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Placing a Concrete Foundation on Rigid Foam Insulation

Careful coordination makes site prep and foundation work proceed smoothly at the Orchards at Orenco project in Oregon

Building your house on foam can raise a few eyebrows. The footing forms for the multifamily Orchards at Orenco project in Oregon were placed over 4 inches of expanded polystyrene insulation.
Image Credit: All photos: Mike Steffen
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Building your house on foam can raise a few eyebrows. The footing forms for the multifamily Orchards at Orenco project in Oregon were placed over 4 inches of expanded polystyrene insulation.
Image Credit: All photos: Mike Steffen
This is the typical base of wall condition, with brick veneer used as a “wainscot” around the building base, running to a height of approximately six feet. Instead of using a “brick ledge” configuration on the footing as is typical to provide support for the brick, a steel ledger angle is used. The angle is thermally isolated from the foundation by steel brackets, spaced intermittently at 4’-0” o.c. We used off-the-shelf “FAST” brackets manufactured by Fero Corp.
Image Credit: Architectural drawings: Ankrom Moisan Architects
Base of wall detail at the few locations where full-height brick occurs. This is a structural brick veneer so a separate footing is required for support of the brick. Again, typically a brick ledge configuration would be used at the footing to provide support for the brick, but here a separate footing allows for thermal isolation of the perimeter footing. The site has been cleared and a rock pad has been placed to accommodate construction traffic. The building visible in the distance (to the west of our site) will be a mixed-use, market rate apartment building when completed in 2015. Initial excavations for footings at the building perimeter. Note the use of compacted gravel to establish a firm and level base for the foam insulation that is placed under the footings. The design documents called for controlled density fill (CDF) at locations where needed; however, the excavation contractor did an excellent job with the subgrade and gravel base preparation and after quality control inspections it was agreed that the CDF was not needed. A Building Envelope Coordination (BEC) meeting. The need for the meeting on the Orchards project was even more critical than it is typically, given the importance of envelope performance to achieving the Passivhaus standard. Here the team can be seen reviewing envelope details with the siding subcontractor. This dialogue helps everyone verify the design requirements and encourages questions about the design intent, conflicts in the design information, or possible omissions. Example of coordination drawings developed by Walsh after the BEC meeting, to clarify design intent and coordinate the work of multiple trades. These drawings were issued to the architect as a Request for Information to facilitate clarification of the requirements and document approval by the architect. The building edge is laid out on the compacted and level gravel base installed over the subgrade, foam is placed, and work begins on the formwork for the perimeter footings. The foam sits firmly on the gravel base. Due to the excellent work by the excavation subcontractor, controlled density fill was not required. Troy and team finish the mud as it is placed. Denise braces the forms. View of footing work in progress. Interior formwork has been stripped and foam is placed on the interior face of the footings. Nick applies adhesive to the foam which is then adhered to the footing. Excavation for “deadman” footings. A number of these footings provide restraint at points of high lateral load in the building. Compacting the gravel base for the foundation slab. On top of this base goes the radon mitigation system, the slab foam insulation layer, the vapor barrier, and then the concrete slab-on-grade. Regarding pipe penetrations of the slab assembly, it is important to space pipes to facilitate sealing of individual pipes to the vapor barrier. This was readily achievable at typical conditions around the building foundation; however, dimensional constraints prevented this at the electrical room. As shown in the photo, several conduit had to be ganged together to meet electrical requirements. View of the west wing showing the underground plumbing pipes (white) and the beginning of the radon mitigation system installation (black pipes). View of the crushed gravel base layer (containing radon piping), prior to installation of the slab insulation. The first layers of EPS foam slab insulation are placed over the gravel base and the deadman footings. Note the layout markings on the gravel, denoting the thickened slab / interior bearing wall footing locations; 1-inch-thick EPS will be placed in those locations. Foam insulation is cut and trimmed to fit around pipes and other slab penetrations. Spray foam is used at voids around penetrations of the slab insulation and at any locations where gaps occur in the insulation. Vapor barrier installation begins. The StegoWrap vapor barrier is installed on top of the foam insulation to avoid any potential of creating a “pond” of water should it rain prior to the concrete placement. The vapor barrier is taped with manufacturer’s sealing tape at all lap seams and penetrations. Where ganged conduits occur, the vapor barrier is sealed with a mastic accessory product. Concrete is placed over the vapor barrier, and finish work begins.

It should go without saying that any high-performance building should be built on a solid foundation. So why would we set our building on a layer of foam insulation?

The answer, of course, is to limit thermal bridging. Those bridging effects can cause a significant amount of heat loss through the mass structure at the base of the building. By thermally isolating the building foundation from the ground, building performance is improved, not only from an energy performance standpoint but also in terms of comfort and moisture management.

We put rigid foam under the footings

In some high-performance building circles, it has become common to place a layer of insulation under a slab on grade. This is especially the case in colder climates. What’s new with Passivhaus design is the idea of completely isolating the building foundation from the ground, not only under the slab, but also under the footings.

As designers and builders working on the Orchards at Orenco multifamily project in Hillsboro, Oregon, we’re getting our feet wet with Passivhaus design for the first time. Our collective common sense suggested that we should be suspicious of this idea. Nearly all of the building’s structural loads are placed on the footings and it strikes many people as possibly a fool’s errand to place building footings on foam.

However, after extensive research it became clear there is a long history of using certain types of very high density expanded polystyrene (EPS) foam insulation underneath major structural works of all kinds, including roadways, bridges, and runways. Our concerns receded based on the evidence and we were swayed — yet still reserved and cautious. The caution persists to this day and will follow us until this is a well-established building practice, without significant drawbacks.

Four inches of EPS is the right amount

Once the team actually started believing this could work, the next concern became how much insulation to use. Together, we landed on the idea of 4 inches of EPS foam, based on a sense this would provide a good balance of cost and constructability. In particular, we were trying to avoid the thicker levels of insulation that have been used on some Passivhaus buildings.

Throughout the design process, PHPP iterations were run that looked at using more or less foam foundation insulation; yet the team kept coming back to the 4 inch foam layer. We looked at the relationship of the foundation R-value to changes in other envelope parameters such as the wall R-value, the window U-factor, and the roof insulation. After numerous iterations, the 4 inch foam thickness was agreed to by the team.

So, how does this work? The foam is placed under the entire slab on grade and wraps around and underneath the footings at the building perimeter. The 4 inch foam thickness is reduced to 1 inch at bearing wall locations, resulting in a thickened slab with reinforcement, to serve as footings for those walls at the building interior.

Due to the seismic design of the project, there are several large, deep footings that serve as the base for hold-downs to resist high lateral loads on the building. These deep footings were actually poured such that the slab insulation runs continuously over the top of the footing.

Coordinating with subcontractors

As clearing and grubbing began at the site, and then the initial excavation activities, the construction team began a detailed coordination process. To properly construct a high-performance Passivhaus design, diligent, proactive coordination of the work is required of the general contractor. There is no substitute for diligence when it comes to this coordination. Even a highly developed and accurate set of design documents does not include all the information needed to build the project, and inevitably there will be some gaps in documentation or a need to modify a detail slightly, or in a major way, to achieve the design intent while accommodating construction variables such as sequencing of the work, manufacturer’s installation instructions, etc.

Coordination of the work is fundamental to all construction projects, but the need is heightened when executing a Passivhaus design, especially when it comes to the detailing of the airtight and thermal-bridge-free building envelope. For example, at some detail conditions there could be four or more trades that impact the airtightness of the building since they each supply and/or install components that are integral to the air barrier system.

An important duty of the general contractor is to actively communicate with the entire group of subcontractors, to let them know about the Passivhaus and requirements on the project, and to educate them about key issues that may impact their scopes of work and the overall Passivhaus certification. Due to the intricacies involved with material specifications and detailing of the Passivhaus design, communication with the subcontractors that impact the building envelope needs extra attention.

On the Orchards project, a full day Building Envelope Coordination (BEC) meeting was held on site during the first month of construction, to gather together all the envelope-related subcontractors and key suppliers and review project requirements, including specifications, detailing, schedule, sequence of trades, etc.

Scheduling this meeting very early during construction allowed the team to work through any gaps or inconsistencies in the scopes of work of various trades, as well as any issues related to the design documents. Upon completion of the BEC meeting, resolved issues were addressed readily and efficiently through the project submittal process. Issues that needed further examination or design work were addressed through the project Request for Information (RFI) process. The coordination work touched all major elements of the design, including the foundation, exterior walls, windows and doors, and the roof.

Does the StegoWrap vapor barrier go above or below the rigid foam?

An important concern that arose during the coordination process was the location and detailing of the subslab vapor barrier. The vapor barrier was not clearly indicated in the architect’s details, although a vapor barrier had been specified. In the slab on grade assembly drawing, the vapor barrier was indicated to be installed below the slab insulation. The Walsh team questioned this location, given our concern that a large amount of water could collect in the slab insulation layer if it should rain prior to a slab pour. The configuration of insulation and vapor barrier essentially created a sealed “bathtub” that could hold a lot of water. Not a good scenario!

Even though we were in the dry summer months in Portland, there is always a chance of rain. When we pointed this out, the architect understood the concern and agreed with relocating the vapor barrier to the top of the slab insulation. Furthermore, the detailing of the vapor barrier at the foundation perimeter was not clear in the design drawings. We discussed this with the architect and sorted out the termination details as part of the coordination process, working with the vapor barrier manufacturer’s standard details and sealing products. With these details resolved, construction on the building foundation began.

The concrete contractors were hesitant to place concrete footings on rigid foam

To move forward from this point we had to overcome a little hesitation from the concrete crew. They’d never prepped a concrete foundation to go on top of insulation before. This idea raised more than a few eyebrows. After explaining the purpose of the foam layer below the footings and slab, resistance was overcome, if only temporarily.

To construct the footings at the building perimeter, 4 inches of Type IX EPS foam were placed on top of the gravel base, the foam was informally tested to ensure solid contact with the base, formwork was constructed on top of the foam, and then concrete was placed. After initial set and curing, the formwork was stripped and EPS was applied to the vertical face of the footing.

Upon completion of the initial footing work at the building perimeter, preparations for the slab on grade were made. A capillary break layer with 6 inches of clean crushed gravel was placed over the rock working pad and compacted to a dense state. A radon mitigation system was installed on top of the gravel base. The system includes 4-inch perforated flexible piping wrapped in filter sock material and embedded in an additional layer of gravel, 6 inches thick, placed on top of the compacted gravel base. The gravel provides a minimum 1 inch cover over the piping.

Four inches of Type II EPS foam was placed on top of the gravel base. The design documents indicated a single layer of foam; however the crew pushed to use two layers of 2-inch foam for the installation. The crew believed the foam would lay flatter and provide more stability on the gravel base going with the two layers. This also had the advantage of allowing for staggered joints in the boards and eliminate direct heat flow paths that would occur at butt joints in the boards had we used only one layer, as is typical with roof insulation installation. The foam was trimmed to fit tightly around penetrations. Any gaps were filled with expanding spray foam sealant.

For the most part, the slab insulation went down well over the gravel base; however, there were a few issues with getting the foam to lie flat and stable. These issues were resolved by reworking the gravel. The geotechnical engineer called for 2”- ¼” gravel whereas a smaller rock or pea gravel would have helped to eliminate the issues we encountered.

A rugged subslab vapor barrier

A cross-laminated plastic vapor barrier was specified. We installed a StegoWrap membrane, 15 mil thick product. All lap joints and seams were sealed with the tape provided by the manufacturer. Typical pipe and conduit penetrations were sealed with tape also. Where penetrations were ganged too closely together to allow detailing with the tape, mastic was used to seal the vapor barrier to the piping/conduit. This is an accessory product offered by the vapor barrier manufacturer. Where the vapor barrier intersects with footings at the building perimeter, a special butyl tape was used to seal the vapor barrier to the footing. This seal was important to provide air barrier continuity between the slab-on-grade assembly and the exterior wall assembly.

It turned out that we were very fortunate to have moved the vapor barrier to the top side of the foam insulation since it rained heavily for a few days in early August. The rain didn’t create any serious water problem as most of it ran off to the edges of the slab area, and what remained dried off relatively quickly. If the vapor barrier had been placed over the gravel, below the insulation, we could have had a major issue on our hands with water retained in the foam insulation layer.

A 4-inch-thick concrete slab was placed on top of the vapor barrier. We paid close attention to the concrete mix design. We utilized a mix design from CalPortland with a 0.42 water-cement ratio, as was specified. When pouring a concrete slab directly over a plastic vapor barrier, the use of a low water-cement ratio mix design is important to help minimize slab curling and also to minimize the potential for moisture-related problems with floor finishes applied over the slab.

Mike Steffen is a builder, architect, and educator committed to making better buildings. He is vice president and general manager of Walsh Construction Company in Portland, Oregon.

21 Comments

  1. Malcolm Taylor | | #1

    Mike
    Thanks for the very comprehensive commentary and illustrations.
    A couple of questions:
    The "deadman" footings for seismic resistance: With the foam running continuously over them how did you end up connecting them to the slab above?
    Are you at all worried about damage to the sub-grade foam from pests and did you take any measures to mitigate against it?

    Looking forward to following the project along through your future blogs.

  2. George Hawirko | | #2

    Anothe WHY?
    The Photos are great.

    I'm sure there is a reason, but why is the EPS being glued to the inside of the Footing and not the outside?

    Why is the foam Glues at all, it would save a bunch if placed into the forms while the concrete was placed?

  3. Peter L | | #3

    Not Time Tested
    No matter what the "experts" say, this approach is NOT time tested and nobody will guarantee that this will not come back to haunt the structural integrity of the building years later. All it takes is for some curious termites or other subterranean insect to chomp away at the EPS. Borate treated or not. Termites have and will tunnel through EPS. If the EPS is removed/damaged, the footing will be suspended without any compact soil underneath.

    Building Science 101 states that a footing needs a solid and stable surface. If you have a void underneath the footing, the house will suffer structurally and can and will shift. Nothing like a failed footing to wreak havoc for the life of the home.

    Is it worth the miniscule R-Value savings to place foam under footings? I don't think so. When your foundation fails, I guarantee you these EPS foam companies and everyone else involved in these projects will run for the hills.

  4. Steven Denbow | | #4

    Another Concern
    What is the compressive strength of the 4" thick EPS underneath the footings? What are the loads placed on the EPS by the building? I would think that the EPS would be compressed over time due to the building loads. This would reduce its thermal resistance (R-value). Was this planned for when originally selecting the EPS thickness/R-Value?

  5. User avater
    Mike Steffen | | #5

    Response to Malcolm and George
    Malcolm, The deadman footings are connected to the thickened slab/footings (and the shear walls) above by steel anchor bolts and holddowns. See detail in the attachment (image credit to Stonewood Structural Engineers). We installed Simpson SB 1x30 and 5/8x24 anchor bolts. These footings and connections are designed to resist uplift/overturning forces. The team was aware that termite damage to foam is a potential issue in some areas of the country. This has not been an issue in the Northwest so the architect specified EPS without treatment. A stainless steel flashing covers the foam at the base of wall typically and should serve as a deterrent to other critters getting access to the foam.

    George, Foam boards were adhered to both the interior and exterior side of the footings. You make a good point about using the foam itself as the form material for the footings. Our concrete gang has done this on numerous projects but encountered difficulty on a few projects with the foam not holding up well to the load when mud was placed, so they went with the conventional forming route on Orchards. Thus the need to apply the vertical pieces later after stripping the forms. We used Type II foam at the vertical, if we used Type IX perhaps it would work better with the approach you suggest.

  6. Malcolm Taylor | | #6

    Mike
    Thanks again. I have had a bit of trouble with carpenter ants and foam, but I live and work right in the PNW rainforest. I wonder if their habits are similar to termites in that, as you say, I've never seen them very far below grade? Most of the havoc they have caused was in upper stories or roofs, and the colonies I have dug up were very close to the surface. The success of using EPS under runways and overpasses does lend some comfort that it isn't too big an issue.

  7. User avater
    Mike Steffen | | #7

    Response to Stephen
    The structural engineer specified Type IX EPS foam under the footings. Compressive strength of Type IX foam is 25 psi, as Len notes above. I don't have the calculated loads off top of my head but will ping off Scott the engineer.

    Martin Holladay addressed many of the technical issues surrounding use of "foam under footings" in a previous GBA blog post. See the "related articles" sidebar above to access his detailed overview of the subject.

  8. User avater
    Mike Steffen | | #8

    Response to Len
    Thanks for adding your notes to address the questions raised. Good to know you have had no unanticipated issues with your foundation set on foam. The structural engineer on Orchards (Scott Nyseth with Stonewood Structural Engineers) has advised the team that he expects some degree of settlement of the foam but expects it to be minimal (1/8"-3/16") and consistent across the extents of foam. This is a manageable degree of movement. On our typical wood frame projects (up to five stories of wood framing) we can see 1/4" of settlement (due to both compression and shrinkage) per floor, and upwards of an inch of cumulative movement for the entire height of the building, so this degree of movement is not out of the norm for wood frame buildings that we are familiar with in the Northwest. Our buildings are not static, we must understand movement and then accommodate it in a variety of ways. We coordinate and plan for this movement for example in how we provide gaps for relief below door sills at the ground floor walkways, at balcony interfaces with walls and doors at the upper levels of buildings, and where claddings and other components such as windows interesect with each other at upper levels.

  9. Trevor Trainor | | #9

    Future Blogs
    Hey Mike - Great summary of the foundation details. What strikes me as extrodinary is the attention to detail on this project. If you're looking for future blog topics, I would love to see a summary of all of the flashing details spec'ed on the project and of course wall monitoring data as it comes in.

    Great job (so far).

    Trevor

  10. Len Moskowitz | | #10

    Termites and Foam
    Here's another data point for rigid foam under footings:

    In our one-family pre-certified PassivHaus in northern NJ, we used 4-inches of Type IX EPS under the poured-concrete footings. We used four inches of Type I EPS to encase the sides of the footings and on the poured-concrete exterior walls of the foundation. (There's a heavy layer of sprayed-on waterproofing between the concrete walls and the exterior foam.) We used eight inches of Type I EPS under the basement slab and over the footings.

    When we researched termite effects on foam we discovered that termites don't go very deep. For example, in their recommendation about anti-termite treatments, the University of Missouri Extension recommends applying anti-termite insecticide only down to a maximum of four feet.

    http://extension.missouri.edu/p/g7420

    Our footings are considerably deeper than that.

    Type IX EPS has a higher compression specification (25 psi) than undisturbed soil. And when it compresses, it settles evenly.

    http://www.epsindustry.org/building-construction/compressive-strength

    I estimate that the entire house weighs around 280,000 pounds (around 100,000 pounds of house and 180,000 pounds of concrete foundation), and we have around 260 square feet of perimeter footings and 30 more square feet of central lally column footing. Assuming the load is evenly distributed, that works out to under 1000 pounds per square foot of footing. That's below the foam's 25 psi rating (3600 pounds per square foot) by a factor of more than 3.

    The footings, foundation walls and slab were poured in September 2013. It's a bit more than one year later and we're not showing any evidence of differential settling, or wall or footing cracks. (The slab has a few cracks, but that was expected.)

    You can see photos of the footings and lots more on our Facebook group: Passive House NJ.

  11. Brian Carter | | #11

    Fo far so good- does anyone sell this stuff?
    I have only come across high density EPS from big wholesale roofing suppliers. It's not the kind of stuff you will ever find at the average building supply. So why should this matter to the average builder? Whats wrong with XPS?

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Brian Carter
    Brian,
    Q. "Does anyone sell this stuff?"

    A. Yes. Here in New England, most builders buy their Type IX EPS from Branch River Plastics in Smithfield, Rhode Island. I'm sure that if you ask around, you'll discover a distributor in your area that can help you.

    Q. "What's wrong the XPS?"

    A. The blowing agents used to manufacture XPS have a very high global warming potential, so green builders try to avoid using XPS. EPS is more environmentally friendly. For more information on this topic, see:

    Avoiding the Global Warming Impact of Insulation

    Calculating the Global Warming Impact of Insulation

    Insulation to Keep Us Warm — Not Warm the Planet

    New Blowing Agent Addresses Climate Impact of Foam Insulation

  13. Len Moskowitz | | #13

    Foam Source in NY/NJ/PA/CT Area
    We bought our foam directly from the manufacturer: Poly Molding LLC in Haskell NJ.

    http://polymoldingllc.com/

    What they didn't have in stock, they made for us with a very short lead time.

    They service from Washington DC to Maine.

    -----

    There's another reason why EPS is preferable to XPS under grade, in addition to those that Martin cited: EPS absorbs less water and suffers less loss of R-rating due t o water.

  14. Brian Carter | | #14

    XPS doesn't absorb water.
    I think you've got that reversed, Len. Even in the above article the question of where to place the moisture barrier pivoted on whether or not it would rain and saturate the EPS. I've pulled XPS off flat roof decks when the roofing failed and it was not saturated the way EPS or Iso can be(ever try to pick up a sheet of foam that weighs over 100 lbs? It doesn't happen in one piece.) There is even a market for reclaimed XPS foam from roofs, and that has more cred than using new stuff, even if you worry about the blowing agents.

  15. Brian Carter | | #15

    Thanks
    Many thanks. But my question remains-if this has a better green profile and works as well why is it something I have to track down? Do we include the greenhouse gases generated by driving all over to find stuff? I will also be surprised if I can buy it in less than a pallet quantity.

    To tell the truth, Martin, I find it difficult to compare the quantity of greenhouse gas used to produce the foam I use to the amount that leaks from even one abandoned oil well or, god knows, the amount in the air around any fracking operation. With a greenhouse potential over 25 times the level of CO2 , methane starts to look just like coal when the cost of sloppy handling, from the well to the household, is added. Sure, I realize we need to do our best in whatever sphere we have , but the overall picture never has justified optimism. I have acted as a saint for decades, now I simply smile and say have a nice day. The war is over, and we lost.

  16. Brian Carter | | #16

    The links you provide are
    The links you provide are helpful, Martin, but really dated. Is XPS still verboten or have the manufacturers switched blowing agents? Thanks

  17. Brian Carter | | #17

    EPS
    Thanks Len and Martin for the links to EPS manafacturers. I checked both and found what I expected-a brief sales pitch and no information. That,apparently , is only done by phone. Why are they doing business this way? It reminds me of when PVC pipe first became available and could only be had from plumbing distributors who provided no information. I first found high density EPS back in the 90's and started the long process of making phone calls and talking to people who had little interest in being helpful. After asking too many questions-such as what the specs were and how much it cost and how could I get it delivered they finally just stopped responding.

    I can see the benefits of this stuff and would use it, but I don't think anyone has any interest in marketing it. If it's that much trouble I don't see it as a realistic alternative to materials that are easily available anywhere.

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Brian Carter
    Brian,
    It's true that some useful building materials aren't available at Home Depot. (You mention Type IX EPS, but the same problem exists when it comes to good triple-glazed windows.) You also mention that choosing specifications for building materials takes research, and that not every question can be answered with a phone call to a distributor.

    You're right, of course. But these aren't new observations, and I'm not sure why this issue bugs you so much.

    If you are happy specifying XPS, go right ahead and keep specifying it.

    If you have learned enough from GBA to discover that Type IX EPS is an interesting material with qualities that make it preferable to XPS, you might want to specify it for your next project.

    If you are daunted by the research associated with creating specifications, hire an architect or an engineer.

  19. Len Moskowitz | | #19

    EPS vs XPS
    I'm sorry that I wasn't as clear as I should've been.

    The spec that I was trying to remember was R-retention value when moisture is absorbed. EPS seems to retain its R-value better than XPS when the foam absorbs water.

    Here are two links that support this conclusion::

    http://blog.insulfoam.com/tag/eps-vs-xps/
    http://blog.insulfoam.com/tag/eps-for-below-grade-insulation/

    And here's one that tries to make the opposite conclusion. You have to read it very closely to figure out exactly what the evidence shows.

    http://www2.owenscorning.com/literature/pdfs/10018681.pdf

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Brian Carter (Comment #16)
    Brian,
    Q. "Is XPS still verboten or have the manufacturers switched blowing agents?"

    A. Manufacturers of XPS in Europe have switched to more environmentally friendly blowing agents; these blowing agents reduce the R-value of the XPS from R-5 per inch to R-4 per inch.

    U.S. manufacturers are reluctant to make the switch out of fears that U.S. consumers care more about R-value per inch than the fate of the planet. So for now, we're stuck with the damaging blowing agents.

    There are changes afoot, however, as evidenced by this recent press release from Honeywell: Honeywell Starts Full-Scale Production Of Low-Global-Warming Propellant, Insulating Agent, And Refrigerant.

  21. User avater GBA Editor
    Martin Holladay | | #21

    Response to Len Moskowitz (Comment #19)
    Len,
    The controversy you allude to has been debated for years. Manufacturers of EPS have sponsored studies that show that their products hold up better when exposed to water than XPS. Similarly, manufacturers of XPS have sponsored studies that show that their products hold up better when exposed to water than EPS.

    I haven't been convinced by either position. Here's my advice: choose a foam that has been rated by the manufacturer for ground contact or burial, and don't worry. EPS uses more environmentally friendly blowing agents than XPS.

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