This is a blog written by Paul Freeman, President of Brooks Post & Beam Inc. This blog will give updates on current projects and thoughts on changes in the post and beam trade, especially on topics relating to sustainable building and energy efficiency.
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” polyurethane 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 building code and figure out what exactly the code is looking at. The code requirements for floor, wall, and ceiling insulation R value were originally written for 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 2x6 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 OR 13+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.”
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)Write comment (0 Comments)
We are pleased to announce our new line of post & beam kits. If you click on the “Post and Beam Packages” tab of the home page, you will see a new section- Timberframe Home Kits. We have put together kit packages which combine our most popular small home designs with pre-cut timber frame and panels. While there are other companies offering kit packages, only Brooks Post & Beam offers easy to assemble draw-bored frames, backed up by our unparalleled experience and expertise to make your building project successful and rewarding.
Why Buy a Timber Frame Kit Home? (by John Fiery)
Every potential client we talk with have their own reasons for wanting a timber frame home kit, but the most common are cost savings, the satisfaction of doing it yourself, flexible scheduling, and minimizing waste.Write comment (0 Comments)
Why should you consider building a new home when there are so many reasonably priced existing homes on the market? This is a good question and not as easy to answer as you would think. In prior articles I have written why you might want to live in a timber frame home in comparison to a conventionally framed home. So let’s assume you have already established that you prefer the aesthetic of heavy timbers and mortise and tenon joinery; you like the open, bright and airy feel of a post and beam home; and you recognize the practicality of the long term savings of a high quality home.Write comment (0 Comments)
Building a new timber frame home is not a job to be undertaken lightly. To help guide you through the many decisions with which you will be faced, we have written the Brooks Post & Beam Owner's Manual.
Whether you plan to contract out the work, be your own general contractor, or do it all yourself, this manual will explain and specify details and various tasksassociated with building a timber frame home.Write comment (0 Comments)
When Phil Brooks first began building new timber frame homes he quickly recognized that contractors all too frequently misunderstood the owner's wishes which resulted in frequent delays and costly changes. As former teachers, both Phil Brooks and Paul Freeman are committed to helping you learn everything you need to know in order to build your dream home. As tradespeople we can communicate effectively with others contracted to work on your home so that they too are able to understand what is necessary to build a quality home that matches your dreams.Write comment (0 Comments)