It took us a while to decide which structural system we should use for our new building. In the end, we went with an “Insulated Concrete Form“ which is made out of cement-bonded wood fiber. Wood fiber, instead of polystyrene! The product is called Durisol -it’s a Dutch invention from 1932 and was widely used to rebuild European cities after WWII.
We were delighted to find out that there is a plant that makes it in Hamilton,Ontario.
Here’s is a list of reasons why we went this way and did not use regular polystyrene ICF’s:
* Most common ICF’s are out of Expanded PolyStyrene. While we have a few concerns about the durability of the foam, we just don’t like the fact that with a regular ICF we have 50% petroleum-based foam in the interior of the building.
* The symmetrical build-up of the insulation of regular ICF’s creates a thermal bridge by tying the floor-slabs structurally into the concrete core. Durisol forms have the majority of the insulation on the exterior where it should be.
* The company claims an R value up to 28 for the 14″ thickness with 5″ mineral wool in the cavities which is sufficient. (Conservative PHPP modeling came closer to 25, but that’s still good enough for this Passive House.)
* The interior surface of Durisol doesn’t have to be covered with a layer of Sheetrock–it can be stuccoed with clay, which should help balancing humidity levels.
* The higher weight compared to EPS blocks seem to help to prevent blow-outs during pouring of the concrete.
Our building has a simple envelope of 51′ long by 25′ wide, and it’s only 4 stories or 40′ in height. Normally this shouldn’t not pose any serious construction challenges but we building a Passiv Haus so certain areas need special attention. Here a (not yet) complete list:
* The cellar is outside the treated envelope but we still need to verify if it is enough to insulate the foundation walls or we still need to add a couple of inches of high-density, insulation-like foam glass under the footings.
* The two parts of the forms are connected by wood fibers and while this is great to avoid thermal bridging we still have to find a solution for applying the air-tight layer on the exterior, which is particularly difficult at the side-lot lines where we have existing buildings. So far, we are leaning towards a liquid applied membrane.
* Balconies built by cantilevering floor slabs are massive thermal bridges and so we are looking into a steel frame with wooden decking solution hung from the exterior walls with aircraft cables and stainless steel anchors.
* Roof parapets should not be filled entirely with concrete but foamed out in the last 4-6 inches (another thermal bridge).
After talking to very helpful person at the company we even came up with an estimated price for the exterior wall of $40,000 including delivery to Brooklyn. Here is a link to the Canadian Durisol website which has detailed technical information.
PS: it seems that this simple building requires some creative solutions from our structural consultants (especially the interior stair) so a loud Thank You to them!
Faswall is a Oregon based competitor with a very similar building system.