Nexcem/Durisol
This product seems to have superior qualities to ICF (in terms of embodied energy, blow outs, and material stability). I do not see much discussion of it here on GBA. My concrete guy says just pour the foundation and footing walls and forget about ICF (or, I assume, Nexcem, though I haven’t mentioned this specifically). When all you have is a hammer, everything looks like a nail…
One main cause of confusion in terms of learning more about this product seems to be that what has long been called Durisol is now called Nexcem. Who knows why? It reminds me of when Rastra, another interesting block product, changed its name unexpectedly. Nexcem describes their product this way:
Nexcem is a modular, insulated, stay-in-place, concrete forming system. Units are made from recycled waste wood (100% natural clean lumber) that is bonded with standard Portland Cement. The wall forms are simply dry-stacked and filled with concrete and reinforcing steel. Our thermal Wall Forms incorporate mineral-fiber (rockwool) insulation that is non-combustible, moisture resistant and positioned toward the exterior of the wall, resulting in additional energy efficiency that is not possible with other insulating concrete forms.
I am considering the Nexcem block for foundation and footing walls, mainly due to my aversion to external ground contact foam (for environmental and termite reasons). So if I’m considering Roxul anyway, perhaps this is a good choice, as it has integral Roxul (rockwool). I also think that an integrally thermally broken block assembly may be preferable to avoid thermal bridging in a thick (larsen truss) wall, though I have not actually drawn the detail yet…
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Here is the Nexcem webpage: https://nexcembuild.com/
Ethan,
There have been quite a few questions and comments on Durisol here at GBA over the past few years. Here are a few relevant comments I have posted in the past:
1. Sept. 5, 2009: "As far as I know, the Oak Ridge National Lab has not tested any Durisol walls for whole-wall R-value. The manufacturer has used a computer modeling program to estimate the clear-wall R-value of a Durisol Wall. ...The R-value of a Durisol wall will depend on whether insulation inserts are used before the concrete is poured. The Durisol WF30 wall form has a clear-wall R-value of R-9.1. If mineral wool inserts are included, the R-value increases to R-13.8 (with a 1.5-inch insulation insert) up to a maximum of R-20.6 (with a 3 1/2-in. insulation insert). As far as I can determine, however, these estimated R-values have not been verified by an independent third-party laboratory."
2. Sept. 3, 2013: "Your analysis of heat flow through a Durisol wall is incomplete. You haven't made a 2-dimensional analysis, much less a 3-dimensional analysis. You have made a modified one-dimensional analysis, which is of limited use.
"Your analysis would be a pretty good way to estimate the R-value of a Durisol wall if the voids were filled with air. Once the voids are filled with concrete, however, your analytical method becomes incomplete.
"I don't know whether Durisol expects the insulation insert to face the interior or the exterior; but for the purposes of thermal analysis, it doesn't matter much. Let's assume the insulation inserts face the interior.
"There is 1.75 inch of material (wood chips plus additives) between the concrete and the exterior. (I'm ignoring the stucco). That material has an R-value of about R-3. That's not much. That's all there is separating the concrete from the outside world. When the outdoor temperature is cold, the concrete will be fairly cold too -- definitely colder than the indoor temperature (although not, of course, as cold as the exterior air).
"The interior heat will leak through the Durisol block and be absorbed by the cold concrete. The maximum heat flow will occur at the inside corners of the cold concrete. At those points, there is only 4.75 of Durisol material between the cold concrete and the interior. That's something -- and it may be enough to avoid cold stripes on your wall. But analyzing these two-dimensional heat flows is tricky. You need a good software program like THERM to see the isotherms created by this type of heat flow.
"Once you introduce three dimensions into the equation, with details at the bottom of the wall and the top of the wall, the thermal analysis becomes still more complicated.
"Most building scientists find the the best way to determine the thermal performance of this type of wall is to measure heat flow through a completed wall assembly measuring 8 feet by 8 feet or 8 feet by 12 feet. Oak Ridge National Laboratory has a calibrated hot box that is large enough for this type of measurement, but as far as I know, no one has ever performed this testing on a Durisol wall."
Ethan,
Under the "Technical Downloads" link on the Nexcem web site, there is an option labeled "Thermal Performance." Interestingly, it's a dead link.
Elsewhere, under "FAQ," the manufacturer notes,"Our typical units range in R-value, between R-8 and R-28. The Nexcem material has an R-value of 1.75 per inch. This means that all Wall Forms regardless of thickness have a basic R-value of R-8.2. We increase the R-value of the 10″, 12″ and 14″ units by incorporating additional insulation inserts into the Wall Form at the time of manufacture. The inserts are made from mineral fiber insulation (Rockwool) and are also completely fire-resistant and made from recycled content. The inserts are placed inside the Wall Form cavity but positioned towards the exterior to maximize the Thermal Mass effects of the wall system.
"We can incorporate non-standard insulation (ex. poly-isocyanurate) in our units for custom orders to achieve up to R-40.
"The Wall System Summary and the Technical Guide provide the steady-state two dimensional R-values for our wall systems. The Thermal Performance document provides information on thermal mass and dynamic effects."
If you try to click the "Technical Guide" link or the "Thermal Performance" link on the FAQ page, you will find that both are dead links.
As the photo from the Nexcem web site makes clear (see below), this system has thermal bridging.
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Hello Martin, Has your opinion on Nexcem changed at all? We're considering using it for all the walls of our new build. Thank you.
I did speak with someone at Nexcem who explained that the dead links are because they are moving over from the Durisol website... the contact at Nexcem was happy to email me the Technical Guide, which I now have. The document says the following
I have attached a screenshot of the thermal bridging report which references "Recent studies by Oakridge National Labs, ASHRAE 90.1 committee and other independent research agencies," though I can't tell if they studied Durisol/Nexcem specifically.
The Tech Guide also states that "Specific test reports on topics such as thermal resistance" are available upon request. I will request these and see what I can find.
To be clear, I am personally not considering this as a replacement for a full wall system, just as a replacement for stem walls and foundation walls, so I am comparing them to solid concrete in terms of performance/cost. I have not even figured out what the cost differential between a durisol wall and a poured concrete wall is, but I have a price list so I will be doing that shortly.
Martin, you posted that picture of the block to point out the thermal bridging, but my understanding/thought is that the wood fiber/concrete itself has some insulative quality, as in an ICF? This assumption of mine is bolstered by the Tech Guides assertion that the "standard Non-thermal Wall Form unit has an insulation value of R8." In other words, the block itself is giving R-8, and the added rockwool is increasing this somewhat... They are also very clear that the poured concrete in the middle of the blocks is what should be counted on in terms of structural capacity.
Ethan,
Q. "You posted that picture of the block to point out the thermal bridging, but my understanding/thought is that the wood fiber/concrete itself has some insulative quality, as in an ICF?"
A. Most ICFs consist of two continuous layers of EPS rigid foam; the only thermal bridging occurs through plastic connectors joining the two EPS layers (see photo below). I'm not saying that the thermal bridging is zero -- it exists, and can be calculated -- but it is insignificant, because (a) the plastic isn't very conductive, and (b) the cross-section of the plastic pieces is so small.
The Durisol block is different. The thermal bridges are wide. You're right that the material has an R-value of R-1.75 per inch, which is something. But we still end up with a complicated R-value calculation involving two or three dimensions. It's not easy to calculate.