathing layer is higher than the assembly without continuous insulation. During colder temperatures, this is a significant difference to
reduce risk of condensation.
There are several different types of insulation that can be used as exterior
insulation, including semi-rigid mineral wool, extruded polystyrene (XPS) rigid
For optimal energy performance,
it is best to go the performance
path when designing the building
enclosure.
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Figure 1: Tem perature of sheathing layer (condensation plane).
board, expanded polystyrene (EPS) rigid board, closed-cell or open-cell spray
polyurethane foam (SPF), and polyisocyanurate (polyiso) rigid board. Each of
these insulation types has different performance characteristics and added benefits. The thickness of the exterior insulation that should be used will depend
on code requirements, the thermal resistance value of the insulation, and the
vapor permeability of the insulation.
• Semi-rigid mineral wool has an approximate R-Value of R4/inch
(RSI 0.70/25.4mm), and is highly vapor permeable.
• XPS rigid board has an approximately R-Value of R5/inch
(RSI 0.78/25.4mm), and is vapor impermeable.
• EPS rigid board has an approximately R-Value of R4/inch
(RSI 0.70/25.4mm), and is vapor impermeable.
• Closed-cell SPF has an approximate R-Value of R6/inch
(RSI 1.05/25.4mm), and is vapor impermeable.
• Open-cell SPF has an approximate R-Value of 3.5/inch
(RSI 0.61/25.4mm), and is vapor permeable.
• Polyiso rigid board has an approximately R-Value of R5.6/inch
(RSI 0.98/25.4mm), and is vapor impermeable.
When is continuous insulation actually continuous?
ASHRAE defines continuous insulation as insulation that is uncompressed and
continuous across all structural members without thermal bridges other than fasteners and service openings; it is installed on the interior or exterior or is integral to
any opaque surface of the building. Although the definition is pretty clear, there
seems to be misconceptions on different attachment methods being classified
as continuous insulation.
Different insulation types will have different fastening requirements. Most
commonly, they are fastened using long fasteners/screws, or can be adhered.
Although fasteners will have an impact on performance, the effects are minimal
and considered negligible. Deflection and other concerns come into effect only
when using fasteners to attach continuous insulation—especially with heavier
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MATERIALS | INSULATION
ASSEMBLY
U-VALUE
(W/M2·K)
RSI
(M2·K/W)
R-VALUE
(H·°F ·FT2/BTU)
REDUCTION
(%)
R20 Exterior Insulation w/ vertical z-girts
@ 400mm o.c.
0.621
1.61
9.15
54%
R20 Exterior Insulation w/ fibreglass clips
@ 400mm o.c.
0.341
2.93
16.65
17%
R20 Exterior Insulation fastened to substrate (5 screws per board)
0.304
3.29
18.71
6%
Notes:
[1] HEAT 3 was used to model the assemblies.
[2] Substrate of the assembly consists of interior gypsum board, 2x6 steel frame, and exterior gypsum sheathing.
[3] R4/inch mineral wool was used as the exterior insulation.
[4] Thermal conductivity of materials used in models based on software database, as indicated by the IEA.
Table 1: Thermal model results, U-Values, R-Values and Reduction (%)
cladding systems, such as terra cotta panels, and with taller buildings that will
experience elevated wind loads. This can lead to more fastener requirements
or the use a grid system (z-girts) to attach the claddings.
This is when the grey area for the term “continuous insulation” begins. How
many fasteners does it take until the thermal bridges are no longer negligible?
If using a grid system or clip and rail system, is the insulation still “continuous,”
and can it still follow the prescriptive path for compliance? Although often not
seen as priority since it doesn’t affect code compliance, the critical question
is how do the significant thermal bridges affect the durability of the building
enclosure? These questions can be addressed using thermal modelling to determine where and how significant the thermal bridges are, and whether or not
they increase the potential for condensation.
As identified in the table and figures below, it is evident that a clip and rail
system has more significant effects than fasteners, and that a z-girt system is the
Figure 2: Thermal model results, R20 exterior insulation installed with z-girts.
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MATERIALS | INSULATION
Figure 3: Thermal model results, R20 exterior insulation installed with fibreglass cli p system.
worst performer. The areas of high thermal bridging will decrease the temperature of the critical areas (note the temperature gradient through the assembly),
which can lead to increased risk of condensation within the assembly. These
thermal bridges also increase the heat transfer through the assembly, which
then affect the energy performance of the building enclosure.
It is critical to note to all of these reductions are considering perfect workmanship, and do not consider other parts of an assembly that can cause thermal
bridging such as openings (windows and doors), metal flashing (if applicable),
Figure 4: Thermal model results, R20 exterior insulation fastened with long screws (5 screws per board).
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Image Copyright of Paolo De Santis | 123rf.com
floor slabs or balconies. It also is important
to note that similar thermal reductions are
going to occur independent of the type of
insulation and its nominal R-Value. The final
overall effective performance may differ,
but the amount of reduction always will
be at par because the steel (or structure)
always will be the driving factor.
With masonry veneer buildings, the
exterior insulation does not have to be
fastened to the substrate. Rather, brick
ties can be used to secure the insulation
in place. However, brick ties—and more
importantly, shelf angles required for masonry veneer—can significantly affect the
thermal performance of the assembly. The
material of the brick ties and shelf angles
will change the performance. With regular ties, switching from galvanized steel to
stainless steel will make a significant difference in reduction (Finch & Higgins, 2013).
Additionally, other solutions exist where
the brick ties are thermally broken to reduce thermal bridging.
What does the science say?
Although we still may not know what the codes consider too much thermal
bridging to no longer consider continuous insulation “continuous,” we can at
least take away the following:
• Thermal bridging matters!
• The performance compliance path for
codes typically will give you a more
efficient building enclosure.
• The more exterior insulation, with
minimal thermal bridging, the better.
• Grid systems (i.e., z-girts) should never be
used as an exterior insulation attachment.
• The fastening material conductivity makes
a significant difference when it relates to
brick ties and angles.
• You can’t account for workmanship
deficiencies. wMD
With masonry veneer buildings,
the exterior insulation does
not have to be fastened to the
substrate.
About the author:
Alejandra Nieto is an Energy Design Centre (EDC)
specialist at Roxul. She is a graduate from the Master
of Building Science program at Ryerson University, with
a background in construction science and management,
and architectural technology from George Brown College.
She has experience in design and research of the methods and materials involved in energy-efficient buildings
and systems. As a specialist for the EDC, she provides
expertise in building envelope design, and moisture and
heat transfer analysis.
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Works cited
Finch, G., & Higgins, J. (2013).
Masonry Veneer Support Details: Thermal
Bridging. 12th Canadian Masonry
Symposium. Vancouver.
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