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I am in the progress of having a home built and we are presently at drywall stage so humidity is fairly high in the house. Our 6’ concrete crawl space will be heated. We have two construction heaters and a fan that was placed in the crawl space to dry it up however I’m wondering if it’s created a problem with condensation. We have spray foam insulation on the walls with fibreglass bats over top. I pulled a piece of fibreglass off to look at the spray foam and noticed condensation on the foam. Will this dry as the basement drys out or should I get a dehumidifier to assist. Is the heat causing this problem?

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    Stock "construction heater" dumps a bunch of moisture into the air as a byproduct of combustion, since they don't have a vent and just dump the exhaust into the space being heated.
    – Ecnerwal
    Commented Mar 10 at 15:46
  • 1.0# of propane becomes CO2 and about 1.3# of water through combustion. You haven't been pumping water into this area, have you?
    – popham
    Commented Mar 11 at 18:18
  • These construction heaters are 220v electric not combustible type
    – Dave
    Commented Mar 12 at 19:34

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Exhausting the moist air to the outside with a fan should be your first instinct. Only if it's very cold outside and the space is heated will a dehumidifier work better. Even in that case, you should also use a fan or fans to accelerate things by blowing moisture saturated air away from wet surfaces. I suppose that high relative humidity outside could change the calculus, but I'm skeptical.

You can't get the heat from combustion, though. Propane is C3H8. By combining with oxygen in the air (complete combustion implies C3H8 + 5O2 ➔ 3CO2 + 4H2O), it becomes water and carbon dioxide. Assuming 80% combustion efficiency and tracking hydrogen through the combustion process, 1.0# of propane transmogrifies into 1.3# of water:

(0.80)(1.0#)(1000g/2.2#)[(1mol C3H8)/(36g+8g)][(8mol H)/(1mol C3H8)][(1mol H2O)/(2mol H)][(2g+16g)/(1mol H2O)](2.2#/1000g) = 1.3#.

That's not going to dry anything.

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