Hi again, Thanks to all who contributed to my last discussion. It died on page 2 and perhaps for good reason. I learned a lot from all of the comments and think that I have some new perspective. Restating the general situation as briefly as possible: 1) finishing basement in 10y/o expose ranch home. Generally very well insulated/built with exception of uninsulated slab and r5 on stem walls at exposure sections. Basement bone dry, soil type likely hard-pan 2) Large project, approx. 1700 sf. to finish. about 65% of exterior wall is below grade. Has one long interior wall that divides finished and unfinished basement 3) Had a HVAC friend do a heat load calc, he came up with approx. 45k and I got similar result online. (NO, not trying to be an engineer by any stretch, hoping this seems reasonable given complexity of basement space.) 4) Floors at coldest of winter around 62 with exception of by double door with outer slab, which stays round 57. Air temp in basement stays a consistent 63 all winter. 5) I have a huge and modern cast iron stove that I burn when home, maybe 2 days a week, more of hobby. Not primary but is a great backup. Will heat the entire house without problem. I realize now that I will need to give up some additional height to do radiant floors. I understand that wall and ceiing panels are a better choice but would really like the warm floors if possible. Hoping to do this and keep floor gain under 2 inches. My thoughts for now: 1) Use a panel system like Roth or RHT with minimum of 1/2" foam under 1/2 pex and 1" between tubes(w foil on slab side), then heat transfer plates, then osb with plank vinyl. 2) Possibly add wall radiant in bedroom with large windows and considering desire for higher temps 3) 2 temp zones, 5 loops estimated. Use a 50 gal direct vent LP water heater as source. 4) don't need 80 degrees, maybe 67 max 5) Considering adding some cold air returns and supplies from the existing furnace (large enough for house and basement) for circulation and possible whole home dehumidifier. 6) likely to insulate floors above for sound, not sure if that matters much but thought I would mention. Also going to redo all exposed 2x6 walls with spray foam to be on safe side. I know that a little knowledge can be dangerous, just hoping some of you are willing to shoot holes in this or ask questions. 1/2" of foam under pex clearly isn't going to give me 95% efficiency but I am looking for a decent result and a compromise on total height gain to get the warm floors. Thanks in advance for any thoughts! (PS - no idea why my line spacing isn't working here?)

Nothing has changed? If you are just looking at not having a cold floor, skip the PEX completely and just place vapor barrier, 3/4-1” EPS, 5/8” plywood and then your choice of floor finish. You already received recommendations from both people selling their products and people knowledgeable about this subject and just providing honest feedback. Nevertheless, you seem bent on doing a HR floor no matter what, so just do it and stop trying to convince us that it isn’t a bad decision. And I am an engineer.

Thanks for the direct response. I thought I added a little more detail and slightly modified approach from the last thread but obviously nothing significant from an engineering perspective. Too long of a story for this forum, but I know that zoning my existing forced are is not an option based on a couple inspections. I would need to add a small second furnace based on some quirks in the layout and don't really have any interest in doing that. I will take a closer look at the radiant wall and ceiling options that Dana suggested earlier to see if I can make something work. Thanks!

Thanks for the direct response. I thought I added a little more detail and slightly modified approach from the last thread but obviously nothing significant from an engineering perspective. Too long of a story for this forum, but I know that zoning my existing forced are is not an option based on a couple inspections. I would need to add a small second furnace based on some quirks in the layout and don't really have any interest in doing that. I will take a closer look at the radiant wall and ceiling options that Dana suggested earlier to see if I can make something work. Thanks!

Is there any issues adding height? Like will it screw up all your steps? (I believe code say all steps have to be with in 3/8's of an inch in USA)

Would it screw-up any door entrances(interior and exterior)? Or would you have to step up and down entering a room?

Good questions - 2 existing doors, one interior and one exterior on opposite ends. Both difficult to change without making a huge mess of things. To avoid problems with these, thought it might be possible to do a small tile entry area with step-up at each door to accommodate height change. 1.5 or below I could sneak under these doors but it would short the bottom stair by that amount. Interior door is wood so could shorten it a little but would only increase impact on bottom stair. Guessing that stair changes aren't the best idea but haven't checked code. I could go as much as 1" foam under pex but that would make the transitions a little more noticeable. Hope that answers the questions.

I am now curious since most of the feedback is that I am headed in the wrong direction. Assuming no height restrictions, how much foam under pex is enough to retrofit over an existing slab? I am guessing that it depends on climate zone, etc. It seems like most other chats are referencing a fixed 2 inches without regard to any other details which surprises me. Are the concrete temps I mention, around 62 degrees, suggestive of anything? I have no comparison.

Perhaps more of an engineering question, but if 2 rooms have the same amount of foam on the concrete with one heated via floor pex above the foam with the other heated by different means (forced air, wall radiant, etc), will both rooms lose the same amount of energy through the floor?

The slab surface temp has little significance as it is largely influenced by the average indoor air temp and to a lesser extent the actual ground temp. The bottom of the slab would be closer to the actual ground temp. The goal is to minimize the heat rate transfer into the much colder ground. The Siegenthaler (see “Modern Hydronic Heating”) recommended minimum HR under-slab R-value is 0.125 times the difference between the indoor design temp (normally 68F) and the local outdoor design temp. This minimum R-value gets you about a 70% efficient HR emitter and many energy efficient conscientious HR designers target 90-95% efficient HR emitters.

Residential floor heat loss rate is typically calculated using either the floor perimeter or the floor area, the R-value between the design indoor air temp and the design outdoor temp. You might want to play with some free heat loss analysis tools to get a feel for this and the other things that affect building heat loss. Perhaps try this software:

Borst Heat Loss Analysis Software

A HR heated floor has a higher heat loss rate (all other parameters being equal) than other heating options. The reason being that the HR heated floor temp is a higher temp than it would otherwise be and because heat loss rate is proportional to the delta temp which is larger for a HR heated floor. This is why HR heated floors require more insulation than non-HR heated floors...at least if you care about getting a high percentage of the heat you put into the PEX back into the building.

So, a typical 99% outside design temp for a random location in WI would be mid single-digits below zero- call it -5F. At an indoor temp of 70F that would be a 75F difference.

Using the Ziggy rule of thumb 0.125 x 75F= R9.375. That would be about 2.25" of Type-II EPS. That's a minimum.

If using expensive heating fuels like propane or resistance electricity doubling that would still be financially rational. Got enough headroom for 4.5"?

Even unheated slabs climate zone 6 would be financially rational with R10 under the slab (2.5" of EPS). In zone 7 (northern WI) R15 can be financially rational (3.5" of EPS), even without the radiant. See Table 2, p10:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

Sailaway and Dana,

Thanks for the additional context. Spent a good amount of time with the Borst Heat Loss Analysis, nice program compared with others I found earlier. This tool suggests 31k btu for the basement. One thing I did notice as I played with the variables...the floor R value doesn't seem to impact the btu calc if the floor type is set as basement but makes a big difference if it is set as slab on grade.

Is the .125 rule of thumb applicable to below grade concrete, or only the edges that are exposed? If it is applicable, it seems like max floor insulation would be a good idea even without floor radiant heat. Thoughts?

Thanks

The basement floor option in our Borst software is for an uninsulated basement floor and the calculation is based on floor area and a Siegenthaler 0.024 U-factor. So you can change the floor R-value and it won’t change the floor heat loss estimate... In reality, basement floor heat loss (insulated and uninsulated) is way more complicated than that and depends on soil isothermals which depend on the depth of the basement floor, the shape of the building and the location of the building. It is true that the largest heat loss rate occurs at the perimeter and increases as the depth of the basement floor approaches the ground level which sees the outdoor temperature.

The Siegenthaler 0.125 rule of thumb is for slab-on-grade concrete floors and he doesn’t provide any additional/separate guidance for below ground level basement floors as far as I know. However, most of the professional HR design software (i.e., LoopCAD and other ACCA Manual J or ASHRAE compliant software) do account for this and my understanding is that the local at depth ground temp is used to some extent to estimate the basement floor heat loss. When we run HR calculations, we increase the under-slab R-value until the floor downward heat loss becomes less than 5-10%. and that’s what we use when constructing the HR emitter.

As an educated guess, I would think your 4’ or greater below ground level basement floor heat loss would only be about 30% of what your slab-on-grade floor heat loss would be. So if you accept this educated guess, that would indicate that about 11/16” of Type-II EPS (R2.8) would get you to the minimum recommended 70% efficient HR emitter...and this assumes you don’t have much R-value above the PEX tube. While I personally wouldn’t be happy with that level of HR emitter performance given the expense to do it, maybe you are okay with it. Nevertheless, my recommendation would still be to insulate the slab to the maximum extent that you can and do the HR radiant panels (ceiling and/or walls).

You are welcome!

Leaning toward ceiling/wall, still learning about options for these. Regardless, thought it would be good info to understand since heat loss through the floor will have impact on any system to some degree. Thanks again, all very helpful!