Have you had any experience with rigid foam used in house construction?

I'd like to see your references - "building science research shows it is a pointless to bad idea". I'd also be curious why you would think you know better than everyone else since this is pretty standard stuff. Maybe you do but maybe you don't.

You are using an example of a failed slab under a shed. I certainly agree that putting foam under a shed slab seems unusual. But that doesn't mean anything in the grand scheme of things.

Certainly possible that the attention to detail for a shed was less than a house regarding sloping, roof overhangs and gutters.

Please explain how you would meet the energy efficiency requirements of the IRC or the IECC. Would you use REScheck? What about heated slabs?

Just a quick search of the internet produces convincing evidence that contradicts the claims of this post. https://www.constructionspecifier.com/out-of-sight-not-out-of-mind-specifying-thermal-insulation-below-grade-and-under-slab/

This University of Alaska report with real world excavated samples shows the loss of R value and water absorption over time in both EPS and XPS. (EPS trade groups claim the reverse.). No mention about insect infestation, which seems to be a particular problem in warmer climates.

I'm omitting under slab insulation from our latest build as it's not Code required and it adds zero to re-sale value as far as I can see. I'm not a Passivhaus competitor!

Suoer Lumen:

You didn't say if you and your friend discovered a manufacturer-required membrane between the gravel and the foam.You can't blame a product for an installation error.

You are wrong.

And the university of AK cold weather research center recommends using ridgid foam insulation under and around the perimeter. With actual science and what and how the tests were done. Along with many technical studies from multiple places.

Coldest cell insulation can loose a small amount of insulation value can be lost, small value after years. You cherry picked a report and if you read the whole thing like some of us you would know. Closed cell and open will not absorb water. Styrofoam floats.

This process has been used all over the world successfully.

My qualifications. Certification for RHVAC. 2 years college degree.

Btw oour new home has 4" under the heated slab, 4" around the perimeter of the slab and 4' wings around the house.

And 2" over the rockwool insulation on the inside.

We essentially did the same - wrapping foundation in high density foam... Been done for decades in cold climate finland using the foam we used. We skipped heated floors since it's so disconnected from the ground.

https://www.finnfoam.com

In most of the climate zones in the US, rigid foam insulation board is required at the edges of slabs on grade and under heated slabs so, in most instances, it is unavoidable.

dont feed the troll

Speaking of trolls...

According to the all knowing GPT.

"There are building scientists who have raised concerns about the use of underslab foam insulation. Here are a few examples:

1. Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, is a renowned building science expert and principal at Building Science Corporation. He has written extensively on building science and has questioned the use of underslab foam insulation, particularly in certain climate zones. In an article titled "Insulation Below Slabs on Ground" published in ASHRAE Journal in 2002, he argued that in some cases, underslab insulation can increase the potential for moisture-related problems in the slab and the building.
2. Dr. John Straube, Ph.D., P.Eng., is a building science professor at the University of Waterloo and a principal at RDH Building Science. He has also expressed concerns about the use of underslab foam insulation. In a blog post titled "Why I Don’t Like Foam Under Slab," he argues that underslab insulation can increase the risk of moisture accumulation, leading to mold and indoor air quality issues.
3. Building science expert William Rose, P.E., is a research architect at the University of Illinois at Urbana-Champaign and a principal at Building Research Council. In an article published in the Journal of Light Construction in 2005, he argued that underslab insulation can create a "bathtub effect" that can trap moisture and lead to problems with slab and foundation durability. He also notes that underslab insulation can make it more difficult to detect and repair leaks in the slab."

Au contraire GPT!

Dr. John Straube

"Comfort levels and moisture related issues including dampness and musty odors, and storage of moisture sensitive materials on the floor will decrease if underslab insulation is used. In some cases when radiant floor heating is used, R20 or greater underslab insulation is necessary to reduce the heat loss to the ground." Source: Building America Special Research Project. High R Foundations

Dr. Joe

"Basement floor slabs are best insulated underneath with rigid insulation: both extruded or expanded polystyrene have been widely used with success."

https://buildingscience.com/documents/digests/bsd-103-understanding-basements

And the article cited by GPT doesn't even exist, according to the ASHRAE index.

William Rose, P.E.

Got me stumped as I can't find either the alleged source of GPT's paraphrase or any on-line contradictory views by this long-time building scientist.

*Inspired by this GPT exercise in omnipotence.

It may be safe to say that rigid foam (open or closed cell we do not know) should not be used under the slab of the OP's friend's shed.

EPS rigid foam- like this- https://liteform.com/insulation-board/flexxboard25/ bonds to concrete, and most importantly, prevents frost heave on unheated concrete slabs. Frost penetrates straight down. By reflecting the ground temperature back down, the earth under that slab will not freeze- you can go out in the spring and the ground around the insulated slab will be higher than the concrete a lot of times due to the differences caused by frost. Some extruded foams will absorb water and lessen it's R-value over time, but a quality EPS is similar to wool- it insulates when wet (water passes through, it doesn't "absorb"). Personally, I would never pour a sidewalk, shed slab, driveway, or home concrete slab without it.