The Attic and Crawlspace, Inside or Outside the House

Why this matters

A roofer cuts a ridge vent into a foamed roof deck because the shingle warranty wants ventilation. A pest tech props open the foundation vents of an encapsulated crawlspace "to dry it out." An HVAC tech finds a supply register blowing into a crawlspace and caps it as a waste of air. Each of them did the right thing for a vented space and the wrong thing for an unvented one, and each one broke a building assembly that was designed and, in most places, permitted as a package. The first question before cutting, venting or sealing above the top floor or below the bottom one is simple: is this space inside the house or outside it?

Two ways to build the same space

Outside the boundary (vented). The traditional attic and crawlspace are deliberately outside the house. The air barrier and insulation sit at the attic floor or the floor over the crawlspace. The space itself is connected to outdoors through soffit, ridge, gable or foundation vents, and runs near outdoor temperature and humidity. Anything in it, ducts, a furnace, water lines, is outdoors for practical purposes.

Inside the boundary (unvented). The air barrier and insulation move to the roof deck, or to the crawlspace walls and ground. The space is sealed from outdoors, has no vents, and runs near indoor conditions. Ducts and equipment in it are inside the house. An unvented attic is usually built with spray foam on the underside of the roof deck, or rigid insulation above the deck. An unvented crawlspace, often called an encapsulated or closed crawlspace, has a sealed ground cover, insulated walls, and a deliberate way of keeping the air dry.

Both are legitimate. What is not legitimate is a hybrid nobody designed: a foamed roof deck with soffit vents still open, or a sealed crawlspace with its vents propped open, which takes on the moisture of outdoors with none of the drying that ventilation was meant to provide.

   VENTED: space outside the boundary

    vent /\ ridge vent          outdoor air moves
        /  \                    through the space
   soffit>  <soffit
   ====insulation + air barrier at attic floor====
          house (conditioned)
   ====insulation + air barrier at floor =========
   vent>  crawlspace, vented to outdoors   <vent

   UNVENTED: space inside the boundary

        /##\  foam or rigid insulation
       /#  #\ at the roof deck = boundary
   no vents   attic is inside, near indoor
          house (conditioned)
   ##wall insulation##  sealed ground cover
   ## crawlspace inside, kept dry by a   ##
   ## supply, exhaust or dehumidifier    ##

What the code calls them

Unvented assemblies are an exception to the default of ventilating attics and crawlspaces, and the exceptions come with conditions. These are model-code sections, binding only as adopted by your jurisdiction, and both have changed between editions.

Unvented attics: IRC R806.5, in the 2021 edition. It permits an unvented attic or unvented enclosed rafter assembly when a package of conditions is met. The main ones are that the attic sits entirely inside the building thermal envelope, that interior vapor retarders are limited as the section describes, and that the insulation is one of the permitted arrangements: air-impermeable insulation directly against the underside of the roof deck, or rigid insulation above the deck at no less than the minimum R-value the section's table sets for the climate zone, alone or combined with insulation below. Colder zones add a vapor retarder requirement for the foam, and the 2021 edition added a vapor diffusion port option for some warm zones under its own conditions. The conditions are a package, not a menu.

Unvented crawlspaces: IRC R408.3, in the 2021 edition. It permits omitting foundation vents when the exposed earth is covered with a continuous Class I vapor retarder with joints overlapped 6 inches and sealed or taped and the edges carried at least 6 inches up the stem wall and sealed, and when the space is handled by one of the listed methods: continuously operated exhaust at 1 cfm for each 50 square feet of crawlspace floor with an air path to the house; a conditioned air supply at the same rate with a return path; use as a supply plenum where the mechanical chapter allows it; or, added in the 2021 edition, dehumidification sized at 70 pints a day for each 1,000 square feet of floor. The exhaust and supply options also require the perimeter walls to be insulated per the energy chapter. Earlier editions omit the dehumidifier option, and some states amend the section.

Read the adopted text before changing anything in one of these spaces. If you cannot tell which assembly you are in, the permit record or the builder's or contractor's paperwork often says.

Worked example: the crawlspace with a capped register

A 1,400 square foot ranch on an encapsulated crawlspace in a humid summer climate. The original contractor met the unvented-crawlspace rule with the conditioned-supply method.

What the rule asks for. 1,400 divided by 50 is 28 cfm of conditioned supply, delivered continuously while the system runs, plus a return path back to the house, plus insulated walls, plus the sealed ground cover. The contractor put one small supply register in the crawlspace and a transfer grille in the floor near the hall.

What happened. An HVAC tech replacing the air handler found the register "blowing into the dirt" and capped it to put that air back into the house. A season later the homeowner calls about a musty smell and cupping in the hardwood over one corner.

Why. With the supply capped, the crawlspace is sealed from outdoors and has no deliberate drying. It still picks up moisture through seams and tears in the ground cover, the stem walls and any gap at the rim, and from the house through the floor. On a humid day the crawl air climbs toward the outdoor dew point. If that is around 70 F and the subfloor over a corner where the supply duct runs sits near 65 F, the subfloor and the duct jacket condense. See Dew Point and Condensation, the Reasoning Behind the Fault.

The other wrong fix. A pest tech on the same house, smelling mold, props open the foundation vents. That makes it worse, not better: humid outdoor air at a 70 F dew point now flows directly across surfaces the house keeps cool, and the conditioned space above is still pulling some of that air up through the floor.

The right fix. Restore a compliant drying method. Uncap and verify the supply at about 28 cfm, or switch methods: exhaust at the same 28 cfm with an air path, or a dehumidifier. At 70 pints per 1,000 square feet, 1,400 square feet calls for 1.4 times 70, or 98 pints a day of rated capacity, under the 2021 method where it is adopted. Then close the vents and have whoever owns mold remediation look at the corner.

What changes it. In a dry climate, or a crawlspace with no ducts and a warm floor, the same mistakes may produce nothing visible for years. In a hot-humid climate they show up in one season.

Before you cut, vent or seal

Roofing. On an unvented attic, do not add ridge, box or soffit vents. If the shingle manufacturer's warranty language is written for vented attics, read its unvented-assembly provisions or ask the manufacturer in writing, rather than cutting through the boundary. On a vented attic, the reverse: do not block soffit intake with insulation or new soffit material.

HVAC. Ducts in an unvented attic or crawlspace are inside the house, so duct leakage there is less of an energy loss, though it still unbalances rooms. In a vented attic the same leakage is outdoors. Do not remove or cap a crawlspace supply, exhaust or dehumidifier without knowing it may be the code method for that space. See related: Duct Leakage Outside the Envelope.

Plumbing and electrical. A hole through the foam at an unvented roof deck, or through a sealed crawlspace liner or wall insulation, is a hole in the boundary. Close it with a compatible material and, for a liner, tape or seal the patch with the overlap the liner system calls for.

Pest control. Bait stations, inspection ports and treatment holes through a crawlspace liner have to be resealed. Do not prop open vents in a closed crawlspace.

Anyone venting a fan. A bath fan or dryer duct ending in an attic or crawlspace is wrong in both assemblies: in a vented space it dumps moisture outside the boundary where it can condense, and in an unvented one it dumps moisture inside the house. Exhaust goes outdoors. See the attic diagnosis in Attic Moisture + Ventilation Diagnosis Reference.

How to tell which one you are in

• Look at the underside of the roof deck. Foam or rigid board covering it, with no soffit or ridge vents: unvented. Bare deck, insulation on the attic floor: vented.
• Look at the crawlspace. Sealed ground cover running up the walls, insulated walls, no open vents, and a register, exhaust fan or dehumidifier: unvented. Vents, a loose ground sheet, insulation between the floor joists: vented.
• Feel it. On a hot afternoon an unvented attic is close to the house temperature; a vented one is far hotter.
• Ask. If what you see is a mix, flag it to the customer as a hybrid that needs a decision, and route it to whoever designs or encapsulates those spaces. See Crawl Space Encapsulation Service.

References

• International Residential Code, R806.5 (unvented attic assemblies) and R408.3 (unvented crawl spaces), 2021 edition, as adopted by your jurisdiction
• See related: Crawl Space Encapsulation Service; Attic Moisture + Ventilation Diagnosis Reference
• See related: Dew Point and Condensation, the Reasoning Behind the Fault; Moisture Control in Residential Construction
• See related: Where the Air Barrier Actually Is in a House