[Greenbuilding] Unvented cathedral ceiling condensation

Sun Dec 11 10:50:20 CST 2016

Lstiburek rocks. Thank you for that article, Jeff.

On Sun, Dec 11, 2016 at 7:58 AM, Jeff Martin <jeff at open2learn.ca> wrote:

> Leslie,
>
> Adding an interior air barrier will almost certainly fix the obvious
> problem of condensation running out of the ceiling and down the walls, but
> it's still not great building science and, unless detailed meticulously,
> may not be enough to avoid future problems with moisture damage to the roof
> assembly. In addition to the great GBA article that you've linked to, you
> might want to check out this Joe Lstiburek article that appeared in Fine
> Homebuilding a few years ago, to get a better understanding of building
> science basics for roof ventilation:
>
> https://buildingscience.com/sites/default/files/migrate/
> pdf/PA_Crash_Course_Roof_Venting_FHB.pdf
>
> If the chosen solution is to just try to achieve a better interior air
> seal, rather than a more robust (and expensive) solution of rebuilding the
> roof assembly so that it is either properly vented (soffit and ridge vents,
> with baffles the full length of the rafter bays) or uses a layer of
> impermeable insulation to provide the air sealing and prevent moisture
> build-up on the underside of the roof deck (e.g., 2 or 3 inches of
> closed-cell spray foam on the underside of the roof deck), the wood
> paneling will need to be removed from the ceiling and a good quality
> membrane carefully installed on the underside of the rafters, prior to
> reinstalling the t&g planks. Since you would be relying on this membrane to
> do all the work of keeping moist air away from the cold roof deck, I think
> you would want to use a really good membrane, like Pro Clima INTELLO Plus.
> CertainTeed's MemBrain is another option that's likely to cheaper and
> easier to source, but we've found that it's somewhat fragile. In any case,
> the membrane should installed using quality sealants and tapes, and as per
> manufacturer recommendations. When reinstalling the wood finish, care will
> need to be taken to minimize damage to the membrane. If there are too many
> breaches in your air barrier, they may lead to decay in the roof assembly
> that won't show up for years.
>
> That said, I think it would be more prudent to go for a more robust
> solution, if budget allows. Pulling the Roxul and adding an adequate layer
> of spray foam to the underside of the roof deck would probably be the
> cheapest and least labor-intensive solution, in this context. Considering
> your climate and the depth of the rafter bays, it appears that 2" of
> closed-cell spray foam would be adequate.
>
> Jeff
>
> On 12/10/2016 9:20 PM, Leslie Moyer wrote:
>
> So it sounds like you're all pretty much in agreement that an air barrier
> will fix the problem.  It seems to me that they have NO air barrier
> now--good, bad or otherwise.  I.e. they don't have penetrations in their
> air barrier--they don't HAVE an air barrier.  The layers in the
> roof/ceiling assembly my friend describes below are the ENTIRETY of the
> construction....he listed everything in order as it is. As far as vapor
> movement goes, they are aware that the Roxul is not a vapor barrier and
> neither is the tongue and groove ceiling. I was leaning toward a "thermal
> bridging" problem & thought rigid foam insulation would fix it...either
> under the boxcar siding or under the roof sheathing.
>
> There are no can lights in the ceiling.  There is wiring in place for one
> fixture, I believe, but no other large holes.  I'm not sure, but I don't
> think they have drywall under the tongue and groove boxcar siding on the
> ceiling--he didn't mention it below and he didn't mention it in his
> conversation with me earlier today.
>
> This is a brand new addition--unfinished still--and they are not looking
> for a short-term fix. The builder just left and is willing to come back to
> fix the problem now, but they need to come to an agreement about what that
> "fix" will entail.
>
> So, for a solution, they need to air seal all holes of any size that go
> from the sidewall plates up into the ceiling; air-seal all holes that
> penetrate into the ceiling. Where, exactly, should the air barrier be
> installed?  "The warm side" doesn't tell me enough--there are several
> layers on the warm side.  They need to know if they should approach this by
> removing the metal roofing & sheathing and go in from the top, or remove
> the tongue and groove siding on the ceiling and fix things from the
> inside-out.
>
> -Leslie Moyer
>
> --- obrien at hevanet.com wrote:
>
> From: "Michael O'Brien" <obrien at hevanet.com> <obrien at hevanet.com>
> To: Green Building <greenbuilding at lists.bioenergylists.org>
> <greenbuilding at lists.bioenergylists.org>
> Subject: Re: [Greenbuilding] Unvented cathedral ceiling condensation
> Date: Sat, 10 Dec 2016 13:38:28 -0800
>
> Hi, Leslie—
>
> Just wanted to chime in to agree about the air leaks across the interior
> side of the cathedral ceiling. Not only recessed can fixtures, but often
> every wiring hole drilled through top plates, every vent stack and flue
> have not been sealed to block air leaks. The walls may be contributing,
> too, if there are penetrations in the top plates.
>
> The long-term fix is to take down the ceiling drywall so the leaks can be
> sealed and a proper vapor rertarder installed, but in the short term they
> could run a dehumidifier to reduce the water vapor in their occupied space.
>
> Sometimes builders will open up the blocks along the eaves and install
> some sort of vent at or near the ridge, but this may have the effect of
> sending cold air under the insulation and cooling down the interior surface
> of the ceiling, it takes some detailing to keep the vent air above the
> insulation.
>
> Best wishes,
>
> Mike O’Brien
>
>
>
> On Dec 10, 2016, at 11:50 AM, Leslie Moyer <unschooler at lrec.org> wrote:
>
> I have some nearby friends having a problem. I think I understand what the
> problem is, and even some possible ways to solve it, but I'm not certain I
> could give them advice that would fix their problem the best or cheapest
> way. I thought you guys could, though!
>
> They read this article & I think they will go ahead and pay to read the
> article referenced within it:
>
> http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-
> insulated-cathedral-ceiling
>
> We are considered a "hot humid" climate--NE Oklahoma
> …………………………….
>
> We have a question regarding condensation problems in a cathedral
> ceiling.  We live in northeastern Oklahoma (zone 3) and just added a
> dinning room (cathedral ceiling) 15 x 19 addition.  The addition was just
> opened up to the main house earlier this week, and we got hit with (what
> are for us) very cold temperatures.  Thursday night had a low of 12 F.  By
> noon on Friday we noticed that water was dripping down the north side
> interior wall  (along the drywall).  The drip lines appeared to be spaced
> every 24 inches, or about where a roof rafter would be.
>
> The ceiling/roof construction was constructed with 2 x 8 rafters and
> insulated with R30 Roxul (rock wool) insulation and is not vented.  The
> interior ceiling is wooden tongue and groove car siding.   The roof decking
> is LP TechShield Radiant Barrier (with the metal foil side facing the
> interior of the house, as described on the boards) with a metal roof (there
> is felt paper in between the decking and the metal roof on the North side,
> but on the South side we used double bubble).   We did not have any
> condensation issues on the south wall.
>
> We have spoken with over half a dozen different experts, and we’re getting
> as many different suggested solutions.  We are desperate to fix this
> problem and would greatly appreciate any help!  Thank you!
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