pavilion radiant heat

Probably not enough to heat to 60° based on a load calculation where the side walls are canvas only. I helped a Wallie last year to get the proper radiator size to heat a glass room that was attached to her home. The room was all glass. It seemed that the plumber that installed the boiler figured that the room just needed a 12 ft section of baseboard radiator to heat a 13 x 25 ft glass room. It turned out that the radiator for the "Solarium" actually caused the ModCon boiler to fail because the return water was too hot. This was discovered after a factory tech support visited the house and found that the 3/4" loop was so short that the water was returning to the boiler return inlet too quickly and offering very little heat to the room.  Not enough temperature drop when only that zone operated.  Which was all the time.

I did the load calculation based on Hydronics Institute Form 1540WH.  I set up the walls as 9’ ’1” x 25’1” then used a window that was 9’ x 25’  the same thing for the two sidewalls that were 9’ x 13’ , and the ceiling was 13 x 25 glass. This made the load calculation of over 35,000 BTU, and that needed more than a 12’ section of baseboard.

You need to do the same thing.  Do a load calculation based on the construction of the roof and insert walls that have no insulation, and no thickness.  No siding, no sheetrock, no nothin.  And the infiltration will be above the charts.  Like maybe 10 or more air changes per hour.

With that load calculation you will find that the water temperature in the slab may need to make the floor 140°F to get the room to stay at 60° room temperature.  I believe that might be too hot to walk on.  

I have actually had experience with this. I built an awning to cover the HotTub on our back paver patio. We had vinyl/canvas side curtains made that rolled up in the summer and were deployed in the winter.  When we decided to use the HotTub on very cold days, that HotTub surface area of about 38 sq.ft. at 110°F warmed up the room by about 15° above the outdoor temperature.   Not very warm for a 17° day.  We also added an electric space heater on the other side of the enclosure (you don't want an electric space heater sitting on the edge of your hot tub) And that made the room go above freezing, but no way warm enough to sit outside and read a book.  

I believe that gas fired infrared heaters hanging from the ceiling may be your better option. 

I have some thoughts. first if the tubing is not too expensive, it won't hurt much to have it in there, though it will freeze if you aren't careful. I personally would not sign off on heating a space with canvas walls outside in the winter via a radiant floor. A draft house in a cold environment might require an 85 degree slab temp (thats on the really high side) to make the space warm. Imagine the slab temp required to heat an outdoor space in the winter, the slab will not like this, your feet probably won't like this, and you will need to keep it running pretty much all the time because to raise the temp in the coldest weather would take a long time. Instead, if you already have the wood heat available, and put the tubing in the slab, just run it like a snow melt. It won't make the space feel warm, but a slab that is 50 degrees or so over the outdoor temp might help somewhat, and in the shoulder seasons it might actually work for your needs. I wouldn't do this if I was paying for the fuel personally though!

If the goal is to feel warm while you are in the space, there will be nothing easier or more efficient than a free standing, or overhead style radiant heater. many electric options are available, and I believe gas models as well. If you ever go to a restaurant that has outdoor seating in the cold fall or winter weather you have seen these. You can turn them on and feel warm almost immediately after, then turn them off when you are not in the space. It would be my first choice if the goal is to just not feel cold outside.

put a large enough wood stove and heat with just that, the radiation of a 300 degree wood stove will transfer more than an 85 degree floor

The idea is to spread the heat source around. As I mentioned, I can have a big fire pit but it will heat one side that's facing the for but not the opposite site. I'm wondering if I could spread the heat around a little by using heating pipe under the"floor".

Thanks for the feedback!

If you don't block the draught at floor level up to chair level, nothing is going to work.

I've already been a nay-sayer on this quite a bit and my opinion hasn't really changed. But if you are determined let me make a few comments. The tubing transfers heat through surface area contact, so if you were to use a metal underlay that touched the entire deck, and groove it so the tubing had more surface contact with the metal it may transfer heat better, the reflective properties are never going to work beyond day 1 as the metal will become tarnished or dirty enough it will not reflect heat like you expect. The tubing will expand and contract, so you need to account for that. The water will need to drain somewhere, if you just leave an end open to drain, heat will move to cold and you will lose all your heat out the open end, remember heat does NOT rise (thats hot air), it moves to cold, you need to fully insulate around any direction you do not want heat directed.

I'd be thinking more like the heated seats in luxury automobiles. Heat the points where people are making contact, the floor, seatback and seat bottom. Don't try to warm the environment. I'd also try to insulate and wind-proof them.

I think with your back and feet kept reasonably warm and your front facing the fire you'd be comfortable. I have no idea how to engineer that though.