Issues with Structural Insulated Panels (SIP)

Dr. Nabil Taha has over 27 years of structural engineering experience. Prior to opening his own engineering firm in Oregon in 1997, he was a Professor of Engineering at Northern Montana State University and at Oregon Institute of Technology. He has structural expertise in a wide range of building systems and can answer questions related to virtually any common building method. His focus is on green design and he is always willing to trying something new. Dr. Taha is dedicated to future sustainability through innovation; he creates solutions for beautiful sustainable and safe structures by melding old and new technologies. He loves a good challenge. He is Licensed in twenty three states and can design buildings and/or consult to assist with structural permitting in these states as well as internationally. As a prior College Professor, Dr. Taha is a teacher at heart. He loves to share his knowledge and offers educational seminars and trainings for the do-it-yourselfers and professionals alike. Dr. Taha's goal is to continue to grow and provide knowledge and services for those trying to make their dream project a reality. No project is too big or too small. For information about Dr. Nabil Taha and his engineering firm see www.structure1.com

Questions and Answers

Q: How is the wood used in a SIP attached to the insulation? Is it nailed, glued?

A: The sheathing of the SIPs itself is glued, Epoxy, to the insulation. On top of that, when the SIPs are installed, the sheathing of the SIPs is screwed to the:
a-Wall top plate.
b-Wall bottom plate.
c-Studs or column at the joints/spline.
Any other lumber/wood is screwed or stapled to the sheathing of the SIPs. So, SIPs are very strong and excellent green/sustainable system.

Q: I came across your site while looking for alternatives for exterior wall finishes on a SIP's house in Troy, NY -- before you go, "Ugh, SIPS!" the project is for low-income housing intended to be built with no skill or low skill labor. We are still experimenting. Originally, the designer called for T-1-11 to go on the outside. There have got to be better solutions that we can afford! Any ideas? Do you supply products yourselves?

A: (Kelly) We don't supply any products ourselves, only information. Depending on available resources, you might consider siding the building with the rounded slabs that are often left over from milling trees; this would produce a log cabin appearance. Another possibility might be to plaster the outside with stabilized adobe, to give it a very natural appearance.

Q: I acquired a series of foam panels I would like to construct a structure with. They are similar to SIP panels, but the outside is thin sheet aluminum. The panels interlock and are held together with sheet metal screws. I believe they are actually freezer/refer panels manufactured by Hussman. My question is who can I work with to design the structure and create some plans I can submit for a building permit?

A: (Kelly) I have just been assembling a new website www.dreamgreenhomes.com, which is devoted to providing sustainable house plans designed by a variety of architects and designers. There are few house plans presented there that are adaptable to SIP construction. If you want to work on a new custom design, let me know, and I can refer you directly to one of the architects.

Q: We live in an 100 yr old cottage that is about 900sf.  Our addition will be approximately 25x20, with a basement below, and connected to the existing cottage by a 8x16 foot transitional area with crawlspace below.  I HATE being cold, and SIPs seem to offer excellent insulation, plus other efficiencies: less timber used, easier/faster construction. We are interested in Hardiboard siding, but I infer from Mr. Frechette's  Q&A's that attaching siding to SIPs could be a problem.  My worry is that, since we are connecting the addition to an older house, that if the plans aren't perfect, there is no wiggle room with an SIP-constructed addition, to make those inevitably necessary tweaks to the project. Help!!!

A: (Kelly) As for attaching hardiboard siding to SIPs, I wouldn't expect this to be a problem. It might be that using deck screws instead of nails to attach the siding would be advised to assure a strong bond with the SIP's OSB skin. Your other concern about there being no wiggle room I suspect would not really end up being a problem. With modern foaming insulation products to fill in any gaps, and the fact that SIPs are inherently dimensionally very stable, your construction should end up seamless in the end.

Q and A (Kelly): I want to build a house that utilizes the simplest approach, as with SIPs. Although I read the technical data I do not totally accept the data on the SIPs. As well, I have issues about OSB and how does the panel breath.

As far as I know these panels do not breath.

Where does the moisture dissipate to with a closed cell panel? What keeps the OSB from deteriorating?

Interior moisture will need to be vented somehow, so this would need to be incorporated into the building design.

Is the panel really a strong I-beam capable of withstanding Texas high wind thunder storms?

My understanding is that these panels are amazingly strong. Obviously a significant system for tying the roof and wall panels together and to the foundation would be essential.

Q: I am very interested in SIPS construction methods. A gentleman, "Hoot" Haddock, builds SIPS at his plant in Florence, about 8 miles from our lot. Everything I read about SIPS discusses OSB on each side. Mr. Haddock's use a cement board cover on each side. Is he just going in a different direction or do you see problems with the cement board.

A: (Kelly) OSB is common with SIPs because it is cheap and it does the job of providing sufficient shear strength to withstand all stresses that the SIP might encounter in its structural life. The question is: does the cement board  you describe also serve this function adequately? I don't know the answer to this, but I would hope that the manufacturer does. I presume he has performed tests that would prove this. Off the top of my head, I would say that the cement board that I am familiar with would not have the shear strength/weight ratio that OSB would have...so weight might also become a factor with this product. I would be cautious, and make sure that adequate testing has been done to your satisfaction. The cement board would have the advantage that it is not vulnerable rot, which cannot be said of OSB.

C: I would also like your comments on the other plans we have for the house. It will have a walkout basement, will use steel framing for the interior walls, and 90+ gas furnace & high efficiency conventional a/c. We will use high efficiency windows and doors. Should we use SIPS panels for the basement walls or use formed/poured concrete (as we use here in Ohio). The house is 2200 to 2400 sf (covenant requirements) and we know it is too big for two people but we expect high visitation rates from friends and family.

A: (Kelly) This is where engineering data related to the SIPs in question would be essential. Can they withstand the vertical load in question? If they can, the added insulation of the SIPs over a conventional poured concrete foundation would be an advantage. Also, I wouldn't even consider SIPs with OSB in such an application, whereas the cement-board ones might work. I think you need some expert engineering advice.

Q & A (Kelly): What are the cons of sip panels?

From a sustainable point of view, the cons of SIPs are that they are rather industrial in nature, so there is a lot of embodied energy in their manufacture and and distribution.

We have a local builder who is gentle in intent toward the environment and can build with sips, offering aluminum sandwich foam. What are the off-gassing issues?

I am not familiar with the aluminum sandwich, but I wouldn't expect much offgassing from these, since the aluminum would seal the foam core; also, my understanding is that there is actually little offgassing from the EPS, if this is what the core is made from.

And then pests-rodents?

I do know that rodents and insects can happily burrow into soft foam products, so it would be important to make sure that they don't have access to do this. It is especially difficult to keep ants and other small insects from doing this, so it may be necessary to deter them with some sort of pesticide if they become prevalent.

How do sips compare with aac's?

AAC is basically a foamed masonry product, so it is inherently much more durable and impervious to the above hazards. It is also fireproof. AAC would not likely provide as much insulation per inch, but this can be overcome by using thicker walls. AAC panels can theoretically be used in similar ways to SIP's, although they would be much heavier and may need special equipment to assemble them. As far as I know there is no off-gassing issue with AAC.

Q: I would like to get your input on our company, it's Techbuilt systems inc. you can get the most information at Techbuilt.com. We have been doing this, with a lot of advancements, for over 20+ years. Having your experience I would love to get your honest thoughts.

A: (Kelly) A brief overview of your systems suggest that you manufacture fairly standard SIPs, using either sheet steel or OSB skins with EPS cores, and that you also use thin gauge steel structural components where necessary. My general comment about SIPs can be seen here. When analyzed from a standpoint of embodied energy, SIPs obviously do represent a fair amount energy both in their manufacture and and their transportation to the construction site. From this standpoint they could never be as "green" as other natural materials such as adobe or earthbags filled with soil. But because they do provide excellent insulation and do reduce the reliance on forest products to some extent, they could be considered moderately green.

C: Thank you for the input, actually we use 18 gauge steel tube (galvanized and welded) on the out side and the inside of our panel, creating a thermal break between the outside and the inside.  There is 1 1/2" 18 gauge angle that connects the  the top and bottom.  There is no plywood or harmful glues in our structures.   I believe we are the only system to use 18 gauge steel tube and that is standard.  We can modify our system to go above 3 story's (above grade) by increasing the gauge of steel.  The steel is 2 ft. on center along the wall (inside and out) and 4 tubes at each corner 8" and 12" from edge.  Our system is documented at 90+ efficient and we won the governor's award in ohio with the most efficient house ever measured in ohio (94 %).   Our roofs are utilizing the same galvanized steel but increasing size and gauge depending on spans.  We also offer on onguard material in our foam that will not allow rodents to burrow or nest in the foam. 

Like you said, the EPS manufacturing has to be addressed.  That is a great point that EPS is not as green as the adobe homes, but I don't know that people are open minded enough to see their benefits, especially, mortgage companies (they're not at all green).  Our mission is to quietly reduce fossil fuel consumption by 75% by building structures that just don't need all that fuel. I love solar and am working on two projects in  long island that will utilize it.  Trust me we struggle financially because we put all the steel in and buy foam that is the best quality we can get.  We have and are trying to go to a soy insulation or there is a gentleman in Idaho who is trying to recycle garbage into an EPS type material.  We told him, we'll be his guinea pig.  We're trying.  Go to a website called sunearth.net; it's for an architect firm called Watrous Associates, read their vision statement.  Sounded great to me.

R: I fully appreciate your interest in providing a reliable building system that appeals to the mainstream public and lending institutions, while creating building shells that are very energy efficient. Your system, when coupled with good passive solar heating and cooling design, could go a long way toward diminishing fuel consumption. I applaud any efforts you make toward utilizing renewable and non-polluting components.

C: Many of the LEED certified architects have not given  a whole lot of consideration to the overall envelope.  It almost seems, if you can't see it you don't need it.  Also I just read an article that will give more points than before to wood.  I hope they are not considering some of the slow growth/rare woods.  I hope the powers that be, will keep raising the bar and not lowering expectations to satisfy the wood councils.  Realistically they have a lot of money on the line, and often times policy is not written by those that are right, but who "invests" the most money.   I don't want to make this political, I just want more people like you.

Q: I am building a home in Kalamazoo, MI very shortly and am building as green as my budget will allow. The house will be a 2 story walkout with the MBR above the 3-car garage. So the house is pretty compact on other words. Do you recommend I use SIP's or ICF's? I'm pretty confused as to the very best for my area. I got to see a Polysteel addition going up this morning and it seemed very cool - but will it be too cold in the middle of January???

A: (Kelly) I think that you would ultimately have better control over the thermal characteristics of your home if you choose a highly insulating SIP system for the shell, use a good passive solar design for the home (see www.dreamgreenhomes.com for some ideas about this), and incorporate sufficient thermal  mass material within the shell. The ICF manufacturers try to convince you that their system will incorporate both insulation and thermal mass, and to some extent this is true, but the technology compromises both aspects in the end.

Q: For a passive solar house in a cold climate, which is better, SIP's or ICF's? If ICF's aren't good because they don't have thermal mass on the inside, are SIP's better, because they don't have thermal mass on the inside either, do they?

A: (Kelly) You are right that SIPs don't have any thermal mass associated with them, but they generally provide better insulation (depending on their thickness) and don't require all of that cement and steel...and thermal mass can always be added to the interior of any house in various ways.

Q: I am interested in building housing for the poor. HUD Say's affordable housing can be no more than 30 percent of income. For a family with only one adult working at minimum wage this equates to a payment under $350.00 a month. After allowing for property tax and maintenance this needs to come in at $30,000.00 a unit. To be sustainable this needs to be done at a modest profit. My Idea is to build shell condominiums with tilt up concrete panels at a rate of about 18 per acre including roads. I need a building system that is durable, inexpensive, fire, flood, and disaster resistant, energy efficient, and environmentally responsible.

What I would like to do is build a wall panel with layers of lightweight concrete similar to a sandwich panel except without using foam sheeting. I'd like to start with a slurry of colored cement at the exterior followed by a few inches of vermiculite concrete for insulation and then 3 to 4 inches of heavier concrete for structure and thermal mass. Do you think that this is a viable idea. Will the panel stay together during drying and lifting. Is 2 inches of vermiculite sufficient for insulation.

A: (Kelly) This is an interesting idea you have for SIPs, and one that I have not heard of before. A key to the value of this would be how well it would insulate the walls (or roof if used this way). According to the Schundler website, 3" of vermiculite concrete is equivalent to 1 1/2" of rigid board insulation, which is to say about R-7 1/2. If you increased this to 4", you would end up with about R-10, which is not really very great for walls or roofs; you need to just about double this to be very effective.

As for whether these panels would hold up to installation and time, I doubt that they would fare well without some form of steel reinforcement embedded in them (probably on both sides) This could be a fairly lightweight mesh. Otherwise I would expect fractures and failures. This whole concept would need some heavy-duty testing to know how well it would work.

Of course, what you are suggesting with these SIPs would only provide the shell for the structure, and the bulk of the cost of building a house is all the stuff that is required besides this. To build such units for less than $30,000 in the US would be a great challenge these days. In Mexico it could be done, because labor costs are so much less. How green this would be is another question. Cement is a major CO2 polluter, and if these panels were not manufactured on-site, there would be transportation costs as well. Obviously, the manufacture and assembly of cheap houses needs to be streamlined as much as possible.

I applaud your interest in providing affordable housing; the need is huge.

Q: What are your thoughts on the product that GreenSteel Technology produces? site is www.greensteelinc.com

A: (Kelly) SIP's, like what they manufacture, do have some advantages in terms of providing good insulation, and the structure can be erected quickly. Also the use of steel for structural support can be considered somewhat green, since much of the steel used these days has been recycled. There is still a lot of embodied energy in these products, both in manufacture and in transportation, so they are far from being the greenest way of building.

Q: My husband and I are trying to plan our retirement home. This has been a 10 year research process and I'm getting really bogged down. We want something that is as energy efficient as possible, but we'd like to get the most "bang for our buck" in construction costs. Here are what we're looking for in our home: as energy efficient as possible - tornado/fire resistant - cost effective to build. I've narrowed it down to either underground, Rastra or SIPS construction. I think that the underground house would be the best bet, but I don't know if it's affordable. Which one would you guys do?

A: (Kelly) It sounds like you have done some good research in realizing that going underground will yield both energy efficiency and safety in a wide range of possible elemental dangers. Most insulated concrete forms such as Rastra can be used underground, at least for the walls. SIPs, on the other hand, are not generally used below grade. Completely earth-sheltered, underground homes often cost a bit more than their above ground counterparts, mainly because of the structural need to support all that weight on the roof. This additional cost can easily be justified over time in greater energy efficiency and in peace of mind. Go for it!

Q: I have found a folding portable house! Called Habitaflex out of Canada. I think its about the size of a shipping container when folded. I like it; what do you think?

A: (Kelly) These fold-up designs are quite cleverly configured and do seem to be almost entirely finished homes, using SIPs. I didn't see the specs for insulation values, so that would be something to check. I've never seen OSB used as flooring; you may or may not like this. They rely on electric heat, which is usually not a very ecological or economical option. The main advantage of these homes that I can see is that they are easily transportable, anywhere around the world. My suggestion is that you compare this package with what your "ideal" green home might cost to have built.

Q: I'm looking into constructing a SIP wall/roof home, on a concrete slab. How can the concrete slab floor be isolated/insulated to eliminate the thermal bridging of the slab floor to the foundation wall (upon which the SIP wall panels will rest?

A: The way SIPs walls are attached to concrete foundations is by installing a pressure treated sill first. Then attach the SIP wall to it. Use sealant under the sill.

Q: In India SIP are made using steel plates since OSB is not readily available. Which skin material is preferred & why? PU is rapidly replacing EPS as core material. What can be done to restore EPS SIP market. Are RCC structural columns a must for SIP structures, especially at corners and load bearing structures.

A: Any skin is fine. The insulation benefits comes from the core, not the skin. You should go with the most economical in your market. That is how the market work. Columns could be of any materials such as wood, steel. Reinforced concrete columns are not a good idea with SIPs.

Q: I was going to make my own "SIP panels" out of a sandwich of cdx plywood/ insulation /bead-board panels. There's so much cdx plywood out there for sheds-- what's the best to used that is on the cheaper side?

A: (Kelly) There are a variety of paneling materials that are sold to be exterior sheathing, and some of these already have a paint or weatherproof coating on them. I suppose most any of these could be used to make your own SIP's. On the inside you could probably use the cheapest plywood you can find, although in a greenhouse I would advise cdx, as it will stand up the the humidity better.

Q: As an owner/builder, I built a home in the Tampa, FL area with galvanized SIP panels. The SIP work was subcontracted out to a company familiar with them. For the exterior I had a painter simply apply an Elastomeric coating. Over the years I have sustained significant rain leaking issues at the joints and must do something to properly seal and cover these panels. I want to consider siding as I have been told that with the expansion and contraction, stucco would crack over time. My concern is that vinyl siding being attached to the panels would need to be attached with screws and not screwed down tightly to allow for expansion and contraction of the siding. Accordingly I have concern that over time the loose screws could back out, particularly with respect to the wind we get on the bay. If I were to go with Hardi or similar siding, my concern is the weight of the panels could, over time, cause the galvanized sheeting of the SIP to delaminate from the foam from the weight of the panels. Do you have any suggestions relative to the best option to resolve my dilemma?

A: I agree with you, Stucco will not be the solution. For the siding, you could have 2x4 applied flat on the outside surface of the SIPs as furring. Then use long screws that go through the SIPs and grip on the structural members in your wall to attach the furring. then attach the felt building paper and the siding to the 2x4 furring. Before you use any product, I would require the manufacturer to guarantee there product for your case.

Q: We just had a SIPs home built. Our contractor attached cypress log siding to the sips with screws. The panels were wrapped first. We are wondering if this was done correctly or if some sort of fastener should have been used.

A: If your contractor used the right screws that are long enough to penetrate the logs and the Sheathing of the SIPs, and the log siding is bearing on the foundation, then most likely you are fine.

Q: I am concerned abut the potential delamination of the SIP panels. What tests have been done. What assurances that in 25 years when we go to sell our hose that the panels will not be delaminating? I have concerns about the glues in the OSB panels being compatible with the foam and the glue they use to glue the perimeter 2 x_s to the panels being compatible and not delaminating. I hope you can shed some reassurances our way.

A: We have never heard about delamination in SIP panels before. However, each manufacturer uses different types of epoxy to glue the foam and the OSB boards. I I would ask for tests that that particular manufacturer did on this SIPs.

It is the floor SIP’s that would concern me the most with the constant flexing as they are walked on or loaded with furniture and such. How long have SIP panels been used in residential construction? As an architect, I have not used SIP panels over the years.  I am seriously thinking of using them on a residential project and would like some outside way to verify the durability of the panels.  In doing research there seems to be a lot of fly by night manufacturers using all sorts of materials.  When I compare the websites they provide little in the way of testing data.  Unlike US Gypsum, Weyerhaeuser, or other manufacturers who have had their products tested to the requirements of the commercial construction.

The tests required for SIPs are included in the International Residential Code, IRC and most likely in your local building Code. Please contact your building plan examiner. If the SIPs panels are designed correctly and detailed correctly, you can walk on them with no problem.

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