Consider insulating the ceiling/roof of your underground space. Even solid sandstone can let heat in during summer or cold in winter, so a layer of insulation will make it much more comfortable without expensive HVAC.
You’re smart to look into it. While you wait on pros, try opening basement windows when possible and seal any visible cracks. It won’t fix everything, but it can help a little short term.
If your radon levels are only slightly elevated and you're handy, DIY may be feasible. But for levels above 4.0 pCi/L, or if you want peace of mind, resale value, or system certification, hiring a professional is often the better long-term choice.
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