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 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.

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) 

Double Vapour Retarders are NOT Fine by Me!

Far too often, a poster on LinkedIn makes comments that defy good building science.  As I am often busy, I try to bite my tongue and just move on, but often the posts push too many of my buttons and I find myself in the position where I MUST comment or commit hari kari!

A resent post titled "Double Vapor Retarders are Fine by Me!" was one such post that pushed me to reply:

I find it interesting that often when I see what (in my view) is bad building science, the individual in the conversation is almost always in the business of selling or pushing foam. The facts are that Physics has not changed – EVER- and the rules are not being ‘smashed’, only disregarded – often to the building owners peril.

Re OP - I believe this is propagating bad language and bad science and (wish) to review my view of the basics.

VB or vapour control layers are to address vapour movement by diffusion. The requirement for the ‘tightness’ of this control layer is based on the vapour gradient across the assembly and location is based on the direction of the pressure. The VB should always be on the high pressure side, so generally inside in heating climates, outside in most cooling climates, and carefully designed and managed in the rest of the climates. Diffusion is a very small vapour driver, and while it cannot be ignored, having a pretty good VB is more than adequate. You do not need to sweat the details on a VB and do not need to worry about holes and untapped seams. A 90% effective or even 80% effective VB is going to be just fine.

An AB on the other hand is critical, and even small holes in AB’s can move large amounts of moisture by means of convection. The AB can pretty much be anywhere in the assembly and many (myself included) like to detail this layer on the exterior of the sheathing, where the number of penetrations are reduced and much easier to detail. When on the exterior, the one issue to address from a thermal performance point of view, is to reduce convection currents within the stud bays by use of a denser insulation.

Can you build a wall assembly with two VB’s? Yes in theory IF you have a perfect AB always. But in practice this is foolish, in my view, and will come back to bite you a significant number of times, because AB’s are almost never perfect in the field and or do not stay so for long even if they start out perfect. So conventional wisdom, based on decades of experience by many, pretty much always recommend that the assembly is generally vapour open from the VB control layer towards the low pressure side of the assembly. Careful design using modeling may show that you get away with a barrier on one side high side and a retarder on the low pressure side, but (in) my opinion, this is only going to consistently work in dryer regions where the load is low anyway. 

The reason a freezer works is because the two metal shells on each side of the foam are perfectly sealed as air barriers and the manufactures go to great lengths to ensure this during assembly with both sealants and gaskets.

The OP mentions “any mistakes are easily corrected by the air leaks”. Really? Are you promoting an ineffective AB? Are you promoting air movement that can move HUGE volumes of moisture into and through a wall??

The only time a dew-point does not exist, is when the temp and humidity of the air on both sides of an assembly never reach each other’s dew-point. Obviously, this is extremely rare. You can however negate a dew-point in various fashions. A) You can remove all of the air in and through an assembly so that there is no moisture to condense. (easy to do in a manufactured fridge – hard in a site built structure) B) You can design your assembly so that your thermal control layer is mostly/all on the low pressure side of the VB and WRB control layers (so that the condensing surface never reaches the dew-point temperature).

The spray foam crowd claim that filling stud bays with foam meets strategy 1. But I have yet to see a spray foam stud cavity where the foam has not pulled away to some degree from the studs (ore probably has been compressed away from the studs as the studs expand and contract into a material with no elasticity). SO, in these circumstances, the ‘air tightness’ has been lost and you better have a Plan B.


In my Linked-IN Post I also commented:

• Both OSB and Plywood are a Vapour retader in dry conditions, but only plywood opens up to over 6 perms in a wet-cup environment, that for instance all sheathing sees in my region. 
• My vapour diffusion holes question was a bit of a trap. Only way they work is when you have air movement through them, which is obviously something you do not want happening. I did a lot or research before posting on this subject http://goo.gl/0SAiSJ

Of course, I should have also added a third method to my post for negating a dew-point, and that is to ensure that all materials down stream of the high pressure side and VB control layer are vapour open enough to allow drying to the low vapour pressure side. 

It really frustrates me, that in order to sell foam (spray or rigid) to the building industry, manufacturers, vendors, and installers all try to twist the science to meet their objectives.  While this will help them sell their product or service, it will often leave the building owners with assemblies containing much higher risk for condensation, rot, and mould. 

I feel it is up to the rest of us to cry foul when we come across these inaccuracies and try to provide some protection to potential victims of bad science.

XPS vs. EPS - Which holds up better in a below grade environment?

To reduce thermal bridging through your slab on ground, it is generally best practice to insulate below your slab.  This becomes even more important when you have hydronic heating pipes running through the concrete slab, as they increases the temperature differential between the slab and soil below increase the rate of heat flow out of the dwelling down into the ground (how much insulation to place below your slab is also under debate and will be discussed on future postings).

EPS rigid foam insulation is commonly used below slabs due to its relatively low cost compared to XPS, however, many in the building science community recommend XPS for its water resistance properties and ability to retain R Value.  Based on ASTM C272 tests, XPS has generally exhibited more resistance to moisture abortion when compared to EPS, but the EPS industry feels the ASTM tests are too short (24 hours/48 hours/and 30 days) and that EPS is actually better are resisting water take-up over XPS long term.

The EPS industry bases their claims on a singe case study performed by a EPS foam company (ACH Foam Case Study) that showed that after 15 years the EPS foam was dryer and retained more of its thermal resistance than XPS in the same environment.  The problem with this case study is that it was performed by a party with invested interest in the outcome, and as a result has very little credibility within the building envelope community.

SENWiEco will try to provide conclusive non-biased results as to whether EPS or XPS is the better choice for below grade installations based only on water absorption and thermal resistance properties (we will not discuss cost or embodied energy of the products). We will also included foam glass in the testing, as it is starting to receive attention on high performance homes with a desire to reduce embodied energy of the insulation products.

To this end, we today started a test that involves monitoring samples of each material in the following conditions:

• Buried below grade in a location that will see regular ground water
• Submerged in a water bath
• Sealed in an airtight zip-lock bag
• Stored in indoor conditioned space on a shelf.

At the end of approx 6 months, the samples will be re weighted to determine the volume of moisture absorbed and also sent to BCIT where they will be tested (ASTM C518 conductivity test) to determine their thermal resistance properties after aging the samples in the various conditions listed above.

Proof the selected site will definitely see ground water.  This was the result after a heavy rain approx 3 days prior.

Samples cut and place in bottom of hole (approx 4ft depth

Gravel added to ensure water can flow around samples and to identify location when dug out in spring.

Samples tucked away for the winter.

Samples to be held under water

Extra pieces of foam to push test samples under water

Lid of test chamber strapped on to keep samples submerged.  Have also now taped seams to prevent evaporation (not shown in photo).

Samples stored in air tight zip-lock bags.

Samples stored on office shelf

We would like to express our gratitude to the following sponsors of this testing:

PlastiFab (EPS)
Pittsburgh Corning (Foam Glass)
Home Depot (Discounted XPS)