SIPs and the IECC-09 Energy Code

The 2009 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. A method to ensure compliance with the IECC-’09 for SIP buildings via the performance path follows.

U.S. Climate Zones based on 2009 International Energy Conservation Code Source: Building Energy Codes Resource Center, Pacific Northwest National Laboratory, U.S. Department of Energy.

U.S. Climate Zones based on 2009 International Energy Conservation Code
Source: Building Energy Codes Resource Center, Pacific Northwest National Laboratory, U.S. Department of Energy.

DETERMINE THE REQUIRED PRESCRIPTIVE R-VALUE

2009 IECC Prescriptive Requirements
Climate Zone Ceiling Wood Frame Floor
1 R-30 R-13 R-13
2 R-30 R-13 R-13
3 R-30 R-13 R-19
4 except Marine R-38 R-13 R-19
5 and Marine R-38 R-19 or R-13+5 R-30
6 R-49 R-19 or R-13+5 R-30
7 and 8 R-49 R-21 R-30

ACTUAL SIP PERFORMANCE, SUPPORTING DOCUMENTATION

The 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. Clearly, the performance of the whole wall somewhat less than the cavity alone. According to research done by the Department Of Energy (DOE) at Oak Ridge National Labs, the “whole wall” R-value of this prescriptive “R-19” wall was actually measured to be R-14.

Typical Stud wall on Left and SIP wall on Right, the lighter colors indicate heat escaping.

Typical Stud wall on Left and SIP wall on Right, the lighter colors indicate heat escaping.

DETERMINE REQUIRED SIP R-VALUE

Section 405 of IECC-’09 outlines the documentation when using the performance method. While a sub-section describes the requirements of any software used for energy modeling, it is assumed that sound mechanical engineering calculations that meet the same requirements are also acceptable.
The following assumptions are made in this engineering proof:
• The proposed (SIP) design is of a house with the same design, surface area, orientation, and construction methods & materials as the standard reference (conventional) design.
• The climate, location, shading, & other outdoor features of the SIP design and conventional design are identical.
• Interpolation and minor extrapolation of the DOE/ORNL Hot Box Surface-To-Surface R-value data is accurate within 2 digits of precision.

For simplicity, this bulletin will use a more convenient form of Eq. 1 with units common in the HVAC industry.
Qws=(A×∆T)/Rws
Equation 2
Where:
Qws = whole surface (ws) heat transfer rate in BTU/hr
Rws = whole surface R-value, 1 R-value = 1 (hr sq-ft deg F)/BTU
Setting the heat loss of the proposed (SIP) design and the standard reference (Conv.) design equal to find the SIP requirement and simplifying results:
Qws.Conv=(A×∆T)/Rws.Conv = Qws.SIP=(A×∆T)/Rws.SIP → Rws.Con=Rws.SIP
Equation 3
Where:
Xws.Conv refers to the whole surface value of conventional construction
Xws.SIP refers to the whole surface value of SIP construction

Using the DOE/ORNL Hot Box data we can establish the relative reduction in performance over a whole surface.
Rws.Conv/Rnom.Conv = (14/19)= 0.73
and
Rws.SIP/Rnom.SIP = (22/23)= 0.96
Equations 4 & 5
Therefore, for a SIP wall, roof, or floor surface to equal it’s conventional equivalent this relationship must be held:
Rnom.Conv × (0.73/0.96)= Rnom.SIP
Equation 6

EXAMPLE

If a roof requires R-30 conventional insulation, the required nominal SIP R-value would be:
30×(0.73/0.96)=23
Equation 7
Using reasonable significant digit precision through the calculations, the following range of Foard Panel over all thicknesses meet or surpass the IECC-’09 for an R-30 roof:
• Polyiso SIPs: 4.50” or greater
• XPS SIP: 5.38” or greater
• EPS SIP: 6.50” or greater

Posted in Technical Bulletin