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Does a house with spray foam roof and BIBS wall insulation need ventilation?

Tony Martinez | Posted in Mechanicals on

Normally a house that is fully spray foamed, some sort of ventilation is done. It is either done by a free air duct or an ERV. The question is for homes with bib insulation in a wall. I understand that bib insulation is not air tight.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Tony, the type of insulation really doesn't influence whether ventilation is required or not. Every wall or roof needs an air barrier to stop the air movement through it. In the case of spray foam insulation it can act as an air barrier. With less air tight insulation, some other material, such as well sealed drywall, poly or the sheathing acts as the air barrier. In all cases there should never be enough air movement through the envelope to act as ventilation to the house.

  2. GBA Editor
    Martin Holladay | | #2

    Tony,
    1. If you aren't sure whether you house has a good air barrier, you should definitely hire a contractor to perform a blower-door test. That test will help you find and seal your air leaks.

    2. Once you've sealed air leaks as well as you possibly can, you should install a mechanical ventilation system.

    For more information on these issues, see:

    Blower Door Basics

    Designing a Good Ventilation System

  3. D Dorsett | | #3

    All houses need ventilation. Relying on accidental randomly located air leaks for ventilation does not ensure that the ventilation air gets to where it is needed, in the necessary volumes or air quality needed. (eg: Drawing in air via an air leak a the transition from a concrete slab to a foundation wall can be of significant volume and extremely poor air quality.)

    All NEW houses need be sufficiently air tight to test at 3 air exhanges per hour @ 50 pascals pressure (3ACH/50) to be compliant with the IRC 2012 building code standard. This level of air tightness is not difficult to reach, but it is tight enough that for optimal air quality the ventilation air paths need to be defined.

    Air sealing the house is the the cheapest, most cost effective energy efficiency upgrade possible. In a wood-framed wood-sheathed house air sealing the walls can be done very robustly by caulking the wood sheathing to the framing at every stud & plate, and putting a bead of caulk (or can-foam) under the bottom plate of the studwall as it is assembled, and between any doubled-up top plates. (With a powered caulk gun this is goes much quicker than you might think.) Then it's a matter of sealing every penetration of that wall for plumbing/electrical/windows/doors/exhuast ducts, etc.

    A studwall detailed in this fashion is more air tight than it would be merely by insulating with spray foam, since the spray foam can only seal the sheathing to the framing in the stud cavities, not under the stud plates, etc.

    Even without that level of detailing dense-packing fiberglass to 1.8lbs per cubic foot density (or higher) or cellulose to 3.0lbs density (or higher) will usually tighten the wall sufficiently to be able to meet the 3ACH/50 requirements, provided there is sufficient air sealing detailing elsewhere, particularly at the wall transitions to the roof & foundation. At 1lbs density fiberglass (a common installation density in budget-limited applications) the air sealing of the sheathing to the framing becomes more critical. Cellulose is still fairly air retardent at any standard wall density.

    But it's still worth caulking to improve both energy efficiency and moisture resilience, since even minor air leakage can move signficant amounts of moisture to layers of the assembly that may be cold enough to condense or adsorb some of that moisture.

    See also:

    http://energy.gov/sites/prod/files/2014/12/f19/ba_innovations_2014_ASHRAE%2062_2.pdf

  4. Expert Member
    Malcolm Taylor | | #4

    Dana, Good advice but why do you suggest caulking between the top plates on walls?

  5. D Dorsett | | #5

    The exterior edges of doubled up top plates are never exactly co-planar where they meet the sheathing, which makes it likely that some air leak will occur between the sheathing and top board of the top plate, and the seam between the boards of the top plate complete the path to the interior.

    Caulking between the top plate boards provides an inspectable stop to that leak, just as the caulk between the sheathing and bottom board of the top plate is inspectable in the open stud bay.

  6. Expert Member
    Malcolm Taylor | | #6

    Ah, thanks. That makes sense.

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