What kind of studies have been done on the cost effectiveness of foam insulation under the basement floor? I don't plan on staying there during the cold winter months (southern Colorado) so that has to figure into the equation. The house will probably be built into the side of a hill so most of the basement will be above grade. My original intent was to put 2" under the slab but now not sure. It's probably cheaper to leave a propane heater on 45 degrees in the basement's Mechanical room to keep the plumbing from freezing than putting 2" under the slab. I will say that here in Texas both of my shop floors get cold in the winter and I hate laying on them to work. If the foam would prevent that it might be worth it. Any comments or suggestions?

There isn't much return on investment in energy terms for installing foam under a slab in a semi-conditioned unfinished basement, especially if the basement is allowed to drop to 45F. The subsoil temp in your area is likely to be 45F or warmer.

A primary reason for insulating under basement slabs is to mitigate against mold or "musty basement smell", which is prone to happen when the slab temperature below the ventilation air's dew point in summer. That's not really an issue in Colorado.

That said, reclaimed roofing EPS is pretty cheap stuff, and once the slab is poured there is no second bite at the apple should you decide to fully heat the basement in winter. And with a couple inches of EPS under the slab the "cold back" issue goes away- the slab's temperature will pretty much track the room temperature-when the room is being kept at 70F, the slab will be north of 65F. Without the foam the slab temp will be somewhere between the subsoil temperature and room temperature. In Texas the subsoil temps are warmer than in Colorado, but without slab-edge insulation slab-on-grade will run pretty cold in winter, especially the 4' band around the perimeter of the slab.

Thanks again!

If we can go with reclaimed roofing EPS we will go that route, otherwise we will leave it out. From Oct to probably June the place will be vacant and I plan on keeping the basement just warm enough that it doesn't freeze.

You are right about the slabs in Texas. In my house I can take my infrared thermometer and see just how far the cold migrates into the slab

Just curious.....why are you laying on the floor?

Anything that won't fit on the welding table gets welded on the floor- lot's of gates for example. I also seem to spend lots of time repairing my Polaris XP Turbo. At the moment i a rebuilding a zero turn mower. Sitting on a milk crate I can feel the cold radiating upwards. After a day in the shop the wood stove will heat the concrete in an area around it but I have to work in there several days in a row to get enough heat in the slab for it to retain any.

Put some cloths on.....working like you are just makes things shrivel up....hence your name "LittleBoss"? hmmmm. JK.

I can't work when I am cold either, but keeping a cement slab warm enough for you to feel it is expensive in the part of the building not used all the time. Even with radiant heat most slabs take at least 6 hours to heat up. Guessing to get the floor to feel warm the room would need to be like 80 degrees.

Unless the slab is above 70 is will feel cold because it is taking heat from your body. I would definitely put in good parameter insulation though.

Putting any under the slab like an inch or more doesn't seem like it is going to do much unless you are doing radiant heat.

I would just get a big heater and make that place toasty when you go down there. Good Luck.

Here in Texas I have wood heat so my upper body is hot and lower is cold from the slab. In Colorado I hope the basement doesn't get too cold. I don't plan on doing a lot of work while there and i won't be there in the winter anyway.

Littleboss was a nickname given to me by my dad. We were partners on the farm for 35 years. Both were hard headed and both wanted to be boss. One day he called me little boss when we were arguing about something and it stuck! Later I put CB radios in the tractors and trucks and my handle was Littleboss and his was BigBoss

Wish you luck. Some of us are very sensitive to temp and I understand. Thanks for sharing the story....

Wish you luck. Some of us are very sensitive to temp and I understand. Thanks for sharing the story....

Wish you luck. Some of us are very sensitive to temp and I understand. Thanks for sharing the story....

Heating a slab on grade with forced air isn't as easy as Dana1 believes. I Have four inches of XPS under a slab that floats inside an ICF frost wall in Pennsylvania. Normally heated hydronically to 75, the floor will lose three degrees a day without added heat and settle in a very uncomfortable place in the mid 50s. While the floor will hold its heat with a two-ton mini split in operation, recovery from a cold slab takes a looong time given the slim delta T between room temp at 70 and 25 tons of concrete at 55. Cheapest basement refit is foam, decking and carpet on top of the slab. Best is insulation underneath and pex runs in the concrete that you can use if your plans change. Gotta say a 75-degree floor spoils you for any other kind of heat.

The lowest cost and highest performance HR floor heating emitter is a concrete slab that you are already pouring. Hence, we always recommend placing PEX anytime you pour a slab if there is any chance that you will want HR floor heating in the future. If you elect to do this, you will want to place EPS insulation under the slab. The other alternative would be to provide sufficient ceiling height such that insulation and another HR floor heating emitter (e.g., Warmboard, etc.) could be placed on top of the slab in the future if desired. While this alternative approach would have a higher acquisition and a higher monthly operational cost if used full-time, it could be used part-time and it would be a much more responsive HR floor heating emitter.

Posted By toddm on 26 Feb 2019 04:18 PM
Heating a slab on grade with forced air isn't as easy as Dana1 believes. I Have four inches of XPS under a slab that floats inside an ICF frost wall in Pennsylvania. Normally heated hydronically to 75, the floor will lose three degrees a day without added heat and settle in a very uncomfortable place in the mid 50s. While the floor will hold its heat with a two-ton mini split in operation, recovery from a cold slab takes a looong time given the slim delta T between room temp at 70 and 25 tons of concrete at 55. Cheapest basement refit is foam, decking and carpet on top of the slab. Best is insulation underneath and pex runs in the concrete that you can use if your plans change. Gotta say a 75-degree floor spoils you for any other kind of heat.

Don't pretend to know what I believe, eh? ;-)

I believe that subsoil temperatures in Texas are more than 10F warmer than Pennsylvania where toddm lives, making slab-edge only insulation far more comfortable in TX than in PA.

http://mb-soft.com/solar/soilmap.gif

I also believe that the slab in Colorado under consideration is a basement slab, not a slab on grade.

I also believe that a 75F floor in a slab on grade is a total spoiler, but too hot for working on your back doing anything more strenuous than lifting the TV remote to pause for snack break while binge-watching K-dramas. :-)

Interesting case good luck Mate!
Best wishes