[Greenbuilding] Unvented cathedral ceiling condensation

Sun Dec 11 09:58:04 CST 2016

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>
> To: Green Building <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 
> <mailto: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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