DIY House build Ep.44 - Unvented attics pros & cons - How to build sealed eaves - Building science, by B e t t e r w A Z e B u i l d i n g on YouTube. 17 min.
- Video length
- 17 min
- Time to do it
- 1 day
- Difficulty
- Takes practice
- Steps
- 7
What you need
Steps
Decide between a vented and an unvented attic before the roof goes on. A vented attic relies on soffit and ridge vents to move air under the roof deck.
Compare the trade-offs. Vented attics are cheap and forgiving but pull humid air, dust and pests into the roof space. An unvented attic brings ducts and equipment inside the sealed envelope.
Know what an unvented attic demands. The roof deck becomes the insulation and air barrier, so you need spray foam or rigid insulation plus a continuous seal.
Choose one of the three ways to seal the eave, then keep that seal continuous with the wall air barrier and the ceiling plane. Gaps at the eave are the most common leak.
Seal around the eave itself. Pack large gaps with rigid blocking or backer, then foam or caulk every joint between the top plate, wall sheathing and roof deck.
Fit raised heel or energy trusses. The extra heel height lets full-depth insulation reach the outside wall and helps prevent ice dams at the eave.
Check the seal before drywall goes up. Walk the attic with a light, feel for drafts, and confirm the result with a blower door test while the framing is still open.
Tips and common mistakes
In a vented attic, keep insulation off the soffit vents with baffles so air can still enter.
Spray foam expands quickly, so apply thin passes and let each one cure before adding more.
Wear gloves, goggles and a respirator when spraying foam, or bring in a contractor for the whole roof deck.
An unvented attic only works when the roof deck is insulated, never the ceiling plane.
Questions
Can I turn an existing vented attic into an unvented one?
Often yes, but only if the roof deck is sealed and insulated to your local code and the assembly is designed for it. Check the code and the insulation maker's spec before you close up the vents.
Do unvented attics still need soffit vents?
No. An unvented attic has no soffit or ridge vents, because the sealed roof deck replaces that airflow. Sealing the eave properly is what keeps moisture out instead.
What insulation goes in an unvented attic?
Usually closed-cell spray foam applied straight to the underside of the deck, or rigid foam above the deck combined with an air-impermeable layer below. Follow the code for your climate zone.
This guide was drafted from the video's details and general task knowledge. It has not been checked against the full video. Check the video and relevant product instructions before following a procedure. Report a correction.
