We are in  zone 3.  Lows in winter of down to -40.  Dry climate.  Winter humidity in the 20-30% at best.  We heat predominantly with wood, using a wood range in the kitchen and a iron box heater in the other half of the house.

The second half of our house has a loft dining room with lots of windows 18 feet at peak.  Two bedrooms on the level above.  Ceiling fan in dining room.  Kitchen shares bottom floor with dining room, but has balcony and 2 bedrooms above.  Bathroom behind kitchen.

This whole floor has baseboard heaters, which are a pain:  You can’t put book cases against the wall.  They are dirt magnets.  They aren’t in the right places to keep the dining room warm without cooking the kitchen.  And the floors are always cold.

This side of the house is a crawl space, with aobut 3 feet of room.  It is insulated with 2 inches of styrofoam insulation, and partially burried outside, about halfway.

In the 20 years of living here, there has been no issues of condensation in this area.

The crawl space has it’s own zone, and a few bare finned radiator tubes.  These can keep the this side above freezing to protect the plumbing, but produce a few warm spots.

The boiler generally circulates water at 140 F, much hotter than normally used for in floor heating.

The house is tight enough that the upstairs zone has never come on.

The main floor one rarely comes on, typically running only 10 hours a year.  

What I would like to do:

A: Remove all the baseboard radiators on the main floor.
B:  Reinstall these in crawlspace  under the windows where there is a major cold floor problem from cold air cascading off the glass.
C:  Run a network of PEX  with one running the length of each floor joist space, fastening these near the top of the joist, NOT to the subfloor.

This means I will have two zones in the crawlspace.  Given the way the kitchen is often warm from the stove, while the dining room is cool, I don’t see this as an issue. 

Questions:

1.  Is this a reasonable approach?  This would put 2 linear feet of PEX tubing per square foot of floor.

2.  I think that there is essentially zero air exchange between this space and the world.   This would mean that  warm air in the cavity between joists would tend to stay there.  How do I test this?  Answer:  Put a set of remote reading thermometers in the space near one of the finned radiators, and put one at the top of the joist space, one level with the radiator but a foot away, and one at the floor.

3.  my current thought is to use the exisiting 3/4″ water lines to this space, then put two manifolds in to scatter/gather the the individual loops.    It works out to about 1000 linear feet of pipe, if I go with the doubled lines between each joist space.  (Doubled lines means few holes in joists.  Note that manifoldes could be just reducing T connectors, and I could do the entire new system in 3/4 inch and 1/2″ Pex.

However the numbers I’m picking up suggest 1 foot pex/sq foot, and that’s in slab.  They are running water that is 15-20F hotter than the intended  temp of the building.  I’m running water 70 F hotter than the intended temp.  But pex to air will not be as efficient as pex to concrete.

So:  Only 1 tube per joist bay?

4.  What have I missed?