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.
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| Proof the selected site will definitely see ground water. This was the result after a heavy rain approx 3 days prior. |
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| Samples cut and place in bottom of hole (approx 4ft depth |
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| Gravel added to ensure water can flow around samples and to identify location when dug out in spring. |
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| Samples tucked away for the winter. |
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| Samples to be held under water |
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| Extra pieces of foam to push test samples under water |
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| Lid of test chamber strapped on to keep samples submerged. Have also now taped seams to prevent evaporation (not shown in photo). |
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| Samples stored in air tight zip-lock bags. |
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| 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)
