The 2012 IECC requires higher levels of insulation than previous energy codes. As with most ICC residential model codes, there are 2 main paths to compliance, 1) prescriptive and 2) performance. Most conventional (stick-frame-and-infill) homes use the prescriptive method. Because SIPs are not conventional in many respects, the performance method is more technically accurate. However, new in the IECC-’12, the prescriptive path includes some notations for continuous insulation. SIPs qualify for the continuous insulation parts of the prescriptive path. Therefore, included in this note is an explanation of applying the prescriptive code for continuous insulation to SIPs. The method to ensure compliance with the IECC-’12 for SIP buildings via the performance path is the same as for the IECC-09 which is covered in a separate blog and will not be repeated in this post.
SIPs AS CONTINUOUS INSULATION, SUPPORTING DOCUMENTATION
The first listed R-value for the prescriptive method is a “center-of-cavity number”. It’s named this way because that is what it is. Conventional construction assumes that there will be studs, rafters, jacks, headers, etc. penetrating all the way through the cavity insulation. A batt of insulation labeled R-19 is just that, the batt itself is R-19. The 2×6 next to it is closer to R-6, and an equivalent metal stud is close to R-1. Clearly, the performance of the whole wall somewhat less than the cavity alone. According to research done by the DOE at Oak Ridge National Labs, the “whole wall” R-value of this prescriptive “R-19” wall was actually measured to be R-14 .
SIPs have very few wood elements penetrating all the way through the insulation this is the reason they are considered continuous insulation. In the same DOE research program, using the same test, a nominal R-23 SIP returned results of R-22 and an R-15 SIP was measured to be R-14 . This is the fundamental reason that SIPs with R-values below IECC table requirements are still compliant.
DETERMINE THE REQUIRED PRESCRIPTIVE R-VALUE
Using Table R402.1.1 of IECC-’12, select the required infill insulation R-value for the building component in question for the correct climate zone. SIPs are most commonly used as roofs, walls, and floor. The “Ceiling R-Value”, “Wood Frame R-Value”, and “Floor R-Value” are the columns that usually apply. In the “Wood Frame R-Value” column there are two options one is a single R-value for the cavity and the other is a cavity R-value + a continuous insulation number. The second combination is intended as if you are going to put a couple of inches of insulation on the wall on continuous over the studs in addition to traditional insulation. Our nailbase and SIPs are often used in this application. For steel buildings there is an additional table 402.2.6 which gives multiple R-value combinations between infill and continuous insulation.
|Climate Zone||Ceiling||Wood Frame||Floor|
|3||R-38||R-20 or R-13+5||R-19|
|4 except Marine||R-49||R-20 or R-13+5||R-19|
|5 and Marine||R-49||R-20 or R-13+5||R-30|
|6||R-49||R-20+5 or R-13+10||R-30|
|7 and 8||R-49||R-20+5 or R13+10||R-38|
|Climate Zones||Wood Frame Wall Requirement||Steel Frame Wall 16"o.c.||Continuous Insulation Option|
|6&7||R-20+5 or R-13+10||R-13+9.5||R-0+14.6|
PRESCRIPTIVE CODE WITH SIPS AS CONTINUOUS INSULATION
In the IECC ‘12 Table R402.2.6 Steel Frame Ceiling, Wall, and Floor Insulation (R-value), there is an explicit listing showing the equivalence of the cavity insulation to various combinations of continuous and cavity insulation. One of the options is R-0 in the cavity. In the table an R-13 cavity is equal to R-9.3 continuous, R-20 cavity is equal to R-14 continuous, and R-21 cavity is equal to R-14.6.
Using those numbers the prescriptive center of cavity number times 0.7 is equivalent to the continuous insulation requirement. Using this information we can find approximate equivalents for the typical prescriptive codes.
Rcavity x 0.7 = Rcontinuous
Example: 49 x 0.7 = 34.3 and 38 x 0.7 = 26.6
Therefore an R-35 or better SIP will meet the R-49 center of cavity code for the IECC ’12, and an R-27 or better SIP will meet the R-38 center of cavity code.
For the R-49 roof code the following Foard Panel standard size panels are the minimum to meet code.
• 10.25” EPS (R-38)
• 8.25” NEO (R-34)
• 8.25” XPS (R-38)
• 6.50” PIR (R-36)
For the R-38 roof code the following Foard Panel standard size panels are the minimum to meet code.
• 8.25” EPS (R-30)
• 8.25” NEO (R-34)
• 6.50” XPS (R-30)
• 6.50” PIR (R-36)
There is another effect at work that isn’t accounted for in IECC-’09: air infiltration. Another DOE test program that built several houses of the same design and location but built with different building systems illustrates this point well. The SIP house had about 1/4 of the air leakage of a conventional house of the same design. Air leakage is widely accepted as a major contributor to energy loss. The IECC ’12 has new performance requirements for air leakage. SIPs with Foard Panel’s typical joinery details will meet all the air leakage requirements. Several of our buildings have been blower door tested and reported very low air infiltration (<1 ACH50 meeting Passive House Standards and well bellow the 2012 Code). While, there are far to many details involved in air sealing for SIPs to claim all the credit for the low air infiltration, SIPs are a major contributor to the performance of the buildings tested. In addition, the foam insulation is both thermal and air barrier so that the thermal envelope and air barrier are aligned as required. With Foard Panel Inc.’s recommended joinery, all our joints are air sealed either with spray foam or mastic and we typically recommend that solid wood joints be covered with SIP tape. We manufacture insulated headers that further contribute to a consistent air control layer. All of which can make achieving air sealing goals very simple.