SENWiEco considers the Durisol CBWF ICF Block for below grade foundations

Anyone who has read my previous blog entries knows by now that I like to verify and do things for myself.  SO it should come as no surprise that I will perform some testing on another ‘new’ product I am considering for my upcoming build.

As mentioned repeatedly on my blog, my focus on this build is a bullet proof and energy efficient building enclosure to lower my impact to this planet.  I am targeting R10/20/40/60 (Slab/Foundation/Walls/Roof) and these are effective values not nominal (so values after taking into consideration all thermal bridging). 

With these targets identified, it makes sense to optimize what ever insulation is installed by placing in locations less effected by thermal bridging.  This usually means putting most/all of the insulation on the interior of the structure or exterior of the wall or roof assemblies.  Now which side you put the insulation on is very important for preventing condensation.  In a Cold-heating-dominated-climate like Vancouver, you want to keep the sheathing above the dew-point potential so that if any interior air leaks into the wall assembly, it will not condense on the back side of the sheathing which can often cause rot and mould. 

Continuous exterior insulation is a great way to prevent thermal bridging (your insulation is firing on all cylinders) and keeps your sheathing, or in this case your foundation walls, warm and dry.  For this reason, an ICF form system makes a lot of sense.  In typical ICF formed walls, there is an interior panel of insulation attached to an exterior panel of insulation with plastic or metal ties.  The concrete is then placed to fill in the gap down the middle. 

From an insulation point of view, the continuous nature of the ICF panels is great and represents a thermal bridge free design.  Your nominal insulation is the same as your effective insulation R values.  However, you do end up with a thickness of insulation on the interior face of the foundation.  This prevents the concrete from acting as a thermal mass that would otherwise allow it to help moderate interior temperatures.  Insulation inboard of the concrete core can also represent a dew-point potential if the concrete pulls away from the foam as it cures and air leakage results.  Finally as the product is made from oil, it can represent strong off gassing potential and a real fire spread and toxic fumes risk if your drywall is not continuous or is damaged and a fire occurs.

But for me, the biggest demerit, against the foam based ICF’s, is that they are made from foam and therefore oil.  If your goal of creating a low energy house is to reduce your impact on the planet, it hardly makes sense to use a product that is the most responsible for human’s impact on the planet today.  We will be no further ahead if we create a demand for foam ICF on a mass scale, as this will just continue the dependence on a product we really need to start considering leaving in the ground.

Now many of you will say the benefits of foam ICF outweigh the use of an oil derived product.  You are at least locking away a lot of the carbon that would be created if the oil was otherwise used for combustion.  At least in a product like this, it will stay buried for probably 50-100 years (and there may even be a potential of recycling the product at the end).  And any increase in insulation decreases the amount of electricity and gas used in homes to provide heat and air conditioning. To an extent, I agree with this rational.  I do not believe you should abandon products just because they are made of oil.  In many categories, the alternative ‘green’ products are not suitable for use and have durability issues.  The regular replacement of an unsuitable component can represent just as large an embodied energy, as using a more suitable oil derived product.  However when you do have a suitable non-oil based alternative, you should do everything possible to incorporate it into your design. It was with this frame of mind, that I started looking at the Durisol ICF block for my upcoming build.

The benefits of a cement-bonded-wood-fibre (CBWF) block are:

• Made from recycled-wood and cement powder,
• Places all of its insulation outboard of the slab,
• Can be left as a final surface within the basement,
• Can be attached to anywhere in the field of wall (do not need to hunt for hidden plastic tabs to fasten drywall or framing to),
• Incorporates a drainage plane within the product,
• Is mold and rot resistant,
• Is bug and rodent proof, and
• Best of all – does not burn easily or give off noxious fumes if it does. 

Now for its negatives:

• Highly air permeable (a benefit of regular ICF is that the concrete core is an air barrier).  The material of these blocks is highly porous and the block has webs that connect the outer and inner panels together THROUGH the concrete core.
• These webs are not just an air path; they are also possible water and likely a vapour path.
• There is at least a passing concern that the block could rot in a below grade application.

In researching this product, I was unable to find any comments on-line that the product had ever broken down below grade from decay.  The manufacturer provided an Ontario MOT testimonial that stated they had never had to repair the product due to decay (the product is used extensively above ground, and partially submerged, as a road side noise abatement fences) after 30 years of use. 

The product has been manufactured since 1953, so certainly has been on the market a long time.  If there were significant failures, it would be readily visible on the web. 

So what’s the catch? 

Well the product has not had a huge uptake for below grade installations to date.  The manufactures claims they have a dozen or so projects a year on average in Ontario and I have found 2-3 blogs on the net describing the use of the product.

What’s the risk?

Well, unlike traditional ICF, the interconnecting webs of each block penetrate through the concrete core.  This provides a path for air, vapour, and possibly moisture travel. 

The air barrier is fairly easy to address with a fully adhered membrane outboard of the block (this still leaves some interesting details at the footing level and will probably require some thinking out of the box to seal on the interior face near the basement floor slab - more on this in the future). An airtight drywall approach (ADA) could also be implemented.

The vapour barrier again will be generally dealt with by the fully adhered exterior membrane. Besides, even regular formed concrete foundations have a considerable moisture movement through them from out to in, which is why you should never have a vapour barrier (just a retarder) beneath the drywall in the below grade basement (NO POLY! EVER!!!).

Now for the real issue: What is the danger of liquid water transport through the webs to the interior face of the block, either under hydraulic pressure or capillary action? 

Durisol partnered with the University of Toronto to study the drainage properties of the CBWF ICF block back in the mid to late 90’s.  This UOFT report confirmed the manufacturer’s claims that the product did not support horizontal capillary movement and that liquid moisture drained readily through the material.  In fact the free draining rate of the product was a whopping .5 gpm through a piece of material that was 3.5” Thick, 8” wide and 11.3’ (yes ft) tall.  In another test, where a sample was fully saturated and then allowed to air dry, the retained moisture after 60 minutes was only 38% and the sample had lost a majority of its moisture after just ten minutes.

So far, this all looks great!

Figure 1: Durisol 12” Thermal Block in the R21 configuration (5.5” Concrete Core)

Figure 2: (LEFT) Semi-rigid mineral wool insulation insert on the outboard side of the block. (RIGHT) Product is created with a mixture of recycled-clean-mineralized-wood-fibre and cement powder.

 

 

 

But unfortunately, I rarely accept others reported results at face value.  I wanted to put the product through a more rigorous testing protocol (in my opinion).  So enter the DBTTC or Durisol Block Torture Testing Chamber!

Figure 3: (Left) 28”l x 19”w x 15”t tub with a selection of Cactus Club takeout containers as standoffs in the bottom.  These support the block without being susceptible to rising damp.  They also isolate the outer and inner webs so that water flowing down the outer web does not flow along the support to the inner web. (Middle) A small water pump is rated for 70 GPH @ 0” head so probably around 40-50 GPH in my configuration. (Right) The containers are 2.5” tall and the water level was set at the halfway point so there is approximately ¾” gap from the top of the water to the bottom of the block (should allow for enough air movement around the bottom of the block so that the humidity does not build up too high and skew the results.

Figure 4: The inboard side of the block surface registered at 11.1% MC.  The block has been sitting in my living room for about a week (which by the way, translates to an indoor air relative humidly of 55% which is what I have the bathroom fan humidistat set to).

Figure 5: I also took the MC of the webs from within the inside journals that the concrete would be placed in.  This will be an easier location to monitor.  I had a reading of 10.4 and 9.3% MC (difference was probably due to a variation in density of the product at the tested location due to the random makeup of the wood fibre).

Figure 6: The Test

Over the next week or so, I will leave the pump on 24/7.  Water flows out of the plastic tube that has been drilled with holes at a regular interval.  The water is draining through the outer panel as fast as it is added at the top where it drips back into the tub and repeats the cycle.

Over time, I will measure the moisture content of the inboard panel exterior face (and the side faces of the internal webs).  The go/no go test will be to see if a piece of paper stapled to the inboard face of the block shows any signs of moisture over time.

I will of course post the results once they have been tabulated.  Here is a video showing the start of the test and another video showing 60 hours into the test.  At this point, there has been no horizontal travel of liquid towards the interior panel, which confirms the testing performed by the University of Toronto.