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Community and Q&A

Ventilate kneewall attic space?

Jeff__W | Posted in Energy Efficiency and Durability on

I finally took care of my second floor kneewalls which had 6 mil. poly on the back as well as open floor joists (2×10) and open ceiling joists (2×6) which communicated with the attic space above the second floor ceiling. All these spaces were insulated with fiberglass.

I used 3/4″ foil-faced polyiso on the kneewalls, and in joist and rafter cavities, all taped, caulked and foamed. I will also be plugging the rafter cavities from the upper attic.

My previously well-ventilated assembly has been plugged! Do I need to concern myself with ventilating the two, now separate, attic spaces? The lower one has full soffit venting. The upper one has two gable vents. I would eventually like to add more blown insulation to the flat ceiling areas.

Should I go with roof vents to exhaust the lower section and maybe ridge vents for the small upper section? I’m in CZ4, bordering on 5 in NJ. Thanks in advance. I learn so much from this site!

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  1. stuccofirst | | #1

    You won't need roof insulation in the kneewall space if the floor is done %100. Your thermal barrier aligns with the floor up to the kneewalls and over the second floor ceiling. Make sure it is well sealed above the first floor wall, and the kneewall floor return. In terms of venting the living space ceiling, I think you would have to remove the sheetrock, and design a vent chase. If you have a scuttle space up there, just hire a very small person to air seal and insulate, leaving a space below the roof.

  2. GBA Editor
    Martin Holladay | | #2

    Installing a tight, continuous air barrier is just as important as insulating, especially if you are insulating with fiberglass.

    Ideally, these triangular attics behind kneewalls should be included in the home's conditioned space. However, you chose not to do it that way. Your way is possible, but meticulous attention to air sealing is necessary to make your approach work. An especially tricky area is the necessary blocking between the floor joists directly under the kneewall.

    If your insulation is fiberglass, you should have included ventilation chutes directly under the roof sheathing, connecting the soffit vents to the ridge vents.

  3. stuccofirst | | #3

    their probably isn't much space between the top of the second floor wall and the roof rafter where you are "plugging" . Maybe run a proper vent through that section and insulate up to it. The gable vents should suffice for the scuttle attic. Insulate and weatherstrip the hatch well. Air seal all penetrations into the attic.

  4. Jeff__W | | #4

    Martin, You said: If your insulation is fiberglass, you should have included ventilation chutes directly under the roof sheathing, connecting the soffit vents to the ridge vents.
    My thinking as I approached this "fix" was to keep the hot/cold air from mingling with my fiberglass, thereby degrading it. My second floor rafters form essentially a catherdral ceiling which, along with the kneewalls and second floor flat ceiling, would radiate tremendous amounts of heat in the summertime (sapping that heat in winter). My approach was to totally isolate that air from these cavities. I'm assuming that the vent chutes you mention would also have to be totally isolated from the cathedralized ceiling space (i.e. continuous and sealed). My scope of work only allowed me to deal with the accessible areas and did not include removing drywall from finished walls and ceilings. Are you saying that my 6' long unvented cathedral ceiling, insulated with fiberglass (existing) could be a problem? From my reading here and elsewhere I'm quite sure we're not worried about roof sheathing, shingles,etc.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Are you saying that my 6' long unvented cathedral ceiling, insulated with fiberglass (existing) could be a problem?"

    A. Yes, and thousands of roof failures around the country bear testament to the riskiness of that method of insulation.

    I have no idea whether your roof installation is particularly risky. It's possible that your roof will avoid problems -- you might get away with it. But you might not. As you apparently understand, limiting air movement from your home into your rafter bays is important. However, air leaks are devious, and it is rarely possible to make a ceiling truly airtight.

    Moreover, the roof assembly you describe is a code violation. If you want to build an unvented insulated sloped roof, you should use spray polyurethane foam, not fiberglass batss.

  6. Jeff__W | | #6

    Thanks for the quick response. Not to beat a dead horse, but for my own edification; when I come across a similar situation which is very typical in knee-wall construction( i.e., open attic side, fiberglass insulation, open floor joists and roof rafters), it's OK to air seal the knee-wall and floor joist areas, but unless I have access to the rafters, (which is almost always much more intrusive) with their existing fiberglass insulation, I need to leave them open to the ambient air.

  7. GBA Editor
    Martin Holladay | | #7

    Whenever I encounter an insulated sloped roof with fiberglass batts between the rafters, alarm bells go off. This is a risky type of construction.

    Ideally, the rafter bays are vented. If you can gain access to the triangular attic behind the kneewall, it's often possible to inspect the rafter bays to see if the builder included ventilation baffles. If these are absent, they can sometimes be added by sliding them in place above the batts, especially if the sloped section of roof is relatively short.

    If ventilation baffles are in place, then it's possible and desirable to install blocking between the rafters (of course, leaving the appropriate ventilation slot at the top), and to seal the blocking in place with caulk or canned spray foam.

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