Editing Openai/6928dffa-2c40-800e-b585-2520d50a2cbb (section)

=== Freeze-thaw behavior (what to worry about) ===
* Concrete (with air entrainment) is a known, code-accepted way to resist freeze–thaw damage in cold climates because air-entraining admixtures create tiny voids that relieve internal ice pressure. For frost zones, footings are usually placed below local frost depth. Wikipedia<ref>{{cite web|title=Wikipedia|url=https://en.wikipedia.org/wiki/Air_entrainment|publisher=en.wikipedia.org|access-date=2025-11-28}}</ref>
* Post-setting polyurethane foam is generally closed-cell and does not absorb water the way plain soil does, so the foam itself isn’t prone to the same internal freeze-thaw cracking mechanisms as un-air-entrained concrete. Many product datasheets list service temperature ranges well below typical freezing temperatures and state suitability for cold-weather use. That said: - Frost heave acts on the whole post/ground system. If the post and its surrounding material are pulled by heaving soil, the anchored system can move. Therefore depth (below frost line) and good drainage still matter. Sika USA<ref>{{cite web|title=Sika USA|url=https://usa.sika.com/retail/en/products-catalog/post-setting/sika-postfix.html|publisher=Sika USA|access-date=2025-11-28}}</ref> - Foam can hold moisture against wood (closed cell traps water at the interface if water is present), which can increase rot risk if the post is not pressure-treated or if the top of the foam/post interface is exposed to wicking. Proper installation (keep hole clear of standing water; use treated posts or metal post bases) mitigates this. Sika USA<ref>{{cite web|title=Sika USA|url=https://usa.sika.com/retail/en/products-catalog/post-setting/sika-postfix.html|publisher=Sika USA|access-date=2025-11-28}}</ref>

Bottom line on freeze-thaw: foam itself tolerates freeze–thaw temperatures; but if you’re in a frost-heave area, you either must set the post below frost depth or use a product/installation method specifically rated for deck/load situations. For small fence posts that rely mostly on lateral resistance, foam often performs acceptably in freezing climates; for heavy loads or poor soils, concrete + below-frost footings is still the safer choice. Sika USA<ref>{{cite web|title=Sika USA|url=https://usa.sika.com/retail/en/products-catalog/post-setting/sika-postfix.html|publisher=Sika USA|access-date=2025-11-28}}</ref>