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PVC vs. KEE: What are the Pros and Cons?

Earlier this year, I discussed the differences between PVC and TPO. That blog post continues to generate a lot of interest, so I thought I’d follow up with a closer look at PVC and the KEE sheets.

Just as a refresher, remember that PVC is a very rigid solid – think about PVC pipes. To make a good membrane, it has to be made flexible, which involves mixing the PVC with what are called plasticizers. When PVC roofing first came out, those plasticizers were fairly volatile and on hot roofs they migrated out and evaporated. The result: the PVC got brittle and often cracked.

Over the years, the plasticizers have gotten better; they are less volatile and stay in the sheet longer. But they do migrate out – just a lot more slowly than was the case for those early formulations.

So, where does KEE come in? Well, conventional plasticizers are liquids, whereas KEE is a very soft and flexible polymer. So, if you used KEE instead of a liquid plasticizer, there’d be no liquid to evaporate out. Sounds great, but a pure PVC-KEE sheet would be very difficult to weld. Therefore, some liquid plasticizer is always needed.

To put it simply, KEE is like an insurance policy – once the roof is down, even if the liquid plasticizers migrate out and are lost, the KEE will ensure that there’s at least some flexibility.

Now that I’ve (hopefully) explained the use of KEE, let’s review the various membranes. PVC is specified by ASTM D4434 which requires the PVC content to be above 50% of the total polymer amount. KEE sheet is specified by ASTM D6754 that requires KEE to be greater than 50% of the polymer content. So, more KEE than PVC – which is why it’s called KEE membrane.

So far this is straightforward – we have PVC and KEE membranes. But, things are never so simple. Many PVC manufacturers add some KEE, but not enough to classify their membranes as KEE. They keep the amount below 50% and are doing it to provide some baseline of flexibility in case the liquid plasticizer is lost. I would argue that we have PVC, PVC-KEE, and KEE membranes on the market.

Let’s take a look at a comparison of the specifications for the PVC and KEE membranes:

Property PVC – ASTM D4434 KEE – ASTM D6754
Heat Aging – key attribute for use in high-heat conditions 176° F for 8 weeks 176° F for 8 weeks
Accelerated Weathering Test – fundamental requirement for UV resistance 6,300 KJ/sq meter at 145° F 6,300 KJ/sq meter at 176° F
Retention of Properties after Heat Aging, % 90 90
Elongation at Break, % 15 18
Tearing Strength, lbf 45 108
Breaking Strength, lbf 200 348
Change in Weight after Immersion in Water, max +/-3.0 +6.0

In terms of heat and UV resistance–in other words, those attributes that are key to weathering performance–these sheets appear to be the same. The marked improvement in tearing and breaking strengths in the KEE specification is worth discussing. There is only one KEE sheet on the market and it was used to develop the ASTM specification. It uses a heavier fabric than the PVC and PVC-KEE sheets and is responsible for most of the performance increase.

Superior chemical resistance of PVC and the KEE variations is often a claimed advantage over membranes such as TPO. However, this is not as simple as it seems. Firstly, there aren’t any standard tests of chemical resistance. Also, if roofs were being exposed to any number of industrial chemicals, there would be other concerns beyond how the roof was performing. Having said that, it’s certainly the case that all of the PVC-based sheets have better resistance to grease, oils, and fats than TPO.

So, there are the PVC and KEE specifications but nothing specific for the PVC-KEE sheets. My advice when looking for a way to differentiate between products is to consider the following:

  • Manufacturer’s reputation – have you had consistently good results using product from the manufacturer, and does that manufacturer have a solid reputation within the industry?
  • Product performance claims – does the manufacturer have solid test data, preferably obtained by an independent third party, to validate their performance? Do the tests follow industry standards, such as ASTM?
  • Do not try to guess about performance based on the formulation, such as the amount of KEE or PVC in the sheet. It is performance that matters, not formulation.

Remember that good installation is critical for the long-term performance of a roof. Thinner sheets can be more easily punctured by falling sharp objects regardless of the reinforcement fabric. Also, sheets containing KEE can be harder to weld, so make sure test-cuts are taken and welder settings validated at the beginning of any installation.

Related articles:

PVC vs. TPO: Is the Debate Over?
How to Clean a TPO or PVC Roof?
What are the Best Attachment Options for PVC and TPO Membranes?

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There are 13 comments

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  1. Sean britt

    Great article very informative I was wondering why fiber tight does not well very well and I’m thinking it’s from KEE

    • Thomas J Taylor, PhD

      Thanks for checking in with our blogs. Yes, even if a particular membrane performs well in testing, it has to weld in a wide range of conditions. Good installation is critical for any roof system and welding is a key part of that. We do test welds over a range of speeds and temperatures with all our membranes to make sure we offer a wide welding window. Whichever membrane you are using, be sure to do a test weld at the beginning of the day and whenever conditions have changed.

  2. Bryan Corpening

    Good and informative article. However, your point that thinner sheets can be more easily punctured regardless of the reinforced fabric seems misleading. We’ve found that FiberTite’s 36 mil membrane is stronger than most if not all other 60 mil membranes due to the superior reinforced fabric that FiberTite uses. Or would this strength be the result of the amount of KEE used with FiberTite membranes? Thanks in advance for your response.

  3. Al Fleetwood

    I have had some consultants and architects tell me KEE sheets have no PVC. Is it not true that you cannot make a KEE sheet without PVC? Also, what is the highest content of KEE you can have in a sheet?

    • Thomas J Taylor, PhD

      Al – thanks for the great question. All the KEE sheets on the market contain PVC. The ASTM D6754 KEE roofing specification states that KEE must be 50% or more of the total polymer content. KEE is a plasticizer and so the other polymer is always PVC. You also asked about the highest possible amount of KEE – my guidance is that the amount of KEE should not be a deciding factor in selecting a membrane. At GAF, we believe that decisions should be based on the performance of the sheet and the reputation of the manufacturer. There are many factors that go into making high performance roofing membranes and the amount of one of the ingredients alone doesn’t really tell you how that sheet will perform. Welding, weathering, and overall handling are the result of careful optimization of all of the ingredient not just the amount of one of them.

  4. Jeff Noss

    We recently specified an ASTM D6754 roofing material for one of our public jobs. The contractor came back with an ASTM D4434 product claiming only one manufacturer provides the ASTM D6754 material, making it a proprietary item and therefore, not permitted on a public job. I find it difficult to believe ASTM would create a proprietary specification but am having trouble finding more than one current manufacturer for the ASTM D6754 material. Would love to hear your input on the matter.

    • Thomas J Taylor, PhD

      Jeff – any manufacturer could produce a sheet to match the D6754 specification, however only one actually does so. That manufacturer worked within ASTM to develop the specification which is part performance and part prescriptive based. I would encourage you to compare the actual weathering performance parts of the two documents – looking at the heat resistance and UV exposure requirements. GAF believes that product performance is the most critical part of any product specification. How it is made and what the ingredients are is something we worry about, so that the customer doesn’t have to. We would like to see standards largely based on heat and UV resistance, along with the physical properties sufficient to give a well performing roof. Thanks for checking in with us.

  5. Allen

    I have heard from various consultants that the best product out in the market today uses Duponts new Formula, KEE-HP. it does better with long term performance they say. Is this something you use in your KEE sheet? Also looking to know when you will be producing it in Cedar City so we can hopefully pickup the material with our trucks if we prefer it over your competition.

    • Thomas J Taylor, PhD

      The KEE-HP material has some advantages and disadvantages compared to regular KEE. As with all our products, when introduced all of our in-house manufactured PVC sheets will have undergone very extensive testing – for long term performance and ease of installation. Stay tuned.

  6. Hooman Aryan

    I am curious too. So much publicity on KEE and yet some manufacturer’s don’t use KEE and have successful installation record. I wonder what they use?

    • Thomas J Taylor, PhD

      You are right – a well formulated “standard” PVC sheet can perform very well. For some people KEE provides extra assurance about long term performance, but in reality PVC and PVC-KEE sheets have the same specifications. KEE is not volatile and stays in the sheet. On the other hand, the liquid plasticizers in PVC membranes can be very slowly lost over time. Thanks for checking in with us.


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