Many older homes, including plenty of additions and porches here in New Jersey, sit on a crawl space with foundation vents, ducts or a furnace underneath, and a dirt or thin-plastic floor. The traditional idea was that vents let fresh air push moisture out. In humid weather the opposite happens: outdoor air is often wetter than the crawl space air, and that moisture ends up in the wood framing, the insulation and the ducts (DOE Building America).
What the research found
The best-known data comes from a DOE-sponsored field study of 12 all-electric homes with the same floor plan on one street in eastern North Carolina. Four had traditional vented crawl spaces, and eight had closed crawl spaces with sealed vents, a sealed ground liner and different drying strategies (Advanced Energy / DOE field study). Over three years, the closed crawl spaces stayed much drier. Homes over them used 15% to 18% less energy for heating and cooling, and crawl space humidity dropped by more than 20% (DOE Building America). The vented crawl spaces stayed above 70% relative humidity through the warm months (DOE Building America).
Other Building America research found similar humidity reductions in Louisiana and Ohio. In the dry West and the marine Pacific Northwest, vented and closed crawl spaces both performed adequately as long as there were no ducts in the crawl space (DOE Building America). Those results helped bring unvented crawl spaces into the International Residential Code in the 2009 and 2012 editions, Section R408.3 (DOE Building America).
What the code requires for an unvented crawl space
Under IRC R408.3, exposed earth must be covered with a continuous Class I vapor retarder. Joints must overlap 6 inches and be sealed, and edges must run at least 6 inches up the wall and be sealed there (GreenBuildingAdvisor). The space also needs a way to dry, such as:
- a small conditioned air supply of 1 cfm per 50 sq ft of floor area, with a return path back to the house; or
- continuous exhaust ventilation at the same rate; or
- a dehumidifier sized to the manufacturer's specifications (GreenBuildingAdvisor).
ENERGY STAR's retrofit guide adds that the supply should be at least 1 cfm per 50 sq ft or 50 cfm total, whichever is greater, with humidity held at 30% to 50% (ENERGY STAR crawlspace guide). Check which edition of the code your town has adopted.
Safety first: never seal the vents around a furnace or water heater that pulls its combustion air from the crawl space. ENERGY STAR calls for identifying how every gas or oil appliance gets its combustion air before closing anything. Natural-draft equipment needs an upgrade, replacement or another combustion-air source, plus depressurization testing, a CO alarm and a sealed-combustion or heat pump option where practical (ENERGY STAR crawlspace guide). Sealing vents around a natural-draft appliance can cause backdrafting and carbon monoxide problems.
Checklist before closing a crawl space
| Check | Why |
|---|---|
| Standing water, grading, gutters, downspouts | Fix bulk water first. Send roof runoff at least 10 ft from the house and slope grade away from the foundation (ENERGY STAR crawlspace guide). |
| Combustion appliances and venting | Natural-draft equipment needs a combustion-air plan before vents are sealed (ENERGY STAR crawlspace guide). |
| Radon | EPA recommends testing every home. Closing a crawl space can change soil-gas flow, so test before and after (EPA; ENERGY STAR crawlspace guide). |
| Ducts in the crawl space | Seal joints, especially returns; leaky returns can contribute to backdrafting (ENERGY STAR crawlspace guide). |
| Wood moisture and pests | Framing moisture should stay below 17%. Leave a 3-inch inspection strip at the top of the wall where termites are a concern (ENERGY STAR crawlspace guide). |
| Drying strategy and condensate | Choose a conditioned supply or a dehumidifier, and pipe its condensate to a proper drain (ENERGY STAR crawlspace guide). |
Where this matters for your HVAC
If your ducts or air handler sit in the crawl space, closing it puts that equipment inside a drier, more moderate space, so duct leaks and heat losses waste less energy. It also helps protect against frozen pipes and condensation on cold ducts in summer (DOE Building America). Once the crawl space is closed, the existing system may be larger than the home needs. Ask for a fresh load calculation before replacing equipment.
Limitations
The 15% to 18% figure comes from new, matched homes in a hot-humid climate. Savings in a New Jersey retrofit depend on the house, the climate, where the ducts are and how well the work is done. Code requirements vary by jurisdiction and edition.
This article names no products and endorses no manufacturer. HVAC Zone Inc is a multi-brand dealer, and the Hub is brand-neutral.
Have a damp crawl space, ducts under the house, or a furnace down there? Request a consultation and we'll check the combustion safety, ducts and moisture before you decide to close it, across all major brands.