Foam Core: EPS, XPS, and Polyurethane

December 20, 2006

I am traveling deep into the heart of SIP country, to the New Englandshire of Brattleboro Vermont. It’s cold, it’s snowing, and it’s progressive. It’s the perfect storm for stress on a residential structure. Why? Because cold conditions create a large temperature difference between indoor and outdoor spaces and this is when condensation happens inside walls (read rot). Also, when it’s snowing we are less likely to be tramping around outside checking on the condition of our houses. And finally, progress is change and change causes us to operate outside our comfort zone and knowledge base.

Of course change is good. Otherwise we would still be building like this.

Brattleboro house 

There are three basic materials used as the foam core of Structural Insulated Panels. Each of the three core materials have different advantages.


Outside Brattleboro, Winterpanel has been making and testing polyurethane panels for over three decades. The main advantage of polyurethane (or the much harder to pronounce derivation, polyisicyanurate) is that it has the highest R-value of any SIP panel. After what’s called thermal drift, it’s about 6 to 6.8 of R-value per inch of panel.

The other advantage is that it has an extremely high melting point. Effectively if your house caught on fire, the polyurethane foam core would be one of the last things left standing.

house on fire

The main detractor of urethane panels is the cost. Doug Anderson from Winterpanel says the difference is about 40 cents a square foot of panel. The other disadvantage in terms of construction is that  polyurethane has a high melting point, hot wire burners (the primary method of modifying EPS and XPS panels on site) can’t be used. This is not insurmountable but still something that needs to be addressed in the planning stage.

Expanded Polystyrene (EPS)

Winterpanel also makes EPS core panels. Over 80% of the SIP panels installed are EPS. EPS core material is widely available. It is easy to modify on sight and most sip installers have experience dealing with EPS. It’s also the least expensive option in terms of material cost.

All good things come at a cost. EPS core material has the lowest R-value and has a low melting point. That means if the fire in your house is intense enough to burn past the gypsum and the OSB–POOF! The good news is that if the fire gets that hot to begin with, most likely everything else inside your house is already charcoal.


Expanded Polystyrene (XPS)

Bo Foard of Foard Panel has been in the SIP business for over twenty years. He has made and installed Polyurethane, EPS and XPS panels and is convinced an XPS core is the best option in most situations.

Bo’s XPS House

XPS has many of the best characteristics of both polyurethane and EPS. It has a high R-value (5 R-value per inch of panel). It is dense and stable yet has a relatively low melting point so on-site modifications are as easy as EPS. Also, like EPS, it bonds easily to OSB and gypsum board.

Unfortunately XPS is expensive and manufactured panels are not widely available (the only manufacturer I could find that makes XPS panels as part of their regular panel line is Foard Panel other companies like Murus will make them when customers requests it.). The good news is that the folks at Foard seem to really know their stuff. We sat down for a good three-hour geek-out session on building science and I get the feeling that Bo and his project engineer Paul Malko could have gone on much longer.

As always I invite you to post a comment with your thoughts and insights about SIPs.

Better yet, go to the link at the top right of this page and Post Your SIP House.

Pictures from today

padel routerBrattleboro churchDON’T TOUCH THE CNC MACHINEBricks in Brattleboro



  1. Have you discussed skin materials much? In particular I’m interested in using cementatious-skinned panels to create board & batten style exteriors and eliminate drywall on the interior of the exterior walls. This doesn’t seem very common practice though. Thoughts?

    • What was the answer to the cementatious skinned panels question?

  2. I presently live in Timberframe home and find the SIPs for insulation, sound, and ease of additional window placements in wall SIPs most helpful. My concern is quality of OSB in roof panels and moisture vulnerabilty in my area NC where we are in a modest rainforest environment. Zip 28717. Any movement in SIP skin towards use of a more engineered OSB like Advantec or similar more moisture resistant, especially for Cedar Shake roofs and drying in roofs during rainy season construction.

  3. About a year ago my builder constructed a wonderful house using SIPS panels.

    My question is: Are there any health hazards associated with these panels? Both my husband and I have severe seep deprivation problems since moving into the house. We are lucky to get just a few hours of sleep a night. Any idea where I might go to find out about this possibility?

  4. Diane,

    The SIPs themselves are pretty benign. The skin material is typically OSB which is the same material used to sheath stud framed houses.

    If the foam core is EPS then that’s not the cause either. EPS is inert and does not off gas at all.

    Check and make sure that the air handler you have is working properly. The American lung association recommends at least .5 air exchanges per hour in a residential home. In a super tight structure like a SIP home there is little or no passive air movement so it has to be actively managed by a HRV or ERV.

    If fresh air is not introduced, gases and allergen particulates can build up to unhealthy levels.

    Your SIP supplier and contractor should be able to get you some more information.

    Good Luck!

    • John I am new to SIP am extremely interested though. I am from Australia. What do the abbreviations HRV and ERV stand for? I am also interested in finding out about tools used for constuction. For example can you buy of the shelf paddle routers. In particular are there models that could be attached to an extractor for collection of the polystyrene? Thanks looking forward to your reply.

  5. The author seems to be ill informed. The SIP I have worked with over the years are listed by ICC-ES. The evaluation criteria for ICC-ES is that the EPS must be treated for flame spread less than 25 and smoke developed of less than 400. It is the lack of air, however, that makes the SIP whether of EPS, XPS or urethane superior in a fire. Without air there can be no fire. If the only thing left standing after a house fire is the insulation- so what? Urethane insulation does not melt but does deteriorate after expose to continuos temperature of greater than 300 F. Check with Dow Chemical and you will see they do not recommend exposure of urethane insulation to continuous temperatures of 300 F or above. Other urethane manufacturers will confirm that this is the threshold.

  6. Gary,
    You’re right. SIPs, no matter what variety, typically don’t burn easily in a standard house fire. I saw a demonstration where a blow torch was held to EPS and the flame vaporized the foam. Then the flame was held against urethane foam and it hardly affected it. Later I found out that the amount of oxygen available is what determines how fast the burn is. By the time the fire goes through the OSB skin, it won’t matter what type of foam is in the core.

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  8. Diane:

    It does seem bizarre to think that the cause of your sleepless nights would be caused by a SIP but you may not rule the though out completely. You may want to look into what type of adhesive the company used to stick the OSB onto the PolyStyrene. I know that in order to glue all of the OSB strands together as well as in other laminating processes they use phenol resourcenol or phenol fermaldehyde both of which are linked to a plethera of health problems.

  9. Good article.
    Unfortunately, when we ask for the facts about a product from the manufacturer, we do not always get “the whole story”.
    Having made choices for products on a number of contruction projects, I can attest to the following “truths”:
    1. Urethanes use a cyanide -cousin molecule for their reaction/expansion. This “isocyanurate” is listed as hazardous with the EPA, and is proven to cause health issues.
    2. Because of the out-gassing of the cyanurate derivatives from the “bubbles” which form the urethane foam (which give it the insulating properties) the product loses 20-25% of it’s insulation value. Industry estimates this process of out-gassing can continue for up to 5 years post manufacture.
    3.This now impacts the HVAC system sizing – and NOT in a positive way. You cannot effectively over-size the HVAC system when the house built with urethane/iso-cyanurate panels is new, nor should you.
    So – as the house loses insulation value, the HVAC system can not make up for that, and thus – now the HVAC is undersized. Not very “efficient” for a material sold to us as “energy efficient”
    4. The stated “40 cents per square foot of panel” is completely false. I have recieved numerous quotes for SIP panels for specific projects showing an average of 32% increase in cost for uerthane/iso-cyanurate panels. Period.
    5.The flame test is as nebulous a test as is possible to even call a test. If one examines the smoke generated by the urethane or iso- panels, you find it is listed – by all governing authorities concerned with toxicity, flame spread, and smoke qualities (EPA, UL, CSA,ETL) – as one of the the most toxic tumescent materials on a jobsite. Flatly – the smoke form urethane or iso-cyanurate panels will kill you far faster than flames from any house fire.
    6.When you do a side by side comparison between EPS foam, and Urethane or Iso-cyanurate what you find is this:
    A. 30 to 40% lower construction cost for EPS

    B. EPS is 1% petroleum and 99% AIR – that is it…AIR. NO out-gassing. Stable, un-changing R-values.

    C. If you have any real world based concerns about burn times, flame sread, and smoke toxicity – do REAL research. Ask the questions of someone OTHER than the manufacturer of the products in question. BUt really – the sheetrok in the home is the burntime fire codes concern themselves with.

    D. Realistically – any of the foams will work for home building. What is the total cost? What is the known stability of the stated R-values? If you are making a buying descision based on either R-value or flame spread – smoke – then YOU REALLY need to do a lot of homework.
    For my family, I chose EPS, because the drywall (gypsum, sheetrock) is the REAL flame spread barrier. Again – when you ask the questions of fire professionals, you get a much better picture of this entire scenario.

    • So why is it that SIP mfgs use an EPI adhesive (emulsified poly isocyanate to stick the XPS or EPS to the OSB? Isn’t that adhesive dangerous too?

  10. In Europe HBCD(D) is used as a flame retardant in EPS and XPS, but it is a PBT substance (persistense, bioaccumulative, toxic). But according to some of these comments above it seems to me that flame retardant is not neessary in EPS/XPS, is it?

  11. It is not necessary, as EPS, and XPS are classified as “self-extinguishing”. That is, once the flame source is removed, they quit burning.


  12. in the sip housing the core materail XPS and PUF is better to use as compare the EPS, if so out of these two what is most cheap and relaible. where it always matter that in PUF with molded process there is always density variation which reflect the quality and cost of panel as compare to XPS as a core materail.

  13. Is XPS a viable option as a moldable insulated shipper for pharmaceutical applications?

    • ya it is, it can bear laod over it skin as another material like wood.. light in weight, fire retardant. good physical properties all together it is good for shipper.

  14. I get confused when you use the terms EPS and XPS aren’t they both foam? Is PUF foam?

  15. quick clarification – I believe you have a typo, which might confuse readers. On your headings for the three insulation types, you have “Expanded Polystyrene” for both EPS and XPS; you should have “Extruded Polystyrene” for the XPS paragraph…thanks – great writeup.

  16. Using GFRC – Glass Fiber Reinforced Concrete for your SIP will help as GFRC acts as a thermo regulator which will distribute the heat.

  17. QUOTE: “Using GFRC – Glass Fiber Reinforced Concrete for your SIP will help as GFRC acts as a thermo regulator which will distribute the heat.”

    While this may or may not be true ( I do not have any real numbers, nor have I seen any ) the fact remains that GFRC is not available in 8′ x 24′ sheets – which maximizes efficiency of manufacturing, with current equipment sized to take advantage of OSB in this dimension.
    Additionally, GFRC is extremely hard on fabrication tooling, thus – has a higher “cost of manufacture” consideration.
    And – lastly – it is practically impossible to get GFRC skinned SIPs built, delivered, and set in place without damaging them – often severly. They are too heavy, and too brittle to accomodate all other considerations of a possible advantage in the use of GFRC

    • Tim GFRC is a very manipulable material as matter of fact the company which i work for, DecoArc has the ability to manufactured any size shape and texture of GFRC panels as per the thermal conductivity is about 0.23 ºF.ft^2.h/Btu (0.04km^2). Also GFRC is not as hard to produce as you describe. As per damaging a GFRC panel that wouldn’t happen the Compressive Strength is 27.6 MPa while the Tensile Strength is at 8.69 MPa

      The real problem of using GFRC as SIP would be cost as do mention it has a higher production cost

  18. Indeed a nice post. I like it. Thanks for sharing it.

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  21. In my county, I also use Expanded Polystyrene for the building. That is cool.

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  24. We live in a subdivision that slopes to a holding pond on the back of our property. It was grades to allow the natural flow of water from our yard through the neighbors into the pond. Well, we replaced the fence and afterwards our neighbor build up her lawn and added grass essentially cutting off the drain flow. Now we have massive puddles where the fence is due to the natural flow of the grade. We are considering putting in a french drain but essentially no way to take it to the street in front of the house due to the already deep slope upwards. Would a flow well be a good alternative with french drain to it. My only concern is the flow well filling up and overflowing causing more problems. Any suggestions. We just had landscaper put in plant and a lot of them are dying from too much water. Thanks.

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