Integrally Insulating Concrete

In building details up to the present time, there often were gaps in the insulation due to the needs of structure.  One common challenge was the bottom of a frame wall meeting a slab on grade.  For structure, the slab wants to reach out and touch the foundation.  Unfortunately this would result in a 4 inch or more band all around the building that had no insulation.  There are ways to jog the top of the foundation and receive the slab with at least a one inch layer of foam between the slab and the wall.  This works out pretty well but it is something to follow during construction because it is not what many of the foundation and construction companies are used to doing.

Another common challenge is a metal building.  The metal panels don’t do well at grade so the common thing is to have a 4 foot or so high band of concrete wall above slab and grade and then start the metal panel and insulation system.  What was common was to not insulate this 4 foot high foundation wall at all so as to keep the wall durable from abuse of vehicles inside and out.  This was a pretty big insulation hole.

There are a couple of systems out there that proactively and simply solve this challenge of keeping structure and insulation sound.  They are both also good means of achieving “continuous insulation,” a new good focus in building insulation.  Continuous insulation recognizes the need for continuity.  Think of dressing yourself for a cold day.  Do you provide yourself with a continuous layer of protection or do you leave bands exposed?  The more well known of these two options is insulated concrete forms or ICF.  With ICF, giant, hollow, snap together, foam blocks are stacked up and then filled with concrete and reinforcing steel bars, rebar.  By having the concrete inside the foam, we wind up with at least one good layer of foam on the outside for thermal protection and sometimes we can keep both layers of foam at the intersections with floors and roofs.

ICF foam blocks come with a plastic frame that holds the foam in place, provides places to snap in rebar, and provides strips along the outside faces for screwing on sheetrock on the inside and siding on the outside.  Electrical and communication lines can be run in the foam and there are shallow electrical and communications boxes designed to set into the foam.

Carriage Hill, the assisted care facility in Madbury, New Hampshire that I designed and recently opened, used ICF.  The owner/builder was familiar with the Nudura ICF brand and I liked the product line too.  They also used the Nudura ceiling insulation system which is two layers of foam on the bottom of the ceiling structure.  By doing this extensive continuous insulation, the 14,000 sf facility was able to install a heating and air conditioning system that was approaching 1/3 the size of the original design with a more standard building.  They also were able to significantly reduce the type and volume of ventilation because of the low tonnage required.

The things to watch with ICF is making sure the blocks are properly braced, the right mix of concrete is used, the right rebar size and spacing is used, and treatment of the outside foam near the grade.  I like covering the foam there with some stone by using a flared out block below to support the stone.  The lower brow way to go is to cover the foam at grade with a parging .  I recommend if you are going to try that, to go with some genuine cement plaster and do a pretty good thickness of it.  I have seen lots of cases of a thin layer of something over foam at grade where the thin layer of something gets destroyed by frost heaving.  The other thing to be aware of with ICF is to be sure the interior of it is covered with 1/2 inch of sheetrock or 1/2 inch of plywood or some other 15 minute thermal barrier to meet fire safety regulations.  In a basement you might want to consider using Dense Armor instead of Sheetrock in the event of unanticipated moisture or water.

The other system is an inside out ICF approach and the only vendor I know that does this is Thermomass.  With Thermomass, standard concrete forms are used and a layer of foam is stuck in the middle of the forms.  This foam is fitted with a “bed of nails” array of polypropylene rebars that stick through the foam and out each side tying the inside layer of concrete to the outside layer of concrete.  This is a really good option for the metal building challenge since the four foot height of foundation wall can have continuous insulation along with exposed concrete on inside and outside ready to take abusive abuse while the continuous insulating foam is well protected in the midst of the concrete wall.  I have also used Thermomass on a house for the basement foundation and I like the end result.  No stone covering is needed outside at grade and the basement wall of exposed concrete is great for residential basements as it endures any humidity, incidental groundwater, shelving brackets, and abuse.  There we used 4 inches of foam with 4 inches of concrete inside and out for an overall 12 inch wide form.

Here are some things to watch with Thermomass.  The concrete mix and rebar are a bit different since the width of concrete is usually just 4 inches on the inside of the foam and 4 inches on the outside of the foam.  Foundation companies are drawn to Thermomass since they are needed because Thermomass requires the forms that they own.  With ICF, other trades can do the foundation.  Also, although Thermomass has been around for 20 years, a lot of foundation companies are not familiar with it, and there is a learning curve.  Some foundation companies complain while some both love and promote Thermomass.

With both Thermomass and ICF there is a not totally tangible benefit that having a concrete wall brings.  The thermal mass of the concrete somehow slows the rate of change of temperature from one side of the wall to the other far beyond the capacities of just the layer(s) of foam for both these systems.  Another thing for both systems is that the lowest concrete slab should have a continuous layer of foam beneath it that touches a vertical foam of the system to provide “continuous insulation” around the corner of wall to floor connection.  Having a continuous layer of foam is another thing a bit new for the trades people to get used to.  Try for similar direct connection of roof insulation to wall insulation.