Are SIPs Necessary?
—Originally Published in Timber Framing, the Journal of the Timber Framers Guild
At least once a year, I have a client in my office who’d like to build a timber frame home and enclose it in strawbales. The first time this happened, we agreed and pushed forward, eager to learn about strawbale enclosures. We raised the frame in January of 2007, and the owner-builders and a strawbale subcontractor set to work enclosing it, followed by plastering. The owners survived (kind of) two weird winter rainstorms, daily wind, and an incompetent contractor, and finally got the thing plastered by late fall. They’ve since replaced the exterior plaster in its entirety twice, and they’re still dealing with plaster problems.
I now promote a dialogue with clients that examines alternative enclosure systems for every project we do. While strawbale enclosure may well be practical or even ideal in other circumstances, for our local conditions (we’re in Fort Collins, Colorado), local contractors and the budgets of our clients, we have never since concluded that the best enclosure system was strawbales.
What about structural insulated panels (SIPs), then? Most timber frame contractors encourage the use of SIPs; They must be the best, right? We were certainly happy for some time to promote them as the best choice. It was easy to dismiss the strawbale alternative because of the bad experience of our clients, and building external stud walls and then insulating them surely couldn’t be energy efficient, could it? Were SIPs the answer?
I accepted as true all the benefits claimed by SIP salesman and manufacturers, taught them to our clients and tried to sell panels to everyone we worked with. I hadn’t done a lot of research and didn’t understand alternatives. I wasn’t even thinking about return on investment at that point, and in sum I wasn’t really able to give my clients enough information. I was a SIP salesman.
The last part of 2008 changed the way Frameworks did business. We trotted along until about October 15 when a sale we thought was a done deal cancelled with a 60-second phone call. Frustrating, but we still had January through March sold; maybe we could start a little early on that project. We could not, that job officially cancelled via a message on my cell phone on Christmas Eve. I became familiar with the phrase “wave of cancellations.”
I laid off staff and tried to find anything to do to keep remaining help in groceries. I set about a project of business introspection that I had never done. Since there was no work to manage, I had time to obsess with what-if questions. A free consultation from the Small Business Development Center introduced me to the concept of gross margin. A light bulb went off about how we’d been estimating, what our estimates might look like under an accurate analysis and, of course, the perpetual what-ifs. What if we lowered our prices and did more volume? What if we sold only timber frames and made no money on panels? What if we offered more services? What if we offered fewer services?
At the root of all those questions was one real goal-to sell more work. I was trying to learn from the past so I did the most examination of the jobs that had cancelled. I naturally wondered whether if we’d somehow been able to charge less, would those jobs not have cancelled? It wasn’t as if we were getting rich or building exorbitant budgets, so how on earth could we reduce the price? The spreadsheet I was using at the time tracked percentage of overall budget next to each line item, and I had by then noticed that the number next to SIPs was consistently around 30 percent. I had all the motivation I needed (hunger) to take a close look at these panels and try to figure out if they were worth their apparent premium.
The alternative I began to consider was a studded wall of 2x6s, 24 in. o.c., filled with sprayed open-cell foam. I went through all of the last ten jobs we hadn’t sold and built estimates for replacing panels with the frame and spray system. I got consistent results. In the ten jobs I studied, using the prices of the day, it would have cost about $10 more per sq. ft. of finished house to use SIPs for walls and roofs. In more recent analyses, this number falls between $5 and $10 per sq. ft. Panel prices in our area have gone down substantially, while the price we’ve been willing to pay to framing carpenters has gone up. Commodity lumber pricing is also fickle. When I ran the original price comparisons, street price for oriented strand board (OSB) sheathing was less than $7 a sheet. This summer (2010), it got as high as $14 a sheet, and right now is at $9.57.
I’ve stopped running panel estimates because of my conclusions but I did go back to analyze a 2065-sq.-ft. house we built this summer. I know what we actually paid to enclose it, and I know current rates for panels here. To have built the roof and exterior walls with panels would have cost an additional $14,000, or $6.78 per sq. ft. That’s less than the premium I’ve cited above but more than the $5.14 per sq. ft. we’ll use below for some payback scenarios. It’s probably right to assume that I might get a different figure for every house I analyzed, but I do believe my research identifies a definite trend. When we look at payback numbers later in this article, we’ll use the bottom of the identified range of savings.
You may come up with different numbers because of your location, your climate, your local suppliers and your building season. In our area, framing carpentry rates are competitive, there are a lot of local lumberyards competing to sell studs and OSB sheathing, and we apparently get pretty good rates on spray foam. For these reasons, a favorable cost-benefit ratio of SIPs just doesn’t appear to pan out here.
It’s important to consider the entire cost of an installed system. Let’s look at panels’ advertised benefits to see if they are real for me or my clients.
SIPs reduce waste. Do they really? Or do they just move it to the manufacturer or fabricator’s warehouse where neither I nor my customers have to look at it? What about the chunks of panel that end up in the dumpster because of imperfect project management? Panel scraps, as far as I know, have almost no alternative use, while the pile of cutoffs from a 2×6 stud wall may have a second life as blocking, backing or, at worst, firewood.
SIPs reduce labor. Again, do they really, or do they just move it to the fabricator’s shop and off my budget line? Even if they do reduce labor because the labor’s more efficient in a factory setting, does that offset the additional cost of the system as a whole?
SIPs install quickly, thereby saving time and money. It’s generally accepted that the actual applying of walls to the exterior of a timber frame takes a few days less with panels. As a general contractor, I’m not sure this benefit is more than academic. First, the installation time (and consequent cost of field labor) saved is outweighed by the cost of the panels compared with the cost of the insulated stud walls. And the time saved as a portion of the whole project is insignificant. Custom residential construction simply isn’t managed tightly enough that a few days of time gained in wall installation provides a notable financial benefit (loan interest).
To take an actual example, the 2065-sq.- ft. house I mentioned above was framed studded out in 11 days by a crew of four. That framing time included the insulating walls, interior partitions, second floor framing and stairs. I’m not convinced you could actually install the panels and do all of the interior framing in less time than that. A good study of total project schedules also recognizes that other subcontractors take longer to perform their work later in the project, potentially nullifying any overall elapsed time advantage of SIPS. If you believe that saving a few days in a construction schedule is of benefit, do the math on the cost of the construction loan (if there is one), and see just how many dollars could be saved by shortening a construction schedule.SIPs save energy and “pay for themselves” in reduced heating and cooling costs. I am not a scientist or a physicist. I am a thinking carpenter turned general contractor who expects to be able to understand science as it applies to what I do. I make no exception for “building science,” specifically in my search to understand the value of R-value. I’ve learned a lot about building energy-efficient buildings in the last couple of years, not least that it’s very difficult to find consistent information about this particular subject. Sorry, but consistent effort as a researcher reveals a snake oil environment. Everybody selling one kind of insulation or another (including SIP manufacturers who use urethane foams instead of polystyrene and vice versa), claims everybody else is wrong about initial R-values, R-value creep, air seal, greenhouse emissions, off gassing, etc. More to the point, though, is this question: “If my clients spend extra money at installation, can I demonstrate that they’ll recover that money over a reasonable time via reduced fuel bills and, if so, how long it will take?”
I would expect building scientists to be able to model the following situation: At R-Values of X for walls and roof, the calculated heat loss of your house is equal to Y BTUhr (British Thermal Units per hour.) At current (and projected) costs of fuel, the cost to generate those BTUhr is $Z. If this calculation exists, we could then change R-values on the input side, and monitor the result in dollars on the output side. (If we can land a man on the moon, we can make Excel generate these values.) I’ve found only one person running anything close to this software, one of our local radiant heating contractors. In an attempt to approach this question scientifically, he and I modeled a sample 1500-sq. foot house three times, the only variable the wall system. (At the time we ran this comparison, I was only considering changing wall systems. I was still assuming SIP roofs were the best choice. I don’t any longer.)
We analyzed three wall systems: 6-in expanded polystyrene (EPS) SIPs; stud-framed 2×6 walls filled with open-cell foam; and 6-in. stud-framed, foamed walls but this time with woven 2×4 studs to eliminate thermal bridging and again filled with open-cell foam. Thermal bridging, for the record, is not the demon that SIP manufacturers would have you believe. The effect of thermal bridging on overall R-value of a wall can be calculated in much the same fashion as for windows in a wall. After modeling the three systems, we achieved the following heat loss values, (in BTUhr): 6-in. EPS, 25,343; 2×6 wall 25,757; and 2×6 wall with staggered 2×4 studs 25,659.
As a point of reference, note that a standing human generates 400-450 BTUhr, and closer to 800 when dancing. The total difference from worst to first here is 414. Run the numbers and you’ll notice that there’s 1.6 percent more heat loss in the 2×6 framed wall than there is in the SIP wall. (Although I can’t prove it, I suspect 1.6 percent is within the range of error in the calculations, and that we’ve actually proven that there is no significant difference in thermal performance between a foam-insulated stud wall and a SIP.)
To put this into a financial perspective, upgrading to panels on the job we were designing at the time, a 2100-sq.-ft. house would have cost an extra $10,800. That includes labor and materials for a complete installation of wall and roof systems, although it does not include additional costs that I believe SIP manufacturers would prefer you ignore, such as for window jamb extensions, extra charges by your electrician for rough-in, the difficulty of hanging cabinets on SIP walls, and in general the fairly constant slight increases in cost when subcontractors are dealing with unfamiliar building systems.
Although the cost of increasing indoor temperature in the winter is not linear (it takes less energy to increase indoor air temperature from 50 to 60 than it does 60-70), let’s look at this question as if it were. To pay back a $10,800 investment in 10 years, we’d need to be able to reduce our heating bills by $1,080 per year or $90 a month. If the supposedly less-efficient 2×6 system in fact requires us to generate 1.6 percent more energy to maintain temperature, $90 needs to be about equal to 1.6 percent of the heating bill. For a ten-year payback on additional SIP expense, then, our heating bills would need to be $67,500 per year or $5625 a month for this 2100-sq.-ft. house!
Using local figures from my area, it would probably cost less than half of $5625 to heat an insulated 2100-sq.-ft. house for a year with propane, the most expensive fuel option here. (I live in 2800-sq.-ft. house built in 1918. The remodeled parts are well insulated with cellulose but the one-third of the house that hasn’t been touched still has old windows that air quite literally blows through. The most expensive heating bill I’ve ever had was $210 in one month; most months are less than $100.) Supposing a conservative total of $2400 a year to heat the 2100-sq.-ft. house we designed, SIPs would then save $38.40 a year and payback of the $10,800 investment would take a staggering 281 years.
If there’s a flaw in my reasoning, I’d like it pointed out by a neutral authority, and I’d also like to make it clear that I’m not suggesting SIPs have no place in the timber frame industry. I am pleading for us to have and to use better research and information when we help our clients make decisions about enclosure. If SIPs are a lot less expensive in your area than in mine, or carpenters are much more expensive or unavailable, or the closest place to find a spray foam installer is a day’s drive away, or your building season is really short, panels may be the best choice for you.
For me, there are additional factors to consider about enclosure systems. The largest one is longevity. I’m not sure when structural insulated panels were first put in service, but I do know that we don’t have a lot of experience in how they endure. I believe it’s worth noting that the entire panel system, from exterior sheathing to interior finish, is dependent on adhesives. How long are those going to last? What will happen to these buildings if the adhesives fail? Are they reparable? We may clad our studded walls in adhesive- dependent OSB, but the connection of cladding to studding is mechanical and thus reversible. If a sheet (or an entire wall) of OSB goes bad on a studded wall, it can be replaced without disturbing the interior. Not possible with SIPs: the whole sandwich has to be removed, including interior finish.
There’s a lot more history available for light wood frames. I grant that a lot of it’s not good in terms of energy consumption. Light wood frames, though, seem to last fairly well, they’re reparable in part if parts are damaged and they can actually be recycled. (Anyone recycling SIPs?) I know roofs shouldn’t leak, but I also know that almost all of them do, sooner or later. One of the principles of really long-term buildings is first to accept that they will all eventually fall into disrepair. When that happens, are they fixable? And when they are ultimately deconstructed, are their materials recyclable? As far as I can tell, SIPs fail these tests. For me, that adds two more reasons for us to not use them on our buildings.