Absolutely will. Each gallon of propane burned will generate 6.8 pounds of water vapor. If combustion is not vented to the outside, this large amount of water vapor will raise the humidity inside. How much the humidity goes up is dependent on a number of factors, basically how leaky the structure is to air.
If no leakage of air occurs to the outside, in a 10,000 cubic foot structure (1000 square feet x 10' ceiling), burning 5 pounds of propane will generate ~30 grains of water vapor per pound of air.
That 30 grains per pound will raise the relative humidity at 70F from 50% to 77%, high enough to grow mold nicely.
Burning 10 pounds of propane would raise RH to 99%.
Oh, boy, will it ever. The house I live in now is a cabin on a lake, and originally was built without any central heating, only an UNVENTED wall propane heater upstairs.
When I bought the place I learned that the insulation in the ceiling (home-built cathedral ceiling, so it was about four inches from the roof) had become completely sodden and was ruined. Had to peel the roof off, replace the spacers according to code, and the insulation, and put the roof back on. Next thing I did was central heat.
It's nice and toasty now, temps outside about 18F. How they survived the winter here I'll never know.
Accck. Do NOT give any credence to the calculations in #61. They suffer from the dreaded misplace decimal syndrome.
Burning 5 pounds of propane will increase the grains of moisture in the air by 10x the amount in the calculations, which means it’s going to be raining inside before you finish the burn.
Burning one pound of unvented propane would bring RH to very nearly 100%.
Sorry bout that.