Insulated Slab: Passive House Foundations with Thermally Broken Air Tight Vapor Barrier

Insulated Slabs and Passive House Foundations

One of the more radical ideas (as believed by conventional builders) is the notion of below (concrete) slab insulation in a buildings foundation.  They simply don't understand the significance of the heat loss--through conduction and thermal bridging--and how it adversely impacts the overall heating and cooling requirements of a building.  Heat loss can range anywhere from 15%-30% relative to the entire conditioned space!  This is especially important in northern or southern climates that present a heat-load (requiring more heating throughout the seasons).

The conventional thinking is that for below-grade foundation slabs in basements, the more moderate temperatures of the ground provide more than enough "warmth" in the colder months to not be an issue.  This could not be further from the truth!

insulated, thermally broken, air tight, vapor barrier concrete slab foundation floor for passive house

Since sub-ground level temperatures typically range from the mid to upper 40s F in our winter climate --certainly more moderate than surface or air temperatures--the thermal bridging (temperature short-circuiting) that occurs between the interior air and the ground through an uninsulated concrete slab (which has an R-value of nearly zero), results in a huge heat-sink, continually rubbing heat from the conditioned space (ie; the entire interior of the building!)  The impact of a non-insulated basement is even more significant for slab on grade and raised floor systems above ground (why would anyone design a building with) crawlspaces.

Insulation of the foundation slab is one critical requirement in obtaining a successful passive house design.  Skip this step, and you might as well forget it. In fact, it is appropriate to treat the building envelope as a six-walled system.

Thanks to Jason Morosko of Ultimate Air, we were guided to ACH Foam Technologies--a manufacture of high-density closed-cell EPS foam that withstand high levels of pressure (ie; weight) with minimal compression.

Treating the Concrete Floor Slab as Another Exterior Wall

We desired an insulating R-value of approximately R45.  To achieve this, we required 10 inches of EPS below slab insulation.

ACH provides varying size and foam densities that impact the compression strength (not thermal performance).  We selected a mid-level material which we calculated could withstand the weight of the slab and the additional load of furniture and people, with an additional margin of course.  ACH provides the physical properties of their various materials on their web site.  It is essential to properly size the material.  Exceeding the strength characteristics of the foam, would cause a "pancaking" effect under the pressures of weight above.  When considering the material, be certain to account for the weight of the concrete slab itself into your calculations.

Panel sizes typically come in 4' x 8' or 32SF each.  Custom sizes can also be pre-cut at their facility.  From a pricing perspective, be sure to account for freight costs.  Even though the material is relatively light, the space it actually consumes is significant.  In our case for our 2300SF basement, we required a full-truck load, which added a substantial cost to the use of the product.

We ordered enough material to insulate our attached garage as well, as our desire was to have an enclosed semi-conditioned space that would naturally be more moderate in temperature during the extremes of our climate--relying on thermal gain from our southern facing windows to naturally heat the slab.  We also incorporated an earth air tube, to feed slightly pressurized pre-conditioned air to the garage as a temperature moderating element for both winter and summer seasons that also provides fresh air (more on earth air tubes in a future article).

Our configuration consisted of a 2", 3",  and a 5" thicknesses.  We ripped the material with a table saw to create specific shapes that we needed to fit.  We broke the 10" into three sections for several reasons.  First, it made if easier to work with and place.  Second, we were able to select their higher-end (and higher priced) termite resistant material in the 2" size as the first panel that would come in contact with the ground.  The remaining two panels were placed above the initial panels.

Thermally Broken Floor Slabs and Passive Houses

Thermally breaking the slab from the ground is an essential step, but we didn't stop there.  We also needed to thermally break the sides of the concrete slabs (the perimeter) of the superior walls, their "footers," and the edges of the slab which make contact with the outside (below doors) and with any penetrations, such as earth air tubes, plumbing, and load bearing columns.   The foam that would otherwise be visible in these areas was eventually covered by the frame of the doors, so the concrete slab appeared contiguous. The final result of this configuration was a concrete slab entirely encased in EPS insulating foam.

Floor Slabs with Vapor Barrier and Passive Houses

Keeping in mind that insulation and air-tightness are two entirely independent things, we needed to ensure minimal air leakage through the floor and the wall system and to design a vapor-barrier below the slab.  We assumed that eventually all concrete foundation slabs will form cracks (all potential sources of air-leakage).  For this we chose a 10 mil vapor barrier.  Now there are two schools of thought on proper placement.  One thought is to place the vapor barrier below the foam panels.  The other is to place it above.

In our case, we chose to put it below the panels and then put a less robust lining (6 mil) directly below the slab (that wasn't taped together) prior to pouring.  We did this in the hot month of August.  As a result, we did encounter a little bit of condensation taking place below the panels initially.  It is important to note that in both the vapor barrier and foam, neither provides a medium for mold growth.  However, the next time around, I would choose to place the vapor barrier above the panels and below the slab (on the warmer side) leaving the insulating foam to directly contact the crushed stone below.

 

 Air-tight Floor Slabs and Passive Houses

To ensure air-tightness, we used special tape to join the seams and attach the vapor barrier to the superior walls. As an added bonus, this configuration significantly reduces the potential for radon gas infiltration.  The concrete we used was approximately 5000 psi high-density to reduce moisture migration through the slab itself.

I expect over time that the taped seams below in certain sections will be compromised by static pressure from water that could potentially accumulate in the stone field below during intense rain storms (like we have been experiencing lately).  We would hope this not be that case with an appropriate grade away from the building.

A nice benefit of this overall design, is that the concrete foundation slab is far less prone to cracking as there is essentially no expansion and contraction that takes place as a result of varying ground temperatures and minimal temperature variation within the concrete as it acts as a thermal battery in equilibrium with the interior temperatures of the house.

Be aware, though, that this whole procedure is very highly labor intensive and time consuming!  Attention to detail is crucial, especially with air-tightening/sealing.