Groundwater (possibly high) and adding thin radiant slab over existing uninsulated slab, below grade

@hot_rod - there's no bulk water issue whatsoever. Years ago we excavated around the home, installed a perimeter drain near and slightly below the footer, etc and dimple mat on the stem walls. That was a BIG project… the crew hand digging it was here probably 6-8 weeks. It drains to a perforated sump inside the home which I installed, 3' deep, maybe perforated down 24"-28". That pump has run only a couple times that I can remember or have seen in the last 7 years - so the groundwater doesn't get within 2' of the slab. Is the water 4' or 12' when the ditch isn't running? I'm not sure.

Of course, the perimeter drain was only slightly below the footer, and 5' more is a long ways down. When I dug the sump, which I am guessing was 2' after the ditch stopped running, the sandy soil I excavated down to 3' was 100% damp, but not to the point you could squeeze any moisture out of it.

In my county there are many of these ditches. Hand dug for 10's of miles in the 1800's. They fill some of the local reservoirs. You would need more like a 5' culvert, or two 4' :) It's probably 4' across at the bottom, 15' across at the top, and the bridge is about 5-1/2' above the bottom of the ditch.

I would go for a post hole digger if they make one that would go that deep…. but we're talking 10' just to hit 6' below the basement. Or 6-1/2' if cutting the slab, which is a bit of a chore.. I'm not sure they will do those depths?

Now, I do have a well near the house I've never opened in 9 years - has been inoperable. I started digging out the cover today to see if I can open the head and see if the top liner is perforated. Original hand written permit mentions 5-60' tan white limestone, which wasn't mentioned in soil studies 200' to the east and 160' to the S. The house is long and narrow… the ends may have entirely different soils… so I agree with your comment on multiple holes.

I came to the conclusion over night that I likely won't know if I have groundwater here, without some tests, which can be expensive. On Jamie and DCContrarian's point, I don't know how significantly ground water or moist soil affects the overall heat loss, other than it's notable. My calcs, which are old and in Uponor's old ADS program jump up about 25% when I check the 'groundwater within 6' checkbox. I also saw postings here about natural gas consumption doubling or tripling. Is one of these closer to the truth than the other? I will ask my radiant guy to also re-run his numbers with groundwater considered.

I spent an hour on the phone with my architect friend today. Since the company doing the soil studies for septic and construction near by is no longer in business, he gave me the name of the guy who had worked for them and took over. I will check with him.

@Jamie Hall and @DCContrarian - The original plan I had built the stairs to was for a slim assembly. 1" EPS or XPS insulation, 1/2" loops, and 1-1/4" concrete over the insulation - so 2-1/4" assembly. My radiant guy said more insulation was better, but that I could get by with an inch. He mentioned 1-1/4 to 1-1/2" on the pour, with the right mixture. In my case, I am basing the 1-1/4" at the high point of the slab, as it has settled 1/4 to 3/4" lower across much of it, and of course we'd pour level. Note: no major cracking of the slab.

Headroom is tight. With the planned 2-1/4" makeup, I will have finished ceiling height AFF of 7' 8-3/4" for most of the basement, and 7' 7" in one bedroom, with one bathroom at 7' 6-1/2". Equally notable is that several of the door headers are structural beams. So, with any increase in height, I'm cutting down 80" doors. Not the end of the world, but would like to minimize.

After talking with my architect friend, we decide it should be OK to go up to 1-1/2" and possibly 2" thick insulation, with some sacrifices.

I know Jaime mentioned 2-4" insulation. Do you all think that getting to 2" insulation beneath the pour, such that regardless of any ground water, my losses should be reasonable?

@hot_rod - It would be great if the water was only present when the ditch runs. The neighbors soil tests 30' from the ditch (I'm 75' away) showed water at -7' in mid-july (2 weeks after the ditch stopped) and at -9' when tested in early october. So the water went down, but not away. They also mention a gravel/sand layer around -9', which is about 3' below the ditch. In their other test, about 100-120' from the ditch, and within 1' of grade of my house. They found no water in the 12' pit, and mineralization from prior water at about -9'. My slab is about 4' below grade.

So, there seems to be some permanent/longer-term water there, just a bit lower. While I feel reasonably confident there's water within 6' when the ditch runs, I would guess 50/50 chance there's water within 6' in October. I'm considering boring a sample hole through the slab just to see if I can see if the soil is damp in October, but first I'll run some relative humidity tests on the slab today/tomorrow.

These ditches were originally dug for farms. In fact, my country neighborhood (built in the 60's) used to be farm land. Some farms still use the water, but many of the shares have been sold to municipalities at this point, and they store it in a reservoir. Water rights are a massive deal here in Colorado, and certain ditches have different priorities than others, ours I think ranked like #23 or so. So regardless of if farmers still want the water, it gets shut off due to agreements.

Other ideas by anyone, I'm all ears.

@hot_rod - yeah, I have an sds and core bit up to 2-1/2" bit but going deeper might be an issue that small. Like you suggest, I might have to rent the bigger max tool and bit. I was looking last night at some hand well drilling equipment. I even saw an auger at harbor freight that is 2" x 36", but I haven't seen yet if I could adapt that funky attachment to something I own and if I could successfully extend. I do have a camera I could send down if I can get a somewhat clean cut.

Yes, radon galore here. I specifically tied in the new foundation drain system to a radon fan, which works great for that, but sucks some air near the sump and may cool the stem wall behind the dimple mat. I wasn't thinking about that at the time.

I've been kicking this part of the project down the road for years now, somewhat because I could not determine how to deal with the moisture issues and not knowing what sealing the rest of the slab will lead to under my existing structural framing. Now it's time to get my house back together :)

As to panel radiators, it is a good thought. I installed a few of the Myson decor rads upstairs in the MBR, along with some fintube under the bath.

Last week I ran the numbers on radiant over slab vs radiators, and radiant won financially - even with the grind and epoxy of the slab and the concrete pour and having a contractor install the tubing/manifolds.

I can still remember when we just had some baseboard and the nylon carpet + pad over the slab, and my feet literally freezing at my desk. I'm not sure radiators will address that, and based on the information coming together, think I may still need to insulate the slab with radiators. And I'd still need to level the old garage area with the floor drain slope.

One of the killers with the radiators is I would have to rebuild the stairs, which was a long, slow process for me, though very much crafted, true, and robust to last a lifetime. I think just the framing materials to rebuild those is probably over $1000 today. I'd rather invest that in making the place comfortable.