Guidance from The Pole Barn Guru
I spent over two decades designing, providing and/or building post-frame buildings before an interesting challenge arose. One of our clients had applied for a building permit and their plans checker was asking how he was going to do below-grade perimeter insulation.
In this instance, I found, a demand for this insulation was all about energy efficiency and had nothing to do with potential frost heave.
Now granted, my experience was primarily in regions where frost (and frost heave) were minor considerations. Just dig holes to below frost depth and call it a day. Since holes were usually 4’ deep and frost depths were never over 3’, insulating to prevent frost heave had never crossed my mind.
This was all well and good in my native Pacific Northwest. However, moving to an area where I have personally witnessed ground frost over 7’ deep — not so much.
Since my first exposure to below-grade perimeter insulation, our industry has dramatically changed. A huge preponderance of our clients are now climate-controlling their structures – whether commercial, industrial, residential or “just” a backyard shop.
By use of frost-protected shallow foundation (FPSF) technology, we can provide for our clients both protection from heave as well as being energy efficient. I am all about achieving results both cost effective, as well as easily implemented. Meet both of these requirements and I become a happy camper.
I am not going to bore anyone here with all sorts of technical details. If you want to know exactly how FPSF works in all of its splendor, Dr. Dave Bohnhoff, P.E. wrote articles on this very subject and they were published in Frame Building News in June and August 2021.
First, let’s take on energy efficiency. Looking at 2021’s International Energy Conservation Code (IECC) Table R402.1.3 (“R” think – International Residential Code or IRC) requires no slab edge insulation in Climate Zones 0 through 2. Zones 3 and over require R-10 to a 2’ depth, and 4’ depth in Zones 4 and higher.
Like most Building Codes, there exists a footnoted exception. With heated slabs, R-5 insulation shall be provided under full area of heated slab plus slab edge insulation is not required to extend below slab. Every heated slab I have been involved with has used at least R-10 below heated slab insulation, while retired Green Building Advisor editor Martin Holladay recommends no less than R-20.
For IBC-governed buildings, it gets a little more interesting and is divided into two parts (heated or unheated slabs) with two sections (R residential use or all others).
Unheated slabs in R use require no slab edge insulation in Climate Zones 0 through 2; R-10 to a 2’ depth in Zone 3; R-15 in Zone 4 except Marine; R-20 in Zone 5 and Marine 4. Beyond Zone 5, depth jumps to 4’ and remains R-20 until Climate Zone 8, where it is R-25.
For other uses with unheated slabs, no slab edge insulation is required up through and including Climate Zone 3. Zones 4 and 5 require R-15 2’ depth; Zones 6 and 7 R-20, and only in Climate Zone 8 is it R-25 and 4’ depth.
Heated slabs in all uses and Climate Zones require R-5 insulation below. For “R”-use buildings, Climate Zones 2 and less require R-7.5 insulation down 12”. Climate Zone 3 jumps to R-10 down 24”. Climate Zone 4 (except Marine) is R-15 down 24”, while Marine and Climate Zone 5 ares down 36”. Climate Zones 6 and higher ask for R-20 down 48”.
Buildings other than “R” use are similar to R with an exception at Climate Zone 6 being allowed to be R-15 down 36”.
FPSF technology recognizes thermal interactions of building foundations with ground. Choices of insulation should be carefully evaluated to consider possible degradation in R values due to water absorption.
For sake of ease of installation for embedded post-frame columns, my recommendation is to dig a trench down 2’ below finished grade. Width of horizontal insulation plus one-half width of required embedment diameter, starting at inside edge of column embedments and working outward. Column holes can then be dug another 16” in depth and column set in poured premix concrete to create a bottom collar. This leaves area above collar free to place vertical insulation.
Vertical insulation is installed first. The top edge of the vertical insulation is attached to the inside face of the properly pressure/preservative treated splash planks, with the top edge of the insulation even with the top of the future concrete slab on grade. Installation, in this manner, allows for insulation boards to be protected from UV rays, without the need for other protective measures. For sake of ease of installation, vertical insulation boards are cut to a 24” width.
Place compacted sharp gravel in the bottom of the excavated trench up to the lowest point of the vertical insulation. Lay the horizontal insulation from the vertical insulation base outward, directly on top of the gravel.
Now – to address what is neglected in every document I have researched: Protection from burrowing critters. At the extreme exterior edge of the horizontal insulation, dig a narrow trench down another foot, or more. Into this trench, up and over the top of the horizontal insulation and up the exterior face of the vertical insulation, place 19-gauge galvanized or stainless steel ¼” mesh wire. Cover this with more compacted sharp gravel. This will protect your insulation boards (as well as under your slab) from rascally rodents.
The inside face of the vertical insulation will be backfilled to match balance the underslab fill.
But, doesn’t edge of slab on grade need to be thickened? In a word, no … not unless your building will somehow have downward-bearing weight upon the edge of the slab. Concrete does not provide any magical properties to resist heat/cold flow if it’s not insulated, nor will it prevent frost heaving unless extensively reinforced.
These very same principles can be applied to buildings using ICC-ESR approved wet-set brackets to attach wood columns to poured concrete piers. Here, the depth of piers can be greatly reduced (often to 24” or 30” depths) by use of FPSF concepts.
Many building departments want to see piers themselves insulated. We have found square forms built from rigid insulation to be an appropriate solution. FBN
