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Attic Insulation

user-7490458 | Posted in Energy Efficiency and Durability on

Considering best attic insulation for a 24×27 wing, which houses full bath, laundry/utility and 2nd kitchen.  Zone 6A New Hampshire with cold metal roof.   The ceiling is just 3/4 inch pine tongue and groove.  Is a vapor barrier a good idea?  And fiberglass or rock wool?

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  1. user-5946022 | | #1

    I'm inclined to avoid fiberglass, so I would prefer cellulose insulation.

    However you have correctly identified the main issue. If the only barrier between the heated 24x27 wing and it's unconditioned attic is 3/4" pine tongue and groove, you need a vapor and air barrier before you consider any type of insulation. Is the interior space tongue and groove ceiling horizontal or sloped?

    1. user-7490458 | | #5


  2. walta100 | | #2

    Most insulation will not stop air from flowing through the insulation and escaping the building taking the heat with it.

    I think you need an air barrier between the T&G ceiling but that is hard to do without removing the ceiling.


  3. Expert Member
    BILL WICHERS | | #3

    Assuming by “cold” you mean that your roof is vented, then you don’t really need a vapor barrier. You DO need an air barrier behind that tongue and groove ceiling though. A vapor barrier can often double as an air barrier, so you could potentially install one and detail it as an air barrier. The regular way to do this would be to put drywall up, mud it, and then install the t and g ceiling planks over the drywall. The drywall acts as your air barrier in this case. I’ve used 1/2” polyiso behind t and g ceiling planks before to act as an air barrier while also gaining a bit of R value.


  4. Expert Member
    Dana Dorsett | | #4

    >"The ceiling is just 3/4 inch pine tongue and groove. Is a vapor barrier a good idea?"

    A vapor barrier is a good idea in a zone 6 vented attic, but not essential if adequately vented.

    An AIR BARRIER between the insulation & living space is ABSOLUTELY ESSENTIAL for controlling both heat & moisture in any cold climate, and a good idea in ANY house.

    Even in warm or temperate climates it would be highly unlikely that the t & g ceiling would be sufficiently air tight to meet current IRC code maximums for whole house air leakage, even if it's only ~650 square feet of ceiling.

    >"And fiberglass or rock wool?"

    With an air barrier between the t & g and insulation, high density fiberglass can work, and is roughly about as air-retardent as rock wool to convection (but not wind or stack effect) pressures to meet it's labeled performance. But cellulose will usually be even better- worth considering despite the lesser R/inch.

    If this is a cathedralized ceiling rather than flat ceiling below an attic you may have no choice but to pull the t & g to insulate properly. If that's the case it would be important to know the depth of the rafter bay cavities, the material of the structural roof deck (OSB? ship lap plank?) the underlayments of the metal roof, and what if any rafter bay venting currently exists.

  5. user-7490458 | | #6

    Would wedging 1/2 inch drywall into the rafter bays and sealing with caulk or tape be a sufficient air break? These are attic trusses so there is a small storage area (maybe 8x10) storage room attaching to the second floor of the main house.

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