When we discuss our timber frame packages with potential customers, we often get questions about the structural insulated panels (SIPs). Probably the most common question is, “What is the R-value of your wall and roof panels?”
Over time foam prices change, but right now the best value is 4-1/2” polyisocyanurate core (PIR) wall panels at R-23 and 8” expanded polystyrene core (EPS) roof panels at R-30. A frequent follow up question is something like: “I’ve checked my local building codes; the required R value for walls is R-20 and ceilings is R-38 or R-49. You wall panels look great in comparison, but the roof panels are lower than the code minimum. What’s up with this? Shouldn’t we use thicker panels to comply with the code?”
And so we come to the topic of this post- what’s the deal with R values and SIP panels? There is some confusion and misunderstanding about SIP panels, building code requirements, and R values in general.
First of all, let’s define some terms. R value is used to measure thermal resistance of walls, floors and ceilings. U value is the inverse of R value- 1/R value = U value, and 1/U value = R value. U value is a more precisely defined unit of measurement; BTUs per hour per degree Fahrenheit per square foot, or more simply understood as the amount of heat that can transmit through a building component (wall, floor, ceiling, window, door and so on) in one hour. It is important for consumers to understand that some building materials and components, such as windows and exterior doors, have thermal performance measured in U value; other materials such as insulation are measured in R value. So just remember- higher R value = better thermal performance, lower U value = better thermal performance.
Next, let’s look at the residential building code and figure out what exactly the code is looking at. The code requirements for floor, wall, and ceiling insulation R value are referencing fiberglass batt insulation fitted into cavities between wood or steel structural members- studs, joists, rafters. And here’s the “dirty little secret” the code writers know and that the general public misses – the concept of “whole wall R value”. According to research conducted by the Oak Ridge National Laboratories, a 2×6 wall with R-19 fiberglass insulation and studs at 24” on center actually performs as low as R-11.7 when the thermal bridging of the wood studs and the inevitable gaps and voids of a typical insulation job are factored in. (Residential Energy, p 274).
The code officials know and understand this concept, and the evidence is right in the code book. According to the 2009 International Energy Conservation Code , Table 402.1.1, the required minimum R value for wood framed walls in Zone 5 and 6 (most of the northern US) is R-20 OR13+5. What’s this all about? 13+5 = 18…..not 20? Well, the R-5 is continuous insulated sheathing over the wood studs. Already, we’re seeing a “discounted” R value by voiding the thermal bridging of the studs. They say R-20 but they’ll take R-18, and in reality the actual whole wall performance is closer to R-15 or less.
Need more proof? Ok, let’s look at ceiling insulation. This is where we get back to the often asked questions about the roof panels. Section 402.2.1 states “When Section 402.1.1 would require R-38 in the ceiling, R-30 shall be deemed to satisfy the requirements for R-38 wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves.” In other words, if the building can be designed or constructed in such a way as to avoid squashing the insulation at the eaves; if you can actually manage to have a consistent thickness of insulation all the way to the outside walls, you get another “discount”! Over 25%! The code seems to be saying “we recognize that fiberglass insulation is almost impossible to get right, and we know that the wall or ceiling thermal performance is subject to thermal bridging, voids, gaps and compressed insulation. We are asking for a certain higher level of insulation, but we’ll be happy if we can get even half of that when it’s all done.” Oh, and there is one final piece of code information that is often overlooked – the Energy Conservation Code (IECC) insulation requirement for commercial and larger residential structures where the roof insulation is entirely above the roof structure is R-30 for continuous insulation.
So, here we are, 700 words in and I haven’t even gotten to SIP panels yet! At this point I would like to think the case has been pretty well made for a continuous, uniform thickness thermal envelope, minimizing or eliminating thermal bridging caused by structural lumber. But just in case you remain skeptical or unconvinced that SIP panels are clearly a superior solution to enclosing a timber frame home, I have one other quote to offer, this from the excellent website www.sips.org. The quote comes from the article “R values in the Real World” (http://www.sips.org/technical-information/r-values-in-the-real-world). “Building with SIPs does not require any insulation to be installed in the field. There is very little dimensional lumber required to build a SIP home because SIPs are structurally sufficient. ORNL tests prove that SIPs maintain their full R-value in whole wall testing”. Brief and to the point. Oh, and one last tidbit of information- EPS foam insulation, which is the type used in our standard roof panels, has been shown to actually increase its R value as the temperature drops! That’s right- the colder it gets, the better it works!
There is one other very important topic that we need to cover when discussing SIPs, and that is- air. Air infiltration, air sealing and ventilation. That’s a topic that has been covered here before, and will be covered again in an upcoming blog post. I’ll just leave you with this teaser….
“One-third of the energy you buy probably leaks through holes in your house”
That the subtitle of a 2012 Fine Homebuilding article by John Straube which examines the effects of air leaks and how they waste energy. We’ll visit that topic soon….
Sources for this blog post:
www.sips.org The Structural Insulated Panel Association
“Residential Energy” 5th Edition; John Krigger/Chris Dorsi; c2009 Saturn Resource Management
IRC 2009, IECC 2009 (https://up.codes)
Author: John Fiery