Filling a fireplace opening is usually done with solid masonry or block, but the vapor control has to match your climate, so the interior side needs an engineer or building science consultant to confirm it, since a sealed poly layer on the wrong side can trap moisture in the wall. For the floor, find the source before replacing anything: a high reading under a floor next to an exterior wall often comes from a leak or condensation there, so check the siding and flashing at that wall, the attic ventilation above it and the outside grade. Take readings with a pin moisture meter at several spots over a few weeks and note whether they rise after rain or in cold weather. Residential building science consultants and home inspectors who handle older homes are often easier to find than commercial envelope firms. Which way does that exterior wall face, and does the darkness show up after rain or only in winter?

Your project is a big step. People pay extra for a home with a fireplace.

I don't know the condition of yours, but if the FP is too much trouble there are gas or electric inserts that can be installed minimize maintenance.

helpmenow, it’s an exterior wall facing NNE. There is no story or attic to speak of above it, just scissor trusses going up to a 15’ vault. The dark spot in the floor is permanent. Immediately outside is a concrete patio and steps and a corner gable, so during heavy rain, a lot of water comes over the gutter. The roof was just replaced and we had them upsize from 4” to 6” gutters to help with that. The fascia underneath them was pretty badly rotted, so we had that replaced as well. That was several months ago and the high moisture reading was well after and during about 70F day temps.

Beverly, it doesn’t fit in our modern aesthetic and it’s positioned right where the only good spot for a TV is. It’s already gas log and we never use it. In the winter you can feel freezing air rolling across the floor and in the summer you can either feel heat coming out of it or cold air from the house being sucked up and out. And yes, the flue is closed.

Water that overshoots the gutter and runs across a sloped concrete patio can sit against the base of that wall for hours, and a corner gable with steps can channel even more toward the same spot, so the wetting may be happening at grade rather than inside the chimney. Check whether the patio pitches toward the wall, where the siding meets the patio or soil, and whether any flashing or siding sits below the finished surface. A reading taken on a dry 70 degree day will miss that pattern, so a reading taken the morning after a heavy storm is more telling. After the next heavy rain, does the reading climb at that spot?

The patio is generally pitched correctly away from the house and generally abuts the house against the top of the basement’s cinder block wall below the finish grade. There is a small gap between the patio and the cinder block.

My question about the chimney is unrelated as far as I know, other than trying to make sure it’s done right, especially where moisture is concerned.

Water should not be coming over the gutter, it should be going down a downspout. At the bottom of the downspout there should be an downspout extension that carries the water well away from your foundation, or else the downspout should be connected to a buried PVC pipe that does the same thing.

Good catch on that gap. A slab that meets block below grade is a common route for water, so a flexible exterior sealant over a backer rod at that joint is the usual fix. Also make sure the patio slopes away from the wall, and extending the downspout or adding a splash block at the overshooting gutter will move water farther from that joint.