Interior Flagstone

Hello. My wife and I have recently purchased a house in the Southwestern United States. This house has interior flagstone floors in some of the rooms. We are having a little trouble with crumbling grout between the stones. Does anyone have any experience with the maintenance and care of interior flagstone floors?
The floors also have radiant heating. I notice the flagstone floors do not seem to conduct the heat as well as some of the other floors (tile, concrete, brick). I suspect there are air gaps between the stones and the heating coils. I have discovered air gaps under locations where the grout has collapsed. Can anyone recommend a good grout?
I would appreciate any guidance. These floors are all quite new to us.
Roy
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I'm certainly no authority when it comes to interior flagstone floors, but I've been passingly familiar with outdoor flagstone floors/patios, which tend to be more harshly treated by the environment than their indoor counterparts. Again, this is your basic knee-jerk stuff, so ...
First, when you say "flagstone," exactly what kind of stone are you talking about? Limestone is different from granite is different from, well, you get the idea. This might help others give you some reasonable advice.
Second -- and this is only if I recall right -- it occurs to me that it isn't *grout* that's filling in the spaces between the stones. Grout is used on bathroom tile. You've got something else there, and I'm afraid only either a mason or a good stone contractor familiar with indoor stone floors/patios can ID what was used in yours and can offer a good, long-lasting solution. Unless of course your house was owned by some dope who was cheap and/or stupid enough to use bath tile grout. If that was the case, you'll probably find yourself chieseling out all the bath grout and resetting with a significantly better material.
AJS
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Is the flagstone itself easily stained?
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"Roy" wrote:

Southwestern
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