Am I reading your post correctly? You have a 7" thick concrete slab, more or less open to the outside, without ANY insulation? If so, I bet that floor is cold!

I like your idea for adding radiant. Your plan seems sound to me. If I were paying the bills, I might consider increasing the insulation to 4" of rigid. That slab will take a lot of energy to heat up, but once it does, it will radiate that heat for a long time. The extra insulation will make sure that heat only goes in the direction you want (up).

Good luck,
Ed

Yes the floor is very cold. Thanks for the reply. I can increase it to 4"without a problem. Any other thoughts?

 

James

None come to mind.  The radiant will ensure the slab is warm, but the insulation alone would make a world of difference.
Ed

Start by just sealing up and insulating the crawl space to a minimum of R10 (R15-20 would be even better- double up the layers if you have a whole barnload), turning it into semi conditioned space. See:  http://www.buildingscience.com/documents/information-sheets/5-thermal-control/crawlspace-insulation/   (Pay attention to sealing the ground vapor retarder to the wall insulation.) It'll take a lot less insulation & expense than insulating & heating the slab. That alone is likely to

A: Cut the heat loss enough that the baseboards can keep up

B: Raise the temp of the floor by 10-25F.

Heating with a 7" thick radiant slab has a huge time lag, and you may run into issues keeping the room temp stable- it may both lag, then overheat the space.  If you go that route use a PID control approach, and use the baseboards as the secondary stage of a 2-stage thermostat. The response time of the slab will be like turning a container ship around- can't do it too nimbly without some help.

Rather than heating the whole slab from below, a WarmBoard(tm) type system placed on top of the slab with at least R3 of insulation would be much more responsive and easier to control. And as long as the crawlspace is insulated at the exterior walls the heat loss to the slab through the fairly skimpy insulation won't be a disasterous heat loss (the way it is currently.)

Thanks for your reply. Are you saying that I should insulate the walls of the crawlspace or just the under floor above? The floor above has nice slate tile so I'd prefer not to put anything over it.

I'm saying insulate (to R10 minimum, R20 better) & air-seal the walls of the crawlspace, and at the same time vapor-seal the floor of the crawlspace (be it dirt or whatever) with polyethylene sheeting for a ground-vapor retarder.  This converts the crawlspace munchkin-height room into conditioned space, and the temperature of the slab will stay much closer to room-temperature of the room above, just as the floors of the rest of the house do. It's highly likely that the existing baseboard will then be able to keep up with the heat load.

Only if that proves inadequate would putting radiant under the slab be worth considering.  Managing the substantial thermal mass of a 7" slab as the primary radiator, and the fact that it would still likely need to use the baseboards as a second stage to keep up with peak heat loads adds complexity & expense to the project.  Also, heating the slab wouldn't be as efficient, since it's edges aren't insulated, and elevating the temp from 60-65F (in the insulated crawlspace, no radiant scenario) to 75F or more increased the heat lost to the outdoors.

2" of foam under the radiant would not be sufficient insulation over an unconditioned crawlspace- you'd need at LEAST 4".  Nor would just running the tubing with surface contact with the concrete be an adequate conductive path.  Getting the heat out of the tubing requires more surface area contact, which is why extruded plates with a firm wrap-around contact on more than 50% of the tube's surface works best.  This starts to get expensive too.

But once you've already insulated the walls of the crawlspace converting it into conditioned space, then 2" of foam under the radiant would be adequate, and a simpler approach could be taken if the slab still feels too cold. Installing the tubing suspended tube style and running it at the same temp as & in parallel with the baseboards would would raise the slab temp a bit for comfort, and could, all be controlled by a single standard room thermostat. That way the slab stays a bit warmer, but isn't the primary radiator, and the room temp won't have over/undershoot issues due to the thermal mass of the slab.

"Installing the tubing suspended tube style and running it at the same temp as & in parallel with the baseboards would would raise the slab temp a bit for comfort, and could, all be controlled by a single standard room thermostat."

Thanks again and I really do appreciate your comments and help. As I undertsand it, I should install 4" (R20) of rigid foam board inside the crawlspace along the exterior walls and add a 6 mil. vapor barrier on the ground. I assume that I do not need to insualte under the slab. Finally, if the room is still too cold I can then add the radiant tubing under the slab and run it with the current zone thermostat. Please let me know if I miss understood anything.

Thanks,

James

Methinks you've got it!

I'm fairly confident that with the crawlspace air-tight & insulated the baseboard will be able to keep up, but the slab will be cool (but no longer ice-cold) to the touch. If you like to hang out in your socks, a bit of suspended tube radiant below will raise it to room temp, maybe slightly above.

Thanks a million.