Tag Archives: Climate Zone

Meeting IRC Slab Edge Thermal Breaks With Post Frame

Meeting IRC Slab Edge Thermal Breaks With Post Frame

Reader CHUCK in MUNCIE writes:

“Morning sir, I read your link in your post about post frame buildings for barn houses… one thing I am wondering, is how does the building pass energy code for residential construction, plus the IRC talks about a building being used for residential occupancy needs a thermal break at the foundation wall…. in a conventional post frame building the posts are on footings, and a slab on grade is poured, so how do you provide the thermal break to meet the building code?”

Mike the Pole Barn Guru says:


Post frame construction for residences has no appearance of going away at any time according to my crystal ball. And why should it? Post frame is more economical than stick frame, very DIY user friendly and can be readily super insulated. Here, I previously expounded upon post frame’s residential virtues: https://www.hansenpolebuildings.com/2022/01/why-your-new-barndominium-should-be-post-frame/

Slab edge thermal breaks (slab perimeter insulation) is only required in Climate Zones 3 and greater. You can look up your Climate Zone at codes.iccsafe.org/content/IECC2021P2/chapter-3-re-general-requirements When required, it must be a minimum of R-10 and down two feet (Climate Zones 4 & higher adds a horizontal R-10 component or becomes down four feet).

A common question with rigid foam insulations is how well it resists water. A number of studies show EPS retains less moisture than XPS. A case in point is a side-by-side analysis of these two rigid foam types installed on a commercial building foundation in St. Paul, MN. When extracted and tested after 15 years in service, EPS had 4.8% moisture content by volume, compared to 18.9% for XPS (a four-fold difference). A testing lab also found  XPS holds water longer than EPS. After 30 days of drying time, XPS still had elevated moisture of 15.7%, while EPS had dried to 0.7%.

For installations where insulation will be exposed to large amounts of water or frequent wetting, rigid foam insulation is available with water-resistant facers or pre-cut drainage grooves. Insulation with polymeric laminate facers keep water from entering insulation and also provide an added barrier to water wicking or diffusing through.

Moisture resistance is also important for below grade and under-slab insulation, since wet products provide much lower thermal resistance. Side-by-side insulation comparison found EPS retained 94% of its specified R-value, while XPS lost nearly half of its insulating capability over 15 years.

In addition to higher moisture resistance, EPS also is not subject to thermal drift. This means its R-value stays same over time. By comparison, XPS’s manufacturing process uses blowing agents diffusing from foam’s cellular structure over product life, thereby reducing its thermal performance. EPS manufacturers typically warrant 100% of published R-value for 20 years or more, while common XPS warranties cover just 90% of published R-value.

Whether selecting EPS or XPS insulation, to ensure performance, confirm product was manufactured to meet requirements of ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. This standard provides a key quality check on rigid insulation.

As insulation becomes increasingly common at slab edges, understanding performance and cost factors of these different materials is important. EPS offers a number of advantages over more commonly installed XPS, including having highest R-value per dollar among rigid insulations, making it a cost effective choice for many jobs.

Full Frost Foundation?

Full Frost Wall Foundation?

Reader AARON in ROSEVILLE writes:

Assuming (not yet verified) that I don’t need a full frost wall foundation for my barndo, wondering about the pros/cons for full frost wall vs pier footings in southern MN. Obviously there is the cost difference, but when it comes to building performance(in the winter), would you recommend frost wall over pier footings? And are there any work arounds to improve the cold weather performance of a pier footing?”

Mike the Pole Barn Guru responds:
When we built our post frame barndominium in NE South Dakota 16 years ago, we could have utilized any foundation type we wanted. We opted for embedded columns and have had absolutely no regrets about our choice.

From a standpoint of both Code requirements, as well as performance over time, there is not a structural reason to use a full frost wall for a post frame barndominium.

Any heated structure should be meeting the most recent edition of ICC’s (International Code Council) 2021 IECC (International Energy Conservation Code), even if not required in your jurisdiction (usually due to either no structural permit requirements or not yet adopted).  You can look up your county’s Climate Zone at: www.codes.iccsafe.org/content/IECC2021P2/chapter-3-re-general-requirements. Once you know your Climate Zone, Chapter 4 will guide you through insulation requirements for roofs/attics, walls and floors.

In Minnesota, Code requires slab edges to be insulated down four feet with R-10 insulation. With post frame construction, you can rip 4′ x 8′ sheets of R-10 EPS in half lengthwise. Attach lengthwise to the inside of pressure preservative treated splash plank (aka skirt board or bottom girt) with top of insulation even with eventual top of concrete slab. Balance of requirement is solved by using another two foot piece horizontally at the bottom of vertical insulation (basically forming an insulation “L”). This does require digging a trench, however one would need to be dug (and far deeper) for a concrete foundation wall.

Now your challenge….how to insulate piers. Whether using embedded columns with a concrete footing/bottom collar, or full concrete piers with wet set brackets, you can build square forms out of EPS. It does mean you will have excess insulation on the under slab side of the pier, but it is a viable and cost effective solution.

Best way to ensure a successful outcome is with really great site preparation. If you would kindly visit our website www.HansenPoleBuildings.com and navigate to SEARCH in the upper right corner. Type in SITE PREP and hit ENTER. Up will come a plethora of relevant articles for your reading pleasure.

Concrete Slabs on Grade for Cold Climates

Concrete Slabs on Grade for Cold Climates

My lovely bride and I have a shouse (shop/house) in Northeast South Dakota, where it can tend to get chilly in Winter. Reader TERRI in DULUTH is in a similar situation and writes:

“What type of slab for cold climates do you recommend?”

Well Terri, thank you for your patience in awaiting a response, your email address was not included with your question, so I was unable to message you back as quickly as I would have liked.

PLEASE – If writing an “Ask the Pole Barn Guru” question include your email address.

Injecting some humor (sadly, I have to point it out as not everyone gets it) – I would recommend a concrete slab.

Before diving into this subject – accept one fact, concrete slabs will crack. If you are expecting otherwise, you are setting yourself up for disappointment.

First key to a successful slab is excellent site preparation: https://www.hansenpolebuildings.com/2017/02/building-site-preparation/

Second – insulate your slab’s perimeter. 2021’s International Energy Conservation Code (IECC) provides guidance to meet energy code requirements for every county within our country. Begin by looking up your Climate Zone: https://codes.iccsafe.org/content/IECC2021P1/chapter-3-re-general-requirements

I will cheat and tell you Duluth (St. Louis County) is climate Zone 7.

Scroll down to Table R402.1.3 to find Insulation Minimum R-Values.

In Climate Zone 7 your slab perimeter must be insulated to R-10 and be four feet in depth. 

Weirdly enough, Minnesota’s Building Code only requires footings to be at 60 inches of depth in your area (https://www.revisor.mn.gov/rules/pdf/1303.1600/2015-01-23%2012:37:31+00:00). 

Although actual frost depth in your area is more like 80” in depth:

I would probably look at augering holes no less than 72 inches in depth, using a bottom collar of 18 to 24 inches (per your engineered building plans). This would allow you to trench between building columns and install R-10 rigid insulation along your building’s perimeter to a depth of four feet.

If you are going to do radiant in-floor heating you should be placing R-10 beneath your slab and on top of a vapor barrier of no less than 6 mil visqueen (I prefer 15 mil thickness to reduce chances of perforations during pouring).