It should go without saying that any high-performance building should be built on a solid foundation. So why would we set our building on a layer of foam insulation?
The answer, of course, is to limit thermal bridging. Those bridging effects can cause a significant amount of heat loss through the mass structure at the base of the building. By thermally isolating the building foundation from the ground, building performance is improved, not only from an energy performance standpoint but also in terms of comfort and moisture management.
We put rigid foam under the footings
In some high-performance building circles, it has become common to place a layer of insulation under a slab on grade. This is especially the case in colder climates. What’s new with Passivhaus design is the idea of completely isolating the building foundation from the ground, not only under the slab, but also under the footings.
As designers and builders working on the Orchards at Orenco multifamily project in Hillsboro, Oregon, we’re getting our feet wet with Passivhaus design for the first time. Our collective common sense suggested that we should be suspicious of this idea. Nearly all of the building’s structural loads are placed on the footings and it strikes many people as possibly a fool’s errand to place building footings on foam.
However, after extensive research it became clear there is a long history of using certain types of very high density expanded polystyrene (EPS) foam insulation underneath major structural works of all kinds, including roadways, bridges, and runways. Our concerns receded based on the evidence and we were swayed — yet still reserved and cautious. The caution persists to this day and will follow us until this is a well-established building practice, without significant drawbacks.
Four inches of EPS is the right amount
Once the team actually started believing this could work, the next concern became how much insulation to use. Together, we landed on the idea of 4 inches of EPS foam, based on a sense this would provide a good balance of cost and constructability. In particular, we were trying to avoid the thicker levels of insulation that have been used on some Passivhaus buildings.
Throughout the design process, PHPP iterations were run that looked at using more or less foam foundation insulation; yet the team kept coming back to the 4 inch foam layer. We looked at the relationship of the foundation R-value to changes in other envelope parameters such as the wall R-value, the window U-factor, and the roof insulation. After numerous iterations, the 4 inch foam thickness was agreed to by the team.
So, how does this work? The foam is placed under the entire slab on grade and wraps around and underneath the footings at the building perimeter. The 4 inch foam thickness is reduced to 1 inch at bearing wall locations, resulting in a thickened slab with reinforcement, to serve as footings for those walls at the building interior.
Due to the seismic design of the project, there are several large, deep footings that serve as the base for hold-downs to resist high lateral loads on the building. These deep footings were actually poured such that the slab insulation runs continuously over the top of the footing.
Coordinating with subcontractors
As clearing and grubbing began at the site, and then the initial excavation activities, the construction team began a detailed coordination process. To properly construct a high-performance Passivhaus design, diligent, proactive coordination of the work is required of the general contractor. There is no substitute for diligence when it comes to this coordination. Even a highly developed and accurate set of design documents does not include all the information needed to build the project, and inevitably there will be some gaps in documentation or a need to modify a detail slightly, or in a major way, to achieve the design intent while accommodating construction variables such as sequencing of the work, manufacturer’s installation instructions, etc.
Coordination of the work is fundamental to all construction projects, but the need is heightened when executing a Passivhaus design, especially when it comes to the detailing of the airtight and thermal-bridge-free building envelope. For example, at some detail conditions there could be four or more trades that impact the airtightness of the building since they each supply and/or install components that are integral to the air barrier system.
An important duty of the general contractor is to actively communicate with the entire group of subcontractors, to let them know about the Passivhaus and requirements on the project, and to educate them about key issues that may impact their scopes of work and the overall Passivhaus certification. Due to the intricacies involved with material specifications and detailing of the Passivhaus design, communication with the subcontractors that impact the building envelope needs extra attention.
On the Orchards project, a full day Building Envelope Coordination (BEC) meeting was held on site during the first month of construction, to gather together all the envelope-related subcontractors and key suppliers and review project requirements, including specifications, detailing, schedule, sequence of trades, etc.
Scheduling this meeting very early during construction allowed the team to work through any gaps or inconsistencies in the scopes of work of various trades, as well as any issues related to the design documents. Upon completion of the BEC meeting, resolved issues were addressed readily and efficiently through the project submittal process. Issues that needed further examination or design work were addressed through the project Request for Information (RFI) process. The coordination work touched all major elements of the design, including the foundation, exterior walls, windows and doors, and the roof.
Does the StegoWrap vapor barrier go above or below the rigid foam?
An important concern that arose during the coordination process was the location and detailing of the subslab vapor barrier. The vapor barrier was not clearly indicated in the architect’s details, although a vapor barrier had been specified. In the slab on grade assembly drawing, the vapor barrier was indicated to be installed below the slab insulation. The Walsh team questioned this location, given our concern that a large amount of water could collect in the slab insulation layer if it should rain prior to a slab pour. The configuration of insulation and vapor barrier essentially created a sealed “bathtub” that could hold a lot of water. Not a good scenario!
Even though we were in the dry summer months in Portland, there is always a chance of rain. When we pointed this out, the architect understood the concern and agreed with relocating the vapor barrier to the top of the slab insulation. Furthermore, the detailing of the vapor barrier at the foundation perimeter was not clear in the design drawings. We discussed this with the architect and sorted out the termination details as part of the coordination process, working with the vapor barrier manufacturer’s standard details and sealing products. With these details resolved, construction on the building foundation began.
The concrete contractors were hesitant to place concrete footings on rigid foam
To move forward from this point we had to overcome a little hesitation from the concrete crew. They’d never prepped a concrete foundation to go on top of insulation before. This idea raised more than a few eyebrows. After explaining the purpose of the foam layer below the footings and slab, resistance was overcome, if only temporarily.
To construct the footings at the building perimeter, 4 inches of Type IX EPS foam were placed on top of the gravel base, the foam was informally tested to ensure solid contact with the base, formwork was constructed on top of the foam, and then concrete was placed. After initial set and curing, the formwork was stripped and EPS was applied to the vertical face of the footing.
Upon completion of the initial footing work at the building perimeter, preparations for the slab on grade were made. A capillary break layer with 6 inches of clean crushed gravel was placed over the rock working pad and compacted to a dense state. A radon mitigation system was installed on top of the gravel base. The system includes 4-inch perforated flexible piping wrapped in filter sock material and embedded in an additional layer of gravel, 6 inches thick, placed on top of the compacted gravel base. The gravel provides a minimum 1 inch cover over the piping.
Four inches of Type II EPS foam was placed on top of the gravel base. The design documents indicated a single layer of foam; however the crew pushed to use two layers of 2-inch foam for the installation. The crew believed the foam would lay flatter and provide more stability on the gravel base going with the two layers. This also had the advantage of allowing for staggered joints in the boards and eliminate direct heat flow paths that would occur at butt joints in the boards had we used only one layer, as is typical with roof insulation installation. The foam was trimmed to fit tightly around penetrations. Any gaps were filled with expanding spray foam sealant.
For the most part, the slab insulation went down well over the gravel base; however, there were a few issues with getting the foam to lie flat and stable. These issues were resolved by reworking the gravel. The geotechnical engineer called for 2”- ¼” gravel whereas a smaller rock or pea gravel would have helped to eliminate the issues we encountered.
A rugged subslab vapor barrier
A cross-laminated plastic vapor barrier was specified. We installed a StegoWrap membrane, 15 mil thick product. All lap joints and seams were sealed with the tape provided by the manufacturer. Typical pipe and conduit penetrations were sealed with tape also. Where penetrations were ganged too closely together to allow detailing with the tape, mastic was used to seal the vapor barrier to the piping/conduit. This is an accessory product offered by the vapor barrier manufacturer. Where the vapor barrier intersects with footings at the building perimeter, a special butyl tape was used to seal the vapor barrier to the footing. This seal was important to provide air barrier continuity between the slab-on-grade assembly and the exterior wall assembly.
It turned out that we were very fortunate to have moved the vapor barrier to the top side of the foam insulation since it rained heavily for a few days in early August. The rain didn’t create any serious water problem as most of it ran off to the edges of the slab area, and what remained dried off relatively quickly. If the vapor barrier had been placed over the gravel, below the insulation, we could have had a major issue on our hands with water retained in the foam insulation layer.
A 4-inch-thick concrete slab was placed on top of the vapor barrier. We paid close attention to the concrete mix design. We utilized a mix design from CalPortland with a 0.42 water-cement ratio, as was specified. When pouring a concrete slab directly over a plastic vapor barrier, the use of a low water-cement ratio mix design is important to help minimize slab curling and also to minimize the potential for moisture-related problems with floor finishes applied over the slab.
Mike Steffen is a builder, architect, and educator committed to making better buildings. He is vice president and general manager of Walsh Construction Company in Portland, Oregon.