Radon-Resistant New Construction in NH: A Builder’s Guide
New Hampshire is in the highest radon-risk band in the United States — most of the state is mapped as EPA Zone 1 — and roughly one in three NH homes tests above the EPA action level for radon. For builders, the practical implication is clear: any home you put up in NH has materially better odds than national average of testing high after move-in unless you build it to handle radon from day one.
Radon-Resistant New Construction (RRNC) is the term of art for that approach. The components are straightforward, the ANSI/AARST consensus standard (CCAH-2020) is well-defined, and the cost during new construction is a small fraction of retrofit cost after the fact. This guide walks NH builders through what RRNC actually requires, what it costs, and how to get it right the first time.
| The cost case for RRNC
Adding RRNC during new construction in NH typically costs $350–$700 in materials and labor. Retrofitting an active mitigation system after the home is occupied typically costs $1,000–$2,500 — and requires drilling slabs, routing pipe through finished spaces, and explaining to a homeowner why the system was not built in. RRNC is a faster, cheaper, better-looking outcome on every front. |
What RRNC actually means
Radon-Resistant New Construction is a passive system installed during the build that reduces soil-gas entry into the home and pre-stages the components needed to add an active fan if post-occupancy testing shows it is needed. The current consensus standard is ANSI/AARST CCAH-2020 (Reducing Radon in New Construction of One- and Two-Family Dwellings and Townhouses), and many code authorities reference EPA's RRNC guidance and the IRC's Appendix F radon control provisions.
In its passive form alone, a properly installed RRNC system reduces indoor radon by a meaningful margin — typically 50% or more compared with an unmitigated equivalent home — because soil gas is given a low-resistance path to the outside instead of the path of least resistance into the basement. If post-occupancy testing shows residual radon above the action level, the system is upgraded to active by adding an inline fan and a manometer. That upgrade is a same-day job and typically runs $500–$800 because all of the heavy plumbing was done during the build.
The five RRNC components
1. Gas-permeable layer
Four inches of clean, washed gravel (#57 stone is standard) under the slab. The gas-permeable layer creates a continuous void where soil gas can collect and be drawn off through the vent pipe. Substituting fines-laden fill defeats the system before the slab is poured.
2. Vapor barrier
A six-mil polyethylene sheet (or heavier) installed over the gravel and lapped a minimum of 12 inches at seams, sealed with compatible tape or sealant, and sealed at the perimeter. The barrier limits soil-gas entry through the slab and isolates the gas-permeable layer for the vent pipe to draw on. Punctures or unsealed laps reduce performance.
3. Sealed slab and openings
All slab cracks, joints, plumbing penetrations, sump cover edges, and other entry points sealed with appropriate flexible caulk. The sub-slab void is intended to be the path radon takes; any open path through the slab competes with that and reduces system efficacy.
4. Vent pipe
A 3- or 4-inch Schedule 40 PVC pipe extending from a tee under the slab (in the gas-permeable layer) up through the building envelope, terminating above the roofline a minimum of 12 inches above the eave and 10 feet from openings. Routing matters: vertical runs are preferred over offsets, and the route should be planned so a future fan installation lands at an accessible attic location.
5. Junction box for future fan
An electrical junction box located near the attic vent pipe run, wired to a switched circuit, ready for an inline fan if post-occupancy testing shows the passive system needs to be activated. Including the junction box and wiring during the build is what makes a $500–$800 fan upgrade trivial later — versus retrofit electrical work that often costs more than the fan itself.
| The two RRNC mistakes we retrofit most often
(1) The vent pipe is routed inside an unconditioned space (e.g., a garage or unheated chase), which causes condensation and reduces draw. Keep the vent pipe in conditioned space whenever possible. (2) The attic junction box is missing, so adding a fan later means a new electrical run. Spec the junction box every time. |
RRNC for crawl spaces and slab-on-grade
Most NH new construction includes a basement, but the same RRNC principles apply to slab-on-grade and crawl-space construction with minor adaptations. Slab-on-grade follows the same five-component approach with attention to perimeter sealing. Crawl spaces use a sub-membrane depressurization design — a sealed polyethylene membrane across the crawl space floor and lapped up the walls, with the vent pipe drawing from beneath. The membrane installation has its own quality bar that matters for both radon and moisture.
If you build commercial or multifamily structures, the relevant standards are different — ANSI/AARST CC-1000 and the multifamily standards, respectively — and they include additional design and verification requirements. ARM is AARST multifamily-certified for that scope.
Code adoption in New Hampshire
New Hampshire adopts the IRC and IBC at the state level, with local jurisdictions sometimes adding amendments. IRC Appendix F (Radon Control Methods) is the radon-related portion of the IRC. Whether Appendix F is enforced varies by jurisdiction in NH; many builders adopt RRNC as a best practice and standard spec regardless of whether the local code official is enforcing it.
The pragmatic position for most NH builders: spec RRNC on every build. The cost is modest, the marketing value is real, and the liability profile is much better than building without it and having a buyer test high six months after move-in.
Builder workflow that actually works
- At foundation rough-in, confirm the gas-permeable layer is clean stone (not fines or recycled fill), order at proper depth, and verify before the vapor barrier goes down.
- Stage the vent-pipe tee and stub above the slab line before the pour. Mark its location on the framing plan so the framer can chase it cleanly.
- Have the framer route the vent pipe vertically in conditioned space wherever possible. Avoid running it through cold attics over long horizontal distances.
- Confirm the attic junction box is installed and wired to a switched circuit during electrical rough-in.
- Seal slab penetrations and cracks at the appropriate stage.
- After move-in, the homeowner should test radon within the first 90 days. If results exceed 4.0 pCi/L, schedule fan activation. The original plumbing makes this easy.
For buyers of new-construction homes
If you are buying a NH new-construction home and the listing or build spec mentions a "passive radon system" or "radon-resistant construction," verify a few things:
- That the vent pipe terminates above the roof, not under the eave.
- That an attic junction box is present and labeled for radon-fan use.
- That the builder will provide a 90-day post-occupancy radon test, or will let you commission one.
- That the builder has a clear process for fan activation if the test comes back above 4.0 pCi/L.
A passive RRNC system is meaningful protection but is not a guarantee. The 90-day post-occupancy test is what closes the loop.
Frequently asked questions
Is RRNC required by code in NH?
Statewide, RRNC is not currently a separate mandate beyond what the adopted IRC includes. Some NH jurisdictions enforce IRC Appendix F; others do not. Best practice is to spec RRNC regardless of code status.
How much does RRNC add to a build?
Typical materials and labor for RRNC during new construction in NH run $350–$700, depending on home size and configuration. That is a fraction of the retrofit cost on the same home post-occupancy.
Does RRNC eliminate the need for radon testing?
No. RRNC reduces the probability of elevated radon and pre-stages the components for a quick upgrade if needed. Every home should still be tested within the first 90 days of occupancy, regardless of whether RRNC was installed.
Can RRNC handle radon in well water?
No. RRNC addresses soil-gas entry. If the home is on a private well, water radon is a separate question and a separate (water-side) treatment if elevated.
What about multifamily and commercial?
Different standards apply (ANSI/AARST CC-1000 for commercial; multifamily standards for apartment and townhouse construction). The components are similar but verification testing and design requirements are more involved. ARM is AARST multifamily-certified for this scope.
If a passive system does not bring radon below 4 pCi/L, what then?
Activate the system by adding an inline fan and manometer. Because the vent pipe and electrical junction are already in place, this is typically a same-day job in the $500–$800 range. Post-fan testing confirms the upgrade has brought levels down.
Build NH homes radon-resistant — and let ARM handle the activation if needed
ARM has worked with NH builders, GCs, and architects for 25 years on RRNC, post-occupancy fan activation, and full retrofits. We are AARST/NRPP certified and AARST multifamily-certified for commercial and multifamily projects. Whether you need a spec sheet for an upcoming build, a same-day fan activation on a pre-piped home, or a full retrofit on an older property in your portfolio, we work on builder schedules.
Call 603-644-1207, or use radonh2o.com/contact for builder partnership inquiries.