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Community and Q&A

Venting between kneewall attic and peak attic

Nick H | Posted in General Questions on

Hi everyone –

First time here. I have a 1956 home that I am gradually upgrading various parts of.

I need advice/help with knee wall to peak attic ventilation.

I am in the process of deciding whether to seal/insulate the kneewall attics for storage/year-round temperature regulation. (Previous owners had a weird mixture of batts and poly, which I aim to rectify with polyiso, etc.)

The upstairs bedrooms are built inside the roof, with a peak attic above the bedroom ceiling, and knee wall attic spaces behind the bedroom walls. The peak attic is insulated above the bedroom ceiling, and has gable end vents at each end.

I read lots of guides and blogs, understanding that roof deck/sheathing needs to be ventilated from soffits at bottom edge of roof, preferable to ridge or gable end vents.

I bought a couple of pieces of polystyrene ventilation chute to try out between my rafters to help plan the job.

Problem is that when I went to feed the polystyrene venting chute between the rafters (over the top edge of the knee wall) to create an air pathway into the peak attic, I found that the gaps between rafters as they pass over the top edge of knee walls are all blocked with framing…due to the way this roof was built in 1956….(note: the channels between rafters are clear between roof-edge soffits and the blocked gaps at the top of the knee walls).

Has anyone encountered this before? I am very hesitant to mess around with the blocked paths between the rafters for fear of compromising structure.

If I keep the blocked pathways between rafters above knee walls:
a) how should I vent the roof deck & attic space if I decide to keep knee wall attic an ‘outside space’?
b) how should I vent the roof deck if I decide to seal and insulate knee wall attic spaces?
(from what I’ve read, I will not need to ventilate the attic spaces themselves, once sealed and made part off the house’s ‘inside’ space.)

Thanks in advance foe any advice you can give.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Kneewalled attics are a PITA to properly air seal & insulate, with myriad thermal-bypass channels to seal up and usually only very limited roof deck venting.

    The best solution is to add sufficient rigid-insulation ABOVE the roof deck, to go UNvented, seal the soffit vents and insulate between the rafters instead, bringing the mini-attics fully inside conditioned space. The amount of exterior-R necessary to get there varies with climate:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

    Fiber faced roofing polyiso runs about R6/inch of thickness and about 10 cents per square foot per R. (So R10 would run you about $1 per square foot, R20 would be ~$2/foot, etc.) It's not cheap, but the thermal benefits and resilience of the approach is worth it, and it makes air-sealing the place much much easier.

  2. Nick H | | #2

    Hey Dana - thanks for taking the time to comment. Much appreciated.
    Just to be clear, you're saying:
    - remove shingles
    - add layer of rigid foam on top of roof deck (mine are ship-lapped)
    - relay shingles (or equiv) on top of rigid foam
    - insulate between rafters
    - seal soffits a roof edge.

    Sounds like quite a job...but it's really good to know what the options are.
    Also - does this solution work best with shingles or would standing seam metal roof work too?

    If I wasn't in a position to do what you suggest immediately, what would your temporary solution be, as we approach fall weather?

    Thanks.

  3. Expert Member
    Dana Dorsett | | #3

    A flash-shot of closed cell foam on the underside of the roof deck limits the rate of moisture getting into the roof decking, and makes it safe to fill the rest of the cavity with a complete fiber fill. That would require demo-ing and replacing the gypsum on the sloped ceilings. If you don't want to replace the ceilngs just yet and could tolerate some bowing should you accidentally overfill, there are low-rise closed cell foam pour product available from outfits like Tiger Foam, etc. (If you're snaking the hose into a 1" gap between fiberglass and roof deck the blowout risk is pretty low.) It's not cheap- about $1.25 per square foot per inch of depth, but it goes in pretty quickly. A 600 board foot kit should be able to handle a 250-300' of ceiling area.

    On the sloped ceilings behind the knee walls you can hit it with a standard spray-on kit foam, and it pays to be skimpy there- it doesn't take much practice to get a 1-1.5" thickness out of it, and it's not a disaster if you have thin spots that are only 3/4". Read the instructions, and keep the tanks in a tub of water at the right temp to avoid the issues with the chemicals running cold, yielding sub-par foam.

    For more on using the flash-foam approach and it's relative moisture risk, see:

    http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

    Note, those simulations are with OSB roof decking which is far more susceptible than plank decking, or even plywood decking, which take on moisture more slowly, and have less rot potential than OSB even if they did take on moisture. (In 1956 you could have either plank or plywood, but probably not OSB.)

    Standing seam mounted on 2x purlins over rigid foam works great. The purlins can be set up on 24" centers and through-screwed to the rafters with pancake head timber screws, and the mounting hardware/fasteners for the roofing applied to the purlins rather than long-screwed through the foam.

    If you have a source of reclaimed roofing foam (insulationdepot.com will ship if you need a lot of it), it peels 60-80% off the cost of virgin-stock. I have multiple vendors of reclaimed roofing iso in my area (some of whom advertise only on craigslist.) There will be some scrap, and you may have to seal some holes with can-foam but most the time it arrives in pretty good shape when it was from stacked layers on a flat-roofed shopping center or warehouse or something. The real beat up & broken stuff with corners missing and deep gouges in the facers is usually priced accordingly, but sometimes worth it if you're cutting it all up to handle the angles of valleys etc.

  4. GBA Editor
    Martin Holladay | | #4

    Nick,
    Dana is right. Fixing this issue isn't cheap or easy. The best way to go is using lots of rigid foam above the roof sheathing, followed by another layer of roof sheathing and new roofing.

    Here are some articles that you may be interested in reading:

    Two ways to insulate attic kneewalls.

    How to Build an Insulated Cathedral Ceiling.

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