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Building Science

Residential Post-and-Frame Construction, Part 2: Foundation

A frost-protected shallow foundation and in-floor hydronic heat ensure optimal interior conditions and occupant comfort

Frost-protected shallow foundation with closed-cell spray foam insulation wing

When I introduced the “barndominium” project I am working on in climate zone 6, I talked about the unique characteristics of this building type, which is part storage facility, part workshop, and part living quarters. Here, I’ll explain key details of the foundation and the thinking behind them.

High-performance cold-climate foundation

This project has a frost-protected shallow foundation (FPSF), which includes cast-in-place post brackets to secure the wall framing to the foundation. The 24-in. by 24-in. reinforced thickened-edge footing is thermally broken from the ground with 3 in. of closed-cell spray foam (CCSF)—that’s roughly R-20 both under the footing and beneath the 5-in. slab.

Hydronic tube floors
Hydronic tubing for in-floor radiant heating system

Why put foam under the footing? Two reasons: climate zone and heating source. Living in a heating-dominated climate can result in a moderate difference between the ground temperature and the desired temperature of the living area. This heating system includes hydronic in-floor heat. The goal is to maximize the amount of heat moving into the conditioned portion of the structure and decrease the amount of heat moving into the earth. A geothermal ground-source heat pump will supply the heat for the slab; this type of system operates at a lower temperature than fossil-fuel boiler systems. In short, measures had to be taken to ensure ground temperatures don’t suck heat from the slab.

When planning the spray foam details, I learned that 1 in. of CCSF will stop approximately 90% of conductive heat flow. At 2 in., this reduction increases to 94% and at 3 in., it’s 96%. It would take 6 in. to get to 98% and 11-1/2 in. to reach 98.9%. This data comes from the spray foam manufacturer Demilec, and is based on a delta T of 40°F—roughly the difference between the barndominium’s conditioned space and wintertime ground temps. Clearly, this is a situation of diminishing returns. That said, code-minimum R-value is necessary to achieve proper insulation levels for an assembly. I’ve seen people in this climate try to install too little, especially in ceilings and walls, only to have it become a problem.

There were a few reasons to use CCSF for insulation under the slab. Not surprisingly, speed of installation was No. 1. The concrete foundation work was performed late in the construction season, and it is important for temperatures to be above a certain level to achieve the product’s best performance; getting the work done as quickly as possible was critical. Other reasons include CCSF’s ability to air-seal all the penetrations through the slab, and act as the vapor control layer. Because of spray foam’s high global warming potential (GWP), Demilec Heatlok HFO was spec’d; with HFO as the blowing agent, it has a lower GWP than more conventional formulas.

FPSFs require sub-slab and slab-edge insulation, as well as an insulation wing to prevent frost from driving under the slab. This is the first project I’ve been involved with that used CCSF for both the concrete edge and the wing. In hindsight, I would go with rigid foam board at the edge for its greater durability. There was some damage to the CCSF from a scissor lift driving too close to the building. It needs to be removed and replaced in several areas. Also, it is uneven in spots. This insulation needs to be protected not only from physical damage but from sunlight. I foresee needing to trim the insulation back in a few areas, so the metal flashing cap can protect the product properly.

Other sub-slab insulation options

EPS or GPS foam in a Type IX density would work in this application too. Either of those products would have been a cheaper option but the time needed to install it and then add the sub-slab vapor retarder would have been much longer. (Side note: Most contractors in my market install the sub-slab vapor retarder under the foam insulation. This traps a lot of the moisture from the concrete in the insulation layer, where it can take years to dry. As Dr. Joseph Lstiburek has been telling us for years, the poly needs to be on top of the insulation.)

Another option for the sub-slab insulation would have been the new XPS foam with the HFO/HFC blended blowing agent, though my understanding of the GWP of that product is it’s twice that of the Demilec CCSF product we used.

Using a rigid foam board as the sub-slab insulation would have been less expensive than CCSF, even with labor costs and the additional cost of adding a sub-slab poly vapor retarder. Again, CCSF was used mainly because of the construction schedule.

Overall, I’m satisfied with how the foundation turned out. The budget, climate, time of year for construction, and heating system all played a role in the decisions that were made. Changes to any one of those things would have resulted in a different foundation insulation strategy. I am curious to see if the decisions made will work as planned with the geothermal ground-source heat-pump system being installed—the subject of a future blog post.

The full series

Residential Post-and-Frame Construction, Part 1: Introduction

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Randy Williams is a builder and energy rater based in Grand Rapids, Minnesota. Photos courtesy of the author.

2 Comments

  1. Doug McEvers | | #1

    Randy,

    This looks like a nice job of foaming, not the first one for the contractor. How even is the thickness of the CCSF? Did the spray foam add any complications to the concrete pour over what rigid foam boards would have?

    Did they use a long span power screed for the concrete pour?

    1. Randy Williams | | #2

      Hey Doug,
      The spray foam did vary some, over the 8700 square foot, 5-inch slab, I believe the concrete contractor had to order an extra truck load of concrete. Not sure if it was all needed. I missed the pour, didn't get a chance to see how they screeded the concrete, they did have a pair of riding finishing trowels. Not often they get used on a residential project. No other complications, the pour went well.

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