Unless you're building to Passivhaus standards or are in an extreme cold climate, the purpose of under-floor foam in an full foundation basement is not so much to keep heat in as to prevent condensation on the concrete during humid weather. (If you're using in-floor heating, the purpose is to retain that heat.)

Building in our cold climate, I've never bothered with under floor insulation as I've finished the flooring in tile or stone or glued-down synthetic carpet with no underpad. (However, for basement baths I'd say insulation or in-floor heating is a must for comfort.)

Our current architectural plan calls for R-10 under the basement floor. If we go that route, we'll use high-strength EPS rather than XPS. It's cheaper and typically has lower GWP.

An insulated slab foundation will also need vertical exterior insulation around the slab.

Hi Dan1888 and Worthy, thanks for explaining it to me. We live in Ontario CA, a cold weather, depending who you ask ( me for example) it is extreme cold weather. (LOL) .

I thought the underslab insulation would give the warm feeling in the basement, thanks to showing me how wrong I am.

Our basement walls will be R5 board and R12 batten. Would it be sufficient to make it feel warm? I am not sure if the basement ceiling ( main dek) will be insulated. Is it a must if we want the room to be comfortable?.

My concern is the current house we live in, the basement is fresh ( cool) during summer and cold in the winter. We had a electric heater on when the kids were playing. I just don't want to make the same mistake. For flooring we are planning on using LVP.

I will check EPS again, I think the last time i checked I would need a thicker board to for the same R value as XPS, now that I am moving to R5, it might work out. Thanks for sharing your experience and knowledge.

Have a great weekend.

@mqcola, Ontario, California (a city) vs Ontario, Canada (a province). Get your cold straight.

@ strategery, Ontario Canada and not California.LOL.

LOL

Summary of Ontario, California weather:

"The cool season lasts for 3.9 months, from November 22 to March 18, with an average daily high temperature below 22 °C. The coldest day of the year is December 25, with an average low of 7 °C and high of 19 °C." *

Just north of Toronto, Ontario, we had snowfall and frost in late May this year.

Basement walls of R17 in Ontario, California are extreme. IECC 2020 calls for R5 continuous, R13 if framed.

Thanks all. Will go wit R5 and save some money on a tentative to bring the cost more aligned with the budget line. Lumber cost is killling me LOL. All the best to you and have a great week.