Sunday 31 August 2014

Sub Slab Insulation - EPS vs XPS

Update November 2017

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

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.

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.
 I dug up the samples March 25, 2014 and the results do not look good for EPS.

Table 1: Weight of buried samples at end of 9 months.
As you can see in table 1, over the same period of time and in the same conditions, EPS absorbed an average of 258% of its original mass in additional water compared to only 31% for XPS.

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.

Table 2: Dry weight of samples measured by BCIT
Table 3: Measured R-Value (using Hot Box) of both dry and wet samples
Table 4: Difference in R-Value between two insulation types both when dry and wet.
Table 5: Loss of thermal resistance when saturated.

The last graphic tells it all - EPS looses 15.7% of its thermal resistance when in a wet environment and saturated compared to only 3% for XPS.

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) 


Charlie said...

for a rebuttal to this--in particular pointing out the error in confusing global warming and ozone depletion, and the fact that the drop in R-value when wet is not significant compared the cost difference.

SENWiEco Designs said...

Charlie - you need to do some more research. First, Owens Corning is producing Zero Ozone Depleting XPS (the link is in my post) and is not using HCFCs. Second, a drop of 16% R value for saturate EPS is significant and could mean the difference between comfortable floors and cold floors (or even worse, floors that condense).

You get what you pay for. If you are going to cheap out and use EPS, then you are going to offset that savings with a much deeper granular layer, a deeper sump, if needing a pump up, now need a bigger pump using more electricity, etc.

Adam F-K said...

Thanks for sharing your findings. I really appreciate all the hard work and effort you have put into it.

I look forward to finding out about your test results with samples installed against your house as this will be much more relevant for my plans.

Adam :)

SENWiEco Designs said...

Thanks for the comment Adam. I appreciate you visiting my site. Since this above discussion, I have evolved my overall opinion of sub-slab and exterior foundation insulation. The challenges from Charlie below made me do a lot more research and at first that research led me to a conclusion that EPS WAS a better choice from an environmental aspect. The GWP of XPS was just too high even with the new formulation. SO you were better off increasing EPS to a thicker depth to account for resistance losses when it wetted up.

But then ROXUL released its products for use below slabs and my path was clear. In my opinion, Roxul makes the most sense in most insulation applications if performance, cost, AND environmental impact is important to you.

I now plan to install a sub-slab lab to test the wet-up/compression/thermal performance over time of EPS/XPS/Roxul. There will be instrumentation, opening portals to physically measure thicknesses, water level gauges, and removable concrete slabs that will allow the removal and testing of various insulation samples.

Logan said...


Have you published your results from this test anywhere?

SENWiEco Designs said...

Hi Logan,

I am still building the house, so I have not started the sub-slab laboratory yet. The only results I have to date is what is published above.


John The Energy Guy said...

I give you credit for trying to figure out what can work best for you. The chemistry of EPS and XPS is fascinating and you're right XPS does wet up slower than EPS, but you are forgetting that XPS has a hard time breathing out the moisture it absorbs. This proves out in long-term testing. The other element is R-value loss. The chemistry of understanding the blowing agent is really key and why some of your numbers and testing is flawed. When applied in an underground application, the XPS blowing agent (now an HFC -still considered a "harmful" chemical) escapes out and is replaced overtime by air and moisture. This is proved out in LTTR consideration and long-term R-Value testing. EPS on the other hand is filled with regular air. EPS takes on moisture and releases it. So depending on the backfills moisture % when dug up can make a huge difference. So to put this in real world relevance - when EPS and XPS are buried long term for insulating" foundations", typically stone is used for back fill. Moisture cycles in and out and XPS takes in the moisture but has a hard time getting it out. EPS takes it in and breaths it out very quickly. The is why all the scientific field studies show EPS maintaining most of its R-Value and XPS losing 30-50% depending on when it is exhumed. It is interest to note that the Germans have figured this out a long time ago and we are just figuring it out now.

SENWiEco Designs said...

Hi John - Thanks for your follow up.

If you review my November 2017 update at the top of this article, you will find I agree that EPS represents a better environmental option over XPS. You will also see I agree that a granular drainage plane is needed for both options.

The test accurately portrays the behaviour, over a 5.5 month period, in a below grade wet environment. The insulation would have seen regular wetting every time it rained and the water table rose. In this torture test, granted during a very short time period, the XPS performed much better at retaining its R-Value. But we expect that these results would start to degrade over time.

The truth is that both products wet up over time in wet environments. Many point out the EPS dries out faster, BUT that is only IF the wetting cycles are infrequent enough to allow drying. So realistically neither are suitable for wet environments which is why in below grade horizontal OR VERTICAL applications, you should ALWAYS include a granular drainage plane. This is NOT "typically" included for foundation walls as you state above (except on commercial or multi residential projects with professional over-site). In residential builds, the native soil is typically back-filled against the rigid insulation panels.

If you do provide a granular drainage plane, the next issue is how fast these products absorb moisture over time due to adjacent humidity levels (100% RH in the granular drainage plane). My sub-slab insulation lab will hopefully try to answer these questions.

Robert N. said...

I am of a similar mind that mineral wool is the better insulation material to use at this time, however, I recently encountered a GPS (Graphite Polystyrene) product which makes claims to be better than both XPS and EPS in virtually all areas. I am wondering if you have encountered it and if you will be adding it to your evaluation/tests and if you could comment on it to give us your take on it. Thanks for the hard work. Here are a few of the sources I have found...

I haven't found a comparison with mineral wool yet so if you know of one can you share it?


Unknown said...

Hi Robert - Thanks for your question. Sorry to say I am not familiar with the product. I generally stay away from the new and shiny products instead going with items with a long track record of reliability like mineral wool. I did test some foam glass insulation, and it did not even make it through the test. I did have a very fast look at the websites you show and would take a lot of their claims with a grain of salt. For instance the insulfoam site states that EPS has a much lower water absorption rate over XPS. Nothing could be further from the truth. EPS wets up VERY fast compared to XPS. But it also dries faster than XPS, so in some circumstances may stay drier on a long term basis vs XPS. This is why I have setup the sub slab insulation lab to monitor the wet-up cycles of XPS, EPS, and mineral wool over time and report the findings. This just caught my eye on their site, I really did not spend a lot of time looking at the rest of the data. Cheers

Matt said...

"I did test some foam glass insulation, and it did not even make it through the test"
Can you please elaborate, I'm considering Glavel for my new full ICF Passivehaus home as well as insulated pool. It's not cheap and few distributors here on the West Coast. Are you still a fan of Roxul/Comfortboard for subgrade insulation?

SENWiEco Designs said...

Hi Matt - thanks for posting. What I mean is that the foam glass insulation was broken to bits when dug up and saturated. It did not survive being buried. It is VERY brittle and absorbs water. Yes I am absolutely still recommending a mineral wool insulation like ROCKWOOL's Comfortboard for both below and above grade exterior insulation. When below grade make sure it is protected by a dimpled drainage board so that there is no hydraulic pressure on it or the foundation. It also works well for below slab bit make sure you get the 110 on not the 80 product as you need to additional stiffness to walk on when pouring the slab. Also make sure a good gravel drainage below slab. Save your money on the Glavel and just install recycled washed crushed concrete as your gravel layer below the slab (not approved for below footings as cannot be certified to meet required engineering consolidation specs - here you will need to use crush rock). I should also say that if I were starting again, I would skip the ICF. Not worth the money and effort to install. Easier, Faster, Cheaper to do a site formed foundation and apply a proper torch on membrane on exterior and then 3" of semi rigid mineral wool insulation followed by dimple sheet/geotextile. This also allows you to ensure the rebar gets in the right location, something that is almost impossible in ICF foundations.

Matt said...

Thanks, this is very helpful, I will likely go this route after a quick review with Roxul and my builder. I'm going with Fox Blocks ICf, my builder has built over 300 ICF homes. An alternative to rebar is Helix metal fiber additive to the concrete, it's been in use a while now and some interesting presentations available online from the founder. The popular YouTube channel "Handeeman" built his ICF home in AZ using Helix.

SENWiEco Designs said...

Matt - very sorry, your follow up post got caught in my spam filter.

I am aware of the HELIX fibre. It is a LOT more expensive than rebar, and on my foundation (due to the depth of my foundation wall - 12.5ft) did not have the strength to replace all my rebar. I would still have to install the horizontal rebar, making the cost even higher.

I am not saying ICF is difficult, just not worth the added cost and effort THROUGHOUT the build in my view.