Of all the things we talk about with homeowners finishing a basement — layout, ceiling height, egress, moisture — the one with actual health consequences gets the least airtime.
Radon is a naturally occurring radioactive gas produced by uranium decaying in soil and rock. You cannot see it, smell it or taste it. It enters through the lowest level of a house: foundation cracks, slab penetrations, sump pits, crawlspace floors. The EPA identifies it as the second leading cause of lung cancer, and the leading cause among people who never smoked.
Colorado's geology puts a large share of the state well above the national average. A great many Front Range homes test above the level at which the EPA recommends action.
Test before you design
Testing is the cheapest step in this entire article and it belongs before the design conversation, not after drywall.
- Short-term tests run a few days and give you a fast read. Useful for a first look and common in real-estate transactions.
- Long-term tests run ninety days or more and give a much better picture, because radon fluctuates with season, weather and how the house is being run.
- Test the lowest livable level — the space you are about to turn into a bedroom or family room.
The EPA's action threshold is 4.0 picocuries per litre, and it notes there is no known risk-free level. Plenty of homeowners choose to mitigate below 4.0, particularly when children will be sleeping down there.
What mitigation actually is
It is less dramatic than people expect. The standard approach is sub-slab depressurisation: a suction point cut into the slab, a sealed pipe run up and out through the roofline, and a continuously running inline fan that draws soil gas from beneath the foundation and discharges it above the house. Add sealing of cracks, the sump lid and slab penetrations, and a manometer so you can see at a glance that the system is working.
It is a pipe and a fan. It runs quietly, uses modest electricity, and it works.
Why timing changes the price
This is the practical part, and it is the reason we raise radon at the design stage.
- Before finishing: the slab is exposed, the walls are open, and the pipe route is easy. Mitigation is straightforward and cheap to conceal.
- After finishing: the same system now has to route through finished space. That can mean a chase built into a corner of your new family room, opened drywall, or a longer exterior run that is less attractive.
- Passive rough-in: if you test low but want insurance, roughing in the pipe during construction costs a fraction of retrofitting, and a fan can be added later if a future test justifies it.
Newer construction increasingly includes passive systems by code. Many older Denver-metro homes have nothing at all.
What radon is not
It is not a moisture problem, and mitigating radon will not fix a wet basement. It is not something you can smell if it is bad. It is not correlated with how nice or new your house is — neighbouring homes on the same street routinely test very differently, because it depends on soil, foundation condition and how air moves through the building.
That last point matters: your neighbour's test result tells you nothing about your house. Test your own.
How we handle it
When we scope a basement, radon is part of the conversation alongside egress and moisture. We will ask whether the house has been tested, and if it has not, we will tell you to test before we frame. If mitigation is needed, we coordinate it at the point in the schedule where it is cheapest and least visible — which is early.
It is not the exciting part of a basement project. It is the part that is very hard to add gracefully once the space is beautiful.
See our basement finishing guide for the full scope, and egress window requirements for the other thing that makes a basement bedroom legal.
Keep reading from the BH Builders blog
Mitigation is a pipe and a fan. It is one of the cheapest things you will do to a basement and the only one that has anything to do with whether your family gets sick.DT · Owner, BH Builders
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