Sloped ceiling insulation

Discussion in 'Remodel Forum & Blog' started by Kubismo, Oct 12, 2010.

  1. Kubismo

    Kubismo New Member

    Messages:
    6
    Location:
    Vancouver, BC
    I'm remodelling my kitchen, and i'm having to redo the ceiling in the dining portion of the room. The original ceiling is a sloped structure following the roof line, made of lath and plaster with no insulation. It is about 10 feet wide and 7 feet long, so not a large space. The ceiling studs are only 2x4s. There isn't any room to lower the studs to create more space, as it would start to impede the window.

    Spray foam insulation would be the best, but they installers here in Vancouver (for the US folks, think Seattle but 200km north) want a minimum charge of $800! That's pretty steep for a tiny roof and blows my budget pretty good.

    I've been reading a bit in these forums, and it seems like XPS foam board may be the best option. I like the idea of a 2" board and a 1.5" board, with all gaps filled with canned foam spay, like for windows, and of course some drywall sheeting. My idea is to not have any air gaps, to avoid the condensation issue altogether.

    Is this the best option? Am i leaning the right way? Any opinions? Thanks!
    Last edited: Oct 12, 2010
  2. dlarrivee

    dlarrivee New Member

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    It will cost you at LEAST $800 to bring this up to code.

    It's a cathedral ceiling, R10 isn't going to cut it.

    When you ask "Is this the best option", you already know the answer to that.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Roofing nails almost certainly penetrate the roof sheathing, so sliding in a panel is likely to be a big pain. A 2x4 doesn't have much strength, hopefully, there is a lot of slope, or things would get really dicey after a big (maybe rare) snowstorm.
  4. Dana

    Dana In the trades

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    First read: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems A little bit of closed cell SPF goes a long way, but $800CDN is a hunk of change.

    Rather than a couple of layers of XPS adding up to R17, get some 3" fiber-faced iso (R19-R20), and leave yourself a ~6mm gap either side for ease of foam-sealing, and ~12mm of vent space between the iso and the roof deck. Fiberfaced goods will still allow the rafter sides to dry (very slowly) through the spray foam & iso into the vent gap, where foil-faced goods won't. Fiber faced iso is usually sold through commercial construction distributors, since it's ususally used on large low-pitch or flat roofs under membrane. A 4x8' sheet of 3" iso runs ~$55USD in my neighborhood, and you'd be looking at 3 sheets, maybe 4 depending on how it slices & dices into your cavity spaces. If you're feeling brave and want to max out the R, 3-1/4" and 3-1/2" goods are also available from some manufacturers, but the extra R2-R3 seems pretty worthless compared to the higher risk of roof-rot.

    [edited to add] The effect of leaving 10-12mm of air between the iso & roof deck is similar to the 10mm rainscreen between siding & sheathing now required by code across Canada. While a roof isn't exactly a tilted wall it's drying needs are somewhat greater than a wall. Siding rarely gets completely saturated with rain the way roofs can, and NEVER get full snow cover for even a day, let alone a week or two, as might happen in Vancouver a few times per decade. When the roof isn't rain/snow/dew wetted the roof deck can and will dry toward the exterior, but when it is, drying will only to the under-deck ventilation gap. Venting that gap to the exterior under the eaves is advisable, even if there aren't a corresponding vents higher on the roof to promote convection-drying. (But consider installing some of those too, if they're not already in place, one per rafter bay.)

    Don't sweat it if roofing nails puncture the facer on the iso when putting it in there- it won't make a heluva lot of difference in performance (thermal or moisture-wise). If the nails would be penetrating the insulation very far it might be tough to get the insulation in without pushing the nails out though, and trimming them would be a better option.

    You'll probably be required by code to use poly as the vapor barrier, but if you can get away with a more modestly vapor-retardent sheeting such as 6mm fan-fold XPS under the gypsm, that would be preferable in Vancouver's relatively mild climate, and would add another R1+ to the stackup.)

    When it's time to re-roof the exterior of that section, adding more iso above the roof deck and sealing up any vents into the vent gap would buy you more R. (Not sure what code-min R is for roofs in Vancouver these days, but figure on ~R1 for every 4mm of thickness for iso. If you need to go pretty thick to bring the R up to code there are several vendors that make it pre-laminated to OSB nailbase. (Hunter, Atlas, JM, etc.)
    Last edited: Oct 13, 2010
  5. Kubismo

    Kubismo New Member

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    6
    Location:
    Vancouver, BC
    Thanks for teh notes gents... question still is, if my first option wasn't the best, what is?

    The 2x4s have been there since 1933, so I'm guessing they are strong enough for the little snow we get in Van. Any advice on how to best insulate this space then? How about adding an inch of wood shim to the ceiling joists (probably have enough room above the window for that), and then adding a layer of Duravent (sp?) to create an inch of space, then adding 2x4 fibreglass under that, then poly and drywall?

    The only problem I have, is that the roof technically isn't vented at the "ridgeline". There really isn't a ridgeline, as the roof meets a wall of the house. I could have venting at the soffits, but how do I add venting to the top pf teh roof? Could I drill a small hole (1 inch?) through the sides of the house, covered with a mesh of some sort, and then drill the same hole though each joist so that air can travel through to all the channels? Woudl this weaken the joists too much? Is this last step necessary or can I just leave the venting to the soffits? Thanks in advance for any advice on this
  6. Kubismo

    Kubismo New Member

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    Location:
    Vancouver, BC
    Dana I just saw your note - looks like we both posted at teh same time. I'm not familiar with iso, but I'll look into it. Thanks
  7. Dana

    Dana In the trades

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    "iso" is short for "polyisocyanurate". See:

    http://www.atlasroofing.com/general2.php?section_url=2

    If you're looking for domestic sources in Canada see:

    http://www.citt.gc.ca/Dumping/inquirie/findings/nq2j005_e.asp#P199_14126

    Any number of the roofing or sheathing products will work- just avoid those with foil or poly facers for this app.

    And yes a few small holes drilled in the soffit-end of each bay under the eave just below the roof deck with some mesh covering will do a world of improvement for the drying capacity of the roof, even without vents higher up.
    Last edited: Oct 13, 2010
  8. Kubismo

    Kubismo New Member

    Messages:
    6
    Location:
    Vancouver, BC
    Thanks Dana - that's a lot of help - much appreciated. The beer is in the mail!
    Last edited: Oct 13, 2010
  9. Kubismo

    Kubismo New Member

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    Location:
    Vancouver, BC
    ha! no more than 5 minutes after my last post. Ecologic Spray Foam called me (I left them a message a couple of days ago). Looks like they can do my ceiling for about $400... so that's gets into the realm of possibility. It'll probably only cost $100-$150 more than the iso you mentioned, but woudl give me better piece of mind. Because it is a small job they want me to wait until they are in my area, but I'm going to go that way even if i have to wait a couple of weeks. Thanks again for you help though. If it all falls through, i'll lean on your advice for the iso stuff.

    For anyone else in the Vanocuver area - these guys seem to provide competitive quotes!
  10. dlarrivee

    dlarrivee New Member

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    What product does he plan to spray?

    There are 1/2lb. and 2lb. density spray foams available, you only want 2lb. in this application, and it must have a CCMC number.
  11. Dana

    Dana In the trades

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    What dlarrivee said- it MUST be closed-cell (1.5lb density MINIMUM). Also note, the 2lb product from Icynene would only yield ~ R15-R16, (and is priced attractively) whereas generic 2lb polyurethanes & isocyanates from other vendors would yield at least R19.

    It occurs to me that this could be a DIY using a 200 board foot kit (TigerFoam, FomoFoam, etc) for about $350-400 as well, but if the pros are willing to do 2lb goods for the same money, their temperature controlled equipment and experience at applying will be lower risk than a newbie using a kit.

    If it's only half-pound or 1lb goods DON'T DO IT! You'd end up with ~ R13-R15, but would have higher wintertime moisture accumulation potential the roof deck.

    Ideally, with 2lb goods installed local code would accept that as it's own vapor barrier and not require poly. The vapor permeability of 3" of closed cell SPF is less than that of kraft facers on batts, but still higher than polyethylene sheeting. It's more than enough to minimize wintertime uptake of moisture from conditioned space air in your climate, and would still provide some amount of drying capacity toward the interior 8 months out of the year. Canadian national building codes requiring poly vapor barriers make a lot more sense in the frigid midwest, but are counterproductive in the temperate climes of western BC.
  12. dlarrivee

    dlarrivee New Member

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    I looked up the outfit "ecologic spray foam" and found a website with no info on the product they use, just a typical website full of stock photos and plagiarized information promoting spray foam.

    They do you have video on youtube, and I'm not sure if it's them spraying or a stolen video but I would avoid hiring them, based on the install practices shown in the video.

    What you have to understand about foam is that it creates a lot of exothermic heat when being installed. It is not recommended to install "lifts" less than 1/2" but it's also not recommended to install them greater than 1" or 2" either. The guy in the video is filling an entire stud cavity in one shot, and not allowing any cooling of the foam prior to second lifts being applied and he's also throwing in 3.5" of material all at once in some cases. Applying foam in this way does not yield 2 pound density foam, and it also affects the dimensional stability of the foam. Worst case scenario, you're not getting the product you paid for.
  13. Dana

    Dana In the trades

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    All good points, and just some of the reasons you USUALLY want to hire a pro as opposed to hacking away with DIY kit. If the "pro" in the vidi is doing 2lb foam that way, he needs some retraining.

    But with half pound foam you can do full 2x4 stud cavities in one shot- hopefully that's what they're doing in the video.
  14. dlarrivee

    dlarrivee New Member

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    Unfortunately it is not, I can tell just by looking at it.

    2lb. foam is done rising before your eyes, 1/2lb you can walk away while it rises, and a fairly light pass in a stud bay will overflow past the face of the studs.
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