XPS v EPS in vegetated roof assembly

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Author Message Robin E. Snyder Senior Member Username: robin Post Number: 718 Registered: 08-2004 Posted on Tuesday, February 27, 2018 - 10:49 pm:    Any reason(s) why one is better than the other in a vegetated roof assembly (thermal performance is not an issue). There are areas requiring build up, so contractor is proposing 2 inches 40 psi XPS over approximately 2 feet of 25 psi EPS over 60 psi XPS. Structural is ok with this. Just not sure if the inter layer of EPS is acceptable versus XPS. Dave Metzger Senior Member Username: davemetzger Post Number: 722 Registered: 07-2001 Posted on Tuesday, February 27, 2018 - 11:12 pm:    Far be it for me to gainsay a structural engineer, but I've always thought that the substrate should be stronger than the topping. So a 40 psi layer over a 25 psi layer raises a red flag for me. In addition, my understanding is that due to its open cell makeup, EPS absorbs water more than does closed cell XPS. I've also read conflicting statements about this but I'm not convinced. William C. Pegues Senior Member Username: wpegues Post Number: 963 Registered: 10-2002 Posted on Tuesday, February 27, 2018 - 11:55 pm:    All, When it comes down to it, it is what is acceptable to the roofing membrane manufacturer and whoever is doing the wind uplift calculations. So, talk to your roofing membrane manufacturer and ask them for wind uplift requirements and also if eps is acceptable (regardless of what the insulation manufactuer’s state, most membrane manufacturers won’t include in a single source warranty for eps). And, typically its someone on the insulation side of things that is doing wind uplift and they won’t go with a mixed system. So, is a single source warranty a concern? If the owner does not care if he has to pay to move the insulation around to find a leak in the membrane, its not. If he wants a no cost for any overburden than a single source is necessary. In 42 years, I never specified anything other than a single source warranty for the entire roof assembly, and after vegetated came online, it was single source from the top of the plants to the membrane and everything else in between. And wind uplift is something you must consider, or you will be in violation of the IBC. Why, and few know this, and even MasterSpec is incorrect (it says to comply with the local requirements for winduplift). However, the code refers directly to SPRI and SPRI specifically states that the Architect is responsible and must provide a design by engineered and certified design for wind uplift. You can’t rely on your landscaper, or the contractor, or the roofing installer, but the membrane manufacturer will work with their insulation manufacturer and provide a wind uplift analysis. If its under 120 feet from the slab to the ground, you can use charts and tables if you have a high enough parapet. But if not, then its required to be designed by an engineer certified in wind uplift. And that includes the type of erosion control, all perimeter conditions, pavers, etc. Ifyou are over 125 feet, I will tell you that what you are asking can be done. However, I will also say that you will likely need a solid ‘parapet’ 36 inches high, or a solid panel or glass handrail with gaps less than 1-1/2 inch. Open railing wil fail. Also, another party needs to be brought into this, and that is the vegetation supplier. They may be ok with the insulation buildups, but they may have qualifications as well. It does not have to be the final insulation, membrane and vegetation supplier, it just needs to be a group that will do single source, and you work with them. The landscaper needs to be brought into the group as well. And also the project architect due to the parapet/handrail requirements. Otherwise, you will be in violation of the code if they don’t follow the recommendations. Or if you get too far down the road. It’s never nice to have to write a letter to the owner stating that you can’t comply with the design requirements unless the Owner is willing to give you a letter stating that he is instructing you to ignore the building code. William William C. Pegues, FCSI, CCS, SCIP Ronald J. Ray, RA, CCS, CCCA, CSI, AIA Senior Member Username: rjray Post Number: 168 Registered: 04-2004 Posted on Wednesday, February 28, 2018 - 08:17 am:    While not your question, I can’t help but wonder what design parameters would result In 2 feet of roof insulation. Perhaps whatever is driving that aspect should be reconsidered. Also, I can’t imagine why a structural engineer would be involved with the selection of roof insulation. To my knowledge, protected roof system manufacturers only accept extruded polystyrene insulation. Getting at least one manufacture of these types of systems involved in the design process was an excellent suggestion here by others. I would think that all layers of insulation should have the same compressive strength. But if a protected roof membrane manufacture states otherwise, wouldn’t they want the upper layer to have the higher compressive strength? As far as wind loads, I believe MasterSpec does have it correct in their recently updated modified bituminous roof sections. If you are using MasterSpec, but not a modified bituminous roofing membrane, you can always utilize the information from the modified bituminous sections into whatever roof membrane you are using. However, all of MasterSpec’s low-sloped roofing Sections appear to be current on wind uplift design. Guest (Unregistered Guest) Unregistered guest Posted on Wednesday, February 28, 2018 - 11:11 am:    Don't call the EPS 'insulation' ... call it 'geofoam' and no one will bat an eye. The only thing I wonder about in your buildup is why you are sandwiching geofoam between layers of XPS. Robin E. Snyder Senior Member Username: robin Post Number: 719 Registered: 08-2004 Posted on Wednesday, February 28, 2018 - 11:31 am:    William - good points, but i am not clear how (or if) the use of XPS or EPS affects wind uplift issues? Does it? This is an assembly the contractor is submitting, based on Architect's design, in conjunction with Structural, and is a single source warranty, and all other issues have been addressed. The only question that anyone has at this point is if EPS has any significant disadvantages over XPS. Ronald J. Ray, RA, CCS, CCCA, CSI, AIA Senior Member Username: rjray Post Number: 169 Registered: 04-2004 Posted on Wednesday, February 28, 2018 - 11:51 am:    "Geofoam" is a brand name. Kind of like "Styrofoam" is a brand name for Dow's (or whatever Dow is called now) extruded polystyrene insulation. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 843 Registered: 10-2007 Posted on Wednesday, February 28, 2018 - 12:03 pm:    With respect to the structural engineers typical scope of work the insulation is a non issue and hence the decision not to object is reasonable. 20 psi corresponds to 288 psf which is in excess of the roof design loads so do not see a code issue. By placing the 40 psi material over the 20 psi material the system will be less sensitive to damage from high concentrated loads. The structural engineer is concerned about the weight of the insulation and the roofing loads. Denser insulation can weight slightly more than less dense insulation. Ronald J. Ray, RA, CCS, CCCA, CSI, AIA Senior Member Username: rjray Post Number: 170 Registered: 04-2004 Posted on Wednesday, February 28, 2018 - 12:40 pm:    The reason the roofing contractor wants to use EPS as the inner layer is because XPS is only available in thicknesses up to 3 inches. EPS can be manufactured in much thicker boards, up to 24 inches, eliminating several layers of insulation in your particular system. The roofing contractor will save a lot of money in both labor and material by using an EPS inner layer. In considering the roofing contractor’s proposal, I would question the need for 3 layers of insulation if EPS is acceptable. 2 layers is adequate to avoid thermal bridging through the gaps between insulation boards. 3 layers will cost approximately one third more in labor to install than 2 layers. Why not use only EPS, provided it is acceptable to the roof membrane manufacturer, in two layers, and if required, use higher strength EPS. The proposed 25 psi EPS is ASTM C578 Type IX. Expanded polystyrene (EPS) insulation is also available in both 40 and 60 psi strength (ASTM C578 Type XIV and XV) ASTM C578 allows for a 10 percent deformation. If this amount of deformation deformation is unacceptable, consider using ASTM D6817/D6817M insulation which limits deformation to 1 percent. The main difference between EPS and EPS in this application is probably water absorption. Type VI XPS has an water absorption of 0.3 percent. Type IX, XIV, and XV have water absorptions of 2.0 percent. ASTM D6817/D6817M does not test for water absorption. Use the values in ASTM C578. Wayne Yancey Senior Member Username: wayne_yancey Post Number: 856 Registered: 01-2008 Posted on Wednesday, February 28, 2018 - 12:58 pm:    DOW DOW Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 844 Registered: 10-2007 Posted on Wednesday, February 28, 2018 - 01:43 pm:    Regarding the references above to what SPRI says. I have referred to SPRI RP-4 – 13 and I do not find where it states that the architect is responsible although I would expect that the Architect would be responsible for the design and specification of the roofing system. Please give section number. I also do not see any reference to an “engineer certified in wind uplift” which means that we can avoid the question of certified by who. I am not aware of any certification of engineers for wind uplift. I believe it is highly inappropriate for the building code or reference standards to talk about the responsibilities of design professionals. Technical provisions yes, saying what needs to be designed by design professionals no since this is an attempt to regulate the practice of engineering or architecture. The entity adopting the building code does not have the authority to regulate engineers and architects and any provisions in the building code to the contrary have been illegally adopted. Any Civil or Structural engineer designing a building should be capable of determining the wind uplift forces. At least in California both Architects and Civil engineers are legally capable of designing for wind uplift forces. I will suggest that in many instances the question of uplift forces has not been adequately addressed since the structural engineer is seldom asked to provide information on wind uplift forces. Guest (Unregistered Guest) Unregistered guest Posted on Wednesday, February 28, 2018 - 01:52 pm:    "'Geofoam' is a brand name" ... I didn't realize that ASTM started using brand names in naming their standards ... ASTM D 6817 "Standard Specification for Rigid Cellular Polystyrene Geofoam." Your situation would be looking to use Type EPS15 (25 psi compressive strength). My point in wondering about the sandwich of EPS between XPS is what does the bottom layer of XPS get you? Why not only use a top layer of XPS over the EPS if your simply looking for higher compressive strength for the top layer? Or better yet, why not remove all the XPS and simply use a cover board over the EPS? William C. Pegues Senior Member Username: wpegues Post Number: 964 Registered: 10-2002 Posted on Wednesday, February 28, 2018 - 10:55 pm:    For Mark Gillingan: The reference in the current version of SPRI RP-4, Sections 3.2 and 3.3, and indirectly elsewhere. References do speak to the local jurisdiction approving the design, but stating that it must ‘comply’ with the requirements of the local jurisdiction would still be incorrect. So it has to pass the approval of the local jurisdiction, but it can’t be passed off as someone else’s responsibility. Specifically this is for buildings over 150 feet to the roof. BUT...review the requirements for how to verify that a design passes (other than the charts) and you might not want to do math for a much shorter building if you are not literally on the charts. 3.3 indicates that the designer must be experienced in wind roof design. I believe that the certified I mentioned was me carrying over from the previous version. In short, unless your design is an exact fit for the charts and tables, most are not likely to be able to do this correctly...and the roof membrane manufacturer or insulation manufacturer will do it at no cost, and fairly quickly as well. If the area is being treated as an amenity space and a landscape architect is involved in selecting materials, or the contractor convinces the owner to go with an open handrail design on an 8 inch ‘parapet’ then the architect needs to know, or be able to refer someone capable of providing an engineered response that it will fail. Either that, or down the road suddenly the landscaper ends up with paver straps all over the place and the entire amenity area is ‘lost’. You can’t move this into realm of deferred design unless, the code won’t let you. It must be done up front as part of the design process, though you can get assistance from membrane manufacturers. ******copy/paste begins (note that ‘registered design professional’ is in bold face in the standard itself) 3.2 Building Height The building height shall be measured from ground level to the roof system surface at the roof edge. When more than one roof level is involved, each shall have its own design per Sections 4.0 and 5.0; or be designed to the criteria required for the most exposed or highest roof level. When building height exceeds 150 ft. (46 m), the roof design shall be designed by a registered design professional using current wind engineering practices consistent with ASCE 7 and the design shall be approved by the authority having jurisdiction. See Commentary. 3.3 Slope This Standard is limited to roof slope designs up to 2 in 12 (10 degrees) as measured at the top side of the roof membrane. For slopes greater than 2 in 12, a registered design professional experienced in roof wind design shall provide design requirements and the design shall be approved by the authority having jurisdiction. ***quote ends. William C. Pegues, FCSI, CCS, SCIP William C. Pegues Senior Member Username: wpegues Post Number: 965 Registered: 10-2002 Posted on Wednesday, February 28, 2018 - 11:26 pm:    For Robin, The entire sandwich from the top of the slab to the top of the vegetation (all in between) affects the wind uplift calculations. SPRI has charts/tables for some fairly typical conditions. But when it comes to amenity areas, these must be considered carefully. As to the insulation being used to build up areas, these areas, the tops of them, count as the ‘height above ground’ for the height of that roof area. And if they are exposed unprotected by a parapet in the analysis of the wind uplift designer (architect or whomever) then they will be calculated separately. And I can tell you time after time where this has been a real issue. I have had situations where the landscape architect and our designers were providing beautiful roof designs, very low parapets, nice amenity areas and vegetated areas, and it all worked just fine because there was a glass handrail. Then the GC proposes a cost reduction using an open handrail. And we had to balk and say you can’t do that. And major developers, as well as the GC, come back and relate how they do this all the time and can show us several local projects where they did it. We show them the results of the wind uplift calculations where it fails, and the requirement in the code where it is not permitted to fail. And as to the local jurisdiction approving it - well what about those other completed projects that were being cited, they would not pass either. It is rarely caught in many jurisdictions except for those under high wind conditions like coastal hurricane and major storm zones. As to insulation that is even under pavers, over 120 feet, and even with a decent parapet, you may have to have multiple rows of pavers, all strapped together, to keep the wind from picking them up. And if its an amenity space, no one wants to see straps and concealed straps are much more expensive. To say nothing of what if the design is carrying vegetation out to just 2 feet from the back of the parapet. Then their whole design is a failure. After sometime in 2012 I developed a process that as soon as a project came into the office and design began I had the project architects provide me with the civil, landscape architect, our designer’s names and contact. I then sent out a coordination email to all of them, and included the contact for the group we used for winduplift analysis and a vegetation supplier. And included that they make contact with each other for whatever the wind uplift analysis would require or alternatives to be considered. Even then, we would get back curious items from landscape architects and developers such as a great design that complied perfectly, but that behind the parapet for the 2 foot perimeter at the vegetation, it was not pavers or crushed stone, it was a specially selected smooth beach stone from Mexico. We asked for samples and found various technical information and rushed that off to the wind uplift analysis group who came back and said, it passed but they needed to increase the size of the stones to the next size larger rather than what the landscape architect wanted to use. Fortunately, they did not have a problem with that. That kind of thing was also a reason that we said that the only thing the landscape architect was permitted to specify or included in their details was the soil and the plants. All ballast, including ornamental ballast, pavers, and erosion control mats that might be integral to the soil and everything part of the vegetated roofing system below the soli was going to be in our sections. They could tell us what they wanted to use, but we would control the actual specification. I do recognize that most independent specifiers don’t have this opportunity. William C. Pegues, FCSI, CCS, SCIP Robin E. Snyder Senior Member Username: robin Post Number: 720 Registered: 08-2004 Posted on Wednesday, February 28, 2018 - 11:31 pm:    ok, but all other things being equal, and assuming the entire assembly is designed appropriately, and either EPS or XPS would work... What are the pros and cons of using XPS versus EPS? Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 845 Registered: 10-2007 Posted on Thursday, March 01, 2018 - 01:59 am:    William You read a lot into the words. All it says is that this shall be done by a registered design professional which already is required by the State statutes that regulate the practice of engineering or architecture. Any references in the standard to registered design professionals are not relevant because the state licensing laws take precedence. If state licensing laws require work be done by an engineer or architect then the membrane or insulation manufacture cannot satisfy the requirement unless the manufacture has a properly licensed person do the work. What the manufacture can do is provide information for the Architect or Engineer of record who can then review the recommendation and proceed as appropriate. When performing these evaluations it is suggested that you touch base with the structural engineer to determine the wind speeds calculated by the engineer since you would not want your assumptions to be inconsistent with those of the engineer. This may be particularly important when considering surface roughness and topographic effects. William C. Pegues Senior Member Username: wpegues Post Number: 966 Registered: 10-2002 Posted on Thursday, March 01, 2018 - 02:17 am:    Robin, The difference is that some membrane manufacturers will not permit the use of EPS in their assemblies...or, they will not warrant it. And some membrane manufacturers will only permit the use of EPS board. Depends on the membrane, and the manufacturer. Also, some roofing membrane manufacturers will only permit the use of their own specific roofing insulation, typically they produce (or have branded for them) EPS board. There is also different opinions, both sides of the argument with supporting white papers, research, etc. that say that EPS board, even special types of it, don’t retain their insulation value as well over time, or don’t resist water absorption over time - and some that say that it does just fine. So, a lot of it boils down to your own personal experience, or the experience of those that you can trust. I started out with my personal experience in IRMA type roofs using XPS board back in the 1970s. I use EPS board under single ply membranes, but really preferred the use of XPS since the EPS back then could stretch the membranes if walked on - and no one stayed on the walking pad surfaces over time. I never in all my 40+ years of projects ranging from Michigan to Florida and from Texas to up and down the east coast, had a problem with either one of those in those particular applications. So, my personal preference is XPS when it is exposed, as well as when it is under the membrane. But, when it is under the membrane I will consider the use of EPS board after cautioning the owner about compression under foot traffic stressing seams and the membrane itself. But then again, we never did very much single ply with insulation under the membrane type projects anyway. Here in your situation you have a mix of the 2 types. Structurally and for compression, it all works just fine. The caution is to consult the membrane manufacturer for compatibility and single source warranty of everything in the ‘sandwich’. You can’t rely on the installer, you have to be talking to the manufacturer or his technical rep directly and have them review the system and specifically ask about a single source warranty for the complete assembly. I know some that would not do this...but I know installers that would say that they would (and have said such) and they turned out to be wrong. I would just be very cautious. Design is one thing, but, did someone actually verify with all the parties. In addition, some membrane manufacturers will not warrant all XPS manufacturers, only a few. The same with EPS, so if you get to the point of listing membrane manufacturers, you may need to tie the insulation manufacturers in the roof insulation spec to being acceptable to the membrane manufacturer. If allowing free choice for membranes as well as insulation, you may void the single source requirement. The same with the vegetation system. Some membrane manufacturers will not single source all vegetation suppliers. Some vegetation suppliers also want to provide all components above the root barrier and including the root barrier, and some membrane manufactuer’s will not single source that. Sandwich is a good analogy, there is bread, lettuce, meat and cheese. Now who is the primary and what brands or types will each manufacturer work with, or do some meat manufacturers also make cheese and only warrant their own cheese, or do they have partnering agreements with only specific cheese manufacturers. But then, edibles are much more fun than roofing membranes -grin! William C. Pegues, FCSI, CCS, SCIP William C. Pegues Senior Member Username: wpegues Post Number: 967 Registered: 10-2002 Posted on Thursday, March 01, 2018 - 02:56 am:    Mark Gillian, I just read the words. Good luck with that idea of ignoring the requirement of SPRI and the ‘registered design professional’. I choose not to make an issue of what a standard specifically referenced by the code wants to say. Following their requirements is not a hardship, so its a fight to take up inside the various standards and codes committees. Not one that needs to be faught in construction documentation for specific projects. I agree with your concept, just not your protest or method of execution. And though it does say a ‘registered design professional’ and in high slope examples requires them to be experienced in wind uplift design, in my experience attempting to get the strucutral engineer to do this results in a higher consulting fee from the engineer, and one that in their contracts is an additional service. And, the membrane manufacturers that I have worked with in my past 40+ years of experience provide the wind uplift study/analysis with a registered engineer specifically experienced in wind uplift - typically in partnership with the insulation manufacturer. Or they use a consultant that is partnered with both of them. And that study/analysis has always been at no cost to the architect even when it has to be repeated to pick up required changes or in the case of design changes...or even in the case of reviewing novel materials (example of Mexican beach stones as perimeter ballast referenced above in the thread). In short, the services of a registered design professional experienced in wind uplift are at no expense, fast, detailed including specifics, signed, and thus not an issue. In Robin’s example, were I to go through my typical source, I would have a general response to any major issue of components in the sandwich in a couple hours, and a completed written report indicating what and how anything needed to be changed in order to comply in 2 to 3 days if it was a rush but typically not more than 5 days. This would be for a building over 150 feet in height, with numerous roof levels, all with different parapets and surface conditions and uses including vegetated systems and amenity spaces (which is the typical scope of spaces in high rise residential over the past several years). It’s just not an issue, though your mileage may vary. William C. Pegues, FCSI, CCS, SCIP Robin E. Snyder Senior Member Username: robin Post Number: 721 Registered: 08-2004 Posted on Thursday, March 01, 2018 - 12:44 pm:    William, all good info. However, the original question, if anyone can assist, is whether there is a performance issue with EPS v XPS in this application. Warranty, wind uplift etc is not part of the question for this specific application. Any issues with moisture retention, or degradation etc of the EPS v XPS. Dewayne Dean Senior Member Username: ddean Post Number: 154 Registered: 02-2016 Posted on Thursday, March 01, 2018 - 12:59 pm:    Robin, My feeble attempt to help From Google: XPS has a lower moisture absorption rating than EPS . https://www.achfoam.com/ACH/media/ACH/docs/Foam-Control/EPS-vs-XPS.pdf Ronald J. Ray, RA, CCS, CCCA, CSI, AIA Senior Member Username: rjray Post Number: 171 Registered: 04-2004 Posted on Thursday, March 01, 2018 - 01:44 pm:    Robin, From my earlier post, a corrected version: The main difference between EPS and EPS in this application is probably water absorption. Type VI XPS has an water absorption of 0.3 percent. Type IX, XIV, and XV EPS have water absorptions of 2.0 percent. ASTM D6817/D6817M does not test for water absorption. Use the values in ASTM C578. So, the question is, are comfortable with the water absoprption difference. Steven Bruneel, AIA, CSI-CDT, LEED-AP, EDAC Senior Member Username: redseca2 Post Number: 636 Registered: 12-2006 Posted on Thursday, March 01, 2018 - 02:51 pm:    Everything in William's excellent posts is true. That said, if you also need a fire rated assembly and the decking is not concrete filled to a sufficient thickness to provide the fire rating, you will need to also throw into the metaphorical sandwich which combinations of products are included in a myriad of UL assemblies. Michael Chusid, RA FCSI CCS Senior Member Username: michael_chusid Post Number: 372 Registered: 10-2003 Posted on Thursday, March 01, 2018 - 03:28 pm:    Perhaps it is easier to raise the roof deck so you don't have to add so much insulation. If it is an existing roof, look at the CURA Adjustable Reroof Framing Systems or other light gage metal framing to build raised, new roof deck. See http://www.reroof-america.com/index.php Full disclosure: Reroof America is a former client of mine. Michael Chusid, RA FCSI CCS 1-818-219-4937 www.chusid.com www.buildingproduct.guru Steven Bruneel, AIA, CSI-CDT, LEED-AP, EDAC Senior Member Username: redseca2 Post Number: 637 Registered: 12-2006 Posted on Thursday, March 01, 2018 - 04:01 pm:    Michael, You do not need very much insulation thickness to satisfy thermal insulation needs or to provide protection for waterproofing. But there are other logical reason reasons to do so. Sometimes a complex vegetated roof design will need extremely thick insulation over parts so that when you omit the thick insulation you are creating deep soil pockets for large trees. This is easier and less costly than trying to build these level differences into the structure. We recently completed a large hospital campus that featured an acre+ landscaped quadrangle on several levels between the buildings. Below the quadrangle is actually a multi-level underground parking garage that connects the buildings. The below grade concrete roof of the garage is sloped, and the quadrangle design includes deep and shallow planting areas over the garage, planting areas over the natural grade, and vehicular driveways and hardscape areas on several levels. The overall site would be considered "hilly". The "Insulation Plan" for the quadrangle was very complex and varied from 2-inches to 8-feet. Brett Scarfino (Unregistered Guest) Unregistered guest Posted on Thursday, March 01, 2018 - 04:25 pm:    For what its worth, EPS is essentially beads fused together, some small (or microscopic) voids at the contact area between the beads. As a result, generally easier for water, liquid or vapor, to jump through the microscopic "barriers"; higher water absorption as well as water vapor permanence relative to cousin XPS (and usually lower strength as well). "bead fusion" is a critical parameter with EPS.