I am building an 1100 ft three floor cottage. The basement is superior walls, the slab is 5.5 to 7 inch slate colored concrete tobeter absorb heat, and will be the finished surface. In the slab is 300 ft of 7/8 pex. We are now abou to order our floor trusses for the second floor and befre i do this I wanted to get some opinions on my best method for the 2nd and 3rd floors. One option would be lightweight concrete. I like this option for sound transmition and the flywheel effect of the thermal mass and lower temps. Sizing up the trusses at this point would not be that bad just need to know what thickness i would need. This would be tiled over. Staple up would be fairly easy with trusses but would require higher temps. Warmboard would be awesome but expensive. This floor area is less than 500 feet for the second floor. Open room, half vaulted ceiling, Half covered with loft. The house is going to be 6 inch sips with lots of southern windows and generous overhangs. All opinions appreciated . Thanks michael

More picts for perspective.

Engineered floor joists speced for 1.5 inch thermal crete 3/4 inch osb glued and nailed to the joists just staple your radiant tubes to the floor and pour your thermal crete on top, solid well built floor with next to nothing for sound transmission

Jimmy, could i use lightweight concrete with structural fibers, maybe 2 inch thick instead of therma crete which i think is like gypcrete. The quote i recieved from gypcrete was waaay hi for my budget. For the second floor 3 yards of concrete would cover the entire floor in 2 inch. this with fibers would be less than $400. Thanks for the reply, i really appreciate the help.

brawler i dont see why you couldnt use a lightweight concrete they use pea stone concrete in my area as well as thermal crete . I used thermal crete in my home because i have solid hardwood flooring in and i had to nail sleepers onto my floor so we had something to nail the hardwood too. Thermal cret was much easier pouring in between the sleepers No doubt concrete is cheaper then thermal crete ( gypcrete) I really dont think you would need any fibre in the concrete you are not useing it for any kind of strength but maybe some one with more concrete expierence could add there imput. Your biggest thing in my opinion is to make sure your floor joists are sized to handle the weight on top of them

Good to hear your using SIPs for your walls. What about the roof? How wide is this building? The span dosen't look very big. That means what ever you use your trusses won't need to be beefed up too much. I've done several projects with SIPs and Radiant both concrete and gypcrete. both worked well. I'm curious myself about Warmboard. I would think that the smaller heat demand that the SIPs and passive solar provide would make a warmboard system work better and in turn more cost effective. I'm thinking you would be able to drasticly down size your hot water source as well.

The outside measurement is 20+28 but the superior walls are 10" thick so nowhere would the span be more than 18'4" This house is real small and will be super tight when finished, i just wonder what would be more comfortable. Since i am i southern NC the house will have either a Hi seer heat pump or a mini split. The radiant is a bonus/redundant system but i think very comfy for what we hope to be a barefoot home. One bonus of concrete with tile is the windows on the main floor with vaulted ceiling would allow for significant solar gain in the winter. I have heard you shouldnt try to do both radiant and passive solar but cant remeber why, seems free btu's are a good thing. I have also heard that small homes with hi r values have problems with radiant so i plan on 4 zones to help prevent overheating. Around here radiant is rare hence expensive so i am trying to do as much as i can myself. With warmboard would i just run lower temps for a longer time, because it seems once the thermast qiut calling for heat a warmboard floor would cool faster causing more fuctuation in floor surface temps? I am just guessing that concrete my "coast" longer and have less fluctuation after the water stopped flowing. I would appreciate hearing from anyone with any exp with any of these types of radiant. Thanks again