Why build a frost protected shallow foundation using prefabricated foam forms ?

Building a frost protected shallow foundation (FPSF) is a well-proven alternative to building deeper, more-costly foundations in cold regions with seasonal ground freezing and the potential for frost heave. Traditional foundations for cold climates always extended down under the frost line onto a strip foundation, they can use lots of CMU block or concrete, take lots of time and excavation, and can also be a source of humidity and mold in the crawlspace or basement because they are built underground of porous materials.  More and more construction professionals and local States and Provinces in cold climates with a frost line and ground heave in winter have started to realize that the simplest way to build a solid and dependable foundation in these harsh conditions is to build using frost protected shallow foundation (FPSF) forms - which are generally made from high density EPS foam or styrofoam.

These foundation forms give many advantages in terms of convenience, longevity, reduced humidity, radon protection and increased insulation levels - leading to a comfortable home with excellent energy efficiency and an efficient use of carbon intensive materials - like concrete - in the heat-stabilizing thermal mass of the engineered foundation slab. Frost protected shallow foundation (FPSF) forms are in our opinion a great way to build a dependable foundation, and we're going to show how easy installing frost protected shallow foundation forms is here.

How to build frost protected shallow foundations - why we chose this system

EcoHome built our demonstration prefabricated low-energy kit home, the s1600, by partnering with a specialist company that manufactures frost-protected shallow foundation forms (slab-on-grade) that are heated with air rather than liquid. This has caused much controversy and discussion ever since, with us often hearing how "air doesn't transfer heat the same as water" and how air doesn't hold as much heat, so can't work as well" - with the people saying this taking air and water in isolation, and forgetting that they are both sat in a big thermal battery - which changes everything. In our experience, the concrete slab in a super-insulated heated frost protected shallow foundation building provides a gentle and even release of comfortable heat throughout the home.

What we've found is there are many advantages to air-heated radiant floors over hydronic radiant floors, but none greater than the ability to inject solar-heated air into the thermal battery which is the concrete slab floor. 

Building a frost protected shallow foundation (FPSF) with radiant heat
Building a frost protected shallow foundation (FPSF) with Passive Solar air-heated radiant floor slab  © Ecohome

Solar air heating panels are a growing trend in green building online, with one notable shortfall – the heat that is generated is difficult to store or control. Many people take advantage of the relatively simple and affordable nature of air-heated solar panels (compared to photovoltaic panels) to harness the energy of the sun to heat garages and workshops, but solar radiant air-heating has yet to be a viable and reliable option for well-balanced home heating. Until now ….

When looking for a heat-storage solution that would allow us to incorporate solar-air heating panels into our design in an "engineered" and reliable way, we approached Legalett, experienced engineers and manufacturers of slab-on-grade frost protected shallow foundation form kits - including for problem soils and air-heated radiant floors, to see if high efficiency solar air panels could feed air directly into their existing in-floor duct work. They welcomed the challenge and engineered a system with the aim of being the main heat source of our new (as close as we can get it to ) Zero Energy home. Needless to say that we will also incorporated high efficiency cellulose insulation and air tight building envelope similar to a Passive House design in this beautiful prefab home kit which was a model from Ecohabitation, our non-profit sister in Quebec. 

Building Legalett's frost protected shallow foundation forms with radiant floor heating consists of:

  1. Set a benchmark level on the build site to establish finished slab elevation. If excavation level needs to be raised, refer to the manufacturers drawing for instructions on the use of compacted fill. NOTE: The final landscaping of ground surface at slab shall slope minimum 5% away from the slab.
  2. Flatten the site and strip the topsoil carefully, putting the soil and any small native plants to one side for reuse.
  3. Place 3/4” or 3/8” clear stone (min. 4” thick) and ensure gravity drainage to daylight. Level to within 1/2” of desired elevation using a vibrating plate compactor and calibrated rotating laser level. For an easy build process for your frost protected shallow foundation forms, getting the stone base level is a critical step to get right.
  4. Set up batter boards to 1½” to 3” (depending on building length) above desired finished FPSF slab elevation and lay out the building perimeter from plans. String lines should NOT be used to set the elevation of components, do this using an experienced laser operator.
  5. Carefully check and double check the diagonals of the batter boards for square on the corners and across the full diagonals of the slab foundation. Your frost protected shallow foundation forms are cut square and level, so it stands to reason that the prepared base needs to be set up as close to perfectly square and level as possible or this will cause problems when building up the forms to be ready for the concrete pour. We can't stress this enough, triple check that the frost protected shallow foundation for your build is in the correct place on the site as per the plans, and then check it is set level and square before all that concrete is poured, cures and it's too late.
  6. Locate, mark, excavate and place plumbing risers and all other underslab services (ie. water, gas, electrical, etc.) If installing the air-heated slab system, also fit the extra sheet of insulation under the 2” 4000 series heater boxes as per manufacturers drawing.
  7. Re-level the frost protected shallow foundation forms after the plumbing and services installation to within ¼” of desired elevation under the edge forming elements and within ½” of desired elevation in the FPSF slab centre. Compaction is not required.
  8. Now it's time to start placing and installing the pre-cut foam frost protected shallow foundation forms. (NOTE: We recommend a calm and wind free day for handling the foam forms!) Like a giant jigsaw, begin with installing the edge forms. Tie the top of edge elements together using the metal caps and check elevation of edge forms regularly using a laser level. Lap the metal cap joint at corners and joints and secure with sheet metal screws securely. If you'd rather skip to the timelapse video showing how to build frost protected shallow foundation forms, click here
  9. Tie the bottom of the precut foam foundation form elements together by simultaneously installing two courses of expanded high density polystyrene (EPS) insulation board, tied together and to elements with 6” nails to keep edge elements in place and square. Backfill gravel against outside of edge element as adjacent foam layers are secured. Pay special attention to corners, making sure they stay square and ensure that corners are well supported by gravel backfill. Back-rake gravel under each sheet just before foam placement. Alternately, follow foam layout on drawing to ensure minimum waste, ie. using the cut-off piece from the previous row to start the next row of foam. For either method, ensure that vertical foam joints do not line up through 2 layers of foam.
  10. Install any in-slab water lines in a groove in the EPS layer and foam the groove after line installation, or use conduits or sleeves above the EPS foam. Under no circumstances should ANY water lines or waste lines be placed directly in the concrete without conduits or sleeves. We recommend that plumbing risers have wrap of rockwool insulation and builder tape wherever they may be encapsulated in the concrete slab. This allows for some setting or movement, and flexibility in siting plumbing fittings.
  11. If installing the in-slab air radiant heating, cut out foam layer(s) and install each furnace box as per drawings and Product Data Sheets.
  12. Time to install the first layer of rebar in your frost protected shallow foundation forms. Install bottom layer of wire mesh on chairs at perimeter and under bearing walls as per drawings. Use one chair per every 2’ at mesh edges. Secure the mesh sheet edges together with rebar ties.
  13. As our FPSF in-slab heating was a custom order to incorporate the solar air heated panel, we will be skipping over this part of the installation that can be seen in the video below. A closed-loop network of 4-inch tubes was installed running through the concrete with a heater box embedded directly in the floor. In this case, added to that system are 6-inch tubes (also seen in video) that feed air to air solar panels on the south wall of the house. Air heated by those solar panels in winter, when the sun's angle is lower, will be fed into the floor heating radiant system, which will reduce the energy demand on the heater coil. Conversely and cleverly, in summer if the heat isn't needed in the house, the heat is diverted to an air/water heat exchanger for heating the hot water tank instead. What this adds up to is Energy savings; ideal for Passive Solar Homes, ZNE, PNE, LEED & Passive House projects. Over the last few years, with everything that's been going on, production and installation of a satisfactory solar heat exchanger has stalled, but once we revive it - we will be the first to let everyone know! 
  14. Almost the final step before concrete pouring in the frost protected shallow foundation forms. Install the top layer wire mesh and other rebar as per drawings. Tie top and bottom mesh layers together around pipe at perimeter with supplied tie wraps and tie interior mesh and rebar together with supplied wire ties.
  15. Install frost skirting if needed and backfill along perimeter. See product datasheet - Excavation and Skirting
  16. Complete inspection (Trained Legalett Installer / Agent only) or call for inspection by LEGALETT personnel. Fill out and submit Inspection Report complete with photographs.
  17. After approval of inspection report by LEGALETT personnel and receipt of authorization code to pour, place concrete in the frost protected shallow foundation forms using pencil vibrator for good consolidation. Install ICF dowels/anchor bolts and confirm their locations with framing contractor and ensure bolts are offset from studs and have a minimum of 3” embedment in concrete.
  18. Steel trowel surface using power trowels.
  19. Keep top of slab moist for three days to minimize shrinkage and cracking. (Concrete chemically sets, then subsequently dries, but if it dries too soon, then it doesn't set properly and strength is compromized.)
  20. A LEGALETT construction heater can be used the day after the concrete is placed for cold weather pours only. In all cases, the construction heater MUST be run for a minimum of 2 weeks after the building is closed in (and insulated if during the heating season) and before permanent heating insert(s) and floor coverings are installed. To obtain effective drying, the entire slab should be heated
    to 27°- 30°C (80°-85°F). Overall reduction in moisture content of the slab is important to prepare the surface for floor covering adhesives and prevent moisture damage to the permanent insert(s). For further information contact LEGALETT and / or refer to the following datasheets: i) Construction Heater and Floor Finishes and ii) Slab Drying and Moisture Test.
  21. Request flushing adaptor (water units only), moisture test kit and thermostats.
  22. Complete start-up inspection and submit form to LEGALETT for approval. Upon approval and receipt of authorization code to install heaters, permanent heating insert can be installed.

If you are interested in getting a cost for a Legalett air heated frost protected shallow foundation forms, see here

When building a frost protected shallow foundation (FPSF) fit heating tubes.
Building Passive Solar radiant air-heated floor slab on grade form kit ductwork © Ecohome

Generating solar domestic hot water when heating is not required also serves to protect the panels from damage that can occur due to overheating in summer.

Our pre-fabricated S1600 model with dense-packed cellulose walls was delivered and installed on the slab in spring time. Below is a short video of the solar heated frost protected shallow foundation floor slab construction, for more on the (close to) zero carbon prefab home that is seated on this frost protected shallow foundation, see here!  

See here for more reading and videos on pre-fabricated engineered insulated slab on grade frost protected shallow foundation forms for problem soils like expansive clay, to read more about Passive House Certification see here, from the EcoHome Green Building Guides

How to build frost protected shallow foundations - Time lapse video of our FPSF build