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You can fill a double stud wall with cellulose, which has a much lower embodied energy than the foam in a SIP panel, if that sort of thing is important to you.
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Thick walls have a tax penalty in that the tax man measures the outside of your home and calculates a square foot number. That number is then used to determine the value of your home. In that all homes in your neighborhood with the same number of bed and bath room are assigned the same value per square foot for the structure.
Walta
Walter,
The difference in square footage between a SIP wall and a double stud wall of equality R-value is going to be pretty small. I think the tax valuation methodology differs from region to another, so this may not even apply. For example, in my area every new house is individually appraised, based on numerous criteria, none of which is external footprint.
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Chris,
Another benefit of conventional wood framing over SIPs is the ease of renovating or adapting the house over time.
Chris,
For green builders, the cellulose-insulated double-stud wall wins every time, due to its lower environmental impact.
SIP walls lack studs, so siding ends up being fastened to 1/2-inch OSB. The fasteners have no studs to penetrate. For most types of siding, this isn't an issue -- although some types of siding can't be used due to this fact.
Air sealing details at SIP joints need to be meticulous for long-term durability. I would never use SIPs without a redundant air-sealing plan; generally, this means canned spray foam at the seams plus a high-quality interior tape. Builders who have been sloppy with these details (especially in cold climates) have ended up with rotten sheathing near the seams. While this problem is more common on SIP roofs than on SIP walls, it sometimes happens with SIP walls.
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While I am not against either option, I wonder about the value of these types of walls in a warm climate, like Charlotte. I suggest you look at this post on EnergyVanguard.com, where Allison Bailes lays out the question of whether going beyond 2x4 walls makes any sense down south.
Be sure to check out his follow up post on an error in this post.
Martin,
I never considered the siding. I think this would be a problem for a lot of different siding types. I know for sure that Hardie panels/planks and the major brands of engineered wood require studs. This is what SIPA has to say about it:
" Builders should consult the siding manufacturer’s installation instructions for how to attach their product to SIPs. Because SIPs use very little solid lumber, an increased fastening schedule is often required when attaching exterior cladding.
If the siding manufacturer does not offer recommendations for attaching their product to SIPs, a licensed architect or engineer can calculate the appropriate fastener frequency by obtaining fastener pullout capacities from the SIP manufacturer."
So, a few of things to consider there. One, have fun getting an engineer to provide that schedule for you. Expensive and just a pain in the butt. Would an architect even be qualified to do so? I'm thinking no. Second, even if you get the go-ahead from an engineer, kiss your siding warranty goodbye. Most siding manufacturers look for anything to void your warranty claim, and this would give them a slam dunk. Lastly, consider the increased cost of siding install if you double, triple or quadruple the number of fasteners required. And have fun finding a siding contractor who'd be willing to do it.
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Chris,
You might want to read this article: "How to Design a Wall."
I agree with Norman: in Charlotte, N.C., investing money in a high-R wall probably doesn't represent the place to focus your attention. In your climate zone, close attention to air sealing or (if your local utility offers net-metering contracts) installing a PV system are likely to be two ways to achieve more bang for your buck than upgrading the R-value of your walls.
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In S.C., Duke is phasing out their net metering plan this year and may have already closed it down. This is due to the S.C. legislature being unable to come up with a means to extend the plan that met both the needs of the consumers and the needs of the utilities. It was a great disappointment, but there is hope for a reasonable compromise in the next legislative session (BTW, I managed to get my application under the wire, barely).
I am not sure you are on the N.C. or S.C. side of metro-Charlotte, but wanted to pass this along.
Chris,
I agree with most of the posts here that SIPS for walls can be a difficult detail to get right, and in the end, my experience has always been that the integrity of the SIPS ends up being compromised by trades. I am in Colorado, and my go to wall lately has been 2x8 plates with staggered studs, and blown in fiberglass (cellulose not readily available in my area, but working on it). I also always include a rigid foam of some sort on the exterior. Lately I have been going with 1-1/2" EPS - less expensive and little less harsh on the environment. Where I do like SIPS is on the roof. Anytime I have an exposed beam/vaulted ceiling I try to go with SIPS. I have a pretty good detail if T&G is involved - let me know if you need that......Good Luck!
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I had not considered the potential issue with hardie siding on sips so I asked Charles at Eco-Panels to read through this post and let me know his thoughts. I am posting his response below but first I want to say that I not affiliated with them at all and have never purchased Sips form them, I have exchanged a few emails and thought posting his response would just add more info to chew on for anyone else thinking about building with a double stud wall vs sips. I am not sure which way I am going to go, I might even go with a single stud wall w/ exterior foam based on info/links provided by GBA members in this thread.
From Charles at Eco-Panels--
Hi Chris,
Thanks very much for the questions. I’m a big fan of GreenBuildingAdvisor and Martin Holladay is one of my “Top Three” building scientists I most look up to. I will also agree that unless you’ve got a big budget, your biggest bang for the buck would be our 4.5” thick R26 rated (at 52degF) wall panels. And you may want to consider our Huber ZIP System exterior sheathing for the outer skin (with no nail penetrations in the field of the panel like stud framing). Our 6.5” R40 at 52degF rated panels are certainly stronger and much more energy efficient, but I’m sure you’ve got other uses for the money. It was good to hear our R40 rated panel was approx. same cost as the double stud wall, so the 4.5” should certainly save you money. Of course we welcome you to come by the factory to check us out if you haven’t already.
We have a tough time seriously considering arguments of embodied energy when comparing most materials to our panels because that incremental amount can be more than offset through long-term energy savings, decreased moisture issues (which cellulose insulation can attract), increased structural strength, fire resistance, labor savings, etc. Plus it’s quite a slippery slope if the preferred “low embodied energy” insulated wall method uses significantly larger amounts of steel fasteners, significantly more small and large scale dimensional lumber & trim materials (which double stud framed walls do). Engineered wood products are almost inherently more “green” than dimensional lumber. 2x12 lumber, a common dimension lumber for double stud wall top & bottom plates (sometimes code allows OSB or plywood), comes from much larger diameter (DBH) trees than standard 2x4 or 2x6 studs and they literally take a generation or more to grow (approx. 25+yrs). That tree is doing much more to address our carbon problem by breathing it in and providing shade and home in a forested habitat for another 100+ years than it ever will in your house. Your time may be better spent working on changing local code to allow plywood or OSB plates for these complex assemblies.
Thick walls of blown cellulose (even assuming you are talking high density (though not blown wet – not in our climate)) are like sponges just waiting to get wet. Their vapor permeability is about 6-8 times that of our closed cell foam. BECAUSE cellulose is so susceptible to holding moisture good building scientists in the south generally encourage builders to sheath with plywood – not osb. That’s more added cost so you’ll need to be sure to put that in your calculations. And most good insulation installers can account for future anticipated settling of blown cellulose at the top of the walls – but more than a few do not and we have thermal camera images that prove it. The orientation of the studs in a double stud wall can make it difficult to get a very even distribution of cellulosic density.
Attaching siding materials has never been an issue with our panels, regardless of siding type or mfgr warranty. If there is a concern we first recommend furring strips over the WRB – as also recommended by Building Science Corp for building with structural insulated panels. And if that is still not enough, we can add scrap lumber flat up against the inside of the exterior panel skin at 16”oc or similar BEFORE we inject our closed cell foam so the beefed up wood is not a thermal bridge but it gives you the “meat” that you are looking for for siding attachment. Or we could use a thicker exterior skin for our panels like we did for our work in the Caribbean where every structure in the development was destroyed by Hurricane Irma except for all of the buildings made with our panels.
Martin makes a few comments about failures at panel seams and noting you should exercise caution with sips, so please let me elaborate on that. I can understand this – I’ve had builders run away from me when I used the word “sips”. I actually hate the term “sips” (ironic, I know) because I think polystyrene (EPS) panels have given the industry a pretty bad name. In fact their trade group magazine actually advertises for Litigation Support for manufacturers – wow – that says something. Though they call it closed cell foam, most of their product actually performs as open cell foam (yes the beads themselves are closed cell, but unless you are approaching a 2 lb/cuft density you’re looking at the performance of an open cell foam product (and when seams leak it can be more of a problem with open cell foam insulation)). I also do not agree that many of their standard assembly instructions have gone far enough to ensure a good quality final assembly (though it is getting better from what I hear as they continuously understand more ways their product can be assembled incorrectly). But we still do not agree with many of their standard practices. See the below photo of our panel edges. We have a patented multi-laminar profile ensuring much more surface area at the panel joint than other panel systems – much stronger in transverse loading than a traditional single male/female or lumber spline joint, and every 2ft along the vertical panel edge you have a cam-lock pulling the panels tight with hundreds of pounds of force – so four cam-locks on an 8ft panel. PLUS, since we understand that moisture drive occurs not just from the outside -> in but from the inside -> out we require TWO beads of joint sealant foam drawn along the panel edge – one on the interior edge and one on the exterior edge (in addition to top & bottom). And we will guarantee until the end of time there will be no thermal or seam leakage at the joint of our single piece corner panels (also creates a much stronger structure than stud frame construction and that’s lab tested) because it is molded in a single piece. We also have some builders that come back and tape not just the exterior seams but also the interior seams (in addition to the sealant foam) and we coach them on when to use semi-permeable or impermeable tapes.
We also offer a LIFETIME warranty against delamination of our panel skins from our foam core. And our foam core will NEVER melt – EVER (unlike EPS panels). This is a significant safety issue if there is ever a fire.
With a degree in applied physics and an almost 20 year career in the hi-tech manufacturing industry before starting Eco-Panels, I spend a lot of time trying to educate clients and builders that a home construction site is indeed a manufacturing site with many, many, many opportunities for error – tens of thousands, actually - and that’s even if you have good weather and a good steady labor crew (which often is not the case). Due to keyed edges most of our panels only go together ONE WAY. You don’t have to figure out sizing a header because our panels are actually headers – fully insulated – no thermal bridging - with no LVLs or similar. We have worked very hard to ELIMINATE opportunities for error (“OFE’s” for any Malcom-Baldridge legacy folks out there) on the jobsite (manufacturing floor) – because sure enough, IF it CAN be assembled incorrectly, IT WILL be assembled incorrectly. There is a significantly greater chance for assembly errors the more complicated the method, and there is no more complex method than stud framing for building a home. Do we have enough lumber and fasteners for structure for where we are building? Is fastener schedule sufficient for the wind loading? Is my header sized properly? What side is the vapor barrier on? How do we address moisture migration through the wall assembly? Do we have COMPLETE filling of cavity with NO air pockets? This stuff is complicated enough already and now people want to build a DOUBLE wall? PLEASE, there is new technology out there that can be assembled on the jobsite in a FRACTION of the time (so much lower labor cost, faster dry-in, more efficient utilization of materials), and all we ask is to allow people to give us a chance to prove that we do have a better way. I think trees are way too valuable in the forest to be cut down as framing lumber for exterior wall systems where it’s most often serving as a thermal bridge every 12 or 16 or 24 inches for the next 30 or 50 years and literally FEEDING our carbon habit. It’s true that some people are slow to change and many never will, but this country has been stud framing homes for almost 200 yrs now, and I think ours is a far better system in any number of ways than a stud framed exterior wall.
Please let me know if you have any additional questions!
Regards,
Charles Leahy
Eco-Panels
Charles,
I agree with your conclusions. Here in the USA it's all about wood stick frame construction. Yet when you go to Israel, Europe, Middle East, South America, etc. you quickly discover that would frame is not the norm and they use other techniques and practices. Stone, concrete and masonry are more commonplace.
I deviated from stick frame with ICF concrete construction for the walls and SIP (polyurethane) roof. My county inspector never inspected an ICF wall before so it was new to him. The SIP roof was maybe his 2nd ever SIP roof. It was interesting to say the least but I am very happy with the results.
Like you mentioned, most GC's hate change, they despise it. Ever attend a meeting on code changes? GCs do what is cheapest and fastest. No doubt that SIPS are fast (my roof went up in just 7 hours) and I was insulated and sheathed in. SIPS do cost more $$ but the savings can be offset by faster assembly times.
People fear change and GC's are people.
OK, so what blowing agents are being used for the polyurethane in the Eco-Panels?
If HFC245fa, how are the lifecycle aspects of that extremely potent greenhouse gas being managed?
The polymer weight per R-foot is...??
Can anyone point to the data or name the building scientists referred to on the comment:
"BECAUSE cellulose is so susceptible to holding moisture good building scientists in the south generally encourage builders to sheath with plywood – not osb."
It seems a very odd assertion to make. How is plywood preferable to OSB when the cavity fill is cellulose? When dry (as it would be unless bulk water moisture isn't properly handled) the vapor permeance of plywood & OSB are similar- both are "smart" vapor retarders, so it would be important to understand the source actual building science being cited here.
Leahy's comments read more like a hatchet job on the competition rather than an insightful discussion of the merits of the product.
Good morning Dana - regarding your question "can anyone point to the data or name the building scientist referred to on the comment: ..." - that building scientist is Martin Holladay. I encourage you to read his article about how to properly double-stud frame a wall, and what to be careful of. The permeance of cellulose really does not need to be discussed much because the numbers are widely available, but Martin mentions the warning, and it was none other than Martin that taught me that plywood might be preferred to OSB for such a large cavity capable of holding so much moisture.
The blowing agent we primarily use is 365, not 245 - much better at GWP. And we have already been experimenting with zero GWP agents and we expect to make that transition next year. It really takes a lot of testing and validation so not a slow process. Market price and availability also of course very key here.
I can appreciate your skepticism - obviously I'm a skeptic too. I welcome further conversation.
"The blowing agent we primarily use is 365, not 245 - much better at GWP."
Per IPCC AR5:
HFC-245fa has GWP 858
HFC-365mfc has GWP 804
I'm not sure how Martin Holladay would be considered among the "...good building scientists in the south...", given that where he lives is north of nearly half the population of Canada. (He's also not a scientist, nor does he pretend to be. Journalist/blogger would be more accurate.)
The theoretical "cold sheathing" problem of fiber insulated high-R wall assemblies sheathed in OSB or plywood is primarily a cold climate issue, not relevant to Charlotte NC. But in fact the hygric buffering capacity of the cellulose is deemed to be a protective factor for OSB sheathing, not an enhanced risk, per work done by Kohta Ueno (a REAL building scientist), et al.
https://www.energyvanguard.com/blog/77957/Is-This-the-End-of-the-Double-Wall-Cold-Sheathing-Scare
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1501_Monitoring_Double_Stud_Moisture_Conditions_Northeast.pdf
I'm also not convinced by the lets leave the big trees alone and use OSB argument. OSB typically comes from small, young trees. If it is the material of choice there will be no big trees just left around to sequester carbon, the forest will all be fast growing softwood species the get harvested a lot sooner.
A more general comment:
I wish posters would stop disparaging the whole construction industry as being somehow corrupt and ascribing bad faith the builders - especially when it comes from those with little experience. Get some skin in the game. See what's involved building house when your livelihood depends on it. Develop some long-term relationships with builders and subs. Then decide if they are all dinosaurs and thieves.
Hi Malcolm, this is Charles from Eco-Panels. Regarding the conversation with big trees and OSB, I'm not sure I follow your response. You start one way then make an opposite claim. In fact I am trying to say that if we use more engineered wood products (think OSB, parallams, LVLs, TGIs, glulams, CLT, etc.) there will be more large trees left standing. I think this would be a good thing - not sure if you understood that and my apologies if I was not clear. A forestry friend of mine told me recently that in fact it is rare that 2x12s come from a 14 DBH tree. More often than not it comes from an 18+inch DBH tree (that's "diameter at breast height"), and that can be 30+ years of growth (hence my "generation" comment).
And regarding "skin in the game" - I've been a hobby woodworker since the 1970s (because my father was one), worked construction in the 80s (though it is not on my professional profile) and then went into the tech industry. I got back into the construction industry in 2006 - just about the WORST time to enter the industry, and I lost pretty much everything I had - including my house that my family and I had hoped to grow old in - over the years 2008-2012. So I was in construction, I got a degree in physics and then learned to a very high degree about heat transfer, permeance, etc., then I worked in high-tech manufacturing and managed world-wide logistics networks and then got back into construction industry but from a product supplier perspective.
And I have met more disingenuous builders than I care to mention. Builders that say one thing and do another. But I keep meeting good and honest ones too, and from each and every one of them I hope to learn something.
As the saying goes, don't kill the messenger. Gallup and other polls showed that 50-75% of people DO NOT trust building contractors. Let's play the conservative side and say 50% so that's half of people have an unfavorable view of the construction industry and those involved in it. Look, I know it's hard work, and I take my hat off to the honest ones who do good work. Sadly, the industry is filled with con artists, hacks and theives. They ruin it for the others. It taints the whole industry. The good ones know their trade and charge a premium for it. The average homeowner can't afford the best so they go with the others. That's when nightmares and horror stories begin. I was my own GC on my build and at times it was very frustrating dealing with the subs. Most of my family are builders and they are good at what they do and they make a lot of money. I have about 8 family members involved as builders and about 7 out of 8 of them are now multi-millionaires. They made a lot of money building homes. It's hard work but those who are good at it can make a lot of money doing it. I've seen it firsthand.
To all - I want to thank Chris Fortune for giving us an opportunity to respond to questions he had on this thread. And I want to thank all of you for your replies and added input.
You will not find Eco-Panels regularly active on any blog other than our own, but there are few online resources out there I think better than Martin's "GreenBuildingAdvisor". We try to learn from the best. But please understand this - building scientists have jobs as experts because "this shit is complicated!" Market forces push us one way - build "tighter" and "more energy efficient" - and the almost 200 yr old methods may not, in fact, be the most efficient way to proceed and may not provide the most efficient outcome. THAT is why this is complicated. Some industries are quick to change and others or not - plain as that. But I believe that the time has come to move more knowledge to the CONSUMER - and let them make the choice. EDUCATE (just like the purpose of this website). I know that many builders say "you can't trust the consumer with this knowledge - we've been doing it this way for so long, we KNOW what we are doing!" OK - stay with that belief - and we'll keep trying to educate the consumer and pointing them to great websites like this one, where they can see articles on how complicated this stuff is, and they can decide for themselves.
I agree with Dana Dorsett: The arguments presented by Charles Leahy (who makes his living selling SIPs) disparaging "thick walls of blown cellulose" is self-serving, and is not based on science.
Suffice it to say than any wall system needs to be carefully installed to function well and to be long-lasting. It's certainly possible to screw up a SIPs installation -- many builders have. It's also possible to screw up the installation of cellulose insulation. But either approach to building a wall will work well, if conscientiously installed by builders who care about airtightness.
In general, cellulose insulation wins, hands down, in any carbon footprint analysis.
For more information, see "How to Design a Wall."
Hello Martin, I would very much appreciate seeing the data back-up for what you are claiming. I myself have never seen a "carbon footprint analysis" for a double-stud framed wall system and would love to see one. But you can not compare the opportunities for error with the field built double stud image you sent (looked like mineral wool) and a good factory made panel system. But I do agree that with COMPETENT professionals it can all end up very high performing wall system either way. The question is - again - at what cost? Short term and long term.
Allison Bailes a few years back did a fair job of debunking much of the faux science around some in the industry's claims about GWP and refrigerants in insulation - I do encourage those who wish to learn more to review it. Most notably you need to understand that closed cell foam panels do not lose refrigerant - the molecular structure is too large to pass through the cell walls - and when you are using your Igloo or Yeti coolers, or your home refrigerator - you should remember that in comfort. On the other hand you could take for example the Mitsubishi high performance mini-split using R-410a, with a GWP of somewhere around 1,700 to over 2,000, depending on which chart you use. The AC system itself will use more blowing agent with a greater GWP impact than the foam in our wall panels.
I will try to attach some substantive data charts from an older BuildingInsight presentation - I believe commissioned by BASF - that used the TUV (German) method of Ecoefficiency Analysis to compare materials, methods and outcomes. Remember you can't just look at a single material - like trying to compare cellulose and PUR closed cell - you have to look at the overall picture and the pros & cons of each system. And the foam they selected in the charts I am attaching had a much higher GWP than the one we are using today and it still beat the single stud stick frame methods. But I would love to see the numbers for a double stud system.
As a scientist I prefer it when people actually use data to back up their claims, but I understand many in the construction industry are not used to that. We are working very hard to educate and we welcome all informed conversations. I do wish all of you the best in your endeavors, which ever way you choose to build.
Charles,
The graphs you posted emphasize the importance of lifetime utility costs. These are significant for most buildings, of course-- no argument there. If you are comparing a cellulose-insulated building to a SIP-insulated building, it's only fair to compare identical size buildings with identical air leakage rates and identical annual energy bills. Needless to say, it's possible to build a 2,000 s.f. building with R-40 walls, R-40 ceilings, an air leakage rate of 1 ach50, and an annual energy bill of $1,000 using either cellulose insulation or polyurethane-insulated SIPs.
So, item 1: Compare apples to apples. The energy bills will be the same, so they are irrelevant to the discussion.
Here is a link to an article that discusses the embodied energy of insulation materials: "All About Embodied Energy."
In that article, I wrote:
"There are several online sources of information on the embodied energy of insulation materials. Environmental Building News lists the following embodied energy values for R-20 insulation covering one square foot:
Cellulose: 600 BTU
Mineral wool: 2,980 BTU
Fiberglass: 4,550 BTU
Polyisocyanurate: 14,300 BTU
EPS: 18,000 BTU
"While Environmental Building News reports that the embodied energy of EPS is somewhat higher than that of polyiso, information compiled by Mike Eliason for his Brute Force Collaborative blog shows that the embodied energy of EPS is somewhat lower than that of polyiso. Let’s call it a tie. Eliason reports that the embodied energy of XPS is higher than either EPS or polyiso.
"One source reports that the embodied energy of spray foam insulation is about the same as that of EPS.
"Every source reports that the insulation material with the lowest embodied energy is cellulose."
A fallacy in the caption of that middle slide is the assertion that "GWP is primarily a result of energy for heating and cooling the home over a lifetime."
That depends entirely on the assumptions. A heat-pump heated/cooled house with 100% renewable power inputs has but a tiny fraction of the lifecycle GWP of the same house with 100% fossil fuel inputs. At the rate of the greening-up of the grid, the error bars on greenhouse emissions over the lifecycle of a house can't be assumed even with the heat pump/dirty grid example.
Cellulose insulation is sequestered carbon, for at least the lifecycle of the house, and usually beyond. If one calculates the emissions of the processing energy from paper to finished insulation product it's comparable to fiberglass or rock wool, but lower than any polymer system (even half pound polyurethane.) When discounting those initial energy input emissions due to the fact that it's recycled material from other uses it tips VERY favorably toward cellulose:
https://www.buildinggreen.com/sites/default/files/live/images/embodied_GWP_with_title.gif
It's true that R410A has large GWP, but the volumes used for small load systems are actually pretty small compared to the amount of blowing agent it takes to do a high-R house with closed cell polyurethane, and small compared to the emissions of heating with fossil fuels. Over the lifecycle of a house it's likely that R410A will give way to R744 (CO2) or an HFO1234__ variants for heat pump applications, and most equipment vendors are actively researching those options right now. (HFO1234yf is has been the standard automotive AC refrigerant in Europe since 2011, though there are a few Daimler/Volkswagen models that have used R744 such as the VW Phaeton, Mercedes E & S Classes, and Audi A8. )
The presumption that all R410A ever gets manufactured will end up in the atmosphere isn't particularly well founded either- it is regularly recaptured during equipment repair/replacement. But it's hard to see how foam blowing agents can be similarly managed at end of service life. Hopefully some of the HFO1234 solutions used in site-applied closed cell polyurethane can be made to work in SIPs too.
(Could be kickin' a dead pony here but...)
Despite the straw-man BASF slides (that presume heating energy is only from natural gas, and a much lower R for fiber insulated houses than the polyurethane insulated house) presented by Charles Leahy in response #28, BASF is now using HFO blowing agents in their Elastospray® LWP closed cell foam products, launched about a year ago:
https://www.basf.com/en/company/news-and-media/news-releases/2017/06/p-17-247.html
But polymer-per-R matters, since the polymer has it's own still-relevant GWP footprint. (That's why in response #19 I asked, "The polymer weight per R-foot is...??" ) It's not clear from the information available on the Eco-Panels website what the density of the polyurethane, is only that it's about R7/inch. It could be 2lbs per cubic foot or 4 lbs, or something in-between, but probably not less than 2lbs.
Even the polymer content of 2lb closed cell polyurethane has a lifecycle global warming footprint per R nearly 10x that of cellulose (when the paper manufacturing inputs are discounted for being recycled material on the cellulose), which is comparable to Type-I EPS. (Half pound polyurethane is quite a bit greener per R than EPS.) That's still quite a lot better than the ~400-500x multiplier when 2lb polyurethane is blown with HFC365mfc or HFC245fa.
Even if the trees were felled and processed solely for purposes of making cellulose insulation, the polymer in 2lb polyurethane blown with HFOs would still have ~2x the lifecycle GWP per R of the cellulose. But that's 2 times a fairly small number, and comparable to some other insulation products.
I look forward to the day any polyurethane SIP manufacturer comes up with a low-GWP solution, which would be in the same order of magnitude lifecycle GWP as EPS core SIPs, but with several favorable characteristics.
Chris,
First post on here, however I've obsessed over GBA for the past 2 years, Just ask my wife... but figured I would chip in my experience with Eco-panels. My wife and I are in process of a 2600sqft (with 3 car oversized garage) self-build using ecopanels in Connecticut. If you go to ecopanels website on the testimonials you can see some pictures of the build there. My wife and I did it all ourselves, and with LOTS of attention to detail we had all the panels erected in less than 4 days - not consecutive, - we both are working full time through the build. It was an exponential effort, honestly the first two corners may have taken more time than the whole second floor. In fact, the last portion I did was the garage (30x36 with 10ft ceilings) and I put the whole thing up in 6hrs on a sunday by myself with no help other than a small single genie man lift that I rigged some 2x4's as forks) I pained long and hard over using sips versus a double stud as you are. In the end I realized there was no way I would be able to frame it myself using the stud wall and work full time and have the whole build done in a year (wife requirement)- we went with Eco-panels. Honestly its a decision I would make 100 times over again. There are things that I would do differently, lessons learned, (like don't put two boxes on the same conduit in the wall during the design phase of the panels) but choosing eco-panels is not one of them. Charles and his crew are good people and willing to talk you through the process and bounce ideas. If you do decide to go the eco panels SIPS route please reach out, I learned a lot and have a lot of tips that I could give to someone doing it. Reading through this thread, I am seeing the same reasons I went with Ecopanels repeated. A double stud wall seems great IF you can find someone to give it the level of detail you expect, (I found that to be nearly impossible- how can I as a homeowner GC with little connections or experience hire someone and truly know if they will live up to my expectations?) or spend the time to do it right yourself (wasn't an option for me). I will say that I had alot of other subcontractors involved for other things and all were either massively impressed or massively suspect. Those that were suspect also gave reasons that made me suspect of their intelligence as well. The classic question: "So your house has no wood in it? So its basically sawdust and foam? You're nuts!" I believe, and my mechanical engineering education and experience tell me, both systems can work right with the proper attention to detail. For me, the panels made it something I can do and spend the proper time to do it right - my opinion. Btw, alot of this house was built with recycled, mis-ordered materials that we collected over the last 18 months (misordered new-in-box windows off craigslist etc) . I'm a ME and always looking for a different way to do things, and I realize that it may not always be better, but I learn something and that's what I'm always about. In the end I think it did turn out substantially better for me to use the panels (as a DIYer) and it seems to be backed up by our blower door test last week at <.6ACH50, prior to sheetrock. And that is with less than ideal Craigslist Andersen double hungs. Honestly, I'm pretty convinced that I could not have achieved that on the first house I have ever built and designed with the double stud method.
I was scared of some of the sealing issues that people mentioned above and did use a redundant system. Probably overkill but it was easy and cheap insurance, I went with Zip outer skin (eco-panels is the only one I know of that offers this), foamed the joint per eco panels instructions and then used a non-hardening acrylic sealant on the inner seam before taping the inside of the OSB with Zip tape as well. Overkill? probably, but it helps me sleep at night and took a couple nights after work to seal it all up. btw, make sure you look at eco panels joint config over others, the cam locks and double groove as well as zip outer skin are what swayed me.
Btw, I did not use eco-panels roof panels, it is a truss roof with CC SPF at the roof deck. Conditioned attic. I did use the Ecopanels as my gable ends, and that worked really slick, the two carpenters that I had help me set the trusses were blown away at how well that worked out. To say they were concerned at first was an understatement.
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Im studying this same situation...but in the embodied energy comparison discussed here is the embodied energy of the wood studs considered? I see its quite high in embodied energy. Or perhaps the wood in the inside layer of the SIP cancels that out?
One thing I consider about this is that the walls have a variety of things going on.. power, perhaps plumbing insulation and structure. Roofs on the other hand are pretty much only there to shed off rain. So doing a double stud for the walls is a great idea and sips a great idea for the roof.
Costs are near the same when you add in labor costs. This said. the cost of subs doing the plumbing and electrical can greatly be reduced with double studs. Blown in insulation is almost nothing in the larger picture when compared to Sip. You will still need exterior sheathing and some sort of bulk water shedding on the walls. Both sip and double stud will require some sort of cladding.
Remember both systems are there to decouple thermal .. eg they are thermal break . If you really want to get a better feel for what I am describing .. go look at you tube videos dealing with building science. look up perfect wall.
Looks like this is the best of both worlds for a bit more cost. https://www.youtube.com/watch?v=mxDSulcLpAE
https://www.greenbuildingadvisor.com/article/another-take-on-tstuds