TO
EMAIL
REGARDING
Mr. Chris Higgins
[email protected]
City of Vancouver Sustainability Group
453 West 12th Avenue, 7th Floor
Vancouver BC V5Y 1V4
5483.105
City of Vancouver
R-Value Consulting
DATE
December 22nd, 2015
Typical Thermal Performance of 2x6 Wood-frame Walls with Closed-Cell Sprayfoam
Dear Mr. Higgins,
We understand that as a result of the requirement for R-22 effective wall thermal
performance for Part 9 buildings, City of Vancouver (CoV) staff have consistently been
receiving questions regarding the ability of different assemblies to provide necessary
performance. In particular, out of a desire to achieve R-22 performance while using
conventional 2x6 framing practices, we understand that many designers and builders are
proposing 2x6 framed wall assemblies filled with closed-cell (i.e. medium density) spray
polyurethane foams (CCSPF). RDH has previously been retained to calculate the thermal
performance of these assemblies on a case-by-case basis; however, this letter aims to
provide more general comment on the thermal performance of these assemblies,
independent of a specific building project.
Calculation of Thermal Performance of 2x6 Walls with CCSPF
To calculate the thermal performance of wall assemblies, it is first necessary to know
properties of the materials in the assembly. The thermal performance of CCSPF is known
to decrease as it ages, and consequently the Vancouver Building Bylaw (VBBL) specifies
that long-term thermal resistance (LTTR) values determined in accordance with
CAN/ULC-S770-00 are to be used in the calculation of building enclosure performance. It
is important to note that LTTR values do not necessarily correspond with “aged” values
sometimes provided by manufacturers, and that it is necessary that the value specifically
reference the S770 standard. There is commonly confusion regarding CCSPF performance
and LTTR, in part because LTTR is not typically reported on American product data sheets.
The Canadian Urethane Foam Manufacturers Association Inc. (CUFCA) provides product
information including the LTTR for CCSPF products from various manufacturers as shown
in the sheets provided in Appendix B. Based on this information, the thermal performance
of CCSPF ranges from R-5.18 per inch to R-5.85 per inch, and this range has been used in
the example calculations provided later in this letter.
In addition to LTTR, installation for CCSPF should also be considered. Due to the method
of installation for this type of expanding spray applied product, it is typically not possible
to entirely fill a stud cavity, and instead approximately a 1/2” air space will likely exist
between the interior face of the CCSPF and the interior gypsum wall board (or other
finish). The presence of this air space is shown with the other layers of the wall assembly
in Figure 1, and is reflected in the example calculations.
To demonstrate the typical thermal performance achieved by these types of assemblies,
calculations were performed in accordance with Part 9 requirements of the VBBL using the
5483_105 2015 12 22 LR GF LTR 2x6 Wood-frame Walls with CC Sprayfoam.docm
Page 1
isothermal planes calculation technique, material properties provided in Section 9.36 of
the British Columbia Building Code, and the LTTR performance values provided by CUFCA.
A detailed description of the isothermal planes calculation is provided in Appendix A for
reference, and Figure 1 illustrates the method.
Figure 1: Typical wall assembly divided into layers for the isothermal planes calculation.
The following table describes the performance of four wall assemblies with the minimum
and maximum R-value/inch of CCSPF as provided by CUFCA (R-5.18/in to R-5.85/in), and
studs spaced at 16” or 24” on centre (23% and 20% framing factors respectively). It is also
possible to use advanced framing techniques to reduce the amount of framing in the wall
assembly; however, we understand from both the CoV and our own experience that these
practices are rarely implemented in Vancouver, and consequently this option has not been
included in the calculations below. These four wall assemblies are intended to indicate the
range of thermal performance that these wall assemblies can typically achieve.
Wall Assemblies Effective R-Values [ft²·hr·°F/Btu]
Exterior Air Film
5/16” Hardi Siding
1/2" Rainscreen Cavity
1/2" Plywood
Framing and Insulation
1/2” Gypsum Wall Board
Interior Air Film
Effective R-Value
R-5.18/in Insulation
R-5.85/in Insulation
16” o.c.
24” o.c.
16” o.c.
24” o.c.
0.17
0.15
0.91
0.63
15.70
0.44
0.68
18.7
0.17
0.15
0.91
0.63
16.59
0.44
0.68
19.6
0.17
0.15
0.91
0.63
16.45
0.44
0.68
19.4
0.17
0.15
0.91
0.63
17.47
0.44
0.68
20.4
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Appendix A: Isothermal Planes and Parallel Paths Method
The following section describes the steps to calculate the thermal performance of a 2x6
stud wall with closed-cell spray polyurethane foam insulation (CCSPF) using isothermal
planes techniques in accordance with the Vancouver Building Bylaw (VBBL). The figure
below shows the layers of the wall assembly, and equation (1) calculates the thermal
performance based on the contribution of each layer in the assembly. All material
properties for this calculation are provided in Table A-9.36.2.4.(1)D of the BCBC, except
for the CCSPF, for which R-5.18/in has been assumed.
1
=
= 0.17 + 0.15 + 0.91 + 0.63 + 15.70 + 0.44 + 0.68 = 18.7 ft² ∙ hr ∙ °F/Btu%
This calculation is summarized in the table below.
Continuous Layers Description
Exterior Air Film
5/16” Hardi Siding
1/2" Rainscreen Cavity
1/2" Plywood
Framing and Insulation
1/2” Gypsum Wall Board
Interior Air Film
Thickness
[mm]
RSI/mm
RSI-Value
[m²·K/W]
R-Value
[ft²·hr·°F/Btu]
0
8.0
12.7
12.7
139.7
12.7
0
n/a
0.0033
n/a
0.0087
0.0198
0.0061
n/a
0.03
0.026
0.16
0.110
2.77
0.08
0.12
0.17
0.15
0.91
0.63
15.70
0.44
0.68
18.7
Effective R-Value
3.29
*Determined using parallel paths calculation as shown below. Note that since CCSPF insulation
cannot fill the entire stud cavity, a half inch air space was incorporated into the calculation.
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At the stud, where multiple components exist in a layer, a parallel paths calculation is
used to determine the effective R-value of the overall layer. The parallel paths method is
also known as an area weighted U-value calculation. The stud layer is broken into two
layers to reflect the incomplete fill of the cavity with CCSPF. One layer includes the
framing and insulation, and the other includes the framing and an air space as shown in
the figure below.
The thermal contribution of these two layers is calculated separately using the parallel
paths method. The first section is 5" of framing and 5" of spray foam insulation as shown
below. A framing factor of 23% was used, corresponding with 16” on center stud walls, as
indicated by Table A-9.36.2.4(1)A of the BCBC. This framing factor accounts for typical
framing practices including double top plates and extra framing at penetrations such as
windows. Equation (2) describes the parallel paths calculation, and equation (3) gives the
overall R-value for this framing and insulation layer.
2
'
0.163 ∙ 23 + 0.039 ∙ 77
() ∙ *) + (+ ∙ *+
, = '
, = (– 0.067
*) + *+
23 + 77
3
=
1
1
=
= 14.9
(
0.067
This calculation is summarized in the table below.
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Insulation & Studs
Thickness
Parallel Paths Description
[mm]
Framing
2x6 Studs at 16” o.c.
127.0
(23%)
Insulation
R-5.18/in Insulation
127.0
(77%)
R-Value for Stud and Insulation Layer
RSI/mm
RSI-Value
[m²·K/W]
R-Value
[ft²·hr·°F/Btu]
0.009
1.08
6.1
0.036
4.57
26.0
2.62
14.9
The remainder of the stud depth is 1/2" of framing and 1/2" of air space as shown in the
following figure. The same framing factor of 23% is used. The parallel paths calculation
for this layer is provided in equation (4), and the R-value given in equation (5).
4
'
1.639 ∙ 23 + 1.099 ∙ 77
() ∙ *) + (+ ∙ *+
, = '
, = (– 1.220
*) + *+
23 + 77
5
=
1
1
=
= 0.8
(
1.220
This calculation is summarized in the table below.
Air Space & Studs
Parallel Paths Description
Framing
½” of 2x6 Studs at
(23%)
16” o.c.
Insulation
½” Air Space
(77%)
Thickness
[mm]
RSI/mm
RSI-Value
[m²·K/W]
R-Value
[ft²·hr·°F/Btu]
12.7
0.009
0.11
0.6
12.7
n/a
0.16
0.9
0.14
0.8
R-Value for Stud and Air Space Layer
To find the total R-value of the entire stud layer including all framing, insulation, and the
air space, the two R-values can be added, as shown in equation (6).
6
. /0 123 4
= 14.9 + 0.8 = 15.7 ft² ∙ hr ∙ °F/Btu%
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Appendix B: Canadian Urethane Foam Manufacturers Association Inc.
(CUFCA) Data Sheet
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