Insulation Formed Concrete - Details

The particular details of Carnation Walling.

R-36 insulation

Carnation Walling provides an 8" thickness of EPS on the outside of the concrete and deliberately no insulation on the inside of the concrete.

The EPS used is Type 2 (50% higher density than regular Type 1 EPS). The polystyrene density is 1.5 lb/ft3 (0.024 g/cm3). The compressive spec is 15 psi which makes it stand up well to abuse on the construction site. Just applying 3/4" stucco is sufficient to avoid dents from most sensible abuse. The use of higher density EPS also increases the R value per inch to about R-4.5 at 40 degrees F, so the house is better insulated. Carnation Walling (as described) provides R-36 insulated walls. You could modify the Carnation Walling implementation to increase or decrease the amount of insulation, but R-36 is about right for the well insulated houses that we should be building.

Note that this is a continuous R-36 thermal break rather than the very inferior batt insulation used in stud wall construction.

Forming concrete

The main principle used to provide a concrete form strong enough to withstand the pressures associated with wet concrete is a vertical 2x4 baton on both sides of the wall and ties that go between the batons through the wall concrete. The ties actually go through the concrete inside 8" (or 12") cut lengths of low cost half inch electrical conduit so they can easily be removed after the concrete has set. The plastic pipe also holds the forming the right distance apart to make the required concrete cavity width. Concrete cavity widths of 12", 8", and 6" are easily accommodated by just using different lengths of plastic conduit pipe.

Forms need to be fairly watertight. Loose forms permit either loss of cement (in thin slurry form), resulting in honeycomb, or loss of water causing sand to streak.

Forming concrete on the outside

The use of two glued together layers of EPS with staggered overlapping joints makes a continuous solid wall of EPS that is both strong to withstand wet concrete pressures as well as being waterproof to stop concrete slurry leaking out.

Regular commodity EPS sheets are used. They are not a special design from a particular manufacturer, so the cost of the EPS is a lot less than buying it in proprietary ICF block form. Type 2 EPS sheets are used because they can withstand being knocked about and Type 2 provides a slightly higher R value per inch.

In my implementation I use 6" thick EPS for the first layer and 2" thick EPS for the second layer. A thinner second layer allows for the option of using higher density EPS if you need to make the wall highly resistant to kicking and denting.

Forming concrete on the inside

The reason of course that half the EPS is on the inside in a traditional ICF system is that it is used to constrain the wet concrete to form the wall. When using traditional ICF, I strip away the interior ICF EPS once the concrete has hardened. This is a lot of work and is also of course very wasteful of EPS. It leaves the wall with only about 2.5" of EPS insulation, so lots of EPS sheets need to be attached to the outside of the wall to get it up to the required insulation value.

With Carnation Walling, on the inside of the wall, two sheets of plywood (one thin and one thick) with staggered overlapping joints (together with 2x4 bracing) withstands the wet concrete pressure and stops concrete slurry leaking out.

Regular construction grade half inch plywood is used on the inside. This is typically known as CDX plywood and in practice 15/32" is often sold as 1/2" plywood. Once the wall concrete has solidified, the wall ties are removed and then the construction grade plywood is removed . The plywood sheets are typically used in their full uncut 8 foot x 4 foot form so can be reused for other concrete wall pours or for building other things. The only need to cut the 8'x4' sheets is to fit around windows and any obstacles or lengths associated with the house you are building. By keeping the plywood as 8'x4' sheets as much as possible it reduces cutting costs and allows it to be better reused after it is stripped from the solidified concrete. Also bigger sheets means less joints for the concrete to try to seep through.

There is also a thin sheet of smooth surface plywood between the concrete and the thick construction grade plywood. Only the thick construction grade plywood is removed after the pour because the thin plywood is designed to stay in place to form a nice wall surface that can be painted.

The plywood (both the thin and the thick) is totally standard and brought from your local supplier of choice, which significantly reduces costs. It is best to choose formaldehyde-free plywood, particularly for the thin plywood as it stays in place inside the house. The use of two layers of plywood with staggered overlapping joints and 2x4 lumber over the joints between 8'x4' sheets means that the plywood does not need to be tongue and groove.

As well as being great from a building science perspective, not having EPS on the inside makes it easy to implement things like elevator shafts and swimming pools without having to strip away the EPS like with ICF systems.

Wall ties

The 8'x4' plywood sheets (with the batons) on the inside and the 8'x4' EPS sheets (with the batons) on the outside attach using concrete wall ties of my own design. An important driver here is keeping everything to a minimal cost. In strategically important places, the ties through the wall are made from 3/8" threaded steel rod plus nuts and washers (total of about $1.50), but the rods are removed after the concrete pour and can be reused.

Keeping the wall square and true

Angled adjustable bracing made from 2x4s is used to ensure the walls are square and perpendicular. The 2x4s are used with the flat face against the plywood.

In addition to the wall bracing, the fact that the concrete ceiling (done with Carnation Flooring) is concrete poured at the same time as the Carnation Walling is very useful for holding the walls vertical and for holding the building square. Without this it would be necessary to use a lot more bracing.

Coping with weather during construction

The outer EPS walls and the EPS Carnation Flooring above are built well before the rebar is added and certainly a long time before the plywood that constrains the concrete on the inside. This is great logistically when building a house because most of the work can be done inside a warm dry enclosure. This is way better for the workers and avoids the plywood and other wood being damaged by rain. Also working inside is much safer than working on ladders up the outside of the building.

Holding the rebar in place

The vertical and horizontal rebar with lots of low cost cable ties is used to hold the wall tie in the right position (both horizontally and vertically). This is useful for getting the plywood on and of course the rebar needs to be securely held in the right locations during the concrete pour.

Because the rebar is added from the inside after the polystyrene wall is built means that there is no problem having large sheets of EPS. In contrast, ICF blocks have to be only 1 foot tall if the horizontal rebar needs to be every 1 foot (which it does in the case of my house in an earthquake zone).

Structural engineering typically calls for the center of rebar above window and door openings to be 2.25" from the edge of the concrete. In the case of 1/2" rebar that means the support needs to be 2" above the concrete edge. Typically the cutting of the opening in the EPS above a window is placed on an integer X number of feet from the slab, but the cavity part of the bucking will extend 1.5" up from that, so the height of the edge of the concrete will be at X' 1.5". That means the rebar support above openings needs to be at X' 3.5". The diameter of the plastic conduit tube is just over 27/32" which can be approximated to 1", so the support height above the center of the wall tie is about 1/2". This means the centers of the wall ties should be at X'3".

Below window openings, structural engineering provides more flexibility in the placement of the rebar. Under window openings it is typical to cut the EPS at a height of X'9". The cavity bucking will mean the concrete edge is at X' 7.5". It's good to have a concrete covering over the rebar of 3.5" below windows. For 1/2" rebar and a support height of 1/2" above the tie center that means the rebar support is best at X' 7.5" - 0.5" - 0.5" - 3.5" = X' 3"

Carnation Walling allows rebar to be placed at X'9" or X'3" above slab height, where X is an integer number of feet up the wall. In most cases it is a wall tie at X'9" that is used to support the rebar. When needed, eg above and below window openings or at the bottom of the wall, a wall tie can be added at an X'3" position to support the rebar.

Carnation Walling actually provides full flexibility in the placement of rebar because the wall ties can be put and used wherever you want, whereas ICF blocks have a fixed plastic web that determines where the rebar can be placed. In the case of an ICF block it is in practice hard to even use the lower web position because the rebar has to be threaded through from the wall end. With Carnation Walling it is easy to use the lower position as the rebar is added from inside the building. If you really wanted to with Carnation Walling you could even put wall ties at whatever height you want, but it is unlikely that you would ever want to deviate from the X'9" and X'3" positions.

Making corners and T pieces

Corners are made on the job site by just gluing the edge of one EPS sheet to the face of the other EPS sheet. It is strengthened by gluing on the second layer of EPS sheet that does the over-lap and under-lap the opposite way round. Even though in practice this provides enough strength, it is typical to also use an L shaped wooden 2-by corner baton to give extra peace of mind.

In the row of 8x4 EPS sheets above it is typical at the corner to reverse the over-lap under-lap pattern as it all adds to the strength and wall squareness.

Concrete seepage at the corner is prevented using foam adhesive and/or foam gap filler. Because the outer EPS is thick it provides a large gluing area. Just do a square cut with a hotwire cutter or serrated calving knife or saw. Put foam adhesive and/or foam gap filler on the cut surface and butt it up to an uncut inside edge of the EPS block on the other wall. You can easily make T pieces as well as make any corner angle you want.

Finishing the wall inside

Even though the thick construction grade plywood is removed after the concrete pour, the thin plywood stays in place. Unlike the thick plywood, the thin plywood used has a nice smooth finish.

The thin plywood will have 1 foot spaced 7/16" holes in it. These are filled with finger dabs of PL-Premium adhesive (which is a great filler compound) and when dry this is sanded to make the whole wall a nice smooth painting surface. The PL-Premium also glues the plywood to the inside of the tie rod plastic conduit tube to further ensure the thin ply remains firmly attached to the concrete.

Finishing the wall on the outside

Typically the outside EPS is stuccoed. There is typically a 5/16" scratch coat, then a 5/16" brown coat, then an 1/8" top coat. That's a total stucco thickness of 3/4".

It is best to apply the stucco within 6 months of putting up the EPS walls so that UV sunlight does not make the EPS surface powdery. The stucco sticks best to a non-powdery EPS surface so you want to avoid the job of having to use a stiff brush or sanding block on the EPS surface to remove the powder.

Fire resistance

Building code requires that any EPS (even fire treated EPS) on the inside of the house envelope is covered with a naturally fire resistant product, eg drywall. There is no requirement for an internal covering when using Carnation Walling because there is no internal EPS used as part of Carnation Walling. On the inside is just concrete with a thin layer of plywood.

Because the only EPS is on the outside of the wall, there is no real need (other than building code) to add fireproofing chemical to the EPS mix even though untreated EPS is flammable. Having said that, it is still some slight benefit to add fire retardant to the EPS on the outside of the wall in case your house is subjected to a forest fire and you want to reduce the amount of exterior damage.

Carnation Walling uses regular commodity EPS sheets and all EPS sheets are treated by the EPS manufacturer with a brominated flame retardant called Hexa-bromo-cyclo-dodecane (HBCD). The HBCD percentage in the EPS is about 0.7%. It is claimed by the EPS industry that emissions in a fire are no more toxic than burning wood, but some people do worry about its long term health effects and it is possible that it may get banned (starting first in Europe). A ban would have a huge negative effect on the traditional ICF industry, but would not effect Carnation Walling. Long term health worries are irrelevant in the case of Carnation Walling given that none of the fire retardant with its associated potential emissions will be inside the house envelope. If HBCD does get banned then it will either just be left out of the Carnation Walling EPS sheets or the EPS sheets will move to another flame retardant such as Polymeric FR as and when it becomes available.