Re: Radiant insulation

Geode

Hello group,

I'll be pouring a new concrete slab soon in my basement. It's complicated, but the gist of the situation is that I can not insulate the slab from the footings. The concrete walls are insulated to R-10 on the outside. Is there a possibility I could still see good performance from the radiant heat system? It seems that by not insulating between the floor and footings, that it might be similar to not insulating the center of the floor to create a heat sink?

Thanks 

Jean-David Beyer

good performance from the radiant heat system

I wondered about this for my house that was built on a slab about 1950. My non-professional examination of the snow melting around the edges of my house, and replies I have obtained here point to the high likelihood that my slab night be insulated with some sand or is not insulated at all. It is quite thick and has 1/2 inch copper tubing in it that has hot water from my boiler pumped through it for heating my downstairs zone.



I calculated the heat loss of the downstairs to be 22761 BTU/hour, partially ignoring the heat loss downwards. The floor is about 700 square feet. I happen to know the water table is about six feet down. This calculation is too high because it assumes a 0F design day, and around here in New Jersey, the design day temperature is 14F.



When I heated with oil, I used between 300 and 600 gallons of oil a year depending on how cold it was. It was closer to 300 gallons in recent years. I converted to gas and if I read the meter right, I used 92,900 cubic feet of gas in two years (meter started at 0). The gas boiler is a mod-con with outdoor reset. The oil burner did not have outdoor reset and condensed only accidently.



I am not sure why you cannot insulate the slab from the footings, but if the footings are insulated on the outside, that should help a lot. Do insulate so the bottom of the slab is insulated though. You do not really want to create a heat sink there. A heat sink is like a money sink. Remember that I am not a heating professional.

Geode

Re: Radiant insulation

Thanks for the info. The footings/foundation is a long sad story

The floor contact is needed to lock the footings in to keep the walls/footings from pushing in. Would be cool If I could find a material which could isolate the two, but I've no clue on what material would be strong enough to oppose the inward forces from the foundation and have a decent r value.

The footings are not insulated, but the concrete wall block is. Sure would like to add radiant, have done a lot of the leg work, heat load calcs etc, but have to put the structural integrity first.



I've done some more reading and it seems isolating the floor from the walls/footings is a lot more important than insulating under the slab for proper heat transfer. Makes sense, because the radiant slab being in contact with the footings and walls will really upset the system.

Jean-David Beyer

I wish a pro would comment in this thread.

It may depend on where you are. The consensus here seems to be that my 1950 house has no insulation under or around my slab. It is possible that it was insulated underneath with sand. This was back when oil was $0.20 to $0.40 per gallon. I bought the house in 1976 when it was about $0.40/ gallon and the heating cost was still acceptable (then). The oil fired boiler had no reset and sent water from the boiler into the floor. The boiler ran at about 140F, but had strange piping that I figured mixed the return water with the boiler water to supply the slab. No thermostatic valve though. It was one-inch pipe to the slab and one inch pipe from the slab through the circulator in a loop. Across the loop was a piece of 3/4 inch pipe through a globe valve to the boiler return port and a piece of 3/4 pipe from the supply port of the boiler to the intake side of the circulator.  In other words, the boiler was across the heating loop. At first, I could not figure out how it worked at all. But eventually I did. If you  closed that valve, the heating loop ran full speed and got no hot water. Fully open you got maximum heat, but probably a little less than the boiler temperature. I ran it sort-of in between, but there was no thermometer to indicate the water temperature in the floor. It did get too hot to walk on barefoot, though. If I was lucky, it put 120F water into the floor. Whatever, it was enough that it was difficult to keep the asphalt tile glued down to the top of the slab.



Now that I have outdoor reset, the water temperature is a lot lower. It is set to run between 75F when it is warm out, and it will not get up to 120F until it gets to 6F outside. Design temp around here in New Jersey is 14F and I have never seen it get to 6F outside, though it gets down to 9F briefly a few times a year. At 14F it puts 112F water into the slab. So I should get less loss into the ground that the former owner got.



My guess (but I hope a pro will comment) is that if your water table is as low as mine and you are in a place like New Jersey, you should be OK. But as they say, IT DEPENDS and YMMV.

NRT_Rob

in general

you want to make an "insulation bowl" and pour the concrete in it.



If you can't do that, do not have a high water table or ledge right under the slab, then continuous insulation on the outside of the foundation/frost wall is acceptable but less ideal.



Otherwise I would consider overpours or overlayments.... you might do it, and it might be ok, but it would be significantly more efficient to do something else. You want an insulation layer somewhere between the heated space and the outside of the frost wall/foundation.

Geode

Re: Another foundation sealing thread

Thanks for the comments, I'll attach a picture of what I have going. My mason suggested I add knee walls to my 100 year old footings to increase the footing area and also allow me to drop my finished floor about 6". The strategy is a kludge, for lack of a better term. We should of, a.) removed the walls and dug a deeper footing and re built the walls, or underpinned the footings. As it is know, the only hope to keep the knee walls stable is by using the floor to lock them in place, hence my dilemma. Hindsight is alive and well here...Keep in mind that the knee walls are a lot deeper than the floor will be. When the finished floor is in place the top of it will be within an inch of the old footings. I was planning for several inches of gravel, insulation and up to 6 " of concrete. One option we are considering is to every couple feet to let the concrete floor contact the walls to "lock" them in place, maybe a 6" section every two feet. The remaining space would be filled with insulation to minimize heat transfer to the knee walls. As I've pretty much decided, it would be crazy to risk the integrity of the foundation for the radiant heat. My wife would be really disappointed without the radiant, but understands the issues (she's the one that dug out the 14 cubic yards of soil so far)....