Since first writing this posting, my viewpoints have updated based on additional gained knowledge. While I still believe that XPS wets up slower than EPS, I now know that both will wet up in the long run in damp environments. So drainage below (sub-slab) or along (vertical foundation) is key to keeping these products performing well.
When choosing between the two products, I also agree with the recommendation by some to just increase the thickness of EPS by 20% to account for R value loss when wet. This is based on the fact that EPS currently has a much better environmental footprint over XPS.
And indeed this was the direction I had planned to take on the house I am building. But then I found out that ROXUL had approved its mineral wool insulation for sub slab installations. This represents an even better alternative to rigid foams. Mineral wool is free draining, has a smaller environmental footprint (especially ROXUL that is produced using electricity from a nearby Hydro Electric dam), and is hated by insects and rodents (relevant for vertical installation on the outside of a foundation). ROXUL recommends their ComfortBoard 110 product for this application.
While I now plan to use this product below my slab, I still feel that long term unbiased testing of the typical sub slab insulation options would still be of value to the building industry. This is why my house currently under construction will now include a sub-slab lab comparing XPS, EPS, and ROXUL. We will look at wet-up, R value loss, and compression of these insulation's over many years under real world conditions. The slab will include removable panels allowing access to the insulation below. Details for the lab can be viewed at theEnclosure.ca
Original Post
As some of my regular readers know, I tested samples of EPS and XPS in an underground wet environment to see which over time absorbed more moisture.
I described the experiment design in my blog posting of Aug 22, 2013 and describe the start of the experiment in my posting of October 6, 2013.
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| Fig 1: Samples at beginning of experiment. These were buried below aprox 4 ft of dirt in a wet environment subjected to regular/constant ground water. |
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| Table 1: Weight of buried samples at end of 9 months. |
Once I finished my on-site testing of the samples, I then took them all down to Fitsum Tariku, an instructor at BCIT and Director of Building Science Centre of Excellence (to name just some of his many accomplishments and titles). Fitsum offered to have some of his Masters students in the Master of Engineering in Building Science program run some experiments to determine the total moisture take-up potential of both products as well as the thermal resistance once saturated.
Unfortunately they were unable to use my buried samples because they were too damaged (I should have bed them in a thicker layer of sand both below and above to protect the integrity of the samples - however it was still a very revealing test based on my results in table 1 above). Instead they used samples I had submerged in a tub of water and others I had on a shelf during the experiment.
In the following tables, you can see that EPS also does poorly from a R-Value retention point of view when saturated compared to XPS.
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| Table 2: Dry weight of samples measured by BCIT |
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| Table 3: Measured R-Value (using Hot Box) of both dry and wet samples |
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| Table 4: Difference in R-Value between two insulation types both when dry and wet. |
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| Table 5: Loss of thermal resistance when saturated. |
So why is EPS used in many 'green' projects. This stems from the EPS industries claims that it represents a lower Global Warming Potential vs XPS due to its use of Pentane as a blowing agent compared to the traditional HCFC agent used by the XPS industry. But XPS manufacturers like Owens Corning have already replaced their blowing agent with a Zero Ozone Depleting formula.
Finally, one positive recorded result is that both products met or exceeded their published thermal resistance per inch of R4.27 for EPS and R5 for XPS (as shown in table 3 - dry state).
The outcome in our view is pretty clear cut - over the extended period representing the lifespan of a dwelling (50+ Years), the lower initial thermal resistance, and then the significant deteriorating of R value if EPS gets wet and stays wet, far out-way any environmental benefits claimed for EPS. The obvious choice for below slab insulation applications is clearly XPS when all factors are taken into consideration.
Sample Specifications:
XPS - Owens Corning Foamular C-300 (30 psi)
EPS - Plasti-Fab PlastiSpan 30 (30 psi)
