Your building

The foundation walls are hollow block

“The basement walls are concrete block rather than poured concrete.”

Hollow masonry units form a connected network of voids inside the wall. Where those voids communicate with the soil, the wall itself can carry gas upward and release it through joints, cracks, or the top course.

That is a genuinely different situation from a poured wall, and it is one of the cases where drawing only from beneath the slab may not be sufficient.

Why this matters

What it changes about the decision in front of you.

The void network can bypass the slab entirely, so a slab-only approach may leave a path open.
The top course is frequently open in older construction, which puts the void in direct contact with the space above.

What it could influence

Where the effect is likely to show, and in which direction where that is knowable.

A design may need to address the wall cavity as well as the sub-slab area, which changes the scope.
Parging, sealing the top course, or drawing from the void are all possibilities depending on what is found.

What this does not settle

What still stands. Noticing a problem is not the same as losing the evidence.

Block walls do not mean the house reads high or that a system will be complicated.
Visible cracks in a block wall are not by themselves a radon finding — the void network matters more than the surface.
It does not establish that wall depressurization is needed here; that follows from diagnostics.

What a professional is likely to ask

Worth having answers ready. None of these is a test you can fail.

Whether the walls are block, poured, or stone, and whether they have been parged or sealed.
Whether the top course is open or capped.
Whether there is a separate reading from a slab-only approach already installed.

What you can check yourself

Things you can establish today, without anyone visiting.

Whether the wall is visibly block, which is usually apparent where unfinished.
Whether the top of the wall is accessible from above.
Whether any prior sealing work is visible.

What genuinely needs somebody on site

Where this platform stops. These are not questions a website can answer about your house.

Whether the void network communicates with the soil, which requires investigation.
Whether the voids are continuous, since block walls are frequently partly filled.

Categories a next step could fall into

Unordered, and none of them is a recommendation. Which applies depends on things this page does not know about your situation.

Request a professional assessment

When it is considered. When the building itself is part of the question — foundation type, an existing system, access, or an unusual layout.

What it would establish. What the building is doing, which no amount of information typed into a website can establish.

Discuss reduction options

When it is considered. When a homeowner considers their result sufficiently actionable and wants to understand what altering the building would involve.

What it would establish. What a design for this specific building would look like, and what it would and would not commit anyone to.

Sources

U.S. EPA — Consumer’s Guide to Radon Reduction

When you are ready to talk to somebody

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